Meet-in-the-middle attack

Not to be confused with man-in-the-middle attack.

The meet-in-the-middle attack is a cryptographic attack which, like the birthday attack, makes use of a space-time tradeoff. While the birthday attack attempts to find two values in the domain of a function that map to the same value in its range, the meet-in-the-middle attack attempts to find a value in each of the range and domain of the composition of two functions such that the forward mapping of one through the first function is the same as the inverse image of the other through the second function — quite literally meeting in the middle of the composed function.

History

It was first developed as an attack on an attempted expansion of a block cipher by Diffie and Hellman in 1977. When trying to improve the security of a block cipher, one might get the idea to simply use two independent keys to encrypt the data twice. Naively, one might think that this would square the security of the double-encryption scheme. Certainly, an exhaustive search of all possible combination of keys would take 2²ⁿ attempts if each key is n bits long, compared to the 2ⁿ attempts required for a single key.

Diffie and Hellman, however, devised a time-memory tradeoff that could break the scheme in only double the time to break the single-encryption scheme.^[1] The attack works by encrypting from one end and decrypting from the other end, thus meeting in the middle.

Example

Assume the attacker knows a set of plaintext and ciphertext: P and C. That is,

$C=E_{K_2}(E_{K_1}(P))$,

where E is the encryption function (cipher), and K₁ and K₂ are the two keys.

The attacker can then compute E_K(P) for all possible keys K and store the results in memory. Afterwards he can decrypt the ciphertext by computing D_K(C) for each K. Any matches between these two resulting sets are likely to reveal the correct keys. (To speed up the comparison, the E_K(P) set is stored in an in-memory lookup table, then each D_K(C) can be matched against the values in the lookup table to find the candidate keys.)

Once the matches are discovered, they can be verified with a second test-set of plaintext and ciphertext. If the keysize is n, this attack uses only 2^{n + 1} encryptions (and O(2ⁿ) space) in contrast to the naive attack, which needs 2²ⁿ encryptions (but only O(1) space).

See also

• Birthday attack
• Triple DES

References

1. ^ Diffie, Whitfield; Hellman, Martin E. (June 1977). "Exhaustive Cryptanalysis of the NBS Data Encryption Standard". Computer 10 (6): 74–84. doi:10.1109/C-M.1977.217750. http://www.computer.org/portal/web/csdl/doi/10.1109/C-M.1977.217750.