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 and think that this would square the security of the double-encryption scheme. Certainly, an exhaustive search of all possible combination of keys would take formula_1 attempts if each key is n bits long, compared to the formula_2 attempts required for a single key. However, Diffie and Hellman, discovered a time-memory tradeoff that could break the scheme in only double the time to break the single-encryption scheme. The attack works by encrypting from one end and decrypting from the other end, thus meeting in the middle.