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/**
	 * X9.63 key derivation function, Derive AES Session key from secret and
	 * sharedInfo.
	 * 
	 * @param zab
	 *            secret
	 * @param sharedInfo
	 *            sharedInfo
	 * @param keydatalen
	 *            the byte length of the keying data to generate.e.g. 16*5
	 * @return 5 keys
	 */
	public static byte[] deriveFunction(byte[] za, byte[] zb, byte[] sharedInfo, int keyDataLen) {
		int len = za.length + zb.length;
		byte[] secret = new byte[len];
		byte[] keys = new byte[keyDataLen];

		short offset = 0;
		System.arraycopy(za, 0, secret, offset, za.length);
		offset += za.length;

		System.arraycopy(zb, 0, secret, offset, zb.length);
		offset += zb.length;

		SHA256Digest hash = new SHA256Digest();
		KDF2BytesGenerator kdf = new KDF2BytesGenerator(hash);
		kdf.init(new KDFParameters(secret, sharedInfo));
		kdf.generateBytes(keys, (short) 0, keyDataLen);

		return keys;
	}