farm9 README.txt for rcrypt
10-07-2000

Thanks for downloading rcrypt

This is a program for encrypting files using the Rijndael
algorithm, which is the AES standard as of Oct 2, 2000.

Included is the source from the NIST site.

The only part contributed by farm9 is the relatively simple
source file: rcrypt.c

There are many options for using the Rijndael algorithm,
none of which are supported by rcrypt.  When you enter a key
on the command line, that key is used to generate a 256 bit
Rijndael key.  CBC mode is the encryption mode.

Rijndael encrypts 16 bytes at time, so the main contribution
of rcrypt is to just turn any file size into a multiple of
16 bytes.  This is done by adding a 16 byte header that includes
the actual size of the file, then padding the final block of
the raw file data if necessary.  

When a file is decrypted, the header is read to determine the 
actual file size.

For example, if a 1 byte file were encrypted, the resulting
encrypted file would be 32 bytes long.  The first 16 bytes
would contain the encrypted form of the text:

  "1 <random numbers...>", 

The next 16 bytes would contain:

  "<byte>\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"

One side effect of the random numbers in the headers is that
encrypting the same file with the same key will usually result
in totally different output.  This is because the CBC mode
XORs the previous block's encryption with the current block
before encrypting, so a different header propagates throughout
the rest of the file.  

jojo@farm9.com