1
$\begingroup$

This is my first question here, so if I ask it to the wrong group or otherwise infringing the (written or unwritten) rules of the community, please bear with me.

My problem is this: In an exercise in futility, I am trying to write a small program (in C) that implements the DES (FIPS 46-3) standard. I stumbled upon that part in which, from each byte in the 8 byte key, the LSB is used for (odd) parity checking, and eventually discarded.

That, I assume, means that a key like "aaaaaaac" (ASCII encoding) is invalid, since "c" (0x63) has even parity.

Now, either this is correct (and my question would be how commercial / publicly - available packages treat this, since I believe I saw plain ASCII used as key) OR it is a wrong train of thoughts, in which case I would like to know what I am missing.

Thanks a lot in advance.

$\endgroup$
2
$\begingroup$

a key like "aaaaaaac" (ASCII encoding) is invalid, since "c" (0x63) has even parity

Right. Notice that restricting to ASCII ($2^7$ values per byte, of which $2^6$ with odd parity) severely reduces the already small keyspace of DES, from $2^{56}$ to $2^{48}$ keys ($<2^{45.5}$ if one restricts to printable ASCII). For this reason, DES keys are usually expressed in hexadecimal.

how commercial / publicly - available packages treat this

A very common practice is to exclude parity checking of DES keys from the functionality offered, and simply ignore the low-order bit, treating a key byte of 0x63 (ASCII c) just as 0x62 (ASCII b).

Another practice is to reject a key as invalid if any of its byte has even parity. With 64-bit x and side-channel protection aside, that could be implemented on the tune of

x = (x>>4) ^ x;
x = (x>>2) ^ x;
if (( (~((x>>1) ^ x)) & 0x0101010101010101) != 0) // error

For why DES keys have parity, see Purpose of DES parity bits.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.