13
votes
1answer
4k views

ElGamal with elliptic curves

I've searched some information on ECC, but so far I have only found Diffie-Hellman key-exchange implementations using ECC, but I don't want to exchange keys, I want to encrypt & decrypt data like ...
13
votes
2answers
1k views

Does the elliptic curve (EC) cryptosystem outperform RSA and DL cryptosystems?

Throughout the literature, it is stated that EC cryptosystems outperform RSA and Discrete logarithm cryptosystems, but I cannot understand how ECC would be more efficient than RSA and DL in terms of ...
13
votes
2answers
449 views

Are safe primes $p=2^k \pm s$ with $s$ small less recommandable than others as a discrete log modulus?

I take the definition of safe prime as: a prime $p$ is safe when $(p-1)/2$ is prime. Safe primes of appropriate size are the standard choice for the modulus of cryptosystems related to the discrete ...
13
votes
2answers
278 views

Is (AES-)GCM parallelizable?

I recentely faced the issue of random access decryption while AES-GCM was being used. I said this person that the underlying CTR should allow parallelization but I have no idea how authentication ...
13
votes
1answer
1k views

Is Encrypt+HMAC stronger than AEAD?

There are a few posts that I've come across that seem to infer that using regular encryption and a MAC might be better than using the newer AEAD (ie: AES/GCM) modes. ...
13
votes
3answers
2k views

How practical are side-channel attacks and how much of a concern are they?

I see a lot of research in very sophisticated side-channel attacks on crypto systems. Most (but definitely not all) seem to follow a trend, namely, the crypto system does something very dumb like ...
13
votes
2answers
9k views

AES in ECB mode weakness

In a project that I'm currently working on, we are encrypting some data using AES with ECB mode in a database. Each piece of data being encrypted is very small, no more than 10 characters long. Very ...
13
votes
3answers
2k views

What is the ideal cipher model?

What is the ideal cipher model? What assumptions does it make about a block cipher? How does it relate to assuming that my block cipher is a pseudo-random permutation (PRP)? When is the ideal ...
13
votes
2answers
5k views

What is wrong with using SHA1 in digital signatures? Why is a robust hash function needed?

For the purposes of signing and verifying signatures, what is the value of the hash function? Why would it matter if SHA1 is later determined to be easy to break? Since a Public/Private key process ...
13
votes
4answers
659 views

Could one construct a cipher that is secure for friendly parties to use but insecure for hostile parties?

Consider the situation of a nation state (Blue) at war with another nation state (Red). Blue wants to deploy a secure cipher that blue currently can not break, but they are considered that Red could ...
13
votes
1answer
5k views

Compare Blockmode CBC (with diffuser) against XTS

I have some problems in understanding the "advantage" of AES-XTS compared to CBC with diffuser. I read something about FileVault, in this paper they mention the two modes of operations XTS and CBC ...
13
votes
1answer
1k views

Why choose an authenticated encryption mode instead of a separate MAC?

What are cryptographic reasons to choose an authenticated-encryption mode of operation (such as GCM) over a traditional encryption mode plus an independent MAC, or vice versa? Assume there is no ...
13
votes
2answers
13k views

Signatures: RSA compared to ECDSA

I'm signing very small messages using RSA, and the signature and public key are added to every message, which requires a lot of space compared to the actual content. I'm considering switching to ...
13
votes
1answer
2k views

How to communicate by email with forward secrecy and deniability?

My correspondents and I would like to communicate privately by email. We used to employ PGP but that might have been shooting ourselves in the foot. PGP doesn't have forward secrecy. If any of our ...
13
votes
2answers
14k views

What's the fundamental difference between Diffie-Hellman and RSA?

What is the difference in the purpose of DH and RSA? Aren't they both public-key encryption?
13
votes
2answers
845 views

Attacks of the MAC construction $\mathcal{H}(m||k)$ for common hashes $\mathcal{H}$?

Consider a common practically-collision-resistant hash function $\mathcal{H}$ (e.g. SHA-1, SHA-256, SHA-512, RIPEMD-160), perhaps based on the Merkle–Damgård construction as are the first three. We ...
13
votes
1answer
3k views

Assuming a 1024qb quantum computer, how long to brute force 1024bit RSA, 256bit AES and 512bit SHA512

Assuming in the future there was a functioning 1024 qubit quantum supercomputer and it could run Shor's algorithm or Grover's algorithm to crack encryption very quickly. I'm interested in how the ...
13
votes
2answers
960 views

GPG/PGP Verification of Revocation

How does one verify a key revocation? After revoking a key and sending the revocation to MIT's keyserver, I noticed that the key is listed as such: ...
12
votes
2answers
2k views

If you hashed a hash an infinite number of times would you end up with a unique hash?

If you took a hashing algorithm for example MD5 and repeatedly passed the output hash back into the algorithm an arbitrarily large number of times would you eventually end up with one unique hash? My ...
12
votes
4answers
2k views

Why RSA encryption key is based on modulo $\varphi(n)$ rather than modulo $n$

While calculating RSA encryption key we take modulo $\varphi(n)$ rather that modulo $n$. I couldn't understand why its so?
12
votes
5answers
3k views

Using CBC with a fixed IV and a random first plaintext block

What if, instead of using CBC mode in the normal way with a random IV, I used this approach: Use a fixed IV (like a block of 0's). Before encrypting, generate a random block and prepend it to the ...
12
votes
3answers
1k views

How long will my encryption remain private?

This is a basic question in cryptography but I have not found a good, comprehensive answer. It is explained that our keys should expire and we should get new, stronger ones with time, reflecting more ...
12
votes
3answers
2k views

Why does the padding in Merkle–Damgård hash functions like MD5 contain the message length?

I understand the need for padding in MD5. But why do we append the message length to the padding? I heard it strengthens the hash but how? Please provide an example if possible and how it applies to ...
12
votes
3answers
2k views

Why is OCB-AES mode not becoming a standard for authenticated encryption?

The OCB mode of authenticated encryption (used for example with AES) is the fastest way to provide authenticity and confidentiality without having to strive into questions like: Encrypt then MAC, MAC ...
12
votes
3answers
2k views

What is the use of REAL random number generators in cryptography?

I understand the use of pseudo-random number generators. I am not getting mixed up between these and "real" random number generators. However, I don't understand for what a real random number ...
12
votes
4answers
1k views

About Cryptography in a Character Language

Suppose I had a message in Chinese (or another non-phonetic language) and I wanted to encipher it. Some of the simplest encryptions in English are substitution ciphers, but such ciphers don't seem ...
12
votes
2answers
1k views

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 ...
12
votes
2answers
8k views

How does a chosen ciphertext attack work, with a simple example?

Can someone please explain - using a simple example - how a chosen ciphertext attack works?
12
votes
2answers
3k views

Unpredictability of X.509 serial numbers

About X.509 certificates serial numbers the RFC 5280 says: The serial number MUST be a positive integer assigned by the CA to each certificate. It MUST be unique for each certificate issued by a ...
12
votes
3answers
2k views

Why does SHA-1 have 80 rounds?

Why does SHA-1 algorithm have exactly 80 rounds? Is it to reduce collisions? If yes, then why do SHA-2 and SHA-3 have a lower number of rounds?
12
votes
3answers
6k views

What is pre-image resistance, and how can the lack thereof be exploited?

What is "preimage resistance", and how can the lack thereof be exploited? How is this different from collision resistance, and are there any known preimage attacks that would be considered feasible?
12
votes
3answers
1k views

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 ...
12
votes
1answer
12k views

Many time pad attack [duplicate]

I've already sent my correct solution to a homework exercise from Dan Boneh's Introduction to Cryptography class on Coursera: "Let us see what goes wrong when a stream cipher key is used more than ...
12
votes
1answer
816 views

Does unbalancing a feistel cipher always improve security? Does it improve security at all?

So according to Wikipedia unbalanced feistel ciphers provide greater provable security. Specifically, they state: The Thorp shuffle is an extreme case of an unbalanced Feistel cipher in which one ...
12
votes
3answers
1k views

What is a Non-Interactive Zero Knowledge Proof?

I understand the concept of a Zero Knowledge Proof thanks to the easy to understand analogy of Alibaba's cave. However, this seems to require interaction between the verifier and the other party. I ...
12
votes
4answers
10k views

What is the effect of the different AES key lengths?

How does a changing key length affects the ciphertext, not only in case of AES, but in general? I know that the key spaces become much larger and the number of rounds in case of AES changes, but is ...
12
votes
1answer
324 views

RSA with probable primes

I am a bit of a newbie to RSA encryption, so please be patient. I understand that for a 4096 bit RSA, the numbers p and q should be prime. And to have the best security, the p and q should both be ...
12
votes
3answers
678 views

Where do I securely store the key for a system where the source is visible?

I have a customer with an Access database (ugh!) in which credit cards are stored in plaintext (yikes!), so amongst other changes I'm doing in the app, I'm applying some encryption in there. I've ...
12
votes
1answer
5k views

What to watch for with openssl generating weak keys? (low entropy)

(Disclaimer: I am regular software engineer with only basic crypto knowledge, so helpful if can be explained for a layman.) I am concerned about generating weak keys on a shared linux box with ...
12
votes
1answer
3k views

Is HMAC-MD5 considered secure for authenticating encrypted data?

I've read something to the effect that the HMAC construct is able to lessen the problem of collisions in the underlying hash. Does that mean that something like HMAC-MD5 still might be considered ...
12
votes
2answers
3k views

Why is MixColumns omitted from the last round of AES?

All rounds of AES (and Rijndael) have a MixColumns step, save the last round which omits it. DES has a similar feature where the last round differs slightly. The rationale, if I recall correctly, ...
12
votes
2answers
1k views

Using same keypair for Diffie-Hellman and signing

Are there any security risks using a single key-pair for both key-exchange and signing? I'm mainly interested in using Curve25519 for key-exchange and Ed25519 for signing. But similar combinations, ...
12
votes
1answer
10k views

Calculating RSA private exponent when given public exponent and the modulus factors using extended euclid

When given $p = 5, q = 11, N = 55$ and $e = 17$, I'm trying to compute the RSA private key $d$. I can calculate $\varphi(N) = 40$, but my lecturer then says to use the extended Euclidean algorithm to ...
12
votes
1answer
678 views

Why did NIST remove The Lempel-Ziv Compression test from the Statistical Test Suite?

NIST removed "The Lempel-Ziv Compression" test from the Statistical Test Suite in revision 2008 and above and has not incorporated it since – see revision 2010. Why was it removed? Does it no longer ...
12
votes
2answers
1k views

Design properties of the Rijndael finite field

So we've already had a question on replacing the Rijndael S-Box. My question is - can we use a different finite field other than the one given by $x^8 + x^4 + x^3 + x + 1$ in $GF(2^8)$. In other ...
12
votes
1answer
496 views

Do recent announcements about solving the DLP in $GF(2^{6120})$ apply to schemes proposed for cryptographic use?

A recent paper by Göloğlu, Granger, McGuire, and Zumbrägel: Solving a 6120-bit DLP on a Desktop Computer seems to "demonstrate a practical DLP break in the finite field of $2^{6120}$ elements, using ...
12
votes
1answer
7k views

How to choose a padding mode with AES

Depending on the framework you are using, there are various padding modes that can be used with AES encryption. For example, with .NET we can choose PKCS7, ISO10126, ANSIX923, Zeros or None. I ...
12
votes
3answers
678 views

What exactly is the impact of the hidden subgroup problem on cryptography?

I understand my group theory (allegedly), so I can make partial sense of The Hidden Subgroup problem: Given a group $G$, a subgroup $H \leq G$, and a set $X$, we say a function $f : G \Rightarrow ...
12
votes
2answers
2k views

Deriving Keys for Symmetric Encryption and Authentication

So here's the concept. Rather than storing 2 keys and using a random IV, which presents its own problems (key rotation, ensuring no key is used in more than 2^32 cycles, sharing the keys, etc), is it ...
12
votes
1answer
418 views

Security of pairing-based cryptography over binary fields regarding new attacks

In the last week, the discrete logarithm problem was broken for the binary fields $\mathbb{F}_{2^{(14 \times 127)}}$ and $\mathbb{F}_{2^{(27 \times 73)}}$. Pairing-based cryptography using binary ...

15 30 50 per page