Cryptography Question (I hope it's not off-topic on this list)
I was thinking about *chaining* remailers and wondered if encryption of a certain strength could be *chained* in order to make it as strong as stronger encryption. e.g. - If only 56-bit encryption becomes legal, is there a method of *chaining* several passes of 48-bit encryption which would make it just as hard to break as 96/192/384-bit (etc.) encryption? If this is indeed impossible, then perhaps the government might pass a law that makes it illegal to encrypt an encrypted file, but experience seems to suggest that any law passed always leaves a loophole or back door for inventive people to circumvent it. CyberDoc
e.g. - If only 56-bit encryption becomes legal, is there a method of *chaining* several passes of 48-bit encryption which would make it just as hard to break as 96/192/384-bit (etc.) encryption?
This is a similar idea to implementing, say DES, with independent subkeys. Layering encryption in this manner makes the plaintext more difficult to determine providing that: a. The involved cryptosystem is not a group, or does not posess strong group like properties (eg. There are no large subgroups). b. Independent keys are used for each encryption For a good example of a particular case of your idea see 3DES
If this is indeed impossible, then perhaps the government might pass a law that makes it illegal to encrypt an encrypted file, but experience seems to suggest that any law passed always leaves a loophole or back door for inventive people to circumvent it.
This law would be difficult to pass, because it is essentially saying some sets of data may be encrypted and some not, and there are lots of ways out to give plausible deniability "It isn`t an encrypted file officer, it`s random bits being encrypted as cover traffic", if the cryptosystem is strong the cyphertext shouldn`t be distinguishable from random values. As for circumventing it, any law such as this should just be ignored. Datacomms Technologies data security Paul Bradley, Paul@fatmans.demon.co.uk Paul@crypto.uk.eu.org, Paul@cryptography.uk.eu.org Http://www.cryptography.home.ml.org/ Email for PGP public key, ID: FC76DA85 "Don`t forget to mount a scratch monkey"
In article <Pine.LNX.3.91.970728211016.884C-100000@fatmans.demon.co.uk>, Paul Bradley <paul@fatmans.demon.co.uk> wrote:
e.g. - If only 56-bit encryption becomes legal, is there a method of *chaining* several passes of 48-bit encryption which would make it just as hard to break as 96/192/384-bit (etc.) encryption?
This is a similar idea to implementing, say DES, with independent subkeys. Layering encryption in this manner makes the plaintext more difficult to determine providing that:
a. The involved cryptosystem is not a group, or does not posess strong group like properties (eg. There are no large subgroups).
b. Independent keys are used for each encryption
For a good example of a particular case of your idea see 3DES
Careful, here. 3DES gives us a benefit because we can chain crypto at the algorithm level. Chaining several passes of DES may not give this benefit if all we have is an opaque program that block-pads the input and does raw DES on that. For example (the values are imaginary), DES("foo") = DES_raw("foo\x05\x05\x05\x05\x05") = "f983hgls" DES(DES("foo")) = DES("f983hgls") = DES_raw("f983hgls\x08\x08\x08\x08\x08\x08\x08\x08") = "d84koqw78452398f" DES(DES(DES("foo"))) = DES("d84koqw78452398f") = DES_raw("d84koqw78452398f\x08\x08\x08\x08\x08\x08\x08\x08") = "ecy34895y34057834985634y"; whereas 3DES("foo") = 3DES_raw("foo\x05\x05\x05\x05\x05") = "ecy34895" To break DES(DES(DES("foo"))), break the outer DES until you find "\x08\x08\x08\x08\x08\x08\x08\x08" at the end of the "plaintext" (actually, the padded value of DES(DES("foo"))). Repeat until you get "foo". [It is left as an exercise to the reader to determine why you might not be able to merely chop the last 8 bytes off the output of the second and third DES iterations to get around this problem.] - Ian
participants (3)
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iang＠cs.berkeley.edu -
nobody＠REPLAY.COM -
Paul Bradley