Here is my

Plain APDU : 00 A4 00 00

kSMac: 2F 34 E0 0D 1C 46 76 67 43 BA 6B 26 AB 20 4F D3

SSC:20 29 73 0A 6D 5E 7C C4

I am working on constructing a secure message and I am following an example and I am getting a wrong Mac value.

Here is what I have done:

 byte[] modifiedAPDU = new byte[] { 0x0C, 0xA4, 0x00, 0x00 };

//pad the APDU
byte[] paddedAPDU = Pad(modifiedAPDU);

//append Apdu to SSC
byte[] apduWithSSC = new byte[SSC.Length + paddedAPDU.Length];
Array.Copy(SSC, apduWithSSC, SSC.Length);
Array.Copy(paddedAPDU, 0, apduWithSSC, SSC.Length, paddedAPDU.Length);

//print the APDU with SSC
Console.WriteLine("APDU with SSC: " + BitConverter.ToString(apduWithSSC).Replace("-", ""));

byte[] mac = CalculateMac(paddedAPDU, KSMac);

CalculateMac()

static byte[] CalculateMac(byte[] data, byte[] key)
{
 // Initialize DES engine
 DesEngine desEngine = new DesEngine();

 // Initialize ISO9797Alg3Mac with DES engine, 64-bit block size, and ISO7816d4Padding
 ISO9797Alg3Mac mac = new ISO9797Alg3Mac(desEngine, 64, new ISO7816d4Padding());

 // Initialize with the key
 KeyParameter keyParameter = new KeyParameter(key);
 mac.Init(keyParameter);

 // Update with the data
 mac.BlockUpdate(data, 0, data.Length);

 // Compute MAC
 byte[] output = new byte[mac.GetMacSize()];
 mac.DoFinal(output, 0);

 return output;

}

Pad

static byte[] PadData(byte[] data)
{
   int blockSize = 8; // Block size for 3DES
   int paddingLength = blockSize - (data.Length % blockSize);
   byte[] paddedData = new byte[data.Length + paddingLength];
   Array.Copy(data, paddedData, data.Length);
   paddedData[data.Length] = 0x80; // Append 0x80
   for (int i = data.Length + 1; i < paddedData.Length; i++)
   {
       paddedData[i] = 0x00; // Append 0x00
   }
   return paddedData;
}

Expected Mac= A8 FB 97 D2 99 C8 05 2B, what could be the reason I not getting it – I am getting 7B 21 7B 67 47 42 3C FD