Java Cryptography Architecture

# JavaTM Cryptography Architecture

# API Specification & Reference

### Last Modified: 4 August 2002

Introduction
  Design Principles
  Architecture
  Concepts
Core Classes and Interfaces
  The `Provider` Class
    How Provider Implementations are Requested and Supplied
    Installing Providers
  The `Security` Class
  The `MessageDigest` Class
  The `Signature` Class
  Algorithm Parameters Classes
  Algorithm Parameter Specification Interfaces and Classes
    The `AlgorithmParameterSpec` Interface
    The `DSAParameterSpec` Class
  The `AlgorithmParameters` Class
  The `AlgorithmParameterGenerator` Class
  `Key` Interfaces
  `Key` Specification Interfaces and Classes
    The `KeySpec` Interface
      The `DSAPrivateKeySpec` Class
      The `DSAPublicKeySpec` Class
      The `RSAPrivateKeySpec` Class
      The `RSAPrivateCrtKeySpec` Class
      The `RSAMultiPrimePrivateCrtKeySpec` Class
      The `RSAPublicKeySpec` Class
      The `EncodedKeySpec` Class
        The `PKCS8EncodedKeySpec` Class
        The `X509EncodedKeySpec` Class
  The `KeyFactory` Class
  The `CertificateFactory` Class
  The `KeyPair` Class
  The `KeyPairGenerator` Class
  Key Management
    Keystore Location
    Keystore Implementation
    The `KeyStore` Class
  The `SecureRandom` Class

Code Examples
  Computing a `MessageDigest` Object
  Generating a Pair of Keys
  Generating and Verifying a Signature Using Generated Keys
  Generating/Verifying Signatures Using `Key` Specifications and `KeyFactory`
  Determining If Two Keys Are Equal
  Reading Base64-Encoded Certificates
  Parsing a Certificate Reply

Appendix A: Standard Names
Appendix B: Algorithms

# Introduction

> The Security API is a core API of the Java programming language, built around the `java.security` package (and its subpackages). This API is designed to allow developers to incorporate both low-level and high-level security functionality into their programs.

> The first release of Security API in JDK 1.1 introduced the "Java Cryptography Architecture" (JCA), a framework for accessing and developing cryptographic functionality for the Java platform. In JDK 1.1, the JCA included APIs for digital signatures and message digests.

> In subsequent releases, the Java 2 SDK significantly extended the Java Cryptography Architecture, as described in this document. It also upgraded the certificate management infrastructure to support X.509 v3 certificates, and introduced a new Java Security Architecture for fine-grain, highly configurable, flexible, and extensible access control.

> The Java Cryptography Architecture encompasses the parts of the Java 2 SDK Security API related to cryptography, as well as a set of conventions and specifications provided in this document. It includes a "provider" architecture that allows for multiple and interoperable cryptography implementations.

> The Java Cryptography Extension (JCE) extends the JCA API to include APIs for encryption, key exchange, and Message Authentication Code (MAC). Together, the JCE and the cryptography aspects of the SDK provide a complete, platform-independent cryptography API. JCE was previously an optional package (extension) to the Java 2 SDK, Standard Edition, versions 1.2.x and 1.3.x. JCE has now been integrated into the Java 2 SDK, v 1.4.

> This document is both a high-level description and a specification of the Java Cryptography Architecture API and its default provider, as shipped in the Java 2 SDK. A separate document describing the JCE API is provided with the JCE release. See the "Java Security Architecture Specification" for information about the Java Security Architecture aspects of the Security API.

> > Note: The most recent version of this JCA specification can be found online at: http://java.sun.com/j2se/1.4/docs/guide/security/CryptoSpec.html.

## Design Principles

> The Java Cryptography Architecture (JCA) was designed around these principles:

> - implementation independence and interoperability

> - algorithm independence and extensibility

> Implementation independence and algorithm independence are complementary; you can use cryptographic services, such as digital signatures and message digests, without worrying about the implementation details or even the algorithms that form the basis for these concepts. When complete algorithm-independence is not possible, the JCA provides standardized, algorithm-specific APIs. When implementation-independence is not desirable, the JCA lets developers indicate a specific implementation.

> Algorithm independence is achieved by defining types of cryptographic "engines" (services), and defining classes that provide the functionality of these cryptographic engines. These classes are called engine classes, and examples are the `MessageDigest`, `Signature`, `KeyFactory`, and `KeyPairGenerator` classes.

> Implementation independence is achieved using a "provider"-based architecture. The term Cryptographic Service Provider (used interchangeably with "provider" in this document) refers to a package or set of packages that implement one or more cryptographic services, such as digital signature algorithms, message digest algorithms, and key conversion services. A program may simply request a particular type of object (such as a `Signature` object) implementing a particular service (such as the DSA signature algorithm) and get an implementation from one of the installed providers. If desired, a program may instead request an implementation from a specific provider. Providers may be updated transparently to the application, for example when faster or more secure versions are available.

> Implementation interoperability means that various implementations can work with each other, use each other's keys, or verify each other's signatures. This would mean, for example, that for the same algorithms, a key generated by one provider would be usable by another, and a signature generated by one provider would be verifiable by another.

> Algorithm extensibility means that new algorithms that fit in one of the supported engine classes can be added easily.

## Architecture

> #### Cryptographic Service Providers

> The Java Cryptography Architecture introduced the notion of a Cryptographic Service Provider (used interchangeably with "provider" in this document). This term refers to a package (or a set of packages) that supplies a concrete implementation of a subset of the cryptography aspects of the Security API.

> For example, in JDK 1.1 a provider could contain an implementation of one or more digital signature algorithms, message digest algorithms, and key generation algorithms. Java 2 SDK adds five additional types of services: key factories, keystore creation and management, algorithm parameter management, algorithm parameter generation, and certificate factories. It also enables a provider to supply a random number generation (RNG) algorithm. Previously, RNGs were not provider-based; a particular algorithm was hard-coded in the JDK.

> As previously noted, a program may simply request a particular type of object (such as a `Signature` object) for a particular service (such as the DSA signature algorithm) and get an implementation from one of the installed providers. Alternatively, the program can request the objects from a specific provider. (Each provider has a name used to refer to it.)

> Sun's version of the Java runtime environment comes standard with a default provider, named `SUN`. Other Java runtime environments may not necessarily supply the `SUN` provider. The `SUN` provider package includes:

> - An implementation of the Digital Signature Algorithm (DSA), described in NIST FIPS 186.

> - An implementation of the MD5 (RFC 1321) and SHA-1 (NIST FIPS 180-1) message digest algorithms.

> - A DSA key pair generator for generating a pair of public and private keys suitable for the DSA algorithm.

> - A DSA algorithm parameter generator.

> - A DSA algorithm parameter manager.

> - A DSA key factory providing bi-directional conversions between (opaque) DSA private and public key objects and their underlying key material.

> - An implementation of the proprietary "SHA1PRNG" pseudo-random number generation algorithm, following the recommendations in the IEEE P1363 standard (Appendix G.7).

> - A certificate path builder and validator for PKIX, as defined in the Internet X.509 Public Key Infrastructure Certificate and CRL Profile (available as a draft from Internet Engineering Task Force at the time of this writing.).

> - A certificate store implementation for retrieving certificates and CRLs from Collection and LDAP directories, using the PKIX LDAP V2 Schema (RFC 2587).

> - A certificate factory for X.509 certificates and Certificate Revocation Lists (CRLs).

> - A keystore implementation for the proprietary keystore type named `JKS`.

> Each SDK installation has one or more provider packages installed. New providers may be added statically or dynamically (see the Provider and Security classes). The Java Cryptography Architecture offers a set of APIs that allow users to query which providers are installed and what services they support.

> Clients may configure their runtime with different providers, and specify a preference order for each of them. The preference order is the order in which providers are searched for requested services when no specific provider is requested.

> #### Key Management

> A database called a "keystore" can be used to manage a repository of keys and certificates. A keystore is available to applications that need it for authentication or signing purposes.

> Applications can access a keystore via an implementation of the `KeyStore` class, which is in the `java.security` package. A default `KeyStore` implementation is provided by Sun Microsystems. It implements the keystore as a file, using a proprietary keystore type (format) named "JKS".

> Applications can choose different types of keystore implementations from different providers, using the `getInstance` factory method supplied in the `KeyStore` class.

> See the Key Management section for more information.

## Concepts

> This section covers the major concepts introduced in the API.

> #### Engine Classes and Algorithms

> An engine class defines a cryptographic service in an abstract fashion (without a concrete implementation).

> A cryptographic service is always associated with a particular algorithm or type, and it either provides cryptographic operations (like those for digital signatures or message digests), generates or supplies the cryptographic material (keys or parameters) required for cryptographic operations, or generates data objects (keystores or certificates) that encapsulate cryptographic keys (which can be used in a cryptographic operation) in a secure fashion. For example, two of the engine classes are the `Signature` and `KeyFactory` classes. The `Signature` class provides access to the functionality of a digital signature algorithm. A DSA `KeyFactory` supplies a DSA private or public key (from its encoding or transparent specification) in a format usable by the `initSign` or `initVerify` methods, respectively, of a DSA `Signature` object.

> The Java Cryptography Architecture encompasses the classes of the Java 2 SDK Security package related to cryptography, including the engine classes. Users of the API request and use instances of the engine classes to carry out corresponding operations. The following engine classes are defined in Java 2 SDK:

> - `MessageDigest`: used to calculate the message digest (hash) of specified data.

> - `Signature`: used to sign data and verify digital signatures.

> - `KeyPairGenerator`: used to generate a pair of public and private keys suitable for a specified algorithm.

> - `KeyFactory`: used to convert opaque cryptographic keys of type `Key` into key specifications (transparent representations of the underlying key material), and vice versa.

> - `CertificateFactory`: used to create public key certificates and Certificate Revocation Lists (CRLs).

> - `KeyStore`: used to create and manage a keystore.A keystore is a database of keys. Private keys in a keystore have a certificate chain associated with them, which authenticates the corresponding public key. A keystore also contains certificates from trusted entities.

> - `AlgorithmParameters`: used to manage the parameters for a particular algorithm, including parameter encoding and decoding.

> - `AlgorithmParameterGenerator`: used to generate a set of parameters suitable for a specified algorithm.

> - `SecureRandom`: used to generate random or pseudo-random numbers.
> In the 1.4 release of the Java 2 SDK, the following new engines were added:
> - `CertPathBuilder`: used to build certificate chains (also known as certification paths).

> - `CertPathValidator`: used to validate certificate chains.

> - `CertStore`: used to retrieve `Certificate`s and `CRL`s from a repository.

> > Note: A generator creates objects with brand-new contents, whereas a factory creates objects from existing material (for example, an encoding).

> An engine class provides the interface to the functionality of a specific type of cryptographic service (independent of a particular cryptographic algorithm). It defines Application Programming Interface (API) methods that allow applications to access the specific type of cryptographic service it provides. The actual implementations (from one or more providers) are those for specific algorithms. The `Signature` engine class, for example, provides access to the functionality of a digital signature algorithm. The actual implementation supplied in a `SignatureSpi` subclass would be that for a specific kind of signature algorithm, such as SHA-1 with DSA, SHA-1 with RSA, or MD5 with RSA.

> The application interfaces supplied by an engine class are implemented in terms of a Service Provider Interface (SPI). That is, for each engine class, there is a corresponding abstract SPI class, which defines the SPI methods that cryptographic service providers must implement.

> An instance of an engine class, the API object, encapsulates (as a private field) an instance of the corresponding SPI class, the SPI object. All API methods of an API object are declared final and their implementations invoke the corresponding SPI methods of the encapsulated SPI object. An instance of an engine class (and of its corresponding SPI class) is created by a call to the `getInstance` factory method of the engine class.

> The name of each SPI class is the same as that of the corresponding engine class, followed by `Spi`. For example, the SPI class corresponding to the `Signature` engine class is the `SignatureSpi` class.

> Each SPI class is abstract. To supply the implementation of a particular type of service, for a specific algorithm, a provider must subclass the corresponding SPI class and provide implementations for all the abstract methods.

> Another example of an engine class is the `MessageDigest` class, which provides access to a message digest algorithm. Its implementations, in `MessageDigestSpi` subclasses, may be those of various message digest algorithms such as SHA-1, MD5, or MD2.

> As a final example, the `KeyFactory` engine class supports the conversion from opaque keys to transparent key specifications, and vice versa. (See the Key Specification Interfaces and Classes section.) The `KeyFactorySpi` subclass supplies an actual implementation for a specific type of keys, for example, DSA public and private keys.

> #### Implementations and Providers

> Implementations for various cryptographic services are provided by JCA Cryptographic Service Providers. Cryptographic service providers are essentially packages that supply one or more cryptographic service implementations. The Engine Classes and Algorithms section includes a list of implemenations supplied by SUN, the Java 2 SDK's default provider.

> Other providers may define their own implementations of these services or of other services, such as one of the RSA-based signature algorithms or the MD2 message digest algorithm.

> #### Factory Methods to Obtain Implementation Instances

> For each engine class in the API, a particular implementation is requested and instantiated by calling a factory method on the engine class. A factory method is a static method that returns an instance of a class.

> The basic mechanism for obtaining an appropriate `Signature` object, for example, is as follows: A user requests such an object by calling the `getInstance` method in the `Signature` class, specifying the name of a signature algorithm (such as "SHA1withDSA"), and, optionally, the name of the provider or the `Provider` class. The `getInstance` method finds an implementation that satisfies the supplied algorithm and provider parameters. If no provider is specified, `getInstance` searches the registered providers, in preference order, for one with an implementation of the specified algorithm. See The `Provider` Class for more information about registering providers.

# Core Classes and Interfaces

> This section discusses the core classes and interfaces provided in the Java Cryptography Architecture:

> - the `Provider` and `Security` classes

> - the `MessageDigest`, `Signature`, `KeyPairGenerator`, `KeyFactory`, `AlgorithmParameters`, `AlgorithmParameterGenerator`, `CertificateFactory`, `KeyStore`, `SecureRandom`, `CertPathBuilder`, `CertPathValidator`, and `CertStore`.
> engine classes

> - the `Key` interfaces and classes

> - the Algorithm Parameter Specification Interfaces and Classes and the Key Specification Interfaces and Classes

>   This section shows the signatures of the main methods in each class and interface. Examples for some of these classes (`MessageDigest`, `Signature`, `KeyPairGenerator`, `SecureRandom`, `KeyFactory`, and key specification classes) are supplied in the corresponding Examples sections. The complete reference documentation for the relevant Security API packages can be found in:

>   - `java.security package summary`
>   - `java.security.spec package summary`
>   - `java.security.interfaces package summary`
>   - `java.security.cert package summary`

## The `Provider` Class

> The term "Cryptographic Service Provider" (used interchangeably with "provider" in this document) refers to a package or set of packages that supply a concrete implementation of a subset of the Java 2 SDK Security API cryptography features. The `Provider` class is the interface to such a package or set of packages. It has methods for accessing the provider name, version number, and other information. Please note that in addition to registering implementations of cryptographic services, the `Provider` class can also be used to register implementations of other security services that might get defined as part of the Java 2 SDK Security API or one of its extensions.

> To supply implementations of cryptographic services, an entity (e.g., a development group) writes the implementation code and creates a subclass of the `Provider` class. The constructor of the `Provider` subclass sets the values of various properties; the Java 2 SDK Security API uses these values to look up the services that the provider implements. In other words, the subclass specifies the names of the classes implementing the services.

> There are several types of services that can be implemented by provider packages; for more information, see Engine Classes and Algorithms.

> The different implementations may have different characteristics. Some may be software-based, while others may be hardware-based. Some may be platform-independent, while others may be platform-specific. Some provider source code may be available for review and evaluation, while some may not. The Java Cryptography Architecture (JCA) lets both end-users and developers decide what their needs are.

> In this section we explain how end-users install the cryptography implementations that fit their needs, and how developers request the implementations that fit theirs.

> > Note: For information about implementing a provider, see the guide How To Implement a Provider for the Java Cryptography Architecture.

> ### How Provider Implementations Are Requested and Supplied

> > For each engine class in the API, a particular implementation is requested and instantiated by calling a `getInstance` method on the engine class, specifying the name of the desired algorithm and, optionally, the name of the provider (or the `Provider` class) whose implementation is desired.

> > If no provider is specified, `getInstance` searches the registered providers for an implementation of the requested cryptographic service associated with the named algorithm. In any given Java Virtual Machine (JVM), providers are installed in a given preference order, the order in which the provider list is searched if a specific provider is not requested. For example, suppose there are two providers installed in a JVM, `PROVIDER_1` and `PROVIDER_2`. Assume that:

> > - `PROVIDER_1` implements SHA1withDSA, SHA-1, MD5, DES, and DES3.
> >   `PROVIDER_1` has preference order 1 (the highest priority).

> > - `PROVIDER_2` implements SHA1withDSA, MD5withRSA, MD2withRSA, MD2, MD5, RC4, RC5, DES, and RSA.
> >   `PROVIDER_2` has preference order 2.
> > Now let's look at three scenarios:
> > 1. If we are looking for an MD5 implementation. Both providers supply such an implementation. The `PROVIDER_1` implementation is returned since `PROVIDER_1` has the highest priority and is searched first.
> > 2. If we are looking for an MD5withRSA signature algorithm, `PROVIDER_1` is first searched for it. No implementation is found, so `PROVIDER_2` is searched. Since an implementation is found, it is returned.
> > 3. Suppose we are looking for a SHA1withRSA signature algorithm. Since no installed provider implements it, a `NoSuchAlgorithmException` is thrown.

> > The `getInstance` methods that include a provider argument are for developers who want to specify which provider they want an algorithm from. A federal agency, for example, will want to use a provider implementation that has received federal certification. Let's assume that the SHA1withDSA implementation from `PROVIDER_1` has not received such certification, while the DSA implementation of `PROVIDER_2` has received it.

> > A federal agency program would then have the following call, specifying `PROVIDER_2` since it has the certified implementation:

> > >     Signature dsa = Signature.getInstance("SHA1withDSA", "PROVIDER_2");

> > In this case, if `PROVIDER_2` was not installed, a `NoSuchProviderException` would be thrown, even if another installed provider implements the algorithm requested.

> > A program also has the option of getting a list of all the installed providers (using the `getProviders` method in the `Security` class) and choosing one from the list.

> ### Installing Providers

> > There are two parts to installing a provider: installing the provider package classes, and configuring the provider.

> > #### Installing the Provider Classes

> > There are two possible ways to install the provider classes:

> > 1. Place a zip or JAR file containing the classes anywhere in your classpath.

> > 2. Supply your provider JAR file as an "installed" or "bundled" extension. For more information on how to deploy an extension, see How is an extension deployed?.

> > #### Configuring the Provider

> > The next step is to add the provider to your list of approved providers. This step can be done statically by editing the `java.security` file in the `lib/security` directory of the SDK; therefore, if the SDK is installed in a directory called `j2sdk1.2`, the file would be `j2sdk1.2/lib/security/java.security`. One of the types of properties you can set in `java.security` has the following form:

> > >     security.provider.n=masterClassName

> > This declares a provider, and specifies its preference order n. The preference order is the order in which providers are searched for requested algorithms (when no specific provider is requested). The order is 1-based: 1 is the most preferred, followed by 2, and so on.

> > `masterClassName` must specify the provider's master class. The provider's documentation will specify its master class. This class is always a subclass of the `Provider` class. The subclass constructor sets the values of various properties that are required for the Java Cryptography API to look up the algorithms or other facilities the provider implements.

> > Suppose that the master class is `COM.acme.provider.Acme`, and that you would like to configure `Acme` as your third preferred provider. To do so, you would add the following line to the `java.security` file:

> > >     security.provider.3=COM.acme.provider.Acme

> > Providers may also be registered dynamically. To do so, call either the `addProvider` or `insertProviderAt` method in the `Security` class. This type of registration is not persistent and can only be done by "trusted" programs. See Security.

> ### `Provider` Class Methods

> > Each `Provider`class instance has a (currently case-sensitive) name, a version number, and a string description of the provider and its services. You can query the `Provider` instance for this information by calling the following methods:

> > >     public String getName()
> > >     public double getVersion()
> > >     public String getInfo()

## The `Security` Class

> The `Security` class manages installed providers and security-wide properties. It only contains static methods and is never instantiated. The methods for adding or removing providers, and for setting `Security` properties, can only be executed by a trusted program. Currently, a "trusted program" is either

> - a local application not running under a security manager, or

> - an applet or application with permission to execute the specified method (see below).
> The determination that code is considered trusted to perform an attempted action (such as adding a provider) requires that the applet is granted permission for that particular action.

> For example, in the Policy reference implementation, the policy configuration file(s) for a SDK installation specify what permissions (which types of system resource accesses) are allowed by code from specified code sources. (See below and the "Default Policy Implementation and Policy File Syntax" and "Java Security Architecture Specification" files for more information.)

> Code being executed is always considered to come from a particular "code source". The code source includes not only the location (URL) where the applet originated from, but also a reference to the public key(s) corresponding to the private key(s) used to sign the code. Public keys in a code source are referenced by (symbolic) alias names from the user's keystore .

> In a policy configuration file, a code source is represented by two components: a code base (URL), and an alias name (preceded by `signedBy`), where the alias name identifies the keystore entry containing the public key that must be used to verify the code's signature.

> Each "grant" statement in such a file grants a specified code source a set of permissions, specifying which actions are allowed.

> Here is a sample policy configuration file:

> >     grant codeBase "file:/home/sysadmin/", signedBy "sysadmin" {
> >         permission java.security.SecurityPermission "insertProvider.*";
> >         permission java.security.SecurityPermission "removeProvider.*";
> >         permission java.security.SecurityPermission "putProviderProperty.*";
> >     };

> This configuration file specifies that only code loaded from a signed JAR file from beneath the `/home/sysadmin/` directory on the local file system can add or remove providers or set provider properties. (Note that the signature of the JAR file can be verified using the public key referenced by the alias name `sysadmin` in the user's keystore.)

> Either component of the code source (or both) may be missing. Here's an example of a configuration file where `codeBase` is missing:

> >     grant signedBy "sysadmin" {
> >         permission java.security.SecurityPermission "insertProvider.*";
> >         permission java.security.SecurityPermission "removeProvider.*";
> >     };

> If this policy is in effect, code that comes in a JAR File signed by `sysadmin` can add/remove providers--regardless of where the JAR File originated.

> Here's an example without a signer:

> >     grant codeBase "file:/home/sysadmin/" {
> >         permission java.security.SecurityPermission "insertProvider.*";
> >         permission java.security.SecurityPermission "removeProvider.*";
> >     };

> In this case, code that comes from anywhere within the `/home/sysadmin/` directory on the local filesystem can add/remove providers. The code does not need to be signed.

> An example where neither `codeBase` nor `signedBy` is included is:

> >     grant {
> >         permission java.security.SecurityPermission "insertProvider.*";
> >         permission java.security.SecurityPermission "removeProvider.*";
> >     };

> Here, with both code source components missing, any code (regardless of where it originates, or whether or not it is signed, or who signed it) can add/remove providers.

> ### Managing Providers

> > The following tables summarize the methods in the `Security` class you can use to query which `Provider`s are installed, as well as to install or remove providers at runtime.

> > Quering Providers
> > Method | Description
> > `static Provider[] getProviders()` | Returns an array containing all the installed providers (technically, the `Provider` subclass for each package provider). The order of the `Provider`s in the array is their preference order.
> > `static Provider getProvider (String providerName)` | Returns the `Provider` named `providerName`. It returns `null` if the `Provider` is not found.

> > Adding Providers
> > Method | Description

> >     static int
> >     addProvider(Provider provider)

> > Adds a `Provider` to the end of the list of installed `Provider`s. It returns the preference position in which the `Provider` was added, or `-1` if the `Provider` was not added because it was already installed.

> > `static int insertProviderAt (Provider provider, int position)` | Adds a new `Provider` at a specified position. If the given provider is installed at the requested position, the provider formerly at that position and all providers with a position greater than `position` are shifted up one position (towards the end of the list). This method returns the preference position in which the `Provider` was added, or `-1` if the `Provider` was not added because it was already installed.

> > Removing Providers
> > Method | Description
> > `static void removeProvider(String name)` | Removes the `Provider` with the specified name. It returns silently if the provider is not installed. When the specified provider is removed, all providers located at a position greater than where the specified provider was are shifted down one position (towards the head of the list of installed providers).

> > > Note: If you want to change the preference position of a provider, you must first remove it, and then insert it back in at the new preference position.

> ### Security Properties

> > The `Security` class maintains a list of system-wide security properties. These properties are accessible and settable by a trusted program via the following methods:

> > >     static String getProperty(String key)
> > >     static void setProperty(String key, String datum)

## The `MessageDigest` Class

> The `MessageDigest` class is an engine class designed to provide the functionality of cryptographically secure message digests such as SHA-1 or MD5. A cryptographically secure message digest takes arbitrary-sized input (a byte array), and generates a fixed-size output, called a digest or hash. A digest has two properties:

> - It should be computationally infeasible to find two messages that hashed to the same value.

> - The digest should not reveal anything about the input that was used to generate it.

> Message digests are used to produce unique and reliable identifiers of data. They are sometimes called the "digital fingerprints" of data.

> ### Creating a `MessageDigest` Object

> > The first step for computing a digest is to create a message digest instance. As with all engine classes, the way to get a `MessageDigest` object for a particular type of message digest algorithm is to call the `getInstance` static factory method on the `MessageDigest` class:

> > >     static MessageDigest getInstance(String algorithm)

> > > Note: The algorithm name is not case-sensitive. For example, all the following calls are equivalent:

> > > >     MessageDigest.getInstance("SHA-1")
> > > >     MessageDigest.getInstance("sha-1")
> > > >     MessageDigest.getInstance("sHa-1")

> > A caller may optionally specify the name of a provider or a `Provider` instance, which guarantees that the implementation of the algorithm requested is from the specified provider:

> > >     static MessageDigest getInstance(String algorithm, String provider)
> > >     static MessageDigest getInstance(String algorithm, Provider provider)

> > A call to `getInstance` returns an initialized message digest object. It thus does not need further initialization.

> ### Updating a Message Digest Object

> > The next step for calculating the digest of some data is to supply the data to the initialized message digest object. This is done by calling one of the `update` methods:

> > >     void update(byte input)
> > >     void update(byte[] input)
> > >     void update(byte[] input, int offset, int len)

> ### Computing the Digest

> > After the data has been supplied by calls to `update` methods, the digest is computed using a call to one of the `digest` methods:

> > >     byte[] digest()
> > >     byte[] digest(byte[] input)
> > >     int digest(byte[] buf, int offset, int len)

> > The first two methods return the computed digest. The latter method stores the computed digest in the provided buffer `buf`, starting at `offset`. `len` is the number of bytes in `buf` allotted for the digest. The method returns the number of bytes actually stored in `buf`.

> > A call to the `digest` method that takes an input byte array argument is equivalent to making a call to

> > >     void update(byte[] input)

> > with the specified input, followed by a call to the `digest` method without any arguments.

> > Please see the Examples section for more details.

## The `Signature` Class

> The `Signature` class is an engine class designed to provide the functionality of a cryptographic digital signature algorithm such as DSA or RSA with MD5. A cryptographically secure signature algorithm takes arbitrary-sized input and a private key and generates a relatively short (often fixed-size) string of bytes, called the signature, with the following properties:
> - Given the public key corresponding to the private key used to generate the signature, it should be possible to verify the authenticity and integrity of the input.

> - The signature and the public key do not reveal anything about the private key.

> A `Signature` object can be used to sign data. It can also be used to verify whether or not an alleged signature is in fact the authentic signature of the data associated with it. Please see the Examples section for an example of signing and verifying data.

> ### `Signature` Object States

> > `Signature` objects are modal objects. This means that a `Signature` object is always in a given state, where it may only do one type of operation. States are represented as final integer constants defined in their respective classes.

> > The three states a `Signature` object may have are:

> > - `UNINITIALIZED`
> > - `SIGN`
> > - `VERIFY`
> > When it is first created, a `Signature` object is in the `UNINITIALIZED` state. The `Signature` class defines two initialization methods, `initSign` and `initVerify`, which change the state to `SIGN` and `VERIFY`, respectively.

> ### Creating a `Signature` Object

> > The first step for signing or verifying a signature is to create a `Signature` instance. As with all engine classes, the way to get a `Signature` object for a particular type of signature algorithm is to call the `getInstance` static factory method on the `Signature` class:

> > >     static Signature getInstance(String algorithm)

> > > Note: The algorithm name is not case-sensitive.

> > A caller may optionally specify the name of a provider or the `Provider` class, which will guarantee that the implementation of the algorithm requested is from the named provider:

> > >     static Signature getInstance(String algorithm, String provider)
> > >     static Signature getInstance(String algorithm, Provider provider)

> ### Initializing a `Signature` Object

> > A `Signature` object must be initialized before it is used. The initialization method depends on whether the object is going to be used for signing or for verification.

> > If it is going to be used for signing, the object must first be initialized with the private key of the entity whose signature is going to be generated. This initialization is done by calling the method:

> > >     final void initSign(PrivateKey privateKey)

> > This method puts the `Signature` object in the `SIGN` state.

> > If instead the `Signature` object is going to be used for verification, it must first be initialized with the public key of the entity whose signature is going to be verified. This initialization is done by calling either of these methods:

> >         final void initVerify(PublicKey publicKey)

> >         final void initVerify(Certificate certificate)

> > This method puts the `Signature` object in the `VERIFY` state.

> ### Signing

> > If the `Signature` object has been initialized for signing (if it is in the `SIGN` state), the data to be signed can then be supplied to the object. This is done by making one or more calls to one of the `update` methods:

> > >     final void update(byte b)
> > >     final void update(byte[] data)
> > >     final void update(byte[] data, int off, int len)

> > Calls to the `update` method(s) should be made until all the data to be signed has been supplied to the `Signature` object.

> > To generate the signature, simply call one of the `sign` methods:

> > >     final byte[] sign()
> > >     final int sign(byte[] outbuf, int offset, int len)

> > The first method returns the signature result in a byte array. The second stores the signature result in the provided buffer outbuf, starting at offset. len is the number of bytes in outbuf allotted for the signature. The method returns the number of bytes actually stored.

> > Signature encoding is algorithm specific. See Appendix B for more information about the use of ASN.1 encoding in the Java Cryptography Architecture.

> > A call to a `sign` method resets the signature object to the state it was in when previously initialized for signing via a call to `initSign`. That is, the object is reset and available to generate another signature with the same private key, if desired, via new calls to `update` and `sign`.

> > Alternatively, a new call can be made to `initSign` specifying a different private key, or to `initVerify` (to initialize the `Signature` object to verify a signature).

> ### Verifying

> > If the `Signature` object has been initialized for verification (if it is in the `VERIFY` state), it can then verify if an alleged signature is in fact the authentic signature of the data associated with it. To start the process, the data to be verified (as opposed to the signature itself) is supplied to the object. The data is passed to the object by calling one of the `update` methods:

> > >     final void update(byte b)
> > >     final void update(byte[] data)
> > >     final void update(byte[] data, int off, int len)

> > Calls to the `update` method(s) should be made until all the data to be verified has been supplied to the `Signature` object. The signature can now be verified by calling one of the `verify` methods:

> > >     final boolean verify(byte[] signature)

> > >     final boolean verify(byte[] signature, int offset, int length)

> > The argument must be a byte array containing the signature. The argument must be a byte array containing the signature. This byte array would hold the signature bytes which were returned by a previous call to one of the `sign` methods.

> > The `verify` method returns a `boolean` indicating whether or not the encoded signature is the authentic signature of the data supplied to the `update` method(s).

> > A call to the `verify` method resets the signature object to its state when it was initialized for verification via a call to `initVerify`. That is, the object is reset and available to verify another signature from the identity whose public key was specified in the call to `initVerify`.

> > Alternatively, a new call can be made to `initVerify` specifying a different public key (to initialize the `Signature` object for verifying a signature from a different entity), or to `initSign` (to initialize the `Signature` object for generating a signature).

## Algorithm Parameters Classes

> ### Algorithm Parameter Specification Interfaces and Classes

> > An algorithm parameter specification is a transparent representation of the sets of parameters used with an algorithm.

> > A transparent representation of a set of parameters means that you can access each parameter value in the set individually. You can access these values through one of the `get` methods defined in the corresponding specification class (e.g., `DSAParameterSpec` defines `getP`, `getQ`, and `getG` methods, to access `p`, `q`, and `g`, respectively).

> > In contrast, the `AlgorithmParameters` class supplies an opaque representation, in which you have no direct access to the parameter fields. You can only get the name of the algorithm associated with the parameter set (via `getAlgorithm`) and some kind of encoding for the parameter set (via `getEncoded`).

> > The algorithm parameter specification interfaces and classes in the `java.security.spec` package are described in the following sections.

> > ### The `AlgorithmParameterSpec` Interface

> > > `AlgorithmParameterSpec` is an interface to a transparent specification of cryptographic parameters.

> > > This interface contains no methods or constants. Its only purpose is to group (and provide type safety for) all parameter specifications. All parameter specifications must implement this interface.

> > ### The `DSAParameterSpec` Class

> > > This class (which implements the `AlgorithmParameterSpec` interface) specifies the set of parameters used with the DSA algorithm. It has the following methods:

> > > >     BigInteger getP()
> > > >     BigInteger getQ()
> > > >     BigInteger getG()

> > > These methods return the DSA algorithm parameters: the prime `p`, the sub-prime `q`, and the base `g`.

> ### The `AlgorithmParameters` Class

> > The `AlgorithmParameters` class is an engine class that provides an opaque representation of cryptographic parameters.

> > An opaque representation is one in which you have no direct access to the parameter fields; you can only get the name of the algorithm associated with the parameter set and some kind of encoding for the parameter set. This is in contrast to a transparent representation of parameters, in which you can access each value individually, through one of the `get` methods defined in the corresponding specification class. Note that you can call the `AlgorithmParameters` `getParameterSpec` method to convert an `AlgorithmParameters` object to a transparent specification (see the following section).

> > ### Creating an `AlgorithmParameters` Object

> > > As with all engine classes, the way to get an `AlgorithmParameters` object for a particular type of algorithm is to call the `getInstance` static factory method on the `AlgorithmParameters` class:

> > > >     static AlgorithmParameters getInstance(String algorithm)

> > > > Note: The algorithm name is not case-sensitive.

> > > A caller may optionally specify the name of a provider or the `Provider` class, which will guarantee that the algorithm parameter implementation requested is from the named provider:

> > > >     static AlgorithmParameters getInstance(String algorithm, String provider)
> > > >     static AlgorithmParameters getInstance(String algorithm, Provider provider)

> > ### Initializing an `AlgorithmParameters` Object

> > > Once an `AlgorithmParameters` object is instantiated, it must be initialized via a call to `init`, using an appropriate parameter specification or parameter encoding:

> > > >     void init(AlgorithmParameterSpec paramSpec)
> > > >     void init(byte[] params)
> > > >     void init(byte[] params, String format)

> > > In these `init` methods, `params` is an array containing the encoded parameters, and `format` is the name of the decoding format. In the `init` method with a `params` argument but no `format` argument, the primary decoding format for parameters is used. The primary decoding format is ASN.1, if an ASN.1 specification for the parameters exists.

> > > > Note: `AlgorithmParameters` objects can be initialized only once. They are not reusable.

> > ### Obtaining the Encoded Parameters

> > > A byte encoding of the parameters represented in an `AlgorithmParameters` object may be obtained via a call to `getEncoded`:

> > > >     byte[] getEncoded()

> > > This method returns the parameters in their primary encoding format. The primary encoding format for parameters is ASN.1, if an ASN.1 specification for this type of parameters exists.

> > > If you want the parameters returned in a specified encoding format, use

> > > >     byte[] getEncoded(String format)

> > > If `format` is null, the primary encoding format for parameters is used, as in the other `getEncoded` method.

> > > > Note: In the default `AlgorithmParameters` implementation, supplied by the "SUN" provider, the `format` argument is currently ignored.

> > ### Converting an `AlgorithmParameters` Object to a Transparent Specification

> > > A transparent parameter specification for the algorithm parameters may be obtained from an `AlgorithmParameters` object via a call to `getParameterSpec`:

> > > >     AlgorithmParameterSpec getParameterSpec(Class paramSpec)

> > > `paramSpec` identifies the specification class in which the parameters should be returned. The specification class could be, for example, `DSAParameterSpec.class` to indicate that the parameters should be returned in an instance of the `DSAParameterSpec` class. (This class is in the `java.security.spec` package.)

> ### The `AlgorithmParameterGenerator` Class

> > The `AlgorithmParameterGenerator` class is an engine class used to generate a set of parameters suitable for a certain algorithm (the algorithm specified when an `AlgorithmParameterGenerator` instance is created).

> > ### Creating an `AlgorithmParameterGenerator` Object

> > > As with all engine classes, the way to get an `AlgorithmParameterGenerator` object for a particular type of algorithm is to call the `getInstance` static factory method on the `AlgorithmParameterGenerator` class:

> > > >     static AlgorithmParameterGenerator getInstance(
> > > >                                        String algorithm)

> > > > Note: The algorithm name is not case-sensitive.

> > > A caller may optionally specify the name of a provider or the `Provider` class, which will guarantee that the algorithm parameter generator implementation is from the named provider:

> > > >     static AlgorithmParameterGenerator getInstance(
> > > >                                        String algorithm,
> > > >                                        String provider)

> > > >     static AlgorithmParameterGenerator getInstance(
> > > >                                        String algorithm,
> > > >                                        Provider provider)

> > ### Initializing an `AlgorithmParameterGenerator` Object

> > > The `AlgorithmParameterGenerator` object can be initialized in two different ways: an algorithm-independent manner or an algorithm-specific manner.

> > > The algorithm-independent approach uses the fact that all parameter generators share the concept of a "size" and a source of randomness. The measure of size is universally shared by all algorithm parameters, though it is interpreted differently for different algorithms. For example, in the case of parameters for the DSA algorithm, "size" corresponds to the size of the prime modulus, in bits. (See Appendix B: Algorithms for information about the sizes for specific algorithms.) When using this approach, algorithm-specific parameter generation values--if any--default to some standard values. One `init` method that takes these two universally shared types of arguments:

> > > >     void init(int size, SecureRandom random);

> > > Another `init` method takes only a `size` argument and uses a system-provided source of randomness:

> > > >     void init(int size)

> > > A third approach initializes a parameter generator object using algorithm-specific semantics, which are represented by a set of algorithm-specific parameter generation values supplied in an `AlgorithmParameterSpec` object:

> > > >     void init(AlgorithmParameterSpec genParamSpec,
> > > >                               SecureRandom random)

> > > >     void init(AlgorithmParameterSpec genParamSpec)

> > > To generate Diffie-Hellman system parameters, for example, the parameter generation values usually consist of the size of the prime modulus and the size of the random exponent, both specified in number of bits. (The Diffie-Hellman algorithm has been part of the JCE since JCE 1.2.)

> > ### Generating Algorithm Parameters

> > > Once you have created and initialized an `AlgorithmParameterGenerator` object, you can use the `generateParameters` method to generate the algorithm parameters:

> > > >     AlgorithmParameters generateParameters()

## `Key` Interfaces

> The `Key` interface is the top-level interface for all opaque keys. It defines the functionality shared by all opaque key objects.

> An opaque key representation is one in which you have no direct access to the key material that constitutes a key. In other words: "opaque" gives you limited access to the key--just the three methods defined by the `Key` interface (see below): `getAlgorithm`, `getFormat`, and `getEncoded`. This is in contrast to a transparent representation, in which you can access each key material value individually, through one of the `get` methods defined in the corresponding specification class.

> All opaque keys have three characteristics:

> > An Algorithm
> >   The key algorithm for that key. The key algorithm is usually an encryption or asymmetric operation algorithm (such as DSA or RSA), which will work with those algorithms and with related algorithms (such as MD5 with RSA, SHA-1 with RSA, etc.) The name of the algorithm of a key is obtained using this method:

> >   >     String getAlgorithm()

> > An Encoded Form
> >   The external encoded form for the key used when a standard representation of the key is needed outside the Java Virtual Machine, as when transmitting the key to some other party. The key is encoded according to a standard format (such as X.509 or PKCS #8), and is returned using the method:

> >   >     byte[] getEncoded()

> > A Format
> >   The name of the format of the encoded key. It is returned by the method:

> >   >     String getFormat()

> Keys are generally obtained through key generators, certificates, key specifications (using a `KeyFactory`), or a `KeyStore` implementation accessing a keystore database used to manage keys.

> It is possible to parse encoded keys, in an algorithm-dependent manner, using a `KeyFactory`.

> It is also possible to parse certificates, using a `CertificateFactory`.

> Here is a list of interfaces which extend the `Key` interface in the `java.security.interfaces` package:

> - DHPrivateKey
> - DHPublicKey
> - DSAPrivateKey
> - DSAPublicKey
> - PBEKey
> - PrivateKey

> - PublicKey
> - RSAMultiPrimePrivateCrtKey
> - RSAPrivateCrtKey
> - RSAPrivateKey
> - RSAPublicKey
> - SecretKey

> ### The `PublicKey` and `PrivateKey` Interfaces

> > The `PublicKey` and `PrivateKey` interfaces (which both extend the `Key` interface) are methodless interfaces, used for type-safety and type-identification.

## Key Specification Interfaces and Classes

> Key specifications are transparent representations of the key material that constitutes a key. If the key is stored on a hardware device, its specification may contain information that helps identify the key on the device.

> A transparent representation of keys means that you can access each key material value individually, through one of the `get` methods defined in the corresponding specification class. For example, `DSAPrivateKeySpec` defines `getX`, `getP`, `getQ`, and `getG` methods, to access the private key `x`, and the DSA algorithm parameters used to calculate the key: the prime `p`, the sub-prime `q`, and the base `g`.

> This representation is contrasted with an opaque representation, as defined by the `Key` interface, in which you have no direct access to the key material fields. In other words, an "opaque" representation gives you limited access to the key--just the three methods defined by the `Key` interface: `getAlgorithm`, `getFormat`, and `getEncoded`.

> A key may be specified in an algorithm-specific way, or in an algorithm-independent encoding format (such as ASN.1). For example, a DSA private key may be specified by its components `x`, `p`, `q`, and `g` (see `DSAPrivateKeySpec`), or it may be specified using its DER encoding (see `PKCS8EncodedKeySpec`).

> In the following sections, we discuss the key specification interfaces and classes in the `java.security.spec` package.

> ### The `KeySpec` Interface

> > This interface contains no methods or constants. Its only purpose is to group and provide type safety for all key specifications. All key specifications must implement this interface.

> ### The `DSAPrivateKeySpec` Class

> > This class (which implements the `KeySpec` interface) specifies a DSA private key with its associated parameters. `DSAPrivateKeySpec` has the following methods:

> > >     BigInteger getX()
> > >     BigInteger getP()
> > >     BigInteger getQ()
> > >     BigInteger getG()

> > These methods return the private key `x`, and the DSA algorithm parameters used to calculate the key: the prime `p`, the sub-prime `q`, and the base `g`.

> ### The `DSAPublicKeySpec` Class

> > This class (which implements the `KeySpec` interface) specifies a DSA public key with its associated parameters. `DSAPublicKeySpec` has the following methods:

> > >     BigInteger getY()
> > >     BigInteger getP()
> > >     BigInteger getQ()
> > >     BigInteger getG()

> > These methods return the public key `y`, and the DSA algorithm parameters used to calculate the key: the prime `p`, the sub-prime `q`, and the base `g`.

> ### The `RSAPrivateKeySpec` Class

> > This class (which implements the `KeySpec` interface) specifies an RSA private key. `RSAPrivateKeySpec` has the following methods:

> > >     BigInteger getModulus()
> > >     BigInteger getPrivateExponent()

> > These methods return the RSA modulus `n` and private exponent `d` values that constitute the RSA private key.

> ### The `RSAPrivateCrtKeySpec` Class

> > This class (which extends the `RSAPrivateKeySpec` class) specifies an RSA private key, as defined in the PKCS #1 standard, using the Chinese Remainder Theorem (CRT) information values. `RSAPrivateCrtKeySpec` has the following methods (in addition to the methods inherited from its superclass `RSAPrivateKeySpec`):

> > >     BigInteger getPublicExponent()
> > >     BigInteger getPrimeP()
> > >     BigInteger getPrimeQ()
> > >     BigInteger getPrimeExponentP()
> > >     BigInteger getPrimeExponentQ()
> > >     BigInteger getCrtCoefficient()

> > These methods return the public exponent `e` and the CRT information integers: the prime factor `p` of the modulus `n`, the prime factor `q` of `n`, the exponent `d mod (p-1)`, the exponent `d mod (q-1)`, and the Chinese Remainder Theorem coefficient `(inverse of q) mod p`.

> > An RSA private key logically consists of only the modulus and the private exponent. The presence of the CRT values is intended for efficiency.

> ### The `RSAMultiPrimePrivateCrtKeySpec` Class

> > This class (which extends the `RSAPrivateKeySpec` class) specifies an RSA multi-prime private key, as defined in the PKCS#1 v2.1, using the Chinese Remainder Theorem (CRT) information values. `RSAMultiPrimePrivateCrtKeySpec` has the following methods (in addition to the methods inherited from its superclass `RSAPrivateKeySpec`):

> > >     BigInteger getPublicExponent()
> > >     BigInteger getPrimeP()
> > >     BigInteger getPrimeQ()
> > >     BigInteger getPrimeExponentP()
> > >     BigInteger getPrimeExponentQ()
> > >     BigInteger getCrtCoefficient()
> > >     RSAOtherPrimeInfo[] getOtherPrimeInfo()

> > These methods return the public exponent `e` and the CRT information integers: the prime factor `p` of the modulus `n`, the prime factor `q` of `n`, the exponent `d mod (p-1)`, the exponent `d mod (q-1)`, and the Chinese Remainder Theorem coefficient `(inverse of q) mod p`.

> > Method `getOtherPrimeInfo` returns a copy of the `otherPrimeInfo` (defined in PKCS#1 v 2.1) or null if there are only two prime factors (`p` and `q`).

> > An RSA private key logically consists of only the modulus and the private exponent. The presence of the CRT values is intended for efficiency.

> ### The `RSAPublicKeySpec` Class

> > This class (which implements the `KeySpec` interface) specifies an RSA public key. `RSAPublicKeySpec` has the following methods:

> > >     BigInteger getModulus()
> > >     BigInteger getPublicExponent()

> > These methods return the RSA modulus `n` and public exponent `e` values that constitute the RSA public key.

> ### The `EncodedKeySpec` Class

> > This abstract class (which implements the `KeySpec` interface) represents a public or private key in encoded format. Its `getEncoded` method returns the encoded key:

> > >     abstract byte[] getEncoded();

> > and its `getFormat` method returns the name of the encoding format:

> > >     abstract String getFormat();

> > See the next sections for the concrete implementations `PKCS8EncodedKeySpec` and `X509EncodedKeySpec`.

> > #### The `PKCS8EncodedKeySpec` Class

> > > This class, which is a subclass of `EncodedKeySpec`, represents the DER encoding of a private key, according to the format specified in the PKCS #8 standard. Its `getEncoded` method returns the key bytes, encoded according to the PKCS #8 standard. Its `getFormat` method returns the string "PKCS#8".

> > #### The `X509EncodedKeySpec` Class

> > > This class, which is a subclass of `EncodedKeySpec`, represents the DER encoding of a public key, according to the format specified in the X.509 standard. Its `getEncoded` method returns the key bytes, encoded according to the X.509 standard. Its `getFormat` method returns the string "X.509".

## The `KeyFactory` Class

> The `KeyFactory` class is an engine class designed to provide conversions between opaque cryptographic keys (of type `Key`) and key specifications (transparent representations of the underlying key material).

> Key factories are bi-directional. They allow you to build an opaque key object from a given key specification (key material), or to retrieve the underlying key material of a key object in a suitable format.

> Multiple compatible key specifications can exist for the same key. For example, a DSA public key may be specified by its components `y`, `p`, `q`, and `g` (see `DSAPublicKeySpec`), or it may be specified using its DER encoding according to the X.509 standard (see `X509EncodedKeySpec`).

> A key factory can be used to translate between compatible key specifications. Key parsing can be achieved through translation between compatible key specifications, e.g., when you translate from `X509EncodedKeySpec` to `DSAPublicKeySpec`, you basically parse the encoded key into its components. For an example, see the end of the Generating/Verifying Signatures Using Key Specifications and `KeyFactory` section.

> ### Creating a `KeyFactory` Object

> > As with all engine classes, the way to get a `KeyFactory` object for a particular type of key algorithm is to call the `getInstance` static factory method on the `KeyFactory` class:

> > >     static KeyFactory getInstance(String algorithm)

> > > Note: The algorithm name is not case-sensitive.

> > A caller may optionally specify the name of a provider or the `Provider` class, which will guarantee that the implementation of the key factory requested is from the named provider.

> > >     static KeyFactory getInstance(String algorithm, String provider)
> > >     static KeyFactory getInstance(String algorithm, Provider provider)

> ### Converting Between a Key Specification and a Key Object

> > If you have a key specification for a public key, you can obtain an opaque `PublicKey` object from the specification by using the `generatePublic` method:

> > >     PublicKey generatePublic(KeySpec keySpec)

> > Similarly, if you have a key specification for a private key, you can obtain an opaque `PrivateKey` object from the specification by using the `generatePrivate` method:

> > >     PrivateKey generatePrivate(KeySpec keySpec)

> ### Converting Between a Key Object and a Key Specification

> > If you have a `Key` object, you can get a corresponding key specification object by calling the `getKeySpec` method:

> > >     KeySpec getKeySpec(Key key, Class keySpec)

> > `keySpec` identifies the specification class in which the key material should be returned. It could, for example, be `DSAPublicKeySpec.class`, to indicate that the key material should be returned in an instance of the `DSAPublicKeySpec` class.

> > Please see the Examples section for more details.

## The `CertificateFactory` Class

> The `CertificateFactory` class is an engine class that defines the functionality of a certificate factory, which is used to generate certificate and certificate revocation list (CRL) objects from their encodings.

> A certificate factory for X.509 must return certificates that are an instance of `java.security.cert.X509Certificate`, and CRLs that are an instance of `java.security.cert.X509CRL`.

> ### Creating a `CertificateFactory` Object

> > As with all engine classes, the way to get a `CertificateFactory` object for a particular certificate or CRL type is to call the `getInstance` static factory method on the `CertificateFactory` class:

> > >     static CertificateFactory getInstance(String type)

> > > Note: The type name is not case-sensitive.

> > A caller may optionally specify the name of a provider or the `Provider` class, which will guarantee that the implementation of the certificate factory requested is from the named provider.

> > >     static CertificateFactory getInstance(String type, String provider)

> > >     static CertificateFactory getInstance(String type, Provider provider)

> ### Generating Certificate Objects

> > To generate a certificate object and initialize it with the data read from an input stream, use the `generateCertificate` method:

> > >     final Certificate generateCertificate(InputStream inStream)

> > To return a (possibly empty) collection view of the certificates read from a given input stream, use the `generateCertificates` method:

> > >     final Collection generateCertificates(InputStream inStream)

> ### Generating CRL Objects

> > To generate a certificate revocation list (CRL) object and initialize it with the data read from an input stream, use the `generateCRL` method:

> > >     final CRL generateCRL(InputStream inStream)

> > To return a (possibly empty) collection view of the CRLs read from a given input stream, use the `generateCRLs` method:

> > >     final Collection generateCRLs(InputStream inStream)

> ### Generating `CertPath` Objects

> > To generate a `CertPath` object and initialize it with data read from an input stream, use one of the following `generateCertPath` methods (with or without specifying the encoding to be used for the data):

> > >     final CertPath generateCertPath(InputStream inStream)

> > >     final CertPath generateCertPath(InputStream inStream,
> > >                                     String encoding)

> > To generate a `CertPath` object and initialize it with a list of certificates, use the following method:

> > >     final CertPath generateCertPath(List certificates)

> > To retrieve a list of the `CertPath` encodings supported by this certificate factory, you can call the `getCertPathEncodings` method:

> > >     final Iterator getCertPathEncodings()

> > The default encoding will be listed first.

## The `KeyPair` Class

> The `KeyPair` class is a simple holder for a key pair (a public key and a private key). It has two public methods, one for returning the private key, and the other for returning the public key:

> >     PrivateKey getPrivate()
> >     PublicKey getPublic()

## The `KeyPairGenerator` Class

> The `KeyPairGenerator` class is an engine class used to generate pairs of public and private keys.

> There are two ways to generate a key pair: in an algorithm-independent manner, and in an algorithm-specific manner. The only difference between the two is the initialization of the object.

> Please see the Examples section for examples of calls to the methods documented below.

> ### Creating a `KeyPairGenerator`

> > All key pair generation starts with a `KeyPairGenerator`. This generation is done using one of the factory methods on `KeyPairGenerator`:

> > >     static KeyPairGenerator getInstance(String algorithm)
> > >     static KeyPairGenerator getInstance(String algorithm,
> > >                                         String provider)
> > >     static KeyPairGenerator getInstance(String algorithm,
> > >                                         Provider provider)

> > > Note: The algorithm name is not case-sensitive.

> ### Initializing a `KeyPairGenerator`

> > A key pair generator for a particular algorithm creates a public/private key pair that can be used with this algorithm. It also associates algorithm-specific parameters with each of the generated keys.

> > A key pair generator needs to be initialized before it can generate keys. In most cases, algorithm-independent initialization is sufficient. But in other cases, algorithm-specific initialization is used.

> > #### Algorithm-Independent Initialization

> > All key pair generators share the concepts of a keysize and a source of randomness. The keysize is interpreted differently for different algorithms. For example, in the case of the DSA algorithm, the keysize corresponds to the length of the modulus. (See Appendix B: Algorithms for information about the keysizes for specific algorithms.)

> > An `initialize` method takes two universally shared types of arguments:

> > >     void initialize(int keysize, SecureRandom random)

> > Another `initialize` method takes only a `keysize` argument; it uses a system-provided source of randomness:

> > >     void initialize(int keysize)

> > Since no other parameters are specified when you call the above algorithm-independent `initialize` methods, it is up to the provider what to do about the algorithm-specific parameters (if any) to be associated with each of the keys.

> > If the algorithm is a "DSA" algorithm, and the modulus size (keysize) is 512, 768, or 1024, then the "SUN" provider uses a set of precomputed values for the `p`, `q`, and `g` parameters. If the modulus size is not one of the above values, the "SUN" provider creates a new set of parameters. Other providers might have precomputed parameter sets for more than just the three modulus sizes mentioned above. Still others might not have a list of precomputed parameters at all and instead always create new parameter sets.

> > #### Algorithm-Specific Initialization

> > For situations where a set of algorithm-specific parameters already exists (such as "community parameters" in DSA), there are two `initialize` methods that have an `AlgorithmParameterSpec` argument. One also has a `SecureRandom` argument, while the source of randomness is system-provided for the other:

> > >     void initialize(AlgorithmParameterSpec params,
> > >                     SecureRandom random)

> > >     void initialize(AlgorithmParameterSpec params)

> > See the Examples section for more details.

> ### Generating a Key Pair

> > The procedure for generating a key pair is always the same, regardless of initialization (and of the algorithm). You always call the following method from `KeyPairGenerator`:

> > >     KeyPair generateKeyPair()

> > Multiple calls to `generateKeyPair` will yield different key pairs.

## Key Management

> A database called a "keystore" can be used to manage a repository of keys and certificates. (A certificate is a digitally signed statement from one entity, saying that the public key of some other entity has a particular value.)

> ### Keystore Location

> > The keystore is by default stored in a file named `.keystore` in the user's home directory, as determined by the "user.home" system property. On Solaris systems "user.home" defaults to the user's home directory. On Win32 systems, given user name uName, "user.home" defaults to:

> > > - `C:\Winnt\Profiles\uName` on multi-user Windows NT systems
> > > - `C:\Windows\Profiles\uName` on multi-user Windows 95/98/2000 systems
> > > - `C:\Windows on single-user` Windows 95/98/2000 systems

> ### Keystore Implementation

> > The `KeyStore` class supplies well-defined interfaces to access and modify the information in a keystore. It is possible for there to be multiple different concrete implementations, where each implementation is that for a particular type of keystore.

> > Currently, there are two command-line tools that make use of `KeyStore`: `keytool` and `jarsigner`, and also a GUI-based tool named `policytool`. It is also used by the `Policy` reference implementation when it processes policy files specifying the permissions (allowed accesses to system resources) to be granted to code from various sources. Since `KeyStore` is publicly available, SDK users can write additional security applications that use it.

> > There is a built-in default implementation, provided by Sun Microsystems. It implements the keystore as a file, utilizing a proprietary keystore type (format) named "JKS". It protects each private key with its individual password, and also protects the integrity of the entire keystore with a (possibly different) password.

> > Keystore implementations are provider-based. More specifically, the application interfaces supplied by `KeyStore` are implemented in terms of a "Service Provider Interface" (SPI). That is, there is a corresponding abstract `KeystoreSpi` class, also in the `java.security` package, which defines the SPI methods that "providers" must implement. (The term "provider" refers to a package or a set of packages that supply a concrete implementation of a subset of services that can be accessed by the Java 2 SDK Security API.) Therefore, to provide a keystore implementation clients must implement a "provider" and supply a `KeystoreSpi` subclass implementation, as described in How to Implement a Provider for the Java Cryptography Architecture.

> > Applications can choose different types of keystore implementations from different providers, using the `getInstance` factory method in the `KeyStore` class. A keystore type defines the storage and data format of the keystore information, and the algorithms used to protect private keys in the keystore and the integrity of the keystore itself. Keystore implementations of different types are not compatible.

> > The default keystore type is "jks" (the proprietary type of the keystore implementation provided by the "SUN" provider). This is specified by the following line in the security properties file:

> > >     keystore.type=jks

> > To have tools and other applications use a keystore implementation other than the default keystore, you can change that line to specify a different keystore type. Another solution would be to let users of your tools and applications specify a keystore type, and pass that value to the `getInstance` method of KeyStore.

> > An example of the former approach is the following: If you have a provider package that supplies a keystore implementation for a keystore type called `pkcs12`, change the line to

> > >     keystore.type=pkcs12

> > > Note: Keystore type designations are not case-sensitive. For example, "JKS" would be considered the same as "jks".

> ### The `KeyStore` Class

> > The `KeyStore` class is an engine class that supplies well-defined interfaces to access and modify the information in a keystore.

> > This class represents an in-memory collection of keys and certificates. `KeyStore` manages two types of entries:

> > > Key Entry

> > >   This type of keystore entry holds very sensitive cryptographic key information, which is stored in a protected format to prevent unauthorized access. Typically, a key stored in this type of entry is a secret key, or a private key accompanied by the certificate chain authenticating the corresponding public key.

> > >   Private keys and certificate chains are used by a given entity for self-authentication using digital signatures. For example, software distribution organizations digitally sign JAR files as part of releasing and/or licensing software.

> > > Trusted Certificate Entry

> > >   This type of entry contains a single public key certificate belonging to another party. It is called a trusted certificate because the keystore owner trusts that the public key in the certificate indeed belongs to the identity identified by the subject (owner) of the certificate.

> > >   This type of entry can be used to authenticate other parties.

> > Each entry in a keystore is identified by an "alias" string. In the case of private keys and their associated certificate chains, these strings distinguish among the different ways in which the entity may authenticate itself. For example, the entity may authenticate itself using different certificate authorities, or using different public key algorithms.

> > Whether keystores are persistent, and the mechanisms used by the keystore if it is persistent, are not specified here. This convention allows use of a variety of techniques for protecting sensitive (e.g., private or secret) keys. Smart cards or other integrated cryptographic engines (SafeKeyper) are one option, and simpler mechanisms such as files may also be used (in a variety of formats).

> > The main `KeyStore` methods are described below.

> > ### Creating a `KeyStore` Object

> > > As with all engine classes, the way to get a `KeyStore` object is to call the `getInstance` static factory method on the `KeyStore` class:

> > > >     static KeyStore getInstance(String type)

> > > A caller may optionally specify the name of a provider or the `Provider` class, which will guarantee that the implementation of the type requested is from the named provider:

> > > >     static KeyStore getInstance(String type, String provider)
> > > >     static KeyStore getInstance(String type, Provider provider)

> > ### Loading a Particular Keystore into Memory

> > > Before a `KeyStore` object can be used, the actual keystore data must be loaded into memory via the `load` method:

> > > >     final void load(InputStream stream, char[] password)

> > > The optional password is used to check the integrity of the keystore data. If no password is supplied, no integrity check is performed.

> > > To create an empty keystore, you pass `null` as the `InputStream` argument to the `load` method.

> > ### Getting a List of the Keystore Aliases

> > > All keystore entries are accessed via unique aliases. The `aliases` method returns an enumeration of the alias names in the keystore:

> > > >     final Enumeration aliases()

> > ### Determining Keystore Entry Types

> > > As stated in The `KeyStore` Class, there are two different types of entries in a keystore.

> > > The following methods determine whether the entry specified by the given alias is a key/certificate or a trusted certificate entry, respectively:

> > > >     final boolean isKeyEntry(String alias)
> > > >     final boolean isCertificateEntry(String alias)

> > ### Adding/Setting/Deleting Keystore Entries

> > > The `setCertificateEntry` method assigns a certificate to a specified alias:

> > > >     final void setCertificateEntry(String alias, Certificate cert)

> > > If `alias` doesn't exist, a trusted certificate entry with that alias is created. If `alias` exists and identifies a trusted certificate entry, the certificate associated with it is replaced by `cert`.

> > > The `setKeyEntry` methods add (if `alias` doesn't yet exist) or set key entries:

> > > >     final void setKeyEntry(String alias,
> > > >                            Key key,
> > > >                            char[] password,
> > > >                            Certificate[] chain)

> > > >     final void setKeyEntry(String alias,
> > > >                            byte[] key,
> > > >                            Certificate[] chain)

> > > In the method with `key` as a byte array, it is the bytes for a key in protected format. For example, in the keystore implementation supplied by the "SUN" provider, the `key` byte array is expected to contain a protected private key, encoded as an `EncryptedPrivateKeyInfo` as defined in the PKCS #8 standard. In the other method, the `password` is the password used to protect the key.

> > > The `deleteEntry` method deletes an entry:

> > > >     final void deleteEntry(String alias)

> > ### Getting Information from the Keystore

> > > The `getKey` method returns the key associated with the given alias. The key is recovered using the given password:

> > > >     final Key getKey(String alias, char[] password)

> > > The following methods return the certificate, or certificate chain, respectively, associated with the given alias:

> > > >     final Certificate getCertificate(String alias)
> > > >     final Certificate[] getCertificateChain(String alias)

> > > You can determine the name (`alias`) of the first entry whose certificate matches a given certificate via the following:

> > > >     final String getCertificateAlias(Certificate cert)

> > ### Saving the KeyStore

> > > The in-memory keystore can be saved via the `store` method:

> > > >     final void store(OutputStream stream, char[] password)

> > > The password is used to calculate an integrity checksum of the keystore data, which is appended to the keystore data.

## The `SecureRandom` Class

> The `SecureRandom` class is an engine class that provides the functionality of a random number generator.

> ### Creating a `SecureRandom` Object

> > As with all engine classes, the way to get a `SecureRandom` object is to call the `getInstance` static factory method on the `SecureRandom` class:

> > >     static SecureRandom getInstance(String algorithm)

> > A caller may optionally specify the name of a provider or the `Provider` class, which will guarantee that the implementation of the random number generation (RNG) algorithm requested is from the named provider:

> > >     static final SecureRandom getInstance(String algorithm,
> > >                                           String provider)
> > >     static final SecureRandom getInstance(String algorithm,
> > >                                           Provider provider)

> ### Seeding or Re-Seeding the `SecureRandom` Object

> > The `SecureRandom` implementation attempts to completely randomize the internal state of the generator itself unless the caller follows the call to a `getInstance` method with a call to one of the `setSeed` methods:

> > >     synchronized public void setSeed(byte[] seed)
> > >     public void setSeed(long seed)

> > Once the `SecureRandom` object has been seeded, it will produce bits as random as the original seeds.

> > At any time a `SecureRandom` object may be re-seeded using one of the `setSeed` methods. The given seed supplements, rather than replaces, the existing seed; therefore, repeated calls are guaranteed never to reduce randomness.

> ### Using a `SecureRandom` Object

> > To get random bytes, a caller simply passes an array of any length, which is then filled with random bytes:

> > >     synchronized public void nextBytes(byte[] bytes)

> ### Generating Seed Bytes

> > If desired, it is possible to invoke the `generateSeed` method to generate a given number of seed bytes (to seed other random number generators, for example):

> > >     byte[] generateSeed(int numBytes)

# Code Examples

> ### Computing a `MessageDigest` Object

> > First create the message digest object, as in the following example:

> > >     MessageDigest sha = MessageDigest.getInstance("SHA-1");

> > This call assigns a properly initialized message digest object to the `sha` variable. The implementation implements the Secure Hash Algorithm (SHA-1), as defined in the National Institute for Standards and Technology's (NIST) FIPS 180-1 document. See Appendix A for a complete discussion of standard names and algorithms.

> > Next, suppose we have three byte arrays, `i1`, `i2` and `i3`, which form the total input whose message digest we want to compute. This digest (or "hash") could be calculated via the following calls:

> > >     sha.update(i1);
> > >     sha.update(i2);
> > >     sha.update(i3);
> > >     byte[] hash = sha.digest();

> > An equivalent alternative series of calls would be:

> > >     sha.update(i1);
> > >     sha.update(i2);
> > >     byte[] hash = sha.digest(i3);

> > After the message digest has been calculated, the message digest object is automatically reset and ready to receive new data and calculate its digest. All former state (i.e., the data supplied to `update` calls) is lost.

> > Some hash implementations may support intermediate hashes through cloning. Suppose we want to calculate separate hashes for:

> > - `i1`
> > - `i1 and i2`
> > - `i1, i2, and i3`

> > A way to do it is:

> > >     /* compute the hash for i1 */
> > >     sha.update(i1);
> > >     byte[] i1Hash = sha.clone().digest();

> > >     /* compute the hash for i1 and i2 */
> > >     sha.update(i2);
> > >     byte[] i12Hash = sha.clone().digest();

> > >     /* compute the hash for i1, i2 and i3 */
> > >     sha.update(i3);
> > >     byte[] i123hash = sha.digest();

> > This code works only if the SHA-1 implementation is cloneable. While some implementations of message digests are cloneable, others are not. To determine whether or not cloning is possible, attempt to clone the `MessageDigest` object and catch the potential exception as follows:

> > >     try {
> > >         // try and clone it
> > >         /* compute the hash for i1 */
> > >         sha.update(i1);
> > >         byte[] i1Hash = sha.clone().digest();
> > >         . . .
> > >         byte[] i123hash = sha.digest();
> > >     } catch (CloneNotSupportedException cnse) {
> > >         // do something else, such as the code shown below
> > >     }

> > If a message digest is not cloneable, the other, less elegant way to compute intermediate digests is to create several digests. In this case, the number of intermediate digests to be computed must be known in advance:

> > >     MessageDigest sha1 = MessageDigest.getInstance("SHA-1");
> > >     MessageDigest sha12 = MessageDigest.getInstance("SHA-1");
> > >     MessageDigest sha123 = MessageDigest.getInstance("SHA-1");

> > >     byte[] i1Hash = sha1.digest(i1);

> > >     sha12.update(i1);
> > >     byte[] i12Hash = sha12.digest(i2);

> > >     sha123.update(i1);
> > >     sha123.update(i2);
> > >     byte[] i123Hash = sha123.digest(i3);

> ### Generating a Pair of Keys

> > In this example we will generate a public-private key pair for the algorithm named "DSA" (Digital Signature Algorithm). We will generate keys with a 1024-bit modulus, using a user-derived seed, called `userSeed`. We don't care which provider supplies the algorithm implementation.

> > #### Creating the Key Pair Generator

> > > The first step is to get a key pair generator object for generating keys for the DSA algorithm:

> > > >     KeyPairGenerator keyGen = KeyPairGenerator.getInstance("DSA");

> > #### Initializing the Key Pair Generator

> > > The next step is to initialize the key pair generator. In most cases, algorithm-independent initialization is sufficient, but in some cases, algorithm-specific initialization is used.

> > > ##### Algorithm-Independent Initialization

> > > All key pair generators share the concepts of a keysize and a source of randomness. A `KeyPairGenerator` class `initialize` method has these two types of arguments. Thus, to generate keys with a keysize of 1024 and a new `SecureRandom` object seeded by the `userSeed` value, you can use the following code:

> > > >     SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");
> > > >     random.setSeed(userSeed);
> > > >     keyGen.initialize(1024, random);

> > > Since no other parameters are specified when you call the above algorithm-independent `initialize` method, it is up to the provider what to do about the algorithm-specific parameters (if any) to be associated with each of the keys. The provider may use precomputed parameter values or may generate new values.

> > > ##### Algorithm-Specific Initialization

> > > For situations where a set of algorithm-specific parameters already exists (such as "community parameters" in DSA), there are two `initialize` methods that have an `AlgorithmParameterSpec` argument. Suppose your key pair generator is for the "DSA" algorithm, and you have a set of DSA-specific parameters, `p`, `q`, and `g`, that you would like to use to generate your key pair. You could execute the following code to initialize your key pair generator (recall that `DSAParameterSpec` is an AlgorithmParameterSpec):

> > > >     DSAParameterSpec dsaSpec = new DSAParameterSpec(p, q, g);
> > > >     SecureRandom random = SecureRandom.getInstance("SHA1PRNG", "SUN");
> > > >     random.setSeed(userSeed);
> > > >     keyGen.initialize(dsaSpec, random);

> > > > Note: The parameter named `p` is a prime number whose length is the modulus length ("size"). Therefore, you don't need to call any other method to specify the modulus length.

> > #### Generating the Pair of Keys

> > > The final step is generating the key pair. No matter which type of initialization was used (algorithm-independent or algorithm-specific), the same code is used to generate the key pair:

> > > >     KeyPair pair = keyGen.generateKeyPair();

> ### Generating and Verifying a Signature Using Generated Keys

> > The following signature generation and verification examples use the key pair generated in the key pair example above.

> > #### Generating a Signature

> > > We first create a signature object:

> > > >     Signature dsa = Signature.getInstance("SHA1withDSA");

> > > Next, using the key pair generated in the key pair example, we initialize the object with the private key, then sign a byte array called `data`.

> > > >     /* Initializing the object with a private key */
> > > >     PrivateKey priv = pair.getPrivate();
> > > >     dsa.initSign(priv);

> > > >     /* Update and sign the data */
> > > >     dsa.update(data);
> > > >     byte[] sig = dsa.sign();

> > #### Verifying a Signature

> > > Verifying the signature is straightforward. (Note that here we also use the key pair generated in the key pair example.)

> > > >     /* Initializing the object with the public key */
> > > >     PublicKey pub = pair.getPublic();
> > > >     dsa.initVerify(pub);

> > > >     /* Update and verify the data */
> > > >     dsa.update(data);
> > > >     boolean verifies = dsa.verify(sig);
> > > >     System.out.println("signature verifies: " + verifies);

> ### Generating/Verifying Signatures Using Key Specifications and `KeyFactory`

> > Suppose that, rather than having a public/private key pair (as, for example, was generated in the key pair example above), you simply have the components of your DSA private key: `x` (the private key), `p` (the prime), `q` (the sub-prime), and `g` (the base).

> > Further suppose you want to use your private key to digitally sign some data, which is in a byte array named `someData`. You would do the following steps, which also illustrate creating a key specification and using a key factory to obtain a `PrivateKey` from the key specification (`initSign` requires a `PrivateKey`):

> > >     DSAPrivateKeySpec dsaPrivKeySpec = new DSAPrivateKeySpec(x, p, q, g);

> > >     KeyFactory keyFactory = KeyFactory.getInstance("DSA");
> > >     PrivateKey privKey = keyFactory.generatePrivate(dsaPrivKeySpec);

> > >     Signature sig = Signature.getInstance("SHA1withDSA");
> > >     sig.initSign(privKey);
> > >     sig.update(someData);
> > >     byte[] signature = sig.sign();

> > Suppose Alice wants to use the data you signed. In order for her to do so, and to verify your signature, you need to send her three things:
> > 1. the data,
> > 2. the signature, and
> > 3. the public key corresponding to the private key you used to sign the data.
> > You can store the `someData` bytes in one file, and the `signature` bytes in another, and send those to Alice.

> > For the public key, assume, as in the signing example above, you have the components of the DSA public key corresponding to the DSA private key used to sign the data. Then you can create a DSAPublicKeySpec from those components:

> > >     DSAPublicKeySpec dsaPubKeySpec = new DSAPublicKeySpec(y, p, q, g);

> > You still need to extract the key bytes so that you can put them in a file. To do so, you can first call the `generatePublic` method on the DSA key factory already created in the example above:

> > >     PublicKey pubKey = keyFactory.generatePublic(dsaPubKeySpec);

> > Then you can extract the (encoded) key bytes via the following:

> > >     byte[] encKey = pubKey.getEncoded();

> > You can now store these bytes in a file, and send it to Alice along with the files containing the data and the signature.

> > Now, assume Alice has received these files, and she copied the data bytes from the data file to a byte array named `data`, the signature bytes from the signature file to a byte array named `signature`, and the encoded public key bytes from the public key file to a byte array named `encodedPubKey`.

> > Alice can now execute the following code to verify the signature. The code also illustrates how to use a key factory in order to instantiate a DSA public key from its encoding (`initVerify` requires a `PublicKey`).

> >         X509EncodedKeySpec pubKeySpec = new X509EncodedKeySpec(encodedPubKey);

> >         KeyFactory keyFactory = KeyFactory.getInstance("DSA");
> >         PublicKey pubKey = keyFactory.generatePublic(pubKeySpec);

> >         Signature sig = Signature.getInstance("SHA1withDSA");
> >         sig.initVerify(pubKey);
> >         sig.update(data);
> >         sig.verify(signature);

> > Note: In the above, Alice needed to generate a `PublicKey` from the encoded key bits, since `initVerify` requires a `PublicKey`. Once she has a `PublicKey`, she could also use the `KeyFactory` `getKeySpec` method to convert it to a `DSAPublicKeySpec` so that she can access the components, if desired, as in:

> >         DSAPublicKeySpec dsaPubKeySpec =
> >             (DSAPublicKeySpec)keyFactory.getKeySpec(pubKey,
> >                 DSAPublicKeySpec.class)

> > Now she can access the DSA public key components `y`, `p`, `q`, and `g` through the corresponding "get" methods on the `DSAPublicKeySpec` class (`getY`, `getP`, `getQ`, and `getG`).

> ### Determining If Two Keys Are Equal

> > In many cases you would like to know if two keys are equal; however, the default method `java.lang.Object.equals` may not give the desired result. The most provider-independent approach is to compare the encoded keys. If this comparison isn't appropriate (for example, when comparing an `RSAPrivateKey` and an `RSAPrivateCrtKey`), you should compare each component. The following code demonstrates this idea:

> > >     static boolean keysEqual(Key key1, Key key2) {
> > >         if (key1.equals(key2)) {
> > >             return true;
> > >         }

> > >         if (Arrays.equals(key1.getEncoded(), key2.getEncoded())) {
> > >             return true;
> > >         }

> > >         // More code for different types of keys here.
> > >         // For example, the following code can check if
> > >         // an RSAPrivateKey and an RSAPrivateCrtKey are equal:
> > >         // if ((key1 instanceof RSAPrivateKey) &&
> > >         //     (key2 instanceof RSAPrivateKey)) {
> > >         //     if ((key1.getModulus().equals(key2.getModulus())) &&
> > >         //         (key1.getPrivateExponent().equals(
> > >         //                                      key2.getPrivateExponent()))) {
> > >         //         return true;
> > >         //     }
> > >         // }

> > >         return false;
> > >     }

> ### Reading Base64-Encoded Certificates

> > The following example reads a file with Base64-encoded certificates, which are each bounded at the beginning by

> >     -----BEGIN CERTIFICATE-----

> > and at the end by

> >     -----END CERTIFICATE-----

> > We convert the `FileInputStream` (which does not support `mark` and `reset`) to a `ByteArrayInputStream` (which supports those methods), so that each call to `generateCertificate` consumes only one certificate, and the read position of the input stream is positioned to the next certificate in the file:

> >     FileInputStream fis = new FileInputStream(filename);
> >     DataInputStream dis = new DataInputStream(fis);

> >     CertificateFactory cf = CertificateFactory.getInstance("X.509");

> >     byte[] bytes = new byte[dis.available()];
> >     dis.readFully(bytes);
> >     ByteArrayInputStream bais = new ByteArrayInputStream(bytes);

> >     while (bais.available() > 0) {
> >        Certificate cert = cf.generateCertificate(bais);
> >        System.out.println(cert.toString());
> >     }

> ### Parsing a Certificate Reply

> > The following example parses a PKCS #7-formatted certificate reply stored in a file and extracts all the certificates from it:

> >     FileInputStream fis = new FileInputStream(filename);
> >     CertificateFactory cf = CertificateFactory.getInstance("X.509");
> >     Collection c = cf.generateCertificates(fis);
> >     Iterator i = c.iterator();
> >     while (i.hasNext()) {
> >        Certificate cert = (Certificate)i.next();
> >        System.out.println(cert);
> >     }

# Appendix A: Standard Names

> The Java 2 SDK Security API requires and uses a set of standard names for algorithms, certificate and keystore types. This specification establishes the following names as standard names.

> In some cases naming conventions are suggested for forming names that are not explicitly listed, to facilitate name consistency across provider implementations. Such suggestions use items in angle brackets (such as <digest> and <encryption>) as placeholders to be replaced by specific message digest, encryption algorithm, and other names.

> > Note: Algorithm names are not case-sensitive.

> This appendix includes corresponding lists of standard names relevant to the various security subareas:
> - Java Certification Path API Programmer's Guide

> - JCE Reference Guide

> - JSSE Reference Guide

> See Appendix B for algorithm specifications.

> ### Message Digest Algorithms

> > The algorithm names in this section can be specified when generating an instance of `MessageDigest`.

> > MD2: The MD2 message digest algorithm as defined in RFC 1319.

> > MD5: The MD5 message digest algorithm as defined in RFC 1321.

> > SHA-1: The Secure Hash Algorithm, as defined in Secure Hash Standard, NIST FIPS 180-1.

> > SHA-256, SHA-384, and SHA-512: New hash algorithms for which the draft Federal Information Processing Standard 180-2, Secure Hash Standard (SHS) is now available. SHA-256 is a 256-bit hash function intended to provide 128 bits of security against collision attacks, while SHA-512 is a 512-bit hash function intended to provide 256 bits of security. A 384-bit hash may be obtained by truncating the SHA-512 output.

> ### Key and Parameter Algorithms

> > The algorithm names in this section can be specified when generating an instance of `KeyPairGenerator`, `KeyFactory`, `AlgorithmParameterGenerator`, and `AlgorithmParameters`.

> > DSA: The Digital Signature Algorithm as defined in FIPS PUB 186.

> > RSA: The RSA encryption algorithm as defined in PKCS #1.

> ### Digital Signature Algorithms

> > The algorithm names in this section can be specified when generating an instance of `Signature`.

> > MD2withRSA: The MD2 with RSA Encryption signature algorithm which uses the MD2 digest algorithm and RSA to create and verify RSA digital signatures as defined in PKCS #1.

> > MD5withRSA: The MD5 with RSA Encryption signature algorithm which uses the MD5 digest algorithm and RSA to create and verify RSA digital signatures as defined in PKCS #1.

> > SHA1withDSA: The DSA with SHA-1 signature algorithm which uses the SHA-1 digest algorithm and DSA to create and verify DSA digital signatures as defined in FIPS PUB 186.

> > SHA1withRSA: The signature algorithm with SHA-1 and the RSA encryption algorithm as defined in the OSI Interoperability Workshop, using the padding conventions described in PKCS #1.

> > <digest>with<encryption>: Use this to form a name for a signature algorithm with a particular message digest (such as MD2 or MD5) and algorithm (such as RSA or DSA), just as was done for the explicitly-defined standard names in this section (MD2withRSA, etc.). For the new signature schemes defined in PKCS #1 v 2.0, for which the <digest>with<encryption> form is insufficient, <digest>with<encryption>and<mgf> can be used to form a name. Here, <mgf> should be replaced by a mask generation function such as MGF1. Example: MD5withRSAandMGF1.

> ### Random Number Generation (RNG) Algorithms

> > The algorithm names in this section can be specified when generating an instance of `SecureRandom`.

> > SHA1PRNG: The name of the pseudo-random number generation (PRNG) algorithm supplied by the SUN provider. This implementation follows the IEEE P1363 standard, Appendix G.7: "Expansion of source bits", and uses SHA-1 as the foundation of the PRNG. It computes the SHA-1 hash over a true-random seed value concatenated with a 64-bit counter which is incremented by 1 for each operation. From the 160-bit SHA-1 output, only 64 bits are used.

> ### Certificate Types

> > The types in this section can be specified when generating an instance of `CertificateFactory`.

> > X.509: The certificate type defined in X.509.

> ### Keystore Types

> > The types in this section can be specified when generating an instance of `KeyStore`.

> > JKS: The name of the keystore implementation provided by the SUN provider.

> > PKCS12: The transfer syntax for personal identity information as defined in PKCS #12.

> ### Service Attributes

> > A cryptographic service is always associated with a particular algorithm or type. For example, a digital signature service is always associated with a particular algorithm (e.g., DSA), and a `CertificateFactory` service is always associated with a particular certificate type (e.g., X.509).

> > The attributes in this section are for cryptographic services. The service attributes can be used as filters for selecting providers.

> > Both the attribute name and value are case insensitive.

> > KeySize: The maximum key size that the provider supports for the cryptographic service.

> > ImplementedIn: Whether the implementation for the cryptographic service is done by software or hardware. The value of this attribute is "software" or "hardware".

# Appendix B: Algorithms

> This appendix specifies details concerning some of the algorithms defined in Appendix A. Any provider supplying an implementation of the listed algorithms must comply with the specifications in this appendix.

> > Note: The most recent version of this document is available online at: http://java.sun.com/j2se/1.4/docs/guide/security/index.html.

> To add a new algorithm not specified here, you should first survey other people or companies supplying provider packages to see if they have already added that algorithm, and, if so, use the definitions they published, if available. Otherwise, you should create and make available a template, similar to those found in this Appendix B, with the specifications for the algorithm you provide.

> ### Specification Template

> > The following table shows the fields of the algorithm specifications.

> > > Field | Description
> > > Name | The name by which the algorithm is known. This is the name passed to the `getInstance` method (when requesting the algorithm), and returned by the `getAlgorithm` method to determine the name of an existing algorithm object. These methods are in the relevant engine classes: `Signature`, `MessageDigest`, `KeyPairGenerator`, and `AlgorithmParameterGenerator`.
> > > Type | The type of algorithm: `Signature`, `MessageDigest`, `KeyPairGenerator`, or `ParameterGenerator`.
> > > Description | General notes about the algorithm, including any standards implemented by the algorithm, applicable patents, etc.
> > > `KeyPair` Algorithm (optional) | The keypair algorithm for this algorithm.
> > > Keysize (optional) | For a keyed algorithm or key generation algorithm: the legal keysizes.
> > > Size (optional) | For an algorithm parameter generation algorithm: the legal "sizes" for algorithm parameter generation.
> > > Parameter Defaults (optional) | For a key generation algorithm: the default parameter values.
> > > `Signature` Format (optional) | For a `Signature` algorithm, the format of the signature, that is, the input and output of the verify and sign methods, respectively.

> ### Algorithm Specifications

> > #### SHA-1 Message Digest Algorithm

> > > Name | SHA-1
> > > Type | `MessageDigest`
> > > Description | The message digest algorithm as defined in NIST's FIPS 180-1. The output of this algorithm is a 160-bit digest.

> > #### MD2 Message Digest Algorithm

> > > Name | MD2
> > > Type | `MessageDigest`
> > > Description | The message digest algorithm as defined in RFC 1319. The output of this algorithm is a 128-bit (16 byte) digest.

> > #### MD5 Message Digest Algorithm

> > > Name | MD5
> > > Type | `MessageDigest`
> > > Description | The message digest algorithm as defined in RFC 1321. The output of this algorithm is a 128-bit (16 byte) digest.

> > #### The Digital Signature Algorithm

> > > Name | SHA1withDSA
> > > Type | `Signature`
> > > Description | This algorithm is the signature algorithm described in NIST FIPS 186, using DSA with the SHA-1 message digest algorithm.
> > > `KeyPair` Algorithm | DSA
> > > Signature Format | ASN.1 sequence of two INTEGER values: `r` and `s`, in that order: `SEQUENCE ::= { r INTEGER, s INTEGER }`

> > #### RSA-based Signature Algorithms, with MD2, MD5 or SHA-1

> > > Names | MD2withRSA, MD5withRSA and SHA1withRSA
> > > Type | `Signature`
> > > Description | These are the signature algorithms that use the MD2, MD5, and SHA-1 message digest algorithms (respectively) with RSA encryption.

> > > `KeyPair` Algorithm

> > > > RSA

> > > Signature Format | DER-encoded PKCS #1 block as defined in RSA Laboratory's Public Key Cryptography Standards Note #1. The data encrypted is the digest of the data signed.

> > #### DSA KeyPair Generation Algorithm

> > > Name | DSA
> > > Type | `KeyPairGenerator`
> > > Description | This algorithm is the key pair generation algorithm described in NIST FIPS 186 for DSA.
> > > Keysize | The length, in bits, of the modulus `p`. This must range from 512 to 1024, and must be a multiple of 64. The default keysize is 1024.

> > > Parameter Defaults

> > > The following default parameter values are used for keysizes of 512, 768, and 1024 bits:

> > > ##### 512-bit Key Parameters

> > >     SEED = b869c82b 35d70e1b 1ff91b28 e37a62ec dc34409b
> > >     counter = 123
> > >     p = fca682ce 8e12caba 26efccf7 110e526d b078b05e decbcd1e b4a208f3
> > >         ae1617ae 01f35b91 a47e6df6 3413c5e1 2ed0899b cd132acd 50d99151
> > >         bdc43ee7 37592e17
> > >     q = 962eddcc 369cba8e bb260ee6 b6a126d9 346e38c5

> > >     g = 678471b2 7a9cf44e e91a49c5 147db1a9 aaf244f0 5a434d64 86931d2d
> > >         14271b9e 35030b71 fd73da17 9069b32e 2935630e 1c206235 4d0da20a
> > >         6c416e50 be794ca4

> > > ##### 768-bit key parameters

> > >     SEED = 77d0f8c4 dad15eb8 c4f2f8d6 726cefd9 6d5bb399
> > >     counter = 263
> > >     p = e9e64259 9d355f37 c97ffd35 67120b8e 25c9cd43 e927b3a9 670fbec5
> > >         d8901419 22d2c3b3 ad248009 3799869d 1e846aab 49fab0ad 26d2ce6a
> > >         22219d47 0bce7d77 7d4a21fb e9c270b5 7f607002 f3cef839 3694cf45
> > >         ee3688c1 1a8c56ab 127a3daf
> > >     q = 9cdbd84c 9f1ac2f3 8d0f80f4 2ab952e7 338bf511
> > >     g = 30470ad5 a005fb14 ce2d9dcd 87e38bc7 d1b1c5fa cbaecbe9 5f190aa7
> > >         a31d23c4 dbbcbe06 17454440 1a5b2c02 0965d8c2 bd2171d3 66844577
> > >         1f74ba08 4d2029d8 3c1c1585 47f3a9f1 a2715be2 3d51ae4d 3e5a1f6a
> > >         7064f316 933a346d 3f529252

> > > ##### 1024-bit key parameters

> > >     SEED = 8d515589 4229d5e6 89ee01e6 018a237e 2cae64cd
> > >     counter = 92
> > >     p = fd7f5381 1d751229 52df4a9c 2eece4e7 f611b752 3cef4400 c31e3f80
> > >         b6512669 455d4022 51fb593d 8d58fabf c5f5ba30 f6cb9b55 6cd7813b
> > >         801d346f f26660b7 6b9950a5 a49f9fe8 047b1022 c24fbba9 d7feb7c6
> > >         1bf83b57 e7c6a8a6 150f04fb 83f6d3c5 1ec30235 54135a16 9132f675
> > >         f3ae2b61 d72aeff2 2203199d d14801c7
> > >     q = 9760508f 15230bcc b292b982 a2eb840b f0581cf5

> > >     g = f7e1a085 d69b3dde cbbcab5c 36b857b9 7994afbb fa3aea82 f9574c0b
> > >         3d078267 5159578e bad4594f e6710710 8180b449 167123e8 4c281613
> > >         b7cf0932 8cc8a6e1 3c167a8b 547c8d28 e0a3ae1e 2bb3a675 916ea37f
> > >         0bfa2135 62f1fb62 7a01243b cca4f1be a8519089 a883dfe1 5ae59f06
> > >         928b665e 807b5525 64014c3b fecf492a

> > #### RSA KeyPair Generation Algorithm

> > > Names | RSA
> > > Type | `KeyPairGenerator`
> > > Description | This algorithm is the key pair generation algorithm described in PKCS #1.
> > > Strength | Any integer that is a multiple of 8, greater than or equal to 512.

> > #### DSA Parameter Generation Algorithm

> > > Names | DSA
> > > Type | `ParameterGenerator`
> > > Description | This algorithm is the parameter generation algorithm described in NIST FIPS 186 for DSA.
> > > Strength | The length, in bits, of the modulus `p`. This must range from 512 to 1024, and must be a multiple of 64. The default size is 1024.

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