Problem - Kevin Tafuro

Both of these functions output the same number of bytes as were input, unless a
memory allocation error occurs, in which case 0 is returned. The decryption routine
is exactly the same as the encryption routine and is implemented by macro.
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_ofb_encrypt(unsigned char *key, size_t kl, unsigned char *nonce,
unsigned char *in, size_t il) {
SPC_OFB_CTX ctx;
unsigned char *out;
if (!(out = (unsigned char *)malloc(il))) return 0;
spc_ofb_init(&ctx, key, kl, nonce);
spc_ofb_update(&ctx, in, il, out);
spc_ofb_final(&ctx);
return out;
}
#define spc_ofb_decrypt spc_ofb_encrypt
Note that the previous code depends on the SPC_OFB_CTX data type and the incremental
OFB interface, both discussed in the following sections.
The incremental API
Let’s look at the SPC_OFB_CTX data type. It’s defined as:
typedef struct {
SPC_KEY_SCHED ks;
int ix;
unsigned char nonce[SPC_BLOCK_SZ];
} SPC_OFB_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 nonce field is really the buffer in which we store the current block of
the keystream.
To begin encrypting or decrypting, we need to initialize the mode. Initialization is
the same operation for both encryption and decryption:
void spc_ofb_init(SPC_OFB_CTX *ctx, unsigned char *key, size_t kl, unsigned char
*nonce) {
SPC_ENCRYPT_INIT(&(ctx->ks), key, kl);
194 | Chapter 5: Symmetric Encryption
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