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bio website github.com/CodesInChaos
location Frankfurt, Germany
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visits member for 3 years, 5 months
seen 14 hours ago

Mar
9
comment Can one detect if two pairs of elements in Zp have the same exponential relation?
I believe there are some groups which have this property. I think the lucre paper briefly mentioned them.
Mar
9
comment Is there a way to use Shamir Secret Sharing with updatable data?
Then why don't you retain the private key, without giving it to the other parties? If you want to be able to read the data without consulting the other parties, then you need to do something that's equivalent to retaining the key.
Mar
9
comment ChaCha cipher + Poly1305
@nightcracker I recommend splitting your implementation into two parts. 1) KeyExchange aka. crypto_box_beforenm which is Hash(scalarMult(b,A)) with HSalsa as the default hash 2) AuthenticatedEncryption aka. crypto_secret_box or crypto_box_afternm which is Salsa20(HSalsa20(key, n1), n2)
Mar
9
comment Is SHA-256 secure as a CTR block cipher?
Using HMAC halves the output speed, and the gain is rather dubious. (Though if you need a fast streamcipher, you wouldn't use SHA-2 in the first place.)
Mar
8
comment Is there a way to use Shamir Secret Sharing with updatable data?
How about simply using asymmetric encryption where you retain the public key, and secret share the private key?
Mar
8
comment How difficult is it to check if a group element is in a sub group?
I guess there is a generic algo that runs in $\sqrt{s}$ time where $s$ is the order of the subgroup, and group specific algos that may be much faster.
Mar
8
comment At the current time, is SHA256 the de facto standard for strong cryptographic hashes?
SHA-256d isn't such a great idea, it has some minor weaknesses. You should use different prefixes for the inner and the outer hash, such as SHA-256(1 || SHA-256(0 || m)), or just HMAC-SHA-256 with a fixed key.
Mar
7
comment ChaCha cipher + Poly1305
Just look at salsa20-poly1305 in nacl and do the same thing with chacha. Or just use nacl. The advantage of chacha over salsa is minor.
Mar
7
comment Strength of Combining Hash functions
@user5195 You're missing our point. We're saying that none of your combiners is secure for all relevant security notions. For example concatenation is at least as strong as the stronger hash concerning collisions and second pre-images. But you can find a pair of hash functions that are secure on their own, but when you concatenate the output you get no first pre-image resistance what-so-ever. The security of each property needs to be investigated individually.
Mar
7
comment Strength of Combining Hash functions
1) Multiplying hashes doesn't really make sense. 2) As I said, hash functions have different properties, such as collision, first pre-image, second pre-image resistance, pseudo randomness, etc. Different combiners affect those differently.
Mar
7
comment Strength of Combining Hash functions
What's H1*H2 supposed to be?
Mar
7
comment Strength of Combining Hash functions
strength isn't a single value. There are many properties a hash function can have, and typical combiners only preserve a few of them. So you should specify which properties you need. Are you just interested in collisions?
Mar
6
comment Is this encryption algorithm build from MD5 secure?
The random value is a nonce/IV. Sending it prepended to the ciphertext is standard. No need to encrypt an IV, it's not secret. In this case it must be different for each message.
Mar
5
comment is AES secure for java application licensing
If you use the standard AES API, then extracting the AES key is trivial. Just put some kind of breakpoint on the AES function.
Mar
3
comment Are there any secure commutative ciphers?
RSA suffers from pretty much the same issues, so we apply randomized padding and use it as hybrid encryption.
Mar
3
comment Are there any secure commutative ciphers?
Can you use a hybrid scheme, where you apply the commutative cipher to a symmetric key? And how long are your messages?
Mar
2
revised Is solving a modular linear equation a hard problem when the coefficient is not an invertible element?
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Mar
2
revised Is solving a modular linear equation a hard problem when the coefficient is not an invertible element?
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Mar
2
answered Is solving a modular linear equation a hard problem when the coefficient is not an invertible element?
Mar
2
revised Can SRP be used with Elliptic Curves?
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