Problem - Kevin Tafuro

These two functions also 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_ctr_encrypt(unsigned char *key, size_t kl, unsigned char *nonce,
unsigned char *in, size_t il) {
SPC_CTR_CTX ctx;
unsigned char *out;
if (!(out = (unsigned char *)malloc(il))) return 0;
spc_ctr_init(&ctx, key, kl, nonce);
spc_ctr_update(&ctx, in, il, out);
spc_ctr_final(&ctx);
return out;
}
#define spc_ctr_decrypt spc_ctr_encrypt
Note that this code depends on the SPC_CTR_CTX data type and the incremental CTR
interface, both discussed in the following sections. In particular, the nonce size varies
depending on the value of the SPC_CTR_BYTES macro (introduced in the next subsection).
The incremental API
Let’s look at the SPC_CTR_CTX data type. It’s defined as:
typedef struct {
SPC_KEY_SCHED ks;
int ix;
unsigned char ctr[SPC_BLOCK_SZ];
unsigned char ksm[SPC_BLOCK_SZ];
} SPC_CTR_CTX;
The ks field is an expanded version of the cipher key (block ciphers generally use a
single key to derive multiple keys for internal use). The ix field is used to determine
how much of the last block of keystream we have buffered (i.e., that hasn’t been
used yet). The ctr block holds the plaintext used to generate keystream blocks. Buffered
keystream is held in ksm.
To begin encrypting or decrypting, you need to initialize the mode. Initialization is
the same operation for both encryption and decryption, and it depends on a statically
defined value SPC_CTR_BYTES, which is used to compute the nonce size.
#define SPC_CTR_BYTES 6
Using a Generic CTR Mode Implementation | 199
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