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bio website github.com/CodesInChaos
location Frankfurt, Germany
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visits member for 2 years, 9 months
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Sep
19
reviewed Approve suggested edit on Why not the one-time pad with pseudo-number generator
Sep
19
comment Which encryption method supports random reads?
While CTR mode produces the key-stream in 16 byte blocks, that only affects performance. You only need to read and decrypt those ciphertext bits you're interested in. On a modern CPU you won't even notice the overhead of generating a few bytes too much.
Sep
19
comment Proper way of doing encryption and authentication (PBKDF2 + AES)
@Joey Since you're already using PBKDF2 with a salt, you can skip the first step of HKDF. Essentially you're using PBKDF2 as extract step. The expand step doesn't need a salt.
Sep
19
comment Which encryption method supports random reads?
I just want to emphasize that modes that are secure for random reads often aren't secure for random writes. For example CTR mode becomes extremely weak when you do random writes(two-time-pad).
Sep
19
comment Explaining weakness of Dual EC DRBG to wider audience?
Matt Green wrote a blog article about this: The Many Flaws of Dual_EC_DRBG
Sep
18
comment Estimating bits of entropy
There are two issues with Shannon entropy: 1) It's only defined for a probability distribution, not for an individual string 2) Shannon entropy and average key-strength aren't exactly the same thing if the probability distribution isn't uniform.
Sep
18
comment Can PBKDF2 be used to create an XOR cipher key to encrypt random plaintext?
You put OTP into the title of the question...
Sep
18
comment PKCS1 and blind RSA signatures
@neubert With blind signatures the blinder/padder pads and then blinds the message. The signer operates on the blinded signature. The blinder then unblinds the signature, giving them a properly padded signature that isn't blinded anymore. The verifier works with the unblinded signature and knows the signed message. So they can verify normal padding with hashes etc.
Sep
18
comment PKCS1 and blind RSA signatures
@neubert When you actually use blind signatures (e.g. to implement Chaumian E-Cash) then the padding is done by a different entity than the private key operation. One important consequence is that the padder wants to forge signatures, so it doesn't seem like a good idea to use a probabilistic padding scheme.
Sep
18
comment Can PBKDF2 be used to create an XOR cipher key to encrypt random plaintext?
At best you'll get a stream cipher, not a one-time-pad. An OTP needs to be truly random, not pseudorandom.
Sep
18
comment PKCS1 and blind RSA signatures
I'd certainly choose a deterministic padding for blind signatures. In the simplest case, full domain hash.
Sep
18
comment Encrypting 8 times with 8-bit key beneficial?
Meet-in-the-middle will break this with cost $2^{64/2}=2^{32}$, which is feasible with a single PC.
Sep
17
comment Proper way of doing encryption and authentication (PBKDF2 + AES)
Run the output of PBKDF2 through HKDF-Expand to produce different values for different purposes.
Sep
16
revised How is the key shared in symmetric key cryptography?
edited tags
Sep
16
comment How to protect from Silver–Pohlig–Hellman algorithm
"if $p−1$ is a smooth number", but if you choose a $p-1$ with large factors (e.g. twice a prime) then it's not smooth and the algo doesn't break it.
Sep
16
comment Security of (cryptographic) padding compared to other methods for blockcompletion for block ciphers
A proper MAC scheme rejects any modified message in constant time before any other code (such as decryption) runs. Oracle attacks rely on the decrypter leaking some information for attacker modified messages, so by only telling the attacker that the MAC was invalid the attacker doesn't learn anything new. In contract with unauthenticated encryption(or mac-then-encrypt) modiefied messages might be treated differently. For example different error messages, some are accepted, different timing,...
Sep
15
comment What stops the Multiply-With-Carry Random Number Generator from being a Cryptographically Secure Pseudo-Random Number Generator?
No idea why people care about period length above say $2^{256}$.
Sep
14
comment Is it dangerous to use plaintext as IV in CBC?
Your "specification" is pretty unclear. How would you use the plaintext as IV? If you use the plaintext as IV, you need to tell the decrypter about it, but since the usual goal is the keep the plaintext secret, that doesn't make sense.
Sep
13
answered Salsa20 in Davies-Meyer mode with a fixed key (message): is it one-way?
Sep
12
comment Can I dynamically calculate an appropriate number of iterations for PBKDF2 based on the system time, rather than using a fixed value?
One problem with your system is that is assumes performance will double every two years. I think it has already slowed down, and will get even slower in a few years.