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Feb
1
comment private keys for multiple devices
The reference implementation of Curve25519 is in C and is extremely portable. If you need more features, take a look at NaCl, which is also in C and provides easy ways to achieve most common cryptographic goals.
Jan
18
comment Combining a keystream generator with a block cipher
@Melab XOR is a bitwise equality test at it's heart, and that's all it's doing here. Blocks of zeros in the XOR operation show that the corresponding plaintext blocks are identical.
Jan
18
comment Combining a keystream generator with a block cipher
@Nova Many practical applications involve messages with identical blocks of plaintext. For example, DNS requests or ARP packets are identical except for a few bytes each.
Jan
18
comment Combining a keystream generator with a block cipher
@Melab Sure! XORing them just makes that easier.
Jan
18
comment Combining a keystream generator with a block cipher
@Melab It will produce contiguous blocks of zeros wherever the ciphertexts are identical, revealing the blocks with identical plaintext.
Jan
15
comment Possible to use an accumulator to “license” or restrict the qty of certificates being used?
@D.W.: You're right, I should have updated the question first.
Jan
15
comment Combining a keystream generator with a block cipher
The first point of the OP's analysis of his new algorithm says that it "does not fall prey to the problems that reusing a key plagues stream ciphers with." The whole point of the exercise was to be able to reuse keys, which, just like stream ciphers, it fails at.
Jan
15
comment Problem with implementing DES in python
@Newbie: Yeah, but I looked at your code, and you don't apply the P permutation after doing the S-boxes in compressBlock. You could do that inside the function called compressBlock, or you could do it after that call in roundFunction, but it's gotta be done somewhere. (As an aside, your function names aren't terribly conducive to debugging!)
Jan
15
comment What kind of attack does the current brokenness of SHA-1 allow?
@v6ak I can generate an MD5 collision using an Amazon EC2 free trial, so it costs $0.00, meaning it would be infinitely harder to find a SHA1 collision. (My point being that that sort of comparison isn't very useful!)
Jan
15
comment private keys for multiple devices
cr.yp.to/ecdh.html is the reference implementation, and it's actually really easy to use by itself, which is what I'd normally recommend. What platform and language are you working with?
Jan
15
comment Possible to use an accumulator to “license” or restrict the qty of certificates being used?
@makerofthings7: In that case, perhaps to avoid confusion you should edit the question with the updated requirements?
Jan
15
comment Possible to use an accumulator to “license” or restrict the qty of certificates being used?
@D.W.: But it does satisfy the revised requirements the OP and I worked out in the comments on my first answer, which complained about the requirements not being specific enough.
Jan
15
comment Possible to use an accumulator to “license” or restrict the qty of certificates being used?
@D.W.: Yeah, sure, it CAN, but by doing so it leaks its private key to the world, making all its signatures useless.
Jan
14
comment Combining a keystream generator with a block cipher
No, XORing two ciphertexts tells you which blocks in the plaintext are identical. That's a huge break.
Jan
14
comment Combining a keystream generator with a block cipher
But his analysis is NOT correct. He says his scheme "Does not fall prey to the problems that reusing a key plagues stream ciphers with," but it doesn't: reusing a key still breaks security, even if it's not as spectacular as getting the whole keystream back.
Jan
13
comment Possible to use an accumulator to “license” or restrict the qty of certificates being used?
This would also be somewhat more space-efficient than a constructed n-time signature scheme. It's not post-quantum, though!
Jan
13
comment Possible to use an accumulator to “license” or restrict the qty of certificates being used?
I know I'm being awfully negative, but this is an interesting problem, and I don't see an obvious solution. Like I said in my answer, the first step is to define the problem properly; then we can work on solving it. From your comments, I think what you're looking for is a way for the CA to pre-issue "tickets" of some type such that client certs have to use one to be validly signed. The CA doesn't need to know anything about the final client cert to issue the ticket. However, if two client certs appear with the same ticket, some or all of the security guarantees break. Is that fair to say?
Jan
13
comment Possible to use an accumulator to “license” or restrict the qty of certificates being used?
Case in point: your description of the CA issuing primes is just a way of saying there should be some mechanism to produce verifiably "approved" keys by controlling the structure of the private key. The problem is that the structure of the private key can't be verified by anyone, because it's private; if you can see the key to verify it, it's useless. This is a lot easier to see if you look at it in terms of primitives and black boxes rather than doing something mysterious, yet ultimately equivalent, by messing around inside them.
Jan
13
comment Possible to use an accumulator to “license” or restrict the qty of certificates being used?
Not gonna work, because knowledge of a single prime allows you to trivially factor the key and determine the other. In general, any solution that involves you, as a software engineer, treading into the domain of the near-mystical mathematical eggheads known as cryptographers will end in catastrophic failure. Don't worry about the primes and the factoring and the modular arithmetic: just use the crypto primitives as black boxes. Everything useful you can do by messing around inside the primitives has been done, and exposed as other kinds of primitives.
Jan
12
comment Unique assignment of X.509 certificate to each client
If you're talking about using just the openssl command line utility as that system, by default, it requires unique subject fields, even without the use of the special field, but there are no cryptographic guarantees of that, and it's easily disabled in the config file.