Why DES can be used only with 56bit key? And why the plaintext must be 64 bits in length?

Question

Why DES can be used only with 56bit key? What happens if we use the longer key? Also, why the plaintext must be 64 bits in length?

Answer 1

US regulations at the time required users of stronger than 56-bit keys, to submit to "key recovery" to enable law enforcement back-door access.

Thus DES, as a standard, was specified at the maximum allowed key length of 56 bits. If you used a longer key, you would not be compatible with other DES systems.

See: http://en.wikipedia.org/wiki/56-bit_encryption

If you are implementing a system & have a choice of encryption, more modern & stronger ciphers would be absolutely recommended. The current standard would be AES (Advanced Encryption System) which is widely available, strong and allows key sizes from 128 - 256 bits.

For desktop or server applications, AES-256 would be a good default choice.

See: http://en.wikipedia.org/wiki/Advanced_Encryption_Standard

When encrypting data, plaintexts must often be "padded" to a minimum size. Ciphers rely on jumbling and interactions between multiple bits, to preserve secrecy of the plaintext & avoid potentially revealing the key. For a short plaintext without padding, jumbling and interactions are removed as a factor & mathematical complexity drops vastly.

Encrypting just a single character without padding, for example a 'y' or 'n' response, could for example reduce a 2^256 keyspace down to possibly 2^24. That could be cracked in minutes. This would enable an attacker to guess large parts of the key, rapidly break it, and then (worst of all) -- decrypt all other traffic on the channel.