IBM's Lucifer cipher was selected in 1974 as the base for what would become the Data Encryption Standard. Lucifer's key length was reduced from 128 bits to 56 bits, which the NSA and NIST argued was sufficient. The NSA has major computing resources and a large budget; some cryptographers including Whitfield Diffie and Martin Hellman complained that this made the cipher so weak that NSA computers would be able to break a DES key in a day through brute force parallel computing. The NSA disputed this, claiming that brute-forcing DES would take them "something like 91 years".[8] However, by the late 90s, it became clear that DES could be cracked in a few days' time-frame with custom-built hardware such as could be purchased by a large corporation or government.[9][10] The book Cracking DES (O'Reilly and Associates) tells of the successful attempt in 1998 to break 56-bit DES by a brute-force attack mounted by a cyber civil rights group with limited resources; see EFF DES cracker. Even before that demonstration, 56 bits was considered insufficient length for symmetric algorithm keys; DES has been replaced in many applications by Triple DES, which has 112 bits of security when used 168-bit keys (triple key).[11] In 2002, Distributed.net and its volunteers broke a 64-bit RC5 key after several years effort, using about seventy thousand (mostly home) computers.
Mainstream symmetric ciphers (such as AES or Twofish) and collision resistant hash functions (such as SHA) are widely conjectured to offer greater security against known quantum computing attacks. They are widely thought most vulnerable to Grover's algorithm. Bennett, Bernstein, Brassard, and Vazirani proved in 1996 that a brute-force key search on a quantum computer cannot be faster than roughly 2n/2 invocations of the underlying cryptographic algorithm, compared with roughly 2n in the classical case.[21] Thus in the presence of large quantum computers an n-bit key can provide at least n/2 bits of security. Quantum brute force is easily defeated by doubling the key length, which has little extra computational cost in ordinary use. This implies that at least a 256-bit symmetric key is required to achieve 128-bit security rating against a quantum computer. As mentioned above, the NSA announced in 2015 that it plans to transition to quantum-resistant algorithms.[12]
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