Problem - Kevin Tafuro

Therefore, we’ll show you LION, built out of the RC4 implementation from Recipe
5.23 and SHA1 (see Recipe 6.7). The structure of LION is shown in Figure 5-6.
While we cover RC4 because it is popular, we strongly recommend
you use SNOW 2.0 instead, because it seems to have a much more
comfortable security margin.
The one oddity of this technique is that files must be longer than the output size of
the message digest function (20 bytes in the case of SHA1). Therefore, if you have
files that small, you will either need to come up with a nonambiguous padding
scheme, which is quite complicated to do securely, or you’ll need to abandon LION
(either just for small messages or in general).
LION requires a key that is twice as long as the output size of the message digest
function. As with regular CBC-style encryption for files, if you’re using a cipher that
takes fixed-size keys, we expect you’ll generate a key of the appropriate length from a
password.
Plaintext
K 0
K 1
Figure 5-6. The structure of LION
L0 R0 0 20
n
RC4
L0 R1 0 20
n
SHA 1
L1 R1 0 20
n
RC4
Ciphertext L1 R2 0 20
n
Round 1
Round 2
Round 3
We also assume a SHA1 implementation with a very standard API. Here, we use an
API that works with OpenSSL, which should be easily adaptable to other libraries.
Performing File or Disk Encryption | 215
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