The Java platform defines a set of APIs spanning major security areas, including cryptography, public key infrastructure, authentication, secure communication, and access control. These APIs allow developers to easily integrate security into their application code. They were designed around the following principles:

1. Implementation independence
   
   Applications do not need to implement security themselves. Rather, they can request security services from the Java platform. Security services are implemented in providers (see below), which are plugged into the Java platform via a standard interface. An application may rely on multiple independent providers for security functionality.
2. Implementation interoperability
   
   Providers are interoperable across applications. Specifically, an application is not bound to a specific provider, and a provider is not bound to a specific application.
3. Algorithm extensibility
   
   The Java platform includes a number of built-in providers that implement a basic set of security services that are widely used today. However, some applications may rely on emerging standards not yet implemented, or on proprietary services. The Java platform supports the installation of custom providers that implement such services.

A Cryptographic Service Provider (provider) refers to a package (or a set of packages) that supply a concrete implementation of a subset of the cryptography aspects of the JDK Security API.

The java.security.Provider class encapsulates the notion of a security provider in the Java platform. It specifies the provider's name and lists the security services it implements. Multiple providers may be configured at the same time, and are listed in order of preference. When a security service is requested, the highest priority provider that implements that service is selected.

Figures 1 and 2 illustrate these options for requesting an MD5 message digest implementation. Both figures show three providers that implement message digest algorithms. The providers are ordered by preference from left to right (1-3). In Figure 1, an application requests an MD5 algorithm implementation without specifying a provider name. The providers are searched in preference order and the implementation from the first provider supplying that particular algorithm, ProviderB, is returned. In Figure 2, the application requests the MD5 algorithm implementation from a specific provider, ProviderC. This time the implementation from that provider is returned, even though a provider with a higher preference order, ProviderB, also supplies an MD5 implementation.

Figure 1: Provider searching Figure 2: Specific provider requested

Figures 1 and 2 illustrate these options for requesting an MD5 message digest implementation. Both figures show three providers that implement message digest algorithms. The providers are ordered by preference from left to right (1-3). In Figure 1, an application requests an MD5 algorithm implementation without specifying a provider name. The providers are searched in preference order and the implementation from the first provider supplying that particular algorithm, ProviderB, is returned. In Figure 2, the application requests the MD5 algorithm implementation from a specific provider, ProviderC. This time the implementation from that provider is returned, even though a provider with a higher preference order, ProviderB, also supplies an MD5 implementation.

Each installation has one or more provider packages installed. Clients may configure their runtimes 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 algorithms when no particular provider is requested.

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.

Who Should Read This Document

Programmers that only need to use the Java Security API to access existing cryptography algorithms and other services do not need to read this document.

This document is intended for experienced programmers wishing to create their own provider packages supplying cryptographic service implementations. It documents what you need to do in order to integrate your provider into Java so that your algorithms and other services can be found when Java Security API clients request them.

Related Documentation

This document assumes you have already read the Java Cryptography Architecture Reference Guide.

It documents the packages which contain the various classes and interfaces in the Security API.

• java.security
• java.security.spec
• java.security.interfaces
• javax.crypto
• javax.crypto.spec
• javax.crypto.interfaces

Notes on Terminology

• Prior to JDK 1.4, the JCE was an unbundled product, and as such, the JCA and JCE were regularly referred to as separate, distinct components. As JCE is now bundled in JDK, the distinction is becoming less apparent. Since the JCE uses the same architecture as the JCA, the JCE should be more properly thought of as a subset of the JCA.
• The JCA within the JDK includes two software components:
  • the framework that defines and supports cryptographic services for which providers supply implementations. This framework includes packages such as java.security, javax.crypto, javax.crypto.spec, and javax.crypto.interfaces.
  • the actual providers such as Sun, SunRsaSign, SunJCE, which contain the actual cryptographic implementations.

Throughout this document, the terms JCA by itself refers to the JCA framework. Whenever this document notes a specific JCA provider, it will be referred to explicitly by the provider name.

Engine Classes and Corresponding Service Provider Interface Classes

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. It either provides cryptographic operations (like those for digital signatures or message digests, ciphers or key agreement protocols ); 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, here are four engine classes:

• The Signature class provides access to the functionality of a digital signature algorithm.
• A DSA KeyFactory class 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 Cipher class provides access to the functionality of an encryption algorithm (such as DES)
• the KeyAgreement class provides access to the functionality of a key agreement protocol (such as Diffie-Hellman)

The Java Cryptography Architecture encompasses the classes comprising the Security package that relate to cryptography, including the engine classes. Users of the API request and utilize instances of the engine classes to carry out corresponding operations. The JDK defines the following engine classes:

• 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.
• 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.
• CertificateFactory - used to create public key certificates and Certificate Revocation Lists (CRLs).
• 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.
• Cipher</<b>: used to encrypt or decrypt some specified data.
• KeyAgreement</<b>: used to execute a key agreement (key exchange) protocol between 2 or more parties.
• KeyGenerator</<b>: used to generate a secret (symmetric) key suitable for a specified algorithm.
• Mac</<b>: used to compute the message authentication code of some specified data.
• SecretKeyFactory</<b>: used to convert opaque cryptographic keys of type SecretKey into key specifications (transparent representations of the underlying key material), and vice versa.
• ExemptionMechanism</<b>: used to provide the functionality of an exemption mechanism such as key recovery, key weakening, key escrow, or any other (custom) exemption mechanism. Applications or applets that use an exemption mechanism may be granted stronger encryption capabilities than those which don't. However, please note that cryptographic restrictions are no longer required for most countries, and thus exemption mechanisms may only be useful in those few countries whose governments mandate restrictions.

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. For example, the Signature engine class provides access to the functionality of a digital signature algorithm. The actual implementation supplied in a SignatureSpi subclass (see next paragraph) would be that for a specific kind of signature algorithm, such as SHA1 with DSA, SHA1 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 Service Provider Interface 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 and 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 Key Specification Interfaces and Classes Required by Key Factories for details. The actual implementation supplied in a KeyFactorySpi subclass would be that for a specific type of keys, e.g., DSA public and private keys.

Steps to Implement and Integrate a Provider

Follow the steps below to implement a provider and integrate it into the JCA framework:

Step 1: Write your Service Implementation Code

The first thing you need to do is to write the code that provides algorithm-specific implementations of the cryptographic services you want to support.

Note that your provider may supply implementations of cryptographic services already available in one or more of the security components of the JDK.

For cryptographic services not defined in JCA (For example; signatures and message digests), please refer to Java Cryptography Architecture Reference Guide.

For each cryptographic service you wish to implement, create a subclass of the appropriate SPI class. JCA defines the following engine classes:

• SignatureSpi
• MessageDigestSpi
• KeyPairGeneratorSpi
• SecureRandomSpi
• AlgorithmParameterGeneratorSpi
• AlgorithmParametersSpi
• KeyFactorySpi
• CertificateFactorySpi
• KeyStoreSpi
• CipherSpi
• KeyAgreementSpi
• KeyGeneratorSpi
• MacSpi
• SecretKeyFactorySpi
• ExemptionMechanismSpi

(See Engine Classes and Corresponding SPI Classes in this document for information on the JCA and other cryptographic classes.)

In your subclass, you need to:

1. Supply implementations for the abstract methods, whose names usually begin with engine. See Further Implementation Details and Requirements for additional information.
2. Ensure there is a public constructor without any arguments. Here's why: When one of your services is requested, Java Security looks up the subclass implementing that service, as specified by a property in your "master class" (see Step 3). Java Security then creates the Class object associated with your subclass, and creates an instance of your subclass by calling the newInstance method on that Class object. newInstance requires your subclass to have a public constructor without any parameters.
3. A default constructor without arguments will automatically be generated if your subclass doesn't have any constructors. But if your subclass defines any constructors, you must explicitly define a public constructor without arguments.

Step 1a: Additional JCA Provider Requirements and Recommendations for Encryption Implementations

When instantiating a provider's implementation (class) of a Cipher, KeyAgreement, KeyGenerator, MAC or SecretKey factory, the framework will determine the provider's codebase (JAR file) and verify its signature. In this way, JCA authenticates the provider and ensures that only providers signed by a trusted entity can be plugged into JCA. Thus, one requirement for encryption providers is that they must be signed, as described in later steps.

In addition, each provider should perform self-integrity checking to ensure that the JAR file containing its code has not been manipulated in an attempt to invoke provider methods directly rather than through JCA. For further information, see How a Provider Can Do Self-Integrity Checking.

In order for provider classes to become unusable if instantiated by an application directly, bypassing JCA, providers should implement the following:

• All SPI implementation classes in a provider package should be declared final (so that they cannot be subclassed), and their (SPI) implementation methods should be declared protected.
• All crypto-related helper classes in a provider package should have package-private scope, so that they cannot be accessed from outside the provider package.

For providers that may be exported outside the U.S., CipherSpi implementations must include an implementation of the engineGetKeySize method which, given a Key, returns the key size. If there are restrictions on available cryptographic strength specified in jurisdiction policy files, each Cipher initialization method calls engineGetKeySize and then compares the result with the maximum allowable key size for the particular location and circumstances of the applet or application being run. If the key size is too large, the initialization method throws an exception.

Additional optional features that providers may implement are

• the engineWrap and engineUnwrap methods of CipherSpi. Wrapping a key enables secure transfer of the key from one place to another. Information about wrapping and unwrapping keys is provided in the Wrapping and Unwrapping Keys section of the Java Cryptography Architecture Reference Guide.
• one or more exemption mechanisms. An exemption mechanism is something such as key recovery, key escrow, or key weakening which, if implemented and enforced, may enable reduced cryptographic restrictions for an application (or applet) that uses it. For information on the requirements for apps that utilize exemption mechanisms, see How to Make Applications "Exempt" from Cryptographic Restrictions in the Java Cryptography Architecture Reference Guide.

Step 2: Give your Provider a Name

Decide on a name for your provider. This is the name to be used by client applications to refer to your provider.

Step 3: Write your Master Class, a subclass of Provider

The third step is to create a subclass of the java.security.Provider class.

Your subclass should be a final class, and its constructor should

• call super, specifying the provider name (see Step 2), version number, and a string of information about the provider and algorithms it supports. For example:

      super("CryptoX", 1.0, "CryptoX provider v1.0, implementing " +
          "RSA encryption and key pair generation, and DES encryption.");
  

• set the values of various properties that are required for the Java Security API to look up the cryptographic services implemented by the provider. For each service implemented by the provider, there must be a property whose name is the type of service (for example; Signature, MessageDigest, Cipher, KeyAgreement) Signature, MessageDigest, KeyPairGenerator, SecureRandom, KeyFactory, KeyStore, CertificateFactory, AlgorithmParameterGenerator, AlgorithmParameters, Cipher, KeyAgreement, KeyGenerator, Mac, SecretKeyFactory, or ExemptionMechanism) followed by a period and the name of the algorithm to which the service applies. The property value must specify the fully qualified name of the class implementing the service.

• The list below shows the various types of JCA services, where the actual algorithm name is substitued for algName:

  • Signature.algName
  • MessageDigest.algName
  • KeyPairGenerator.algName
  • SecureRandom.algName
  • AlgorithmParameterGenerator.algName
  • AlgorithmParameters.algName
  • KeyFactory.algName
  • CertificateFactory.algName
  • KeyStore.algName
  • Cipher.algName
  • KeyAgreement.algName
  • KeyGenerator.algName
  • Mac.algName
  • SecretKeyFactory.algName
  • ExemptionMechanism.algName

  In all of these except ExemptionMechanism and Cipher, algName, certType , or storeType is the "standard" name of the algorithm, certificate type, or keystore type. See Appendix A of the Java Cryptography Architecture Reference Guide for the standard names that should be used.)

  In the case of ExemptionMechanism, algName refers to the name of the exemption mechanism, which can be one of the following: KeyRecovery, KeyEscrow, or KeyWeakening. Case does not matter.

  In the case of Cipher, algName may actually represent a transformation, and may be composed of an algorithm name, a particular mode, and a padding scheme. See Appendix A of the Java Cryptography Architecture Reference Guide for details.

  The value of each property must be the fully qualified name of the class implementing the specified algorithm, certificate type, or keystore type. That is, it must be the package name followed by the class name, where the two are separated by a period.

  As an example, the default provider named SUN implements the Digital Signature Algorithm (whose standard name is SHA1withDSA) in a class named DSA in the sun.security.provider package. Its subclass of Provider (which is the Sun class in the sun.security.provider package) sets the Signature.SHA1withDSA property to have the value sun.security.provider.DSA via the following:

      put("Signature.SHA1withDSA", "sun.security.provider.DSA")
  

  The list below shows more properties that can be defined for the various types of services, where the actual algorithm name is substitued for algName, certificate type for certType, keystore type for storeType, and attribute name for attrName:

  • Signature.algName [one or more spaces] attrName
  • MessageDigest.algName [one or more spaces] attrName
  • KeyPairGenerator.algName [one or more spaces] attrName
  • SecureRandom.algName [one or more spaces] attrName
  • KeyFactory.algName [one or more spaces] attrName
  • CertificateFactory.certType [one or more spaces] attrName
  • KeyStore.storeType [one or more spaces] attrName
  • AlgorithmParameterGenerator.algName [one or more spaces] attrName
  • AlgorithmParameters.algName [one or more spaces] attrName
  • Cipher.algName [one or more spaces] attrName
  • KeyAgreement.algName [one or more spaces] attrName
  • KeyGenerator.algName [one or more spaces] attrName
  • Mac.algName [one or more spaces] attrName
  • SecretKeyFactory.algName [one or more spaces] attrName
  • ExemptionMechanism.algName [one or more spaces] attrName

In each of these, algName, certType, storeType, or attrName is the "standard" name of the algorithm, certificate type, keystore type, or attribute. (See Appendix A of the Java Cryptography Architecture Reference Guide for the standard names that should be used.)

For a property in the above format, the value of the property must be the value for the corresponding attribute. (See Appendix A of the Java Cryptography Architecture API Specification & Reference for the definition of each standard attribute.)

As an example, the default provider named "SUN" implements the SHA1withDSA Digital Signature Algorithm in software. In the master class for the provider "SUN", it sets the Signature.SHA1withDSA ImplementedIn to have the value Software via the following:

    put("Signature.SHA1withDSA ImplementedIn", "Software")

For further master class property setting examples, see Appendix A to view the current Sun.java source file or Appendix B to see the SunJCE provider. These files show how the Sun and SunJCE providers set properties.

For each Message Digest and MAC algorithm, indicate whether or not your implementation is cloneable. This is not technically necessary, but it may save clients some time and coding by telling them whether or not intermediate Message Digests or MACs may be possible through cloning. Clients who do not know whether or not a MessageDigest or Mac implementation is cloneable can find out by attempting to clone the object and catching the potential exception, as illustrated by the following example:

    try {
        // try and clone it
        /* compute the MAC for i1 */
        mac.update(i1);
        byte[] i1Mac = mac.clone().doFinal();

        /* compute the MAC for i1 and i2 */
        mac.update(i2);
        byte[] i12Mac = mac.clone().doFinal();

        /* compute the MAC for i1, i2 and i3 */
        mac.update(i3);
        byte[] i123Mac = mac.doFinal();
    } catch (CloneNotSupportedException cnse) {
        // have to use an approach not involving cloning
    } 

where:

• mac is the MAC object they received when they requested one via a call to Mac.getInstance,
• i1, i2 and i3 are input byte arrays, and
• they want to calculate separate hashes for:
  • i1
  • i1 and i2
  • i1, i2, and i3

Key Pair Generators

For a key pair generator algorithm, in case the client does not explicitly initialize the key pair generator (via a call to an initialize method), each provider must supply and document a default initialization. For example, the Diffie-Hellman key pair generator supplied by the SunJCE provider uses a default prime modulus size (keysize) of 1024 bits.

Key Factories

A provider should document all the key specifications supported by its (secret-)key factory.

Algorithm Parameter Generators

In case the client does not explicitly initialize the algorithm parameter generator (via a call to an init method in the AlgorithmParameterGenerator engine class), each provider must supply and document a default initialization. For example, the SunJCE provider uses a default prime modulus size (keysize) of 1024 bits for the generation of Diffie-Hellman parameters, the Sun provider a default modulus prime size of 1024 bits for the generation of DSA parameters.

Signature Algorithms

If you implement a signature algorithm, you should document the format in which the signature (generated by one of the sign methods) is encoded. For example, the SHA1withDSA signature algorithm supplied by the "SUN" provider encodes the signature as a standard ASN.1 SEQUENCE of two integers, r and s.

Random Number Generation (SecureRandom) Algorithms

For a random number generation algorithm, provide information regarding how "random" the numbers generated are, and the quality of the seed when the random number generator is self-seeding. Also note what happens when a SecureRandom object (and its encapsulated SecureRandomSpi implementation object) is deserialized: If subsequent calls to the nextBytes method (which invokes the engineNextBytes method of the encapsulated SecureRandomSpi object) of the restored object yield the exact same (random) bytes as the original object would, then let users know that if this behaviour is undesirable, they should seed the restored random object by calling its setSeed method.

Certificate Factories

A provider should document what types of certificates (and their version numbers, if relevant), can be created by the factory.

Keystores

A provider should document any relevant information regarding the keystore implementation, such as its underlying data format.

Step 12: Make your Class Files and Documentation Available to Clients

After writing, configuring, testing, installing and documenting your provider software, make documentation available to your customers.

Alias Names

For many cryptographic algorithms and types, there is a single official "standard name" defined in Appendix A of the Java Cryptography Architecture Reference Guide.

For example, "MD5" is the standard name for the RSA-MD5 Message Digest algorithm defined by RSA DSI in RFC 1321. DiffieHellman is the standard for the Diffie-Hellman key agreement algorithm defined in PKCS3.

In the JDK, there is an aliasing scheme that enables clients to use aliases when referring to algorithms or types, rather than their standard names. For example, the "SUN" provider's master class (Sun.java) defines the alias "SHA1/DSA" for the algorithm whose standard name is "SHA1withDSA". Thus, the following statements are equivalent:

    Signature sig = Signature.getInstance("SHA1withDSA", "SUN");

    Signature sig = Signature.getInstance("SHA1/DSA", "SUN");

Aliases can be defined in your "master class" (see Step 3). To define an alias, create a property named

    Alg.Alias.engineClassName.aliasName

where engineClassName is the name of an engine class (e.g., Signature), and aliasName is your alias name. The value of the property must be the standard algorithm (or type) name for the algorithm (or type) being aliased.

As an example, the "SUN" provider defines the alias "SHA1/DSA" for the signature algorithm whose standard name is "SHA1withDSA" by setting a property named Alg.Alias.Signature.SHA1/DSA to have the value SHA1withDSA via the following:

    put("Alg.Alias.Signature.SHA1/DSA", "SHA1withDSA");

Note that aliases defined by one provider are available only to that provider and not to any other providers. Thus, aliases defined by the SunJCE provider are available only to the SunJCE provider.

Service Interdependencies

Some algorithms require the use of other types of algorithms. For example, a PBE algorithm usually needs to use a message digest algorithm in order to transform a password into a key.

If you are implementing one type of algorithm that requires another, you can do one of the following:

1. Provide your own implementations for both.
2. Let your implementation of one algorithm use an instance of the other type of algorithm, as supplied by the default Sun provider that is included with every Java SE Platform installation. For example, if you are implementing a PBE algorithm that requires a message digest algorithm, you can obtain an instance of a class implementing the MD5 message digest algorithm by calling

       MessageDigest.getInstance("MD5", "SUN")
   
   

3. Let your implementation of one algorithm use an instance of the other type of algorithm, as supplied by another specific provider. This is only appropriate if you are sure that all clients who will use your provider will also have the other provider installed.
4. Let your implementation of one algorithm use an instance of the other type of algorithm, as supplied by another (unspecified) provider. That is, you can request an algorithm by name, but without specifying any particular provider, as in

       MessageDigest.getInstance("MD5")
   
   

   This is only appropriate if you are sure that there will be at least one implementation of the requested algorithm (in this case, MD5) installed on each Java platform where your provider will be used.

Here are some common types of algorithm interdependencies:

Signature and Message Digest Algorithms

A signature algorithm often requires use of a message digest algorithm. For example, the SHA1withDSA signature algorithm requires the SHA-1 message digest algorithm.

Signature and (Pseudo-)Random Number Generation Algorithms

A signature algorithm often requires use of a (pseudo-)random number generation algorithm. For example, such an algorithm is required in order to generate a DSA signature.

Key Pair Generation and Message Digest Algorithms

A key pair generation algorithm often requires use of a message digest algorithm. For example, DSA keys are generated using the SHA-1 message digest algorithm.

Algorithm Parameter Generation and Message Digest Algorithms

An algorithm parameter generator often requires use of a message digest algorithm. For example, DSA parameters are generated using the SHA-1 message digest algorithm.

KeyStores and Message Digest Algorithms

A keystore implementation will often utilize a message digest algorithm to compute keyed hashes (where the key is a user-provided password) to check the integrity of a keystore and make sure that the keystore has not been tampered with.

Key Pair Generation Algorithms and Algorithm Parameter Generators

A key pair generation algorithm sometimes needs to generate a new set of algorithm parameters. It can either generate the parameters directly, or use an algorithm parameter generator.

Key Pair Generation, Algorithm Parameter Generation, and (Pseudo-)Random Number Generation Algorithms

A key pair generation algorithm may require a source of randomness in order to generate a new key pair and possibly a new set of parameters associated with the keys. That source of randomness is represented by a SecureRandom object. The implementation of the key pair generation algorithm may generate the key parameters itself, or may use an algorithm parameter generator to generate them, in which case it may or may not initialize the algorithm parameter generator with a source of randomness.

Algorithm Parameter Generators and Algorithm Parameters

An algorithm parameter generator's engineGenerateParameters method must return an AlgorithmParameters instance.

Signature and Key Pair Generation Algorithms or Key Factories

If you are implementing a signature algorithm, your implementation's engineInitSign and engineInitVerify methods will require passed-in keys that are valid for the underlying algorithm (e.g., DSA keys for the DSS algorithm). You can do one of the following:

1. Also create your own classes implementing appropriate interfaces (e.g. classes implementing the DSAPrivateKey and DSAPublicKey interfaces from the package java.security.interfaces), and create your own key pair generator and/or key factory returning keys of those types. Require the keys passed to engineInitSign and engineInitVerify to be the types of keys you have implemented, that is, keys generated from your key pair generator or key factory. Or you can,
2. Accept keys from other key pair generators or other key factories, as long as they are instances of appropriate interfaces that enable your signature implementation to obtain the information it needs (such as the private and public keys and the key parameters). For example, the engineInitSign method for a DSS Signature class could accept any private keys that are instances of java.security.interfaces.DSAPrivateKey.

KeyStores and Key and Certificate Factories

A keystore implementation will often utilize a key factory to parse the keys stored in the keystore, and a certificate factory to parse the certificates stored in the keystore.

Default Initializations

In case the client does not explicitly initialize a key pair generator or an algorithm parameter generator, each provider of such a service must supply (and document) a default initialization. For example, the Sun provider uses a default modulus size (strength) of 1024 bits for the generation of DSA parameters, and the "SunJCE" provider uses a default modulus size (keysize) of 1024 bits for the generation of Diffie-Hellman parameters.

Default Key Pair Generator Parameter Requirements

If you implement a key pair generator, your implementation should supply default parameters that are used when clients don't specify parameters. The documentation you supply (Step 11) should state what the default parameters are.

For example, the DSA key pair generator in the Sun provider supplies a set of pre-computed p, q, and g default values for the generation of 512, 768, and 1024-bit key pairs. The following p, q, and g values are used as the default values for the generation of 1024-bit DSA key pairs:

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

(The p and q values given here were generated by the prime generation standard, using the 160-bit

SEED:  8d515589 4229d5e6 89ee01e6 018a237e 2cae64cd

With this seed, the algorithm found p and q when the counter was at 92.)

The Provider.Service Class

Since its introduction, security providers have published their service information via appropriately formatted key-value String pairs they put in their Hashtable entries. While this mechanism is simple and convenient, it limits the amount customization possible. As a result, JDK 5.0 introduced a second option, the Provider.Service class. It offers an alternative way for providers to advertise their services and supports additional features as described below. Note that this addition is fully compatible with the older method of using String valued Hashtable entries. A provider on JDK 5.0 can choose either method as it prefers, or even use both at the same time.

A Provider.Service object encapsulates all information about a service. This is the provider that offers the service, its type (e.g. MessageDigest or Signature), the algorithm name, and the name of the class that implements the service. Optionally, it also includes a list of alternate algorithm names for this service (aliases) and attributes, which are a map of (name, value) String pairs. In addition, it defines the methods newInstance() and supportsParameter(). They have default implementations, but can be overridden by providers if needed, as may be the case with providers that interface with hardware security tokens.

The newInstance() method is used by the security framework when it needs to construct new implementation instances. The default implementation uses reflection to invoke the standard constructor for the respective type of service. For all standard services except CertStore, this is the no-args constructor. The constructorParameter to newInstance() must be null in theses cases. For services of type CertStore, the constructor that takes a CertStoreParameters object is invoked, and constructorParameter must be a non-null instance of CertStoreParameters. A security provider can override the newInstance() method to implement instantiation as appropriate for that implementation. It could use direct invocation or call a constructor that passes additional information specific to the Provider instance or token. For example, if multiple Smartcard readers are present on the system, it might pass information about which reader the newly created service is to be associated with. However, despite customization all implementations must follow the conventions about constructorParameter described above.

The supportsParameter() tests whether the Service can use the specified parameter. It returns false if this service cannot use the parameter. It returns true if this service can use the parameter, if a fast test is infeasible, or if the status is unknown. It is used by the security framework with some types of services to quickly exclude non-matching implementations from consideration. It is currently only defined for the following standard services: Signature, Cipher, Mac, and KeyAgreement. The parameter must be an instance of Key in these cases. For example, for Signature services, the framework tests whether the service can use the supplied Key before instantiating the service. The default implementation examines the attributes SupportedKeyFormats and SupportedKeyClasses as described below. Again, a provider may override this methods to implement additional tests.

The SupportedKeyFormats attribute is a list of the supported formats for encoded keys (as returned by key.getFormat()) separated by the "|" (pipe) character. For example, X.509|PKCS#8. The SupportedKeyClasses attribute is a list of the names of classes of interfaces separated by the "|" character. A key object is considered to be acceptable if it is assignable to at least one of those classes or interfaces named. In other words, if the class of the key object is a subclass of one of the listed classes (or the class itself) or if it implements the listed interface. An example value is "java.security.interfaces.RSAPrivateKey|java.security.interfaces.RSAPublicKey" .

Four methods have been added to the Provider class for adding and looking up Services. As mentioned earlier, the implementation of those methods and also of the existing Properties methods have been specifically designed to ensure compatibility with existing Provider subclasses. This is achieved as follows:

If legacy Properties methods are used to add entries, the Provider class makes sure that the property strings are parsed into equivalent Service objects prior to lookup via getService(). Similarly, if the putService() method is used, equivalent property strings are placed into the provider's hashtable at the same time. If a provider implementation overrides any of the methods in the Provider class, it has to ensure that its implementation does not interfere with this conversion. To avoid problems, we recommend that implementations do not override any of the methods in the Provider class.

Signature Formats

If you implement a signature algorithm, the documentation you supply (Step 11) should specify the format in which the signature (generated by one of the sign methods) is encoded.

For example, the SHA1withDSA signature algorithm supplied by the Sun provider encodes the signature as a standard ASN.1 sequence of two ASN.1 INTEGER values: r and s, in that order:

SEQUENCE ::= {
        r INTEGER,
        s INTEGER }

DSA Interfaces and their Required Implementations

The Java Security API contains the following interfaces (in the java.security.interfaces package) for the convenience of programmers implementing DSA services:

• DSAKey
• DSAKeyPairGenerator
• DSAParams
• DSAPrivateKey
• DSAPublicKey

The following sections discuss requirements for implementations of these interfaces.

DSAKeyPairGenerator Implementation

This interface is obsolete. It used to be needed to enable clients to provide DSA-specific parameters to be used rather than the default parameters your implementation supplies. However, in Java it is no longer necessary; a new KeyPairGenerator initialize method that takes an AlgorithmParameterSpec parameter enables clients to indicate algorithm-specific parameters.

DSAParams Implementation

If you are implementing a DSA key pair generator, you need a class implementing DSAParams for holding and returning the p, q, and g parameters.

A DSAParams implementation is also required if you implement the DSAPrivateKey and DSAPublicKey interfaces. DSAPublicKey and DSAPrivateKey both extend the DSAKey interface, which contains a getParams method that must return a DSAParams object. See DSAPrivateKey and DSAPublicKey Implementations for more information.

Note: there is a DSAParams implementation built into the JDK: the java.security.spec.DSAParameterSpec class.

DSAPrivateKey and DSAPublicKey Implementations

If you implement a DSA key pair generator or key factory, you need to create classes implementing the DSAPrivateKey and DSAPublicKey interfaces.

If you implement a DSA key pair generator, your generateKeyPair method (in your KeyPairGeneratorSpi subclass) will return instances of your implementations of those interfaces.

If you implement a DSA key factory, your engineGeneratePrivate method (in your KeyFactorySpi subclass) will return an instance of your DSAPrivateKey implementation, and your engineGeneratePublic method will return an instance of your DSAPublicKey implementation.

Also, your engineGetKeySpec and engineTranslateKey methods will expect the passed-in key to be an instance of a DSAPrivateKey or DSAPublicKey implementation. The getParams method provided by the interface implementations is useful for obtaining and extracting the parameters from the keys and then using the parameters, for example as parameters to the DSAParameterSpec constructor called to create a parameter specification from parameter values that could be used to initialize a KeyPairGenerator object for DSA.

If you implement a DSA signature algorithm, your engineInitSign method (in your SignatureSpi subclass) will expect to be passed a DSAPrivateKey and your engineInitVerify method will expect to be passed a DSAPublicKey.

Please note: The DSAPublicKey and DSAPrivateKey interfaces define a very generic, provider-independent interface to DSA public and private keys, respectively. The engineGetKeySpec and engineTranslateKey methods (in your KeyFactorySpi subclass) could additionally check if the passed-in key is actually an instance of their provider's own implementation of DSAPrivateKey or DSAPublicKey, e.g., to take advantage of provider-specific implementation details. The same is true for the DSA signature algorithm engineInitSign and engineInitVerify methods (in your SignatureSpi subclass).

To see what methods need to be implemented by classes that implement the DSAPublicKey and DSAPrivateKey interfaces, first note the following interface signatures:

In the java.security.interfaces package:

   public interface DSAPrivateKey extends DSAKey,
                java.security.PrivateKey

   public interface DSAPublicKey extends DSAKey,
                java.security.PublicKey

   public interface DSAKey 

In the java.security package:

   public interface PrivateKey extends Key

   public interface PublicKey extends Key

   public interface Key extends java.io.Serializable 

In order to implement the DSAPrivateKey and DSAPublicKey interfaces, you must implement the methods they define as well as those defined by interfaces they extend, directly or indirectly.

Thus, for private keys, you need to supply a class that implements

• the getX method from the DSAPrivateKey interface.
• the getParams method from the java.security.interfaces.DSAKey interface, since DSAPrivateKey extends DSAKey. Note: The getParams method returns a DSAParams object, so you must also have a DSAParams implementation.
• the getAlgorithm, getEncoded, and getFormat methods from the java.security.Key interface, since DSAPrivateKey extends java.security.PrivateKey, and PrivateKey extends Key.

  Similarly, for public DSA keys, you need to supply a class that implements:

  • the getY method from the DSAPublicKey interface.
  • the getParams method from the java.security.interfaces.DSAKey interface, since DSAPublicKey extends DSAKey. Note: The getParams method returns a DSAParams object, so you must also have a DSAParams implementation.
  • the getAlgorithm, getEncoded, and getFormat methods from the java.security.Key interface, since DSAPublicKey extends java.security.PublicKey, and PublicKey extends Key.

The Java Security API contains the following interfaces (in the java.security.interfaces package) for the convenience of programmers implementing RSA services:

• RSAPrivateKey
• RSAPrivateCrtKey
• RSAPublicKey

The following sections discuss requirements for implementations of these interfaces.

RSAPrivateKey , RSAPrivateCrtKey , and RSAPublicKey Implementations

If you implement an RSA key pair generator or key factory, you need to create classes implementing the RSAPrivateKey (and/or RSAPrivateCrtKey) and RSAPublicKey interfaces. (RSAPrivateCrtKey is the interface to an RSA private key, using the Chinese Remainder Theorem (CRT) representation.)

If you implement an RSA key pair generator, your generateKeyPair method (in your KeyPairGeneratorSpi subclass) will return instances of your implementations of those interfaces.

If you implement an RSA key factory, your engineGeneratePrivate method (in your KeyFactorySpi subclass) will return an instance of your RSAPrivateKey (or RSAPrivateCrtKey) implementation, and your engineGeneratePublic method will return an instance of your RSAPublicKey implementation.

Also, your engineGetKeySpec and engineTranslateKey methods will expect the passed-in key to be an instance of an RSAPrivateKey, RSAPrivateCrtKey, or RSAPublicKey implementation.

If you implement an RSA signature algorithm, your engineInitSign method (in your SignatureSpi subclass) will expect to be passed either an RSAPrivateKey or an RSAPrivateCrtKey, and your engineInitVerify method will expect to be passed an RSAPublicKey.

Please note: The RSAPublicKey, RSAPrivateKey, and RSAPrivateCrtKey interfaces define a very generic, provider-independent interface to RSA public and private keys. The engineGetKeySpec and engineTranslateKey methods (in your KeyFactorySpi subclass) could additionally check if the passed-in key is actually an instance of their provider's own implementation of RSAPrivateKey, RSAPrivateCrtKey, or RSAPublicKey, e.g., to take advantage of provider-specific implementation details. The same is true for the RSA signature algorithm engineInitSign and engineInitVerify methods (in your SignatureSpi subclass).

To see what methods need to be implemented by classes that implement the RSAPublicKey, RSAPrivateKey, and RSAPrivateCrtKey interfaces, first note the following interface signatures:

In the java.security.interfaces package:

    public interface RSAPrivateKey extends java.security.PrivateKey

    public interface RSAPrivateCrtKey extends RSAPrivateKey

    public interface RSAPublicKey extends java.security.PublicKey

In the java.security package:

    public interface PrivateKey extends Key

    public interface PublicKey extends Key

    public interface Key extends java.io.Serializable

In order to implement the RSAPrivateKey, RSAPrivateCrtKey, and RSAPublicKey interfaces, you must implement the methods they define as well as those defined by interfaces they extend, directly or indirectly.

Thus, for RSA private keys, you need to supply a class that implements:

• the getModulus and getPrivateExponent methods from the RSAPrivateKey interface.
• the getAlgorithm, getEncoded, and getFormat methods from the java.security.Key interface, since RSAPrivateKey extends java.security.PrivateKey, and PrivateKey extends Key.

Similarly, for RSA private keys using the Chinese Remainder Theorem (CRT) representation, you need to supply a class that implements:

• all the methods listed above for RSA private keys, since RSAPrivateCrtKey extends java.security.interfaces.RSAPrivateKey.
• the getPublicExponent, getPrimeP, getPrimeQ, getPrimeExponentP, getPrimeExponentQ, and getCrtCoefficient methods from the RSAPrivateKey interface.

For public RSA keys, you need to supply a class that implements:

• the getModulus and getPublicExponent methods from the RSAPublicKey interface.
• the getAlgorithm, getEncoded, and getFormat methods from the java.security.Key interface, since RSAPublicKey extends java.security.PublicKey, and PublicKey extends Key.

JCA contains a number of AlgorithmParameterSpec implementations for the most frequently used cipher and key agreement algorithm parameters. If you are operating on algorithm parameters that should be for a different type of algorithm not provided by JCA, you will need to supply your own AlgorithmParameterSpec implementation appropriate for that type of algorithm.

    public interface DHPrivateKey extends DHKey, java.security.PrivateKey

    public interface DHPublicKey extends DHKey, java.security.PublicKey

    public interface DHKey 

    public interface PrivateKey extends Key

    public interface PublicKey extends Key

    public interface Key extends java.io.Serializable 

    public BigInteger getP()

    public BigInteger getQ()

    public BigInteger getG()

Key Specification Interfaces and Classes Required by Key Factories

A key factory provides bi-directional conversions between opaque keys (of type Key) and key specifications. If you implement a key factory, you thus need to understand and utilize key specifications. In some cases, you also need to implement your own key specifications.

Further information about key specifications, the interfaces and classes supplied in Java, and key factory requirements with respect to specifications, is provided below.

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, java.security.spec.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 is contrasted with an opaque representation, as defined by the Key interface, in which you have no direct access to the parameter 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).

Java defines the following key specification interfaces and classes in the java.security.spec and javax.crypto.spec packages:

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.

Java supplies several classes implementing the KeySpec interface: DSAPrivateKeySpec, DSAPublicKeySpec, RSAPrivateKeySpec, RSAPublicKeySpec, EncodedKeySpec, PKCS8EncodedKeySpec, and X509EncodedKeySpec.

If your provider uses key types (e.g., Your_PublicKey_type and Your_PrivateKey_type) for which the JDK does not already provide corresponding KeySpec classes, there are two possible scenarios, one of which requires that you implement your own key specifications:

1. If your users will never have to access specific key material values of your key type, you will not have to provide any KeySpec classes for your key type.
   
   In this scenario, your users will always create Your_PublicKey_type and Your_PrivateKey_type keys through the appropriate KeyPairGenerator supplied by your provider for that key type. If they want to store the generated keys for later usage, they retrieve the keys' encodings (using the getEncoded method of the Key interface). When they want to create an Your_PublicKey_type or Your_PrivateKey_type key from the encoding (e.g., in order to initialize a Signature object for signing or verification), they create an instance of X509EncodedKeySpec or PKCS8EncodedKeySpec from the encoding, and feed it to the appropriate KeyFactory supplied by your provider for that algorithm, whose generatePublic and generatePrivate methods will return the requested PublicKey (an instance of Your_PublicKey_type) or PrivateKey (an instance of Your_PrivateKey_type) object, respectively.
2. If you anticipate a need for users to access specific key material values of your key type, or to construct a key of your key type from key material and associated parameter values, rather than from its encoding (as in the above case), you have to specify new KeySpec classes (classes that implement the KeySpec interface) with the appropriate constructor methods and get methods for returning key material fields and associated parameter values for your key type. You will specify those classes in a similar manner as is done by the DSAPrivateKeySpec and DSAPublicKeySpec classes provided in JDK 6. You need to ship those classes along with your provider classes, for example, as part of your provider JAR file.

The DSAPrivateKeySpec Class

This class (which implements the KeySpec Interface) specifies a DSA private key with its associated parameters. It has the following methods:

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. It has the following methods:

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. It has the following methods:

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. It has the following methods (in addition to the methods inherited from its superclass RSAPrivateKeySpec ):

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 RSAPublicKeySpec Class

This class (which implements the KeySpec Interface) specifies an RSA public key. It has the following methods:

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.

JDK 6 supplies two classes implementing the EncodedKeySpec interface: PKCS8EncodedKeySpec and X509EncodedKeySpec. If desired, you can supply your own EncodedKeySpec implementations for those or other types of key encodings.

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 or private 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". DHPrivateKeySpec, DHPublicKeySpec, DESKeySpec, DESedeKeySpec, PBEKeySpec, and SecretKeySpec.

The DHPrivateKeySpec Class

This class (which implements the KeySpec interface) specifies a Diffie-Hellman private key with its associated parameters.

The DHPublicKeySpec Class

This class (which implements the KeySpec interface) specifies a Diffie-Hellman public key with its associated parameters.

The DESKeySpec Class

This class (which implements the KeySpec interface) specifies a DES key.

The DESedeKeySpec Class

This class (which implements the KeySpec interface) specifies a DES-EDE (Triple DES) key.

The PBEKeySpec Class

This class implements the KeySpec interface. A user-chosen password can be used with password-based encryption (PBE); the password can be viewed as a type of raw key material. An encryption mechanism that uses this class can derive a cryptographic key from the raw key material.

The SecretKeySpec Class

This class implements the KeySpec interface. Since it also implements the SecretKey interface, it can be used to construct a SecretKey object in a provider-independent fashion, i.e., without having to go through a provider-based SecretKeyFactory.