A key bundle requires 24 bytes for option 1, 16 for option 2, or 8 for option 3. Įach DES key is 8 odd-parity bytes, with 56 bits of key and 8 bits of error-detection. ISO/IEC 18033-3 never allowed this option, and NIST no longer allows K 1 = K 2 or K 2 = K 3. This is backward compatible with DES, since two operations cancel out. Keying option 3 All three keys are identical, i.e. This is an improvement over "double DES" which only requires 2 56 steps to attack. This provides a shorter key length of 56*2 or 112 bits and a reasonable compromise between DES and Keying option 1, with the same caveat as above. Sometimes known as 2TDEA or double-length keys. Keying option 2 K 1 and K 2 are independent, and K 3 = K 1. It is still vulnerable to meet-in-the-middle attack, but the attack requires 2 2 × 56 steps. This is the strongest, with 3 × 56 = 168 independent key bits. Sometimes known as 3TDEA or triple-length keys. Keying option 1 All three keys are independent. The standards define three keying options: This improves the strength of the algorithm when using keying option 2 and provides backward compatibility with DES with keying option 3. In each case the middle operation is the reverse of the first and last. Įach triple encryption encrypts one block of 64 bits of data. Triple DES provides a relatively simple method of increasing the key size of DES to protect against such attacks, without the need to design a completely new block cipher algorithm.Ī naive approach to increase strength of a block encryption algorithm with short key length (like DES) would be to use two keys ( K 1, K 2 ). The original DES cipher's key size of 56 bits was generally sufficient when that algorithm was designed, but the availability of increasing computational power made brute-force attacks feasible.
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