Problem - Kevin Tafuro

eturn the number of valid bytes in the final block of data, which could be anything
from zero to one less than the block length.
5.12 Precomputing Keystream in OFB, CTR, CCM,
or CWC Modes (or with Stream Ciphers)
Problem
You want to save computational resources when data is actually flowing over a network
by precomputing keystream so that encryption or decryption will consist
merely of XOR’ing data with the precomputed keystream.
Solution
If your API has a function that performs keystream generation, use that. Otherwise,
call the encryption routine, passing in N bytes set to 0, where N is the number of
bytes of keystream you wish to precompute.
Discussion
Most cryptographic APIs do not have an explicit way to precompute keystream for
cipher modes where such precomputation makes sense. Fortunately, any byte XOR’d
with zero returns the original byte. Therefore, to recover the keystream, we can
“encrypt” a string of zeros. Then, when we have data that we really do wish to
encrypt, we need only XOR that data with the stored keystream.
If you have the source for the encryption algorithm, you can remove the final XOR
operation to create a keystream-generating function. For example, the spc_ctr_
update( ) function from Recipe 5.9 can be adapted easily into the following keystream
generator:
int spc_ctr_keystream(SPC_CTR_CTX *ctx, size_t il, unsigned char *out) {
int i;
if (ctx->ix) {
while (ctx->ix) {
if (!il--) return 1;
*out++ = ctx->ksm[ctx->ix++];
ctx->ix %= SPC_BLOCK_SZ;
}
}
if (!il) return 1;
while (il >= SPC_BLOCK_SZ) {
SPC_DO_ENCRYPT(&(ctx->ks), ctx->ctr, out);
ctr_increment(ctx->ctr);
il -= SPC_BLOCK_SZ;
Precomputing Keystream in OFB, CTR, CCM, or CWC Modes (or with Stream Ciphers) | 207
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