https://stackoverflow.com/questions/19214810
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RussianUS 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.
