Javax.crypto.spec.SecretKeySpec seems to ignore the last bit in every byte

Question

Hi All,
I am having a problem with the SecretKeySpec class in JCE 1.2.2. I am using DES as algorithm, and a 8 byte long key. I am passing a hex array of 16 characters (each 2 representing a byte) and create a byte array of length 8 from that hex array. For example, the following "A12C3D4E5F6A7B8E" is equivalent to a byte array, having the values of the bytes (in signed byte) [-95, 44, 61, 78, 95, 106, 123, -114]. It works perfectly but what I observed is, if one of the bytes looks like "5E" and I change it to "5F," the code still works, the encrypted text using the key created with "5E" key is decrypted correctly using the "5F" key. It doesn't work if the byte is "E5" and I change it to "F5," and this led me to think that the SecretKeySpec implementation ignores the last bit of every byte (uses just the high 7 bits.)
Anybody came accross this before? Any help appreciated.
THanks,
Marius

Answer 1

As I said, "every implementation I'm aware of ignores them". They're not used during en/decryption, they exist only to provide (marginal) evidence that the key might have been mangled in-transit. Many implementations just skip that step. Note that i'm not saying this is a good thing - it's just been my experience.
The 64-bit quntity you're playing with is NOT "the key" - the DES key is the 56 bits that do not include the parity bits. Ignoring the parity-bits doesn't yield more keys that will do the same decryption, since the parity bits aren't used by DES itself.
The net? Don't worry about it. It's the 56 bits that are the important ones.
Grant