Unable to calculate correct C-MAC for doing EXTERNAL AUTHENTICATE

Question

I’m facing a problem successfully executing the EXTERNAL_AUTHENTICATE command for establishing a secure session. I get a 69 82 response meaning “Security status not satisfied”. I have concluded that the C-MAC I’m computing must somehow be wrong, because when I walked through the a tutorial file given to us, I’m not able to arrive at the same C-MAC value illustrated in the document .
Command : 84 82 01 00 10
Input Data : F6 17 A4 CB 36 80 F3 37 (Host cryptogram)
For the above data the C-MAC computed is shown as “3F 67 D7 B5 0C 0D 9C 60” using the S-MAC key AD79403568BE1B46250E389475D2BD7E but I’m arriving at a S-MAC key value of “98 18 0c 25 38 ad 54 8c” using the same key and a padding of 8000000000000000000000
I’m pasting below a sample program that shows how I’m arriving at this value. Can you please help me understand what they are doing differently to arrive at the values that only work with the card?
Thanks
Kannan
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
public class Cryptogram
public static byte[] hexToBuffer(String hexString) throws NumberFormatException
          int length = hexString.length();
          byte[] buffer = new byte[(length+1)/2];
          boolean evenByte = true;
          byte nextByte = 0;
          int bufferOffset = 0;
          if ((length%2)==1)
               evenByte = false;
          for (int i=0; i<length; i++) {
               char c = hexString.charAt(i);
               int nibble;
               if ((c>='0')&&(c<='9'))
                    nibble = c - '0';
               else if ((c>='A')&&(c<='F'))
                    nibble = c - 'A' + 0x0A;
               else if ((c>='a')&&(c<='f'))
                    nibble = c - 'a' + 0x0A;
               else throw new NumberFormatException("Invalid hex digit '"+c+"'.");
               if (evenByte) {
                    nextByte = (byte)(nibble<<4);
               } else {
                    nextByte += (byte)nibble;
                    buffer[bufferOffset++] = nextByte;
               evenByte = !evenByte;
          return buffer;
private static SecretKeySpec getKeySpec(byte[] key) {
if (key.length == 16) {
byte[] key24 = new byte[24];
System.arraycopy(key,0,key24,0,16);
System.arraycopy(key,0,key24,16,8);
return new SecretKeySpec(key24,"DESede");
} else {
return new SecretKeySpec(key,"DESede");
public static byte[] cbcMac(byte[] data,byte[] sessKey) throws NoSuchAlgorithmException, NoSuchPaddingException, InvalidKeyException, InvalidAlgorithmParameterException, IllegalBlockSizeException, BadPaddingException {
byte[] temp;
Cipher cbcDES = Cipher.getInstance("DESede/CBC/NoPadding");
byte[] DEFAULT_ICV = new byte[8];
IvParameterSpec params = new IvParameterSpec(DEFAULT_ICV);                         
cbcDES.init(Cipher.ENCRYPT_MODE, getKeySpec(sessKey), params);
temp = cbcDES.doFinal(data);
byte[] signature = new byte[8];
System.arraycopy(temp, temp.length - 8, signature, 0, signature.length);
return signature;
public static void main(String args[]) throws Exception
byte[] toSign = HexString.hexToBuffer("8482010010f617a4cb3680f3378000000000000000000000");
          byte[] sessKey = HexString.hexToBuffer("AD79403568BE1B46250E389475D2BD7E");
          byte[] cryptogram = cbcMac(toSign,sessKey);
          System.out.println("Cryptogram=" + HexString.bufferToHex(cryptogram));
}

Answer 1

user4928701 wrote:
I don't know the format you are mentioning, I'm using oberthur v7 card.
I realized that the mistake was in the padding of "8482010010f617a4cb3680f3378000000000000000000000";
I'm supposed to pad only with 800000 as that gives a number of bytes in multiple of 8, but I had added yet another 0 sequence of 8 bytes.
The format for external authenticate is 5 byte command + host cryptogram + C-MAC value for the padded sequence computed using S-MAC key.Then you are using the wrong code. The code you posted is for generating the host cryptogram. The code for the MAC for each command (including EXT AUTH) is different.
Cheers,
Shane