BCS
Reputation
Next privilege 250 Rep.
 Dec29 comment Can an RSA public key be generated without ever knowing the factors? For what I'm thinking of, see the context I linked (IIRC down near the bottom). Jun19 comment How hard is it to generate a partial RSA fingerprint collision? @poncho point. That said, I'm assuming the SSH client is diligent enough to spot gratuitously weak keys, which requires the generation (I assume) of at least modestly sized primes, so you can't go to far down that path. Jun19 comment How hard is it to generate a partial RSA fingerprint collision? If it's assumed that finding the large primes is a major cost, then it would be reasonable to economize on them: i.e. generate $n$ reasonably sized primes and check all $\frac{n(n-1)}{2}$ combinations. Jun19 comment How hard is it to generate a partial RSA fingerprint collision? Reading between the lines, anyone with more than a passing interest could forge 10-12 selected hex digits of a fingerprint. So if you are going to even bother looking you had better check at least 20. Jun18 comment How hard is it to generate a partial RSA fingerprint collision? If you have money to throw at it, buying time on commercial VM service is a real option. I've heard real stories of academics running GNFS on Amazons S3. At ~0.6 USD/hr (aws.amazon.com/ec2/#pricing) and your guesses, that's $2^{42}$ to $2^{43}$ for 1k cash. Jun18 comment How hard is it to generate a partial RSA fingerprint collision? See clarification. Jan30 comment Can nesting different ciphers with different keys ever decrease security? I guess then, the question should then be (and I'm not expecting an answer to this ): would it increase the complexity more for the attacker or the defender? -- As you point out, it given more surface area to attack but OTOH, it limits the value of any given attack as it limits the number of rounds that it would apply to. Jan28 comment Can nesting different ciphers with different keys ever decrease security? @fgrieu: so you would never use a composed rounds cipher without careful analyses, but I haven't seen any suggestion that it would be inherently worse than a uniform round cipher. Jan28 comment Can nesting different ciphers with different keys ever decrease security? I assume those modes are with respect to the block chaining and such. (And that is interesting and important information. +1) However, the problem I was thinking of is only with regards to the non-chained bare block cipher. (E.g. AES rather than AES-*.) Jan25 comment Designing a key expander out of ciphers Yup, that looks correct to me un-trained eyes. OTOH the programmer in me sees the risk that comes from not having the second k change as well: if you somehow end up with an off-by-one error and generate "k prime -1", you are back where you started. Jan25 comment Designing a key expander out of ciphers If cipher_1 is just xor, all the k primes will be exactly k. Or am I missing something? Jan24 comment Can nesting different ciphers with different keys ever decrease security? The only surprising result there is the elegance of the sketch. -- OTOH the 2nd result leads to the question of if there are any common ciphers that have more than one unrelated round function and/or a non-regular round structure. (I'm sure if I knew the right terms I could answer that myself but I have no idea what to Google for. :)