Meet-in-the-middle attack

A meet -in-the -middle attack is a generic cryptographic attack.

Triple-DES example

In the Triple-DES encryption is three times the same procedure, but using different encryption keys. Each data block is encrypted using a DES key, then decrypts encrypted with and comprising:

Since the key length is 56 bits, there are a total of possible key combinations for the first two stages.

The idea of ​​the meet-in -the- middle attack is to encrypt a known plaintext - ciphertext pair of the plaintext with all key combinations and store the intermediate results. Then one generated from the ciphertext, the intermediate result. Now you have to compare the intermediate result with the intermediate results. Once you have found a match, one knows the key.

The total cost of finding the key is thus reduced to DES transformations.