Problem - Kevin Tafuro

we ignore that because it will always be zero on failure or the size of the input buffer
on success. Also note that PKCS #11–based calls tend to order their arguments differently
from the way we do, and they will not generally wipe key material, as we do
in our initialization and finalization routines.
Because this API is developed with PKCS #11 in mind, it’s somewhat
more low-level than it needs to be, and therefore is a bit difficult to use
properly. First, you need to be sure the output buffer is big enough to
hold the input; otherwise, you will have a buffer overflow. Second,
you need to make sure the out argument always points to the first
unused byte in the output buffer. Otherwise, you will keep overwriting
the same data every time spc_ctr_update( ) outputs data.
Here’s our implementation of spc_ctr_update( ), along with a helper function:
static inline void ctr_increment(unsigned char *ctr) {
unsigned char *x = ctr + SPC_CTR_BYTES;
while (x-- != ctr) if (++(*x)) return;
}
int spc_ctr_update(SPC_CTR_CTX *ctx, unsigned char *in, size_t il, unsigned char
*out) {
int i;
if (ctx->ix) {
while (ctx->ix) {
if (!il--) return 1;
*out++ = *in++ ^ 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);
for (i = 0; i < SPC_BLOCK_SZ / sizeof(int); i++)
((int *)out)[i] ^= ((int *)in)[i];
il -= SPC_BLOCK_SZ;
in += SPC_BLOCK_SZ;
out += SPC_BLOCK_SZ;
}
SPC_DO_ENCRYPT(&(ctx->ks), ctx->ctr, ctx->ksm);
ctr_increment(ctx->ctr);
for (i = 0; i < il; i++)
*out++ = *in++ ^ ctx->ksm[ctx->ix++];
return 1;
}
To finalize either encryption or decryption, use the spc_ctr_final( ) call, which
never needs to output anything, because CTR is a streaming mode:
Using a Generic CTR Mode Implementation | 201
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