Problem - Kevin Tafuro

if (condition) ctx->c1[SPC_BLOCK_SZ - 1] ^= 0x87;
/* Compute L * u ^ -1, storing it in c2 */
condition = L[SPC_BLOCK_SZ - 1] & 0x01;
i = SPC_BLOCK_SZ;
while (--i) ctx->c2[i] = (L[i] >> 1) | (L[i - 1] c2[0] = L[0] >> 1;
L[0] >>= 1;
if (condition) {
ctx->c2[0] ^= 0x80;
ctx->c2[SPC_BLOCK_SZ - 1] ^= 0x43;
}
spc_memset(L, 0, SPC_BLOCK_SZ);
return 1;
}
void spc_omac_update(SPC_OMAC_CTX *ctx, unsigned char *in, size_t il) {
int i;
if (il < SPC_BLOCK_SZ - ctx->ix) {
while (il--) ctx->iv[ctx->ix++] ^= *in++;
return;
}
if (ctx->ix) {
while (ctx->ix < SPC_BLOCK_SZ) --il, ctx->iv[ctx->ix++] ^= *in;
SPC_DO_ENCRYPT(&(ctx->ks), ctx->iv, ctx->iv);
}
while (il > SPC_BLOCK_SZ) {
for (i = 0; i < SPC_BLOCK_SZ / sizeof(int); i++)
((unsigned int *)(ctx->iv))[i] ^= ((unsigned int *)in)[i];
SPC_DO_ENCRYPT(&(ctx->ks), ctx->iv, ctx->iv);
in += SPC_BLOCK_SZ;
il -= SPC_BLOCK_SZ;
}
for (i = 0; i < il; i++) ctx->iv[i] ^= in[i];
ctx->ix = il;
}
int spc_omac_final(SPC_OMAC_CTX *ctx, unsigned char *out) {
int i;
if (ctx->ix != SPC_BLOCK_SZ) {
ctx->iv[ctx->ix] ^= 0x80;
for (i = 0; i < SPC_BLOCK_SZ / sizeof(int); i++)
((int *)ctx->iv)[i] ^= ((int *)ctx->c2)[i];
} else {
for (i = 0; i < SPC_BLOCK_SZ / sizeof(int); i++)
((int *)ctx->iv)[i] ^= ((int *)ctx->c1)[i];
}
SPC_DO_ENCRYPT(&(ctx->ks), ctx->iv, out);
return 1;
}
For those interested in the algorithm itself, note that we precompute two special values
at key setup time, both of which are derived from the value we get from encrypt-
284 | Chapter 6: Hashes and Message Authentication
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