Cryptography Stack Exchange
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How does RSA compute such enormous numbers?

So I have been reading and learning a lot about cryptography lately and in particular asymmetric ciphers such as RSA. One thing that I am curious about but never seems to be mentioned is how the cipher algorithm manages to compute such enormous…
Luke
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Why would anyone use an elliptic curve with a cofactor > 1?

In cryptography, an elliptic curve is a group based on a finite field $GF(p^k)$; this group has $n$ elements on it, and we work on a prime-sized subgroup of size $q$. We denote the value $h = n/q$ as the cofactor of the curve. My question is: why…
poncho
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Disadvantage AES-GCM

What are the disadvantages and weaknesses of AES-GCM mode for authenticated encryption? Why does the CAESAR competition say that it’s one of the goals to "find an AE scheme that offers an advantage over AES-GCM"? What advantage they are talking…
user2035863
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Should I use the first or last bits from a SHA-256 hash?

I have the need for a hexadecimal token that is smaller than the normal length of the hexadecimal representation of a SHA-256 hash. Should I take the first bits or the last bits? Which of them contain the most entropy?
Peter Smit
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Is 80 bits of key size considered safe against brute force attacks?

I came across the KATAN family of ciphers for small domain input blocks. They cipher arbitrary block lengths: 32, 48 and 64, but their key size is 80 bits only. Is 80 bits of key size considered safe against brute force attacks with current state…
sashank
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Reason why “XOR” is a linear operation, but ordinary “addition” isn’t?

I'm new in cryptography and try to read some articles in this field. Many of these articles talk about non-linear S-boxes, and nothing more on what they mean by their non-linearity. I have a simple question which I think will guide me through my…
Shnd
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In RSA, why is it important to choose e so that it is coprime to φ(n)?

When choosing the public exponent e, it is stressed that $e$ must be coprime to $\phi(n)$, i.e. $\gcd(\phi(n), e) = 1$. I know that a common choice is to have $e = 3$ (which requires a good padding scheme) or $e=65537$, which is slower but safer. I…
Martin
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Why do we need special key-wrap algorithms?

Wikipedia says: Key Wrap constructions are a class of symmetric encryption algorithms designed to encapsulate (encrypt) cryptographic key material. We are using these algorithms to encrypt (and authenticate) a key, using a symmetric algorithm…
Paŭlo Ebermann
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ECDSA Compressed public key point back to uncompressed public key point

From the ECDH demo here, if I generate a private key for Alice I can get _ P = 1175846487558108474218546536054752289210804601041 Which gives the following public key point. X = 583857549063195252150226340830731484791130788759 Y =…
Ian Purton
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Theoretically, what if I were to change some magic numbers in, say, AES?

Purely theoretically. I know it's a bad idea to try to invent your own encryption and that's not the intention here. Just a thought experiment. Say, I change some or all of the magic numbers used in, say, AES (but this would also apply to other…
RobIII
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What is the practical impact of using System.Random which is not cryptographically random?

I recently noticed a .NET software using PBKDF to derive an encryption key from a password string. This password string was dynamically generated using System.Random. Now, I know that System.Random is not really cryptographically random and should…
learnerX
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Is a hash a zero-knowledge proof?

I’m trying to wrap my head around zero knowledge proofs, but I’m having trouble understanding it. In my current understanding, zero-knowledge proofs prove to the recipient that the sender has a certain knowledge without disclosing it. Like trying to…
vrwim
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Prevent double-spending with decentralized digital currencies without all transactions being public?

A recent approach to creating a decentralized online currency, called Bitcoin, has been generating some interest. The goal is to have a way to transfer currency without a central authority and without double spending or counterfeiting. Their…
Artem Kaznatcheev
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Is it bad to expose the public key?

I am studying about blockchain and have been focusing on encryption protocols that allow authentication. The digital signature of public and private key seems solid, I have focused on the algorithm ED25519. But in some places, some people say about…
Victor França
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Applicability of IBM's projected 50-qubit quantum computer Q to cryptanalysis?

IBM announced Q, a project for a 50-qubit universal quantum computer, according to the press realease. Here is more PR spin, and the research sub-page. What would be the applicability of that to cryptanalysis?
fgrieu
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