10,367 reputation
12453
bio website github.com/CodesInChaos
location Frankfurt, Germany
age
visits member for 3 years, 5 months
seen 15 hours ago

Oct
31
comment How are IVs used in association with RSA Encryption?
What's funny about using random padding is that we usually encrypt random symmetric keys (as part of hybrid encryption), a scenario where randomness isn't actually required.
Oct
31
comment If I have the unencrypted text and the encrypted text, can I calculate the key?
This is a "known plaintext attack" and if you use a modern algorithm properly you'll be immune to it.
Oct
31
comment How to develop a public key cryptosystem based on a hard problem?
@DrLecter I didn't say that you need that specific property. But that you need a non trivial mathematical property beyond one-way-ness. Commutativity is one example, many other schemes rely on a trapdoor. The chance that an arbitrary hard to invert function has such a property is very small.
Oct
30
comment How to develop a public key cryptosystem based on a hard problem?
We know many many hard to invert functions, but only a handful of fundamentally different asymmetric encryption/key-exchange schemes.
Oct
30
comment How to develop a public key cryptosystem based on a hard problem?
For asymmetric encryption or key-exchange your system needs lots of mathematical structure. Almost all one-way functions are not suitable. For example with diffie-hellman key-exchange you not only need the property that $A=G^a \mod p$ is hard to invert, you also have the property that $A^b=(G^a)^b = G^{ab} = (G^b)^a = B^a$ i.e. you are able to compute the same shared secret using $A$ and $b$ or $B$ and $a$. Where lowercase letters are private keys and upper case letters the corresponding public keys.
Oct
30
comment How to develop a public key cryptosystem based on a hard problem?
You only described a one-way function so far, something similar to a hash function. You didn't say where the asymmetric component comes in.
Oct
30
comment How does the IV or initial counter increase internally for each block in AES CTR mode?
There isn't really a one true CTR mode. There are many different variants of CTR mode which differ by how they compute the per-block input from IV and counter.
Oct
29
comment Making ECDSA public keys one bit shorter
For a fixed generator there cannot be a significant subset of weak keypairs since you could use that to break all of them.
Oct
29
comment How good is using AES CTR mode with initial counter as 0?
One problem with constant IV is that it enables multi-target attacks. When using AES-128 an attacker you can break all messages encrypted with this scheme with $2^{128}$ work and break one random message cost only $2^{128}/t$ for $t$ known ciphertext. So I'd avoid constant IVs with 128 bit keys, but 256 bit keys it doesn't really matter.
Oct
29
comment Deterministic nonces in CTR mode
One problem with constant IV is that it enables multi-target attacks. When using AES-128 an attacker you can break all messages encrypted with this scheme with $2^{128}$ work and break one random message cost only $2^{128}/t$ for $t$ known ciphertext. So I'd avoid constant IVs with 128 bit keys, but 256 bit keys it doesn't really matter.
Oct
28
comment Definition of a CSPRNG
If an application wants to offer forward secrecy it needs a PRNG that can't be run backwards.
Oct
26
comment The weak link is the password?
For password based encryption the password is usually the weakest link. It's one of the reasons for avoiding passwords as much as possible.
Oct
26
comment Timing Attacks on ECDSA, ECDHE, AES and SHA2
I'd expect typical implementations of SHA-2 to resist timing attakcs, but naive implementations of all others should suffer from timing attacks. What's tricky is figuring out in which situations they're practical and in which they are not.
Oct
25
comment RSA with modulus product of many primes
A related question: Are there any standards of multi-prime RSA key generation? and AGL's blog post about how security and performance scale with the number of primes
Oct
25
comment RSA with modulus product of many primes
You can use multiple primes as long as they're large enough. It gives you less security for a given modulus size, but it can give you a better performance/security trade-off.
Oct
24
comment Knowing pre-encryption data, can we find the private key CryptoLocker used?
No, if they implemented asymmetric encryption properly, such attacks are not possible.
Oct
24
comment RSA decrypt long messages (javascript)
Please don't cross-post
Oct
24
comment RSA decrypt long messages (javascript)
Your formatting problem was that you attempted to use LaTeX ($$) formatting instead of code formatting (indent by four spaces).
Oct
23
comment Is there a “brainwallet” for GPG keys?
@ralu The coding part is easy. Typical password quality is a problem, especially since you can't use salts.
Oct
23
comment Having 4096 bit keys and short (< 256 byte) messages, can I simply use the key as OTP?
1) You still need a MAC, luckily for you polynomial MACs are provably secure (with a certain probability of not detecting the error which is independent of attackers computation resources) 2) The problem with OTP is exchanging those keys and that you need state to ensure that you never reuse a key.