SRTPCipherCTR.java

/**
 * 
 * Code derived and adapted from the Jitsi client side SRTP framework.
 * 
 * Distributed under LGPL license.
 * See terms of license at gnu.org.
 */
package org.restcomm.media.rtp.crypto;

import java.nio.ByteBuffer;

import org.bouncycastle.crypto.BlockCipher;

/**
 * SRTPCipherCTR implements SRTP Counter Mode AES Encryption (AES-CM).
 * Counter Mode AES Encryption algorithm is defined in RFC3711, section 4.1.1.
 * 
 * Other than Null Cipher, RFC3711 defined two two encryption algorithms:
 * Counter Mode AES Encryption and F8 Mode AES encryption. Both encryption
 * algorithms are capable to encrypt / decrypt arbitrary length data, and the
 * size of packet data is not required to be a multiple of the AES block 
 * size (128bit). So, no padding is needed.
 * 
 * Please note: these two encryption algorithms are specially defined by SRTP.
 * They are not common AES encryption modes, so you will not be able to find a 
 * replacement implementation in common cryptographic libraries. 
 *
 * As defined by RFC3711: Counter Mode Encryption is mandatory..
 *
 *                        mandatory to impl     optional      default
 * -------------------------------------------------------------------------
 *   encryption           AES-CM, NULL          AES-f8        AES-CM
 *   message integrity    HMAC-SHA1                -          HMAC-SHA1
 *   key derivation       (PRF) AES-CM             -          AES-CM 
 *
 * We use AESCipher to handle basic AES encryption / decryption.
 * 
 * @author Werner Dittmann (Werner.Dittmann@t-online.de)
 * @author Bing SU (nova.su@gmail.com)
 */
public class SRTPCipherCTR {

    private final static int BLKLEN = 16;
    private final static int MAX_BUFFER_LENGTH = 10*1024;
    private final byte[] cipherInBlock = new byte[BLKLEN];
    private final byte[] tmpCipherBlock = new byte[BLKLEN];
    private byte[] streamBuf = new byte[1024];

    public SRTPCipherCTR() {
    }

    public void process(BlockCipher cipher, ByteBuffer data, int off, int len, byte[] iv) {
		assert off + len <= data.limit();

		// if data fits in inner buffer - use it. Otherwise allocate bigger
		// buffer store it to use it for later processing - up to a defined
		// maximum size.
		byte[] cipherStream = null;
		if (len > streamBuf.length) {
			cipherStream = new byte[len];
			if (cipherStream.length <= MAX_BUFFER_LENGTH) {
				streamBuf = cipherStream;
			}
		} else {
			cipherStream = streamBuf;
		}

		getCipherStream(cipher, cipherStream, len, iv);
		for (int i = 0; i < len; i++) {
			data.put(i + off, (byte) (data.get(i + off) ^ cipherStream[i]));
		}
    }

    /**
     * Computes the cipher stream for AES CM mode. See section 4.1.1 in RFC3711
     * for detailed description.
     * 
     * @param out
     *            byte array holding the output cipher stream
     * @param length
     *            length of the cipher stream to produce, in bytes
     * @param iv
     *            initialization vector used to generate this cipher stream
     */
    public void getCipherStream(BlockCipher aesCipher, byte[] out, int length, byte[] iv) {
        System.arraycopy(iv, 0, cipherInBlock, 0, 14);

        int ctr;
        for (ctr = 0; ctr < length / BLKLEN; ctr++) {
            // compute the cipher stream
            cipherInBlock[14] = (byte) ((ctr & 0xFF00) >> 8);
            cipherInBlock[15] = (byte) ((ctr & 0x00FF));

            aesCipher.processBlock(cipherInBlock, 0, out, ctr * BLKLEN);
        }

        // Treat the last bytes:
        cipherInBlock[14] = (byte) ((ctr & 0xFF00) >> 8);
        cipherInBlock[15] = (byte) ((ctr & 0x00FF));

        aesCipher.processBlock(cipherInBlock, 0, tmpCipherBlock, 0);
        System.arraycopy(tmpCipherBlock, 0, out, ctr * BLKLEN, length % BLKLEN);
    }
}