Cryptography Reference
In-Depth Information
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4.9.4 Notes on Rijndael
Rijndael is a modern cipher. Since it was created in the late 1990's, after many of the standard cryptanalytic
techniques that I discuss in this topic were known, it was tested against these techniques. The algorithm was
tuned so that it was susceptible to none of the techniques, as they were known at the time.
4.10 Block Cipher Modes
Although I have limited the discussion to block ciphers up until this point, I should probably have a few words
on some of the different ways they are used, besides just straight block-for-block encryption, which has been
the assumed method of using the ciphers. (In the previous discussions, I never had the output of one block's
encryption affect a different block's encryption.)
4.10.1 Electronic Code Book
Thenormalmethodisnormally called electroniccodebook(ECB) .Itsimplymeansthateachblockofplaintext
is used as the normal input to the block cipher, and the output of the cipher becomes the block of ciphertext, just
as we would expect. Hence, for each block of plaintext, P , we calculate a block of ciphertext by simply applying
C = Encrypt(P)
However, there are some issues that can easily arise using a cipher in ECB mode. For example, a lot of the
structure of the original data will be preserved, because identical plaintext blocks will always encrypt to identic-
al ciphertext blocks. Figure 4-15 shows how this can occur. In this figure, we encrypt every grayscale (8-bit)
pixel (padded with 15 bytes of zeros) with AES in ECB mode, and then take the first byte of the output block
as the new grayscale value. (This same method is also used for the CBC example in the next section.) 1
 
 
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