Problem - Kevin Tafuro

*out++ = ctx->ctbuf[i];
if (ol) *ol = SPC_BLOCK_SZ - pad;
spc_memset(ctx, 0, sizeof(SPC_CBC_CTX));
return 1;
}
See Also
• PKCS #11 web page: http://www.rsasecurity.com/rsalabs/pkcs/pkcs-11/
• Recipes 4.9, 5.4, 5.5, 5.11, 5.16, 13.2
5.7 Using a Generic CFB Mode Implementation
Problem
You want a more high-level interface for CFB mode than your library provides. Alternatively,
you want a portable CFB interface, or you have only a block cipher implementation
and would like to use CFB mode.
Solution
CFB mode generates keystream by encrypting a “state” buffer, which starts out being
the nonce and changes after each output, based on the actual outputted value.
Many libraries provide a CFB implementation. If you need code that implements this
mode, you will find it in the following “Discussion” section.
Discussion
You should probably use a higher-level abstraction, such as the one
discussed in Recipe 5.16. Use a raw mode only when absolutely necessary,
because there is a huge potential for introducing a security vulnerability
by accident. If you still want to use CFB, be sure to use a
message authentication code with it (see Chapter 6).
CFB is a stream-based mode. Encryption occurs by XOR’ing the keystream bytes
with the plaintext bytes, as shown in Figure 5-2. The keystream is generated one
block at a time, and it is always dependent on the previous keystream block as well
as the plaintext data XOR’d with the previous keystream block.
CFB does this by keeping a “state” buffer, which is initially the nonce. As a block’s
worth of data gets encrypted, the state buffer has some or all of its bits shifted out
and ciphertext bits shifted in. The amount of data shifted in before each encryption
operation is the “feedback size,” which is often the block size of the cipher, meaning
186 | Chapter 5: Symmetric Encryption
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.