Meet-in-the-middle attack
An attack that makes double encryption nearly pointless by working forwards and backwards and looking for a match in the middle.
Whitfield Diffie and Martin Hellman described it in 1977 as part of their case against DES's short key. Encrypt twice with two 56-bit keys and you might expect a 112-bit keyspace. But given one known plaintext–ciphertext pair, an attacker can encrypt the plaintext under every possible first key, store the results, then decrypt the ciphertext under every second key until an intermediate value matches.
The work drops to roughly 257 operations instead of 2112, at the cost of a vast amount of memory. That is why "double DES" never existed and the industry jumped straight to 3DES, which delivers about 112 bits of effective security from a 168-bit key. Not to be confused with man-in-the-middle, which is a different animal entirely.