Problem - Kevin Tafuro

This implementation is only for the case where the feedback size is
equal to the cipher block size. This is the most efficient mechanism
and is no less secure than other feedback sizes, so we strongly recommend
this approach.
The high-level API
This implementation has two APIs. The first is a high-level API, which takes a message
as input and returns a dynamically allocated result.
unsigned char *spc_cfb_encrypt(unsigned char *key, size_t kl, unsigned char *nonce,
unsigned char *in, size_t il);
unsigned char *spc_cfb_decrypt(unsigned char *key, size_t kl, unsigned char *nonce,
unsigned char *in, size_t il)
Both of the previous functions output the same number of bytes as were input,
unless a memory allocation error occurs, in which case 0 is returned.
These two functions erase the key from memory before exiting. You
may want to have them erase the plaintext as well.
Here’s the implementation of the interface:
#include
#include
unsigned char *spc_cfb_encrypt(unsigned char *key, size_t kl, unsigned char *nonce,
unsigned char *in, size_t il) {
SPC_CFB_CTX ctx;
unsigned char *out;
if (!(out = (unsigned char *)malloc(il))) return 0;
spc_cfb_init(&ctx, key, kl, nonce);
spc_cfb_encrypt_update(&ctx, in, il, out);
spc_cfb_final(&ctx);
return out;
}
unsigned char *spc_cfb_decrypt(unsigned char *key, size_t kl, unsigned char *nonce,
unsigned char *in, size_t il) {
SPC_CFB_CTX ctx;
unsigned char *out;
if (!(out = (unsigned char *)malloc(il))) return 0;
spc_cfb_init(&ctx, key, kl, nonce);
spc_cfb_decrypt_update(&ctx, in, il, out);
spc_cfb_final(&ctx);
return out;
}
188 | Chapter 5: Symmetric Encryption
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