Triple DES - Security

Security

In general, Triple DES with three independent keys (keying option 1) has a key length of 168 bits (three 56-bit DES keys), but due to the meet-in-the-middle attack, the effective security it provides is only 112 bits. Keying option 2 reduces the key size to 112 bits. However, this option is susceptible to certain chosen-plaintext or known-plaintext attacks, and thus, it is designated by NIST to have only 80 bits of security.

The best attack known on keying option 1 requires around 232 known plaintexts, 2113 steps, 290 single DES encryptions, and 288 memory (the paper presents other tradeoffs between time and memory). This is not currently practical and NIST considers keying option 1 to be appropriate through 2030. If the attacker seeks to discover any one of many cryptographic keys, there is a memory-efficient attack which will discover one of 228 keys, given a handful of chosen plaintexts per key and around 284 encryption operations.

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