Following packages from java are useful:
java.security.*;
java.util.Random;
javax.crypto.*;



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Code for RSA (encryption/decryption):
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KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA");
generator.initialize(keysize);		//keysize = 1024;


//encryption 
Cipher c = Cipher.getInstance("RSA/ECB/PKCS1Padding");
c.init(Cipher.ENCRYPT_MODE, keyPair.getPublic());

byte[] encr = c.doFinal(message);		//gives encrypted data



//decryption
Cipher c1 = Cipher.getInstance("RSA/ECB/PKCS1Padding");
c1.init(Cipher.DECRYPT_MODE, keyPair.getPrivate());

c1.doFinal(encr);


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Code for DES:
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KeyGenerator gen = KeyGenerator.getInstance(DesEncrypter.algorithm);
gen.init(keysize);			//keysize = 56;

SecretKey key = gen.generateKey();
Cipher ecipher = Cipher.getInstance(algorithm);		//algorithm = "DES"
Cipher dcipher = Cipher.getInstance(algorithm);
ecipher.init(Cipher.ENCRYPT_MODE, key);
dcipher.init(Cipher.DECRYPT_MODE, key);


//encryption
byte[] encr = ecipher.doFinal(message);		//message is in byte array format
byte[] plain = dcipher.doFinal(encr);



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Code for Digest:
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MessageDigest sha1 = MessageDigest.getInstance("SHA-1");

md.digest(message);		//returns digest in byte[] format
				//message should be in byte[] format


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Code for RSA signature generation/verification:
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KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");

kpg.initialize(keySize);			//keysize = 1024
KeyPair keyPair = kpg.generateKeyPair();

Signature sgSign = Signature.getInstance(algorithm);	//algorithm is "MD5withRSA" .. to use for encryption/decryption
sgSign.initSign(keyPair.getPrivate());

sgSign.update(data,0,len);		//data is in byte[] format & its 'len' is length of array 
byte[] signature = sgSign.sign();		//returns signature 

Signature sgVer;sgVer = Signature.getInstance(algorithm);
sgVer.initVerify(keyPair.getPublic());

sgVer.verify(signature);		//returns boolean value whether corrctly verified or not.