Question 1

What is TDES CBC-MAC?

Accepted Answer

TDES CBC-MAC (Triple Data Encryption Standard Cipher Block Chaining Message Authentication Code) is a cryptographic algorithm that uses Triple DES encryption in CBC mode to generate authentication tags for verifying message integrity and authenticity. It processes data in 8-byte blocks, chaining each block with the previous encrypted output. The final block becomes the MAC value, ensuring any tampering with the message is detected.

Question 2

What is the difference between 2-key and 3-key TDES?

Accepted Answer

2-key TDES (112-bit effective security) uses 16 bytes (32 hex characters) with K1 and K2, where K3=K1 in the encrypt-decrypt-encrypt sequence. 3-key TDES (168-bit effective security) uses 24 bytes (48 hex characters) with three independent keys K1, K2, K3. 3-key TDES provides stronger security and is recommended for high-value transactions and modern payment systems complying with PCI DSS standards.

Question 3

What are ISO 9797-1 padding methods?

Accepted Answer

ISO 9797-1 defines two padding methods: Method 1 pads with zero bytes (0x00) to reach a multiple of 8 bytes, simple but vulnerable if the original data ends with zeros. Method 2 (recommended) appends 0x80 followed by zero bytes, always distinguishing padded data from original data. Method 2 ensures unambiguous padding removal and prevents padding oracle attacks in MAC verification systems.

Question 4

When should I use TDES CBC-MAC?

Accepted Answer

Use TDES CBC-MAC for legacy financial systems requiring Triple DES compatibility, payment terminal authentication (POS, ATM), EMV chip card transactions, key management MAC generation, and secure messaging in banking networks. While AES-based MACs (like CMAC) are preferred for new systems, TDES CBC-MAC remains widely deployed in existing payment infrastructure due to backward compatibility requirements and regulatory acceptance.

Question 5

What is the typical MAC truncation for TDES CBC-MAC?

Accepted Answer

TDES CBC-MAC generates an 8-byte (16 hex characters) MAC from the final encrypted block. Common truncation values are 4 bytes (8 hex characters) for standard security in payment systems, 6 bytes (12 hex characters) for enhanced security, or full 8 bytes (16 hex characters) for maximum protection. Truncation balances security with message efficiency - shorter MACs reduce bandwidth but provide fewer bits of security against brute-force attacks.

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