Problem - Kevin Tafuro

that the state function is always replaced by the ciphertext of the previous block. See
Figure 5-2 for a graphical view of CFB mode.
1
2
Figure 5-2. CFB mode
key
leftmost
byte
Buffer
Block
cipher
p c
Encrypted buffer
Buffer
= XOR
3 p = input plaintext byte
byte shift
c = output ciphertext byte
c
The block size of the cipher is important to CFB mode because keystream is produced
in block-sized chunks and therefore requires keeping track of block-sized portions
of the ciphertext. CFB is fundamentally a streaming mode, however, because
the plaintext is encrypted simply by XOR’ing with the CFB keystream.
In Recipe 5.4, we discuss the advantages and drawbacks of CFB and compare it to
other popular modes.
These days, CFB mode is rarely used because CTR and OFB modes (CTR mode in
particular) provide more advantages, with no additional drawbacks. Of course, we
recommend a higher-level mode over all of these, one that provides stronger security
guarantees—for example, CWC or CCM mode.
Many libraries already come with an implementation of CFB 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 CFB
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.
Using a Generic CFB Mode Implementation | 187
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