Problem - Kevin Tafuro

Because we assume the message is available up front, all of the information we need
to operate on a message is passed into the function spc_pctr_setup( ), which requires
a context object (here, the type is SPC_CTR2_CTX), the key, the key length in bytes, a
nonce SPC_BLOCK_SZ - SPC_CTR_BYTES in length, the input buffer, the length of the
message, and the output buffer. This function does not do any of the encryption and
decryption, nor does it copy the input buffer anywhere.
To process the first block, as well as every second block after that, call spc_pctr_do_
odd( ), passing in a pointer to the context object. Nothing else is required because the
input and output buffers used are the ones passed to the spc_pctr_setup( ) function.
If you test, you’ll notice that the results are exactly the same as with the CTR mode
implementation from Recipe 5.9.
This code requires the preliminaries from Recipe 5.5, as well as the spc_memset( )
function from Recipe 13.2.
#include
#include
typedef struct {
SPC_KEY_SCHED ks;
size_t len;
unsigned char ctr_odd[SPC_BLOCK_SZ];
unsigned char ctr_even[SPC_BLOCK_SZ];
unsigned char *inptr_odd;
unsigned char *inptr_even;
unsigned char *outptr_odd;
unsigned char *outptr_even;
} SPC_CTR2_CTX;
static void pctr_increment(unsigned char *ctr) {
unsigned char *x = ctr + SPC_CTR_BYTES;
while (x-- != ctr) if (++(*x)) return;
}
void spc_pctr_setup(SPC_CTR2_CTX *ctx, unsigned char *key, size_t kl,
unsigned char *nonce, unsigned char *in, size_t len,
unsigned char *out) {
SPC_ENCRYPT_INIT(&(ctx->ks), key, kl);
spc_memset(key,0, kl);
memcpy(ctx->ctr_odd, nonce, SPC_BLOCK_SZ - SPC_CTR_BYTES);
spc_memset(ctx->ctr_odd + SPC_BLOCK_SZ - SPC_CTR_BYTES, 0, SPC_CTR_BYTES);
memcpy(ctx->ctr_even, nonce, SPC_BLOCK_SZ - SPC_CTR_BYTES);
spc_memset(ctx->ctr_even + SPC_BLOCK_SZ - SPC_CTR_BYTES, 0, SPC_CTR_BYTES);
pctr_increment(ctx->ctr_even);
ctx->inptr_odd = in;
ctx->inptr_even = in + SPC_BLOCK_SZ;
ctx->outptr_odd = out;
ctx->outptr_even = out + SPC_BLOCK_SZ;
ctx->len = len;
}
210 | Chapter 5: Symmetric Encryption
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.