How to calculate Retail-MAC(Single DES Plus Final Triple DES MAC) in SCP02 (Secure channel Protocol 02) with ICV encryption?

Question

I have seen multiple people asking for a help on C-MAC generation(Retail MAC). This question contains the answer as well. This will help your enough time. I have tested this function with real card and it worked fine.

Answer 1

Note:

1. One can improve the efficiency of function if like.
2. If you find any improvement please suggest.
3. Before you start work on SCP 02 with communication in Ext_Atuh as CMAC please check SCP i value.
   4. This function supports ICV encryption for next command.

public static byte[] generateCmac(byte []apdu,byte[]sMacSessionKey,byte[]icv) throws Exception {

    if(sMacSessionKey.length == 16) {
    byte []temp  = sMacSessionKey.clone();
    sMacSessionKey = new byte[24];
    System.arraycopy(temp,0,sMacSessionKey,0,temp.length);
    System.arraycopy(temp,0,sMacSessionKey,16,8);
    }

    byte []cMac = new byte[8];
    byte []padding = {(byte)0x80,0x00,0x00,0x00,0x00,0x00,0x00,0x00,};
    int paddingRequired  = 8 - (apdu.length) %8;
    byte[] data = new byte[apdu.length + paddingRequired];
    System.arraycopy(apdu, 0, data, 0, apdu.length);
    System.arraycopy(padding, 0, data, apdu.length,paddingRequired);

            Cipher cipher = Cipher.getInstance("DESede/CBC/NoPadding");

            Cipher singleDesCipher = Cipher.getInstance("DES/CBC/NoPadding",
                "SunJCE");
            SecretKeySpec desSingleKey = new SecretKeySpec(sMacSessionKey, 0, 8,
                "DES");
            SecretKey secretKey = new SecretKeySpec(sMacSessionKey, "DESede");
            // Calculate the first n - 1 block. For this case, n = 1
            IvParameterSpec ivSpec = new IvParameterSpec(icv);
            singleDesCipher.init(Cipher.ENCRYPT_MODE, desSingleKey, ivSpec);
            // byte ivForLastBlock[] = singleDesCipher.doFinal(data, 0, 8);

            int blocks = data.length / 8;

            for (int i = 0; i < blocks - 1; i++) {
                singleDesCipher.init(Cipher.ENCRYPT_MODE, desSingleKey, ivSpec);
                byte[] block = singleDesCipher.doFinal(data, i * 8, 8);
                ivSpec = new IvParameterSpec(block);
            }

            int offset = (blocks - 1) * 8;

            cipher.init(Cipher.ENCRYPT_MODE, secretKey, ivSpec);
            cMac = cipher.doFinal(data, offset, 8);

            ivSpec = new IvParameterSpec(new byte[8]);

            singleDesCipher.init(Cipher.ENCRYPT_MODE, desSingleKey, ivSpec);
            icvNextCommand = singleDesCipher.doFinal(cMac);
            System.out.println("icvNextCommand"+Utility.bytesToHex(icvNextCommand, icvNextCommand.length));

            return cMac;

    }