de.flexiprovider.core.elgamal
Class ElGamal

java.lang.Object
  |
  +--javax.crypto.CipherSpi
        |
        +--de.flexiprovider.api.Cipher
              |
              +--de.flexiprovider.api.AsymmetricBlockCipher
                    |
                    +--de.flexiprovider.core.elgamal.ElGamal

public class ElGamal

extends AsymmetricBlockCipher

This class implements ElGamal encryption. A ciphertext block consists of the two numbers B = g^bmod p and c = A^b * m mod p (m = message). For details about the parameters g, p and A see ElGamalKeyPairGenerator.

To encrypt a message, the following steps have to be performed:

 // The message which should be encrypted
 String message = "secret message";
 byte[] messageBytes = message.getBytes();
 
 // The source of randomness
 SecureRandom secureRandom = Registry.getSecureRandom();
 
 // Obtain a ElGamal Cipher Object
 Cipher elGamalCipher = Cipher.getInstance("ElGamal");
 
 // Obtain the corresponding key pair generator
 KeyPairGenerator elGamalKPG = KeyPairGenerator.getInstance("ElGamal");
 
 // Initialize the key pair generator with the desired strength
 elGamalKPG.initialize(1024);
 
 // Generate a key pair
 KeyPair elGamalKeyPair = elGamalKPG.genKeyPair();
 
 // Initialize the cipher
 cipher.init(Cipher.ENCRYPT_MODE, elGamalKeyPair.getPublic(), secureRandom);
 
 // Finally encrypt the message
 byte[] ciphertextBytes = cipher.doFinal(messageBytes);
 

Decrypting a message is similar to encryption, except the Cipher must be initialized with Cipher.DECRYPT_MODE and the private key ( elGamalKeyPair.getPrivate()).

On an AMD Athlon 550 MHz with a 1024 bit key the encryption rate is about 5 kB/sec, the decryption rate is about 17 kB/sec.

Author:

Thomas Wahrenbruch

See Also:

ElGamalKeyPairGenerator, ElGamalPublicKey, ElGamalPrivateKey

Field Summary

static java.lang.String	ALG_NAME The algorithm name.
static java.lang.String	OID The OID of ElGamal.

Fields inherited from class de.flexiprovider.api.AsymmetricBlockCipher

buf, cipherTextSize, maxPlainTextSize, paramSpec

Fields inherited from class de.flexiprovider.api.Cipher

DECRYPT_MODE, ENCRYPT_MODE, opMode

Constructor Summary

ElGamal()

Method Summary

int	getKeySize(Key key) Return the key size of the given key object in bits.
java.lang.String	getName() Return the name of this cipher.
protected void	initCipherDecrypt(Key key, AlgorithmParameterSpec params) Initialize the block cipher with a key for data encryption.
protected void	initCipherEncrypt(Key key, AlgorithmParameterSpec params, SecureRandom secureRandom) Initialize the block cipher with a key for data encryption.
protected byte[]	messageDecrypt(byte[] input) Decrypt a ciphertext.
protected byte[]	messageEncrypt(byte[] input) Encrypt a message.

Methods inherited from class de.flexiprovider.api.AsymmetricBlockCipher

checkLength, doFinal, doFinal, getBlockSize, getIV, getOutputSize, getParameters, initDecrypt, initDecrypt, initEncrypt, initEncrypt, initEncrypt, initEncrypt, setMode, setPadding, update, update

Methods inherited from class de.flexiprovider.api.Cipher

doFinal, doFinal, engineDoFinal, engineDoFinal, engineGetBlockSize, engineGetIV, engineGetKeySize, engineGetOutputSize, engineGetParameters, engineInit, engineInit, engineInit, engineSetMode, engineSetPadding, engineUpdate, engineUpdate, update

Methods inherited from class java.lang.Object

, clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

ALG_NAME

public static final java.lang.String ALG_NAME

The algorithm name.

OID

public static final java.lang.String OID

The OID of ElGamal.

Constructor Detail

ElGamal

public ElGamal()

Method Detail

getName

public java.lang.String getName()

Return the name of this cipher.

Overrides:

getName in class Cipher

Returns:

"ElGamal"

getKeySize

public int getKeySize(Key key)
               throws InvalidKeyException

Return the key size of the given key object in bits. Checks whether the key object is an instance of ElGamalPublicKey or ElGamalPrivateKey.

Overrides:

getKeySize in class Cipher

Parameters:

key - the key object

Returns:

the key size of the given key object.

Throws:

InvalidKeyException - if key is invalid.

initCipherEncrypt

protected void initCipherEncrypt(Key key,
                                 AlgorithmParameterSpec params,
                                 SecureRandom secureRandom)
                          throws InvalidKeyException

Initialize the block cipher with a key for data encryption. Parameters are currently not supported.

Overrides:

initCipherEncrypt in class AsymmetricBlockCipher

Parameters:

key - the key which has to be used to encrypt data.

params - the algorithm parameters.

secureRandom - a source of randomness.

Throws:

InvalidKeyException - if the given key is inappropriate for initializing this cipher.

initCipherDecrypt

protected void initCipherDecrypt(Key key,
                                 AlgorithmParameterSpec params)
                          throws InvalidKeyException

Initialize the block cipher with a key for data encryption. Parameters are currently not supported.

Overrides:

initCipherDecrypt in class AsymmetricBlockCipher

Parameters:

key - the key which has to be used to decrypt data.

params - the algorithm parameters.

Throws:

InvalidKeyException - if the given key is inappropriate for initializing this cipher.

messageEncrypt

protected byte[] messageEncrypt(byte[] input)

Encrypt a message.

Overrides:

messageEncrypt in class AsymmetricBlockCipher

Parameters:

input - the message

Returns:

the encrypted message

messageDecrypt

protected byte[] messageDecrypt(byte[] input)
                         throws BadPaddingException

Decrypt a ciphertext.

Overrides:

messageDecrypt in class AsymmetricBlockCipher

Parameters:

input - the ciphertext

Returns:

the decrypted ciphertext

Throws:

BadPaddingException - if the ciphertext is invalid.