Stephen Zander <gibreel@pobox.com> wrote: | | But wasn't that the gist of Rivest's paper: he's not encrypting the | message, he's just obscuring it really, really well. His point is that the message packets start out readable, and then by adding other packets (not altering the originals) you gain security, whether intentional or not - apparently encryption is performed by accident and without a key. So he argues that since this technique transforms a cleartext stream to a secure one without use of any cryptographic technique or algorithm, no act of encryption has happened. However it is not that simple - for a start to gain real security you have to be careful to mingle streams in very precise ways, to lose the temporal statistics that give away the origins of each packet - you have to match wheat to chaff on a packet by packet basis to get good security. Furthermore you have to use a CSRNG or true random source to generate fake MACs, or have another MAC key for the complementary stream(s) - It is not so easy to say that these precautions could be an accidental act, or that they are entirely non-cryptographic. However I view the process rather differently. There are two channels - the message is carried in the MAC and in the plaintext bits. Chaffing simply serves to obliterate the plaintext channel. The recipient doesn't need to get the plaintext bits at all - they can simply try the MAC against both 0 and 1, and choose the correct one. (although this doubles the workload) Furthermore an "attacker" can't tell, without breaking the MAC scheme, whether the plaintext is genuine or a blind, and so this makes chaffing/winnowing an ideal carrier of steganography. It's like sending a plaintext file and a ciphertext file together, with an assertion that they correspond - unless you can prove this assertion how can an outsider be convinced you are not hiding information in the ciphertext file? How can you prove this assertion without giving away your MAC key? How can you demonstrate you are using a MAC and not simply triple-DES? __Mark [ markt@harlequin.co.uk | http://www.harlequin.co.uk/ | +44(0)1954 785433 ]