Problem - Kevin Tafuro

one block at a time, by encrypting the previous keystream block. * The first block is
generated by encrypting the nonce.
P 0
IV
E K
KS 0
C 0
Figure 5-3. OFB mode
P 1
E K
KS 1
C 1
P 2
This mode shares many properties with counter mode (CTR), but CTR mode has
additional benefits. OFB mode is therefore seeing less and less use these days. Of
course, we recommend a higher-level mode than both of these modes, one that provides
stronger security guarantees—for example, CWC or CCM mode.
In Recipe 5.4, we discuss the advantages and drawbacks of OFB and compare it to
other popular modes.
Many libraries already come with an implementation of OFB mode for any ciphers
they support. However, some don’t. For example, you may only get an implementation
of the raw block cipher when you obtain reference code for a new cipher.
In the following sections we present a reasonably optimized implementation of OFB
mode that builds upon the raw block cipher interface presented in Recipe 5.5. It also
requires the spc_memset( ) function from Recipe 13.2.
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_ofb_encrypt(unsigned char *key, size_t kl, unsigned char *nonce,
unsigned char *in, size_t il);
unsigned char *spc_ofb_decrypt(unsigned char *key, size_t kl, unsigned char *nonce,
unsigned char *in, size_t il)
* As with CFB mode, the “feedback size” could conceivably be smaller than the block size, but such schemes
aren’t secure.
Using a Generic OFB Mode Implementation | 193
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E K
KS 2
C 2
. . .
= XOR