10,282 reputation
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bio website github.com/CodesInChaos
location Frankfurt, Germany
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visits member for 3 years, 5 months
seen 4 hours ago

Nov
13
comment Is there a proof for showing any cryptogram is crackable?
@evening When you try all possible keys you get back all possible messages. Nothing gained. The point of a one-time-pad is that for any message with the same length as the ciphertext there is exactly one key that decrypts the ciphertext to that message. So an attacker doesn't learn anything about the real message, no matter how much computing power they have.
Nov
13
comment How can we get CA's public key?
CA public keys are shipped with OS or applications like browsers.
Nov
12
comment MD5 theoretical question
There is a difference between collisions existing and actually being able to find them.
Nov
11
comment Are there public $p$ and $q$ numbers for use in DSA?
NIST has published a bunch of them.
Nov
10
comment Can two people with different one time pads securely exchange a message like this?
The general scheme is called Three-pass protocol and it's secure for some commutative ciphers, but simple xor is not one of them.
Nov
10
comment Can two people with different one time pads securely exchange a message like this?
An attacker who sees all the messages can trivially extract the plaintext with a bit of xor-ing.
Nov
10
comment Is it true for Java that the transformation mode and padding is ignored when using RSA?
It's even harder since PKCS#1v1.5 encryption padding is broken in a way that requires the higher level protocol to carefully work around its weaknesses. If you use RSA by yourself, use OAEP padding, not v1.5.
Nov
10
comment How can two (or more) parties share and agree upon a common random seed?
@rath Without collision resistance a cheater can pick which of them to reveal and the commitment becomes non binding. So collision resistance if an essential part of the protocol.
Nov
10
comment How can two (or more) parties share and agree upon a common random seed?
Since collision resistance is required, the security level of 64 bits provided by a 128 bit hash is insufficient.
Nov
10
comment Why don't we use MACs to store passwords?
For smaller websites a HSM is too annoying to deploy. But I'd expect big websites like google to encrypt their password hashes with HSMs.
Nov
10
comment Can Diffie-Hellman generate values in a specified range?
btw. DH never returns 0 if both sides follow the protocol. Only way to obtain 0 is if one side uses 0 as public key $A$, which is obviously broken and doesn't result from computing $A=g^a$ for any private key $a$.
Nov
10
comment Can Diffie-Hellman generate values in a specified range?
As I said, simply hash the shared secret with e.g. SHA-2. Then truncate to one byte.
Nov
9
comment Can Diffie-Hellman generate values in a specified range?
@JaimeASV Deriving an 8 bit key is trival using a hash function. But what's the point of using such a small key? Symmetric keys should be at least 80 bits, preferably 128. (and obviously 17 is much too small as well, you need primes larger than a thousand bits to be secure)
Nov
9
comment Decryption or attacking DES encrypted file
Short answer: brute force is feasible thanks to the small key-space
Nov
9
comment How do I test my encryption? (absolute amateur)
The absolute minimum is a specification using typical notation and a reference implementation in c. You should also consider an attacker who knowns or even chooses both message and ciphertext. If such an attacker can break your scheme then it's very weak by modern standards.
Nov
8
comment How do I sign data that's been encrypted using public key? (RSA)
You should sign with your own private key, not with the receiver's public key. You need to create your own key-pair to do so.
Nov
8
comment What is the difference between various modes in FPE?
It's even worse, FFX contains several different modes plus a bunch of parameters.
Nov
7
comment Are there any bijective one-way functions not based on number-theoretic hardness assumptions?
I don't see a requirement in the OP that it should be easy to invert when knowing a secret parameter.
Nov
7
comment Are there any bijective one-way functions not based on number-theoretic hardness assumptions?
@D.W. Some people place a much higher trust in unstructured problems than in highly structured number theoretic problems.
Nov
6
comment Would this program be useful in cryptography?
@user8911 You shouldn't just benchmark multiplication time. Part of key-gen with unknown primes is the primality test. You can't use well known primes like mersenne primes, those could be trivially factored. But just obtaining the key isn't the problem. You actually need to be able to sign/decrypt data with it. Even with karatsuba multiplication that will be pretty expensive. With numbers that size you need to use fancy algorithms, for example a fourier transform based one.