Cryptography Stack Exchange
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3 answers

Can you create a strong blockcipher with small blocksize, given a strong blockcipher of conventional blocksize?

Suppose I want a strong 20-bit blockcipher. In other words, I want a function that takes a key (suppose the key is 128 bits), and implements a permutation from 20 bits to 20 bits. The set of permutations should be close to a randomly-chosen subset…
Fixee
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How does ECC go from decimals to integers?

I realise that elliptical curves are tricky, but there's one aspect that no one seems to explain. I've looked, and it's towards the beginning. This is the traverse over a red curve:- This is only three steps (n=3). For cryptography n must be…
Paul Uszak
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Finding $x$ such that $g^x\bmod p

In a Schnorr group as used for DSA, of prime modulus $p$, prime order $q$, generator $g$ (with $p/g$ small), how can we efficiently exhibit an $x$ with $0
fgrieu
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How does the attack on MD5 work that allows a file to show its own (full) hash?

I've recently stumbled across this "moment" on Twitter, where there are three files, that show their own MD5 hashes. As an example, this GIF (screen-shotted in the following image), has the hash: f5ca4f935d44b85c431a8bf788c0eaca They obviously…
SEJPM
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18
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6 answers

Why do key fobs/garage doors openers use rolling codes instead of PGP or SSL encryption?

Wouldn't they be more secure if they used the same encryption technology computers use for remote access?
Bernard Igiri
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18
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7 answers

Isn't AES-NI useless because now the key length need to be longer?

AES-NI speeds up both encryption and decryption. The encryption is as secure as the time needed to brute force the decryption. Doesn't it mean that the application developers will just need to increase key length until the time taken is almost the…
cnvzmxcvmcx
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1 answer

What does a "real" quantum computer need for cryptanalysis and/or cryptographic attack purposes?

The cryptographic world has been buzzing the word "quantum" for a while now (even the NSA is currently preparing itself for a post-quantum crypto world) and quantum-related hardware engineering is evolving constantly. For example: the 5-qubit…
e-sushi
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18
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Where do Windows applications get entropy from?

Where does the entropy that cryptographic .NET libraries use for encryption come from? I know with Linux you can use /dev/random, but does a similar pool exist within Windows?
Verbal Kint
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Is it possible to obtain AES-128 key from a known ciphertext-plaintext pair?

I have a file, which was encrypted with AES-128 in ECB mode. I know the format of the original file and know that all files in this format have the same headers. So, I have an encrypted block and the original block. Can I get the encryption key,…
Denis Bezrukov
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18
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What is the relation between RSA & Fermat's little theorem?

I came across this while refreshing my cryptography brain cells. From the RSA algorithm I understand that it somehow depends on the fact that, given a large number (A) it is computationally impossible to derive at the two prime numbers (B & C) whose…
PlanetUnknown
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Is there a key for AES-128, 256 etc, where cleartext equals ciphertext?

Is there a key for AES-128, 256 etc, where cleartext equals ciphertext? (regardless of modes like chaining or ctr-cbc)
Malte K.
18
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1 answer

Can you give me a summary of cryptographic hardness assumptions?

Until recently, I had a link to a website which summarizes up-to-date cryptographic hardness assumptions. But, unfortunately I cannot find it. The webpage is categorized well problems such as, DL problem, Factorization, Finite field, DH…
Arious Oh Nine
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Is there any difference between NIST and SECP curves in-terms of their algorithms and implementation?

I'm implementing ECDSA for NIST P-256 curve. I just want to know if the same implementation will also work for SECP curves? If it doesn't, can you point me to one or more references of algorithms for SECP 256? To clarify: I specifically want to know…
abejoe
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How does encryption work in elliptic curve cryptography?

So I think I understand a good amount of the theory behind elliptic curve cryptography, however I am slightly unclear on how exactly a message in encrypted and then how is it decrypted. So my questions are How are messages encrypted and…
Ali
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2 answers

AES - What is the advantage of a 256-bit key with a 128-bit block cipher?

I'm just trying to wrap my mind around the decision to select the 128-bit Rijndael as the AES cipher, even with 192-bit and 256-bit keys. Even with a 256-bit key, you only get 2^128 possible outputs per 128-bit block, thus kind of rendering the…
Scott Arciszewski
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