If you use a system with block size 128 bits (say) that is 16 bytes. If you encrypt a character stream (1 char = 1 byte) going block by block (same key each block) then you are going to repeat if and when author repeats a 16-character phrase. For example, the phrase "Adam P. Goucher\n" where \n is the newline character, occurs often if we were encrypting the math-fun archives. BZZZZ. This indicates to me, despite my complaining before about just XORing text with output of a secure psu-random stream, that this XOR approach should be COMBINED with the block-by-block approach. This is a simple/stupid point, but I have not seen it mentioned in crypto manuals! -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)