Problem - Kevin Tafuro

typedef struct {
SPC_KEY_SCHED ks;
int ix;
int pad;
unsigned char iv[SPC_BLOCK_SZ];
unsigned char ctbuf[SPC_BLOCK_SZ];
} SPC_CBC_CTX;
The ks field is an expanded version of the cipher key. The ix field is basically used to
determine how much data is needed before we have processed data that is a multiple
of the block length. The pad field specifies whether the API needs to add padding or
should expect messages to be exactly block-aligned. The iv field is used to store the
initialization vector for the next block of encryption. The ctbuf field is only used in
decryption to cache ciphertext until we have enough to fill a block.
Incremental initialization
To begin encrypting or decrypting, we need to initialize the mode. Initialization is
different for each mode. Here are the functions for initializing an SPC_CBC_CTX object:
void spc_cbc_encrypt_init(SPC_CBC_CTX *ctx, unsigned char *key, size_t kl,
unsigned char *iv) {
SPC_ENCRYPT_INIT(&(ctx->ks), key, kl);
spc_memset(key, 0, kl);
memcpy(ctx->iv, iv, SPC_BLOCK_SZ);
ctx->ix = 0;
ctx->pad = 1;
}
void spc_cbc_decrypt_init(SPC_CBC_CTX *ctx, unsigned char *key, size_t kl,
unsigned char *iv) {
SPC_DECRYPT_INIT(&(ctx->ks), key, kl);
spc_memset(key, 0, kl);
memcpy(ctx->iv, iv, SPC_BLOCK_SZ);
ctx->ix = 0;
ctx->pad = 1;
}
These functions are identical, except that they call the appropriate method for keying,
which may be different depending on whether we’re encrypting or decrypting.
Both of these functions erase the key that you pass in!
Note that the initialization vector (IV) must be selected randomly. You should also
avoid encrypting more than about 240 blocks of data using a single key. See Recipe 4.9
for more on initialization vectors.
Now we can add data as we get it using the spc_cbc_encrypt_update( ) and spc_cbc_
decrypt_update( ) functions. These functions are particularly useful when a message
comes in pieces. You’ll get the same results as if the message had come in all at once.
When you wish to finish encrypting or decrypting, you call spc_cbc_encrypt_final( )
or spc_cbc_decrypt_final( ), as appropriate.
Using a Generic CBC Mode Implementation | 179
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