The strength of cryptography lies in the choice and management of the encryption keys.

While mainly of historical interest, this paper, by "an ad hoc group of cryptographers and computer scientists", shows that longer keys will resist attack better than shorter keys, that the cost of very strong encryption is not significantly greater than that of weak encryption, and that it is prudent to require that encrypted data should still be secure in 20 years.

The sizes of encryption keys are measured in bits and the difficulty of trying all possible keys grows exponentially with the number of bits used. Adding one bit to the key doubles the number of possible keys, adding ten increases it by a factor of more than a thousand.”