9,506 reputation
12347
bio website github.com/CodesInChaos
location Frankfurt, Germany
age
visits member for 3 years, 1 month
seen 8 hours ago

Sep
23
comment Extracting only the entropy
Compression gives you an upper bound, but for security you need a lower bound. Generic compression routines will probably overestimate entropy so much that they're useless.
Sep
23
comment What is the security loss from reducing Rijndael to 128 bits block size from 256 bits?
Block size doesn't affect security much, for most modes of operations the only consequence is that you should encrypt significantly less than 2^(b/2) blocks of data using a single key. We currently encrypt nowhere near 2^68 bytes under one key, so 128 bit blocks are fine.
Sep
23
comment What is the security loss from reducing Rijndael to 128 bits block size from 256 bits?
There is no reason for matching key and block size. You can't really compare Rijndael with ThreeFish, the latter was designed as a building block for a hashfunction (Skein) and its design choices reflect that goal.
Sep
23
comment md5sum collision question
The block size of MD5 is 512 bits or 64 bytes not 64 bits.
Sep
23
comment Assuming a 1024qb quantum computer, how long to brute force 1024bit RSA, 256bit AES and 512bit SHA512
AFAIK you need about 2 qbits per RSA modulus bit to run Shor's algo which then breaks RSA with negligible cost. So you'd need a ~2048 bit QC to break RSA-1024.
Sep
23
comment Extracting only the entropy
@Blaze It's impossible to estimate the entropy of an input without context, so you can't limit the output size to the entropy, no matter which extractor you use.
Sep
23
comment Extracting only the entropy
Why do you want to avoid hashes?
Sep
23
comment Is it safe to use GZIP to avoid padding related attacks
Padding oracles are just one example of a larger class of attacks where the decrypter leaks information about the plaintext. I bet that gzip by itself is already enough of an oracle, with similar capabilities as a padding oracle.
Sep
21
comment With a true random number generator at hand, how to implement one-time pad?
You output the pad, transfer it through some secure channel (out of scope), and then simply xor it with the message to encrypt/decrypt. With which part do you have a problem?
Sep
20
comment Is it safe to use GZIP to avoid padding related attacks
1) I'd use a deterministic padding scheme 2) It's essential to apply a MAC after encryption. Else active attacks, like padding oracles will break it.
Sep
19
comment What is the lowest level of mathematics required in order to understand how encryption algorithms work?
Depends a lot of which parts of crypto you want to understand. For asymmetric crypto you need number theory, for symmetric crypto you need a combination of probability theory and cryptography specific knowledge. For cryptographic protocols you need very little math, mainly logical thinking and knowledge of the properties different primitives have.
Sep
19
comment Why not the one-time pad with pseudo-number generator
@MichaelKjörling The channel might be secure but high latency. For example consider a trusted courier who takes several hours to fly around the world. Or some conspirators who meet in person for the key-setup, and then later on exchange encrypted messages. A one-time-pad allows you to seperate the time of message exchange from the time you have the trusted channel.
Sep
19
comment Can we design a public-key infrastructure without certificate authorities?
@ddddavidee So you replace a trusted third party by an even more powerful trusted third party. The only advantage of IBE is that you don't need to be connected to the internet to encrypt a message with their private key. That advantage is obviously irrelevant for applications that inherently require internet connectivity.
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.