Problem - Kevin Tafuro

output
Buffer into which the plaintext is written. This buffer must always be at least
ctlen - 16 bytes in size because the message shrinks by 16 bytes when the
authentication value is removed.
The cwc_cleanup( ) function simply wipes the contents of the cwc context object
passed into it.
See Also
• CWC implementation from Brian Gladman: http://fp.gladman.plus.com/AES/
cwc.zip
• CWC home page: http://www.zork.org/cwc
• Recipes 5.4, 5.5
5.11 Manually Adding and Checking Cipher
Padding
Problem
You want to add padding to data manually, then check it manually when decrypting.
Solution
There are many subtle ways in which padding can go wrong, so use an off-the-shelf
scheme, such as PKCS block cipher padding.
Discussion
Padding is applied to plaintext; when decrypting, you must check for
proper padding of the resulting data to determine where the plaintext
message actually ends.
Generally, it is not a good idea to add padding yourself. If you’re using a reasonably
high-level abstraction, padding will be handled for you. In addition, padding often
isn’t required, for example, when using a stream cipher or one of many common
block cipher modes (including CWC, CTR, CCM, OFB, and CFB).
Because ECB mode really shouldn’t be used for stream-based encryption, the only
common case where padding is actually interesting is when you’re using CBC mode.
If you are in a situation where you do need padding, we recommend that you use a
standard scheme. There are many subtle things that can go wrong (although the
Manually Adding and Checking Cipher Padding | 205
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