Problem - Kevin Tafuro

*out++ = *in++ ^ ctx->nonce[ctx->ix];
ctx->nonce[ctx->ix++] = c;
}
return 1;
}
To finalize either encryption or decryption, use spc_cfb_final( ), which never needs
to output anything, because CFB is a streaming mode:
int spc_cfb_final(SPC_CFB_CTX *ctx) {
spc_memset(&ctx, 0, sizeof(SPC_CFB_CTX));
return 1;
}
See Also
Recipes 4.9, 5.4, 5.5, 5.16, 13.2
5.8 Using a Generic OFB Mode Implementation
Problem
You want a more high-level interface for OFB mode than your library provides.
Alternatively, you want a portable OFB interface, or you have only a block cipher
implementation and you would like to use OFB mode.
Solution
OFB mode encrypts by generating keystream, then combining the keystream with
the plaintext via XOR. OFB generates keystream one block at a time. Each block of
keystream is produced by encrypting the previous block of keystream, except for the
first block, which is generated by encrypting the nonce.
Many libraries provide an OFB implementation. If you need code implementing 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 OFB, be sure to use a
message authentication code with it.
OFB mode is a stream-based mode. Encryption occurs by XOR’ing the keystream
bytes with the plaintext bytes, as shown in Figure 5-3. The keystream is generated
192 | Chapter 5: Symmetric Encryption
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.