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Jan
27
comment Is there a strong cryptographic reason for GCM's 2^39 - 256 bit limit?
With concatenation the caller only has to ensure the nonce is unique. For example they can use a counter. If you use xor or add nonce and counter you get overlaps, so a counter as nonce would be fatally broken.
Jan
26
comment How does BLAKE2 ensure that hash(A) != hash(B) when B = A||0 and both A & B have the same number of blocks?
My blog entry Alternative Blake Padding might be relevant, where I propose a simplified padding, which with some minor tweaks turned into the Blake2 padding.
Jan
26
comment How does BLAKE2 ensure that hash(A) != hash(B) when B = A||0 and both A & B have the same number of blocks?
@Nova The specification document is the first link in the Downloads section.
Jan
23
comment Finding strong primes
How big it has to be depends on what you want to use it for. For Diffie-Hellman I'd consider 1024 bits the minimum with 2048 bits recommended. Choosing p=2*q+1 is standard practice when generating Diffie-Hellman groups.
Jan
23
comment Number of keys when using symmetric and asymmetric encryption?
I wouldn't count an asymmetric keypair as two keys.
Jan
22
comment How does RSA padding work exactly?
PKCS#1 describes two paddings, v1.5 padding (which is weak) and OAEP padding (which is recommended). Don't try to invent your own padding.
Jan
20
comment Practical Attack on RSA
Key size is of little relevance when considering side channel attacks. Some kinds of side channel attacks can be prevented by constant time implementations, for some you need to shield the hardware.
Jan
20
comment Why is EdDSA collision-resilient with SHA-512?
It's important do distinguish between collisions of SHA-512 itself and collisions of SHA-512 mod q.
Jan
19
comment Public key encryption and big files with NaCL
Do you know how big the file is before you encrypt it?
Jan
19
comment Why are some x-coordinates unsuitable for an ECDSA generator point?
For an elliptic curve, about half the x values have two associated points, the other half of the x values has none (they correspond to the twist of the curve).
Jan
18
comment Performing HMAC with random key before MAC comparison
I've seen suggestions to use this as constant time comparison by people who're afraid that the compiler will optimize the usual constant time comparison code into variable time code.
Jan
16
comment Pollard's Rho - Restricting the random function to the exponents
You want the cheapest possible function that's still good enough. Exponentiation is expensive.
Jan
16
comment Help converting sha256 to 32-bit stream
I think this is just a weird formulation for "SHA256 outputs a 32 bytes/256 bits which you should then encode with BASE64".
Jan
16
comment Can AES in CCM or GCM counter mode interoperate with AES in “plain” counter mode (CTR)?
Implementing CTR on top of ECB is probably easier than hacking GCM to become CTR.
Jan
16
comment Is ASCON cipher broken?
It only talks about round reduced versions, so it's not a break.
Jan
15
comment Public key encryption and big files with NaCL
1) Salsa20 is a stream cipher, so nonce reuse behaves like one-time-pad key-reuse. crypto.stackexchange.com/… 2) The Poly1305 MAC is a one-time MAC (wegman-carter), so nonce reuse leaks the MAC key for that nonce.
Jan
15
comment Public key encryption and big files with NaCL
1) Do you use a two part nonce, so you can use a different counter as nonce input for each chunk? Reusing the full nonce is fatal. 2) It's essential to use a special counter/nonce for the chunk containing the hash 3) I'd replace the hash by the file size
Jan
14
comment Programming language for modular arithmetic over large numbers
Most major languages either ship with built in bigintegers or have third party implementations. I've heard good things about NTL and it has the additional advantage that it's not merely a biginteger library but contains support for a lot of advanced mathematical features that are useful for cryptography.
Jan
14
comment Programming language for modular arithmetic over large numbers
This question appears to be off-topic because it is about 1) a programming problem, not a crypto problem. 2) A recommendation question with many possible answers, and thus not suitable for stackoverflow either.
Jan
14
comment What is a tweakable block cipher?
@figlesquidge That construction is trivially distinguishable from a proper tweakable cipher. There is a reason that XTS and OCB are more complex.