Henno Brandsma

539 reputation

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	age	43
visits	member for	1 year, 9 months
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programs in C and Perl, mostly

	539 reputation	bio	website		visits	member for	1 year, 9 months
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Oct 7	comment	When is each key used when encrypting an email using OpenPGP? @Xeoncross Indeed.
Oct 2	comment	Could the Enigma algorithm be classified as a Feistel network? Every position of the wheels (plus the Stecker-board) defines a unique mono-alphabetic substitution, that has no fixed points and is self-inverse. It's applied to one letter and then one wheel moves, sometimes triggering another wheel movement as well. We then get the next monoalphabetic etc. After all wheels have gone full circle, the cipher returns to the initial state (but this never happened in practice, as messages were shorter than the cycle length). Indeed a very different structure.
Oct 2	comment	When is each key used when encrypting an email using OpenPGP? You can sign your message using only your own (private) key (it could be a public message, say) but your public key should be known to someone that wants to verify your signature. You cannot encrypt for someone without knowing that person's public key, in a scheme like this.
Oct 2	answered	When is each key used when encrypting an email using OpenPGP?
Oct 1	answered	What are the main weaknesses, if any, of a Playfair cipher?
Aug 19	answered	Cryptanalysing Affine cipher
Aug 17	awarded	Yearling
Nov 12	answered	What is pre-image resistance, and how can the lack thereof be exploited?
Nov 9	answered	Is it reasonable to assure that p-1 and q-1 aren't smooth?
Nov 7	comment	How to choose constants in a cryptographic function? it seems to be that this is hidden in the rka-advantage used in the bound of lemma 5.2 of that paper (the original Crypto 96 paper, to which I do not have access right now, might have more info, and could be my source of recollection); the homogeneity property that I mentioned might give a sharper security bound; and I cannot think the constants are irrelevant, I'd think that constants 0 and 0xff would give worse results, intuitively.
Nov 7	comment	How to choose constants in a cryptographic function? As to S-boxes: there was already a theorem proven that inversion in a field is almost optimally non-linear in a certain sense, and this was the reason the transform was used; the affine transform afterwards is to get rid of the fix point 0 --> 0. The precise form of the constants in the S-box then follows directly from the polynomial in the definition of the Galois field.
Nov 7	comment	How to choose constants in a cryptographic function? As to 0x36 and 0x5c: the requirement for these constants is that there are 4 types of bit positions in a byte: where both constants have 0-bits, both have 1-bits, constant1 has a 1 and constant2 a 0, and vice versa. This means that (when we xor this with a key byte) we toggle half the key bits every time. The security proof depends on the constants having these properties, IIRC.
Oct 30	revised	How to attack a classical cipher using known partial plaintext? added 6 characters in body
Oct 14	revised	How to attack a classical cipher using known partial plaintext? edited body
Oct 12	revised	How to attack a classical cipher using known partial plaintext? corrected square, added observations
Oct 12	answered	How to attack a classical cipher using known partial plaintext?
Sep 27	comment	What is the difference between a stream cipher and a one-time-pad? Also note that a computationally unbounded adversary could break a stream cipher (having enough known plaintext) but not a one-time pad.
Sep 27	awarded	Supporter
Sep 18	awarded	Editor
Sep 18	revised	How to forge Schnorr signatures if you can guess the challenge deleted 2 characters in body