I'm having a bit of difficulty getting an understand of key length requirements in cryptography. I'm currently using DES which I believe is 56 bits... now, by converting an 8 character password to a byte[] my cryptography works. If I use a 7 digit password, it doesn't.
Now, forgive me if I'm wrong, but is that because ASCII characters are 7 bits, therefor 8 * 7 = 56bits?
That just doesn't seem right to me. If I want to use a key, why can I not just pass in a salted hash of my secret key, i.e. an MD5 hash?
I'm sure this is very simple, but I can't get a clear understanding of what's going on.
Each algorithm is designed to accept a certain key length. The key is used as part of the algorithm, and as such, can't be whatever your heart desires.
Common key sizes are:
A number, such as 1234567 is only a 4-byte variable. The key is expected to be a byte array, such as "1234567" (implicitly convertible to one in C) or `{ '1', '2', '3', '4', '5', '6', '7' }.
If you wish to pass the MD5 hash of your salted key to DES, you should use some key compression technique. For instance, you could take the top 7 bytes (somewhat undesirable), or perform DES encryption on the MD5 hash (with a known constant key), and take all but the last byte as the key for DES operation.
edit: The DES I'm talking about here is the implementation per the standard released by NIST. It could so be (as noted above), that your specific API expects different requirements on the length of the key, and derives the final 7-byte key from it.