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I know that with brute force there are 2^56 possible keys to check (56 bits, each either a 1 or 0). But let's say I know the message itself is only made up of letters (a-z, A-Z).

Would knowing things (like the limitation to just letters) about the plaintext make breaking the encryption easier?

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The answer is a vague YES, as this depends on the encryption algorithm. This is usually also a function of the number of cipher and plain-text messages available (in most cases the more such available, the easier it is to break the code).

This is the reason that 56-bits is today counted as very weak protection, especially in view of the computing-power available to government organizations.

For the RSA method quoted by bbaja42, see RSA Algorithm section "Weaknesses in RSA", as well as Cracking RSA and RSA: Hacking and Cracking.

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thanks for info –  bbaja42 Dec 10 '10 at 21:23

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