# All Questions

5k views

### RSA with small exponents?

Just to establish notation with respect to the RSA protocol, let $n = pq$ be the product of two large primes and let $e$ and $d$ be the public and private exponents, respectively ($e$ is the inverse ...
8k views

### Types of Cryptography for a 4-8 bit microcontroller

This is more of a research question. I was wondering what types of crypto algorithms would work best on a small 4-8 bit micro controller. I recently read a paper called Security Considerations for ...
4k views

### Why are primes important for encryption

Why are primes so important? Why can't we just use a random number? My guess is that it's because finding a random prime require more computing power, than finding a random number. Can anybody confirm ...
11k views

### How can one securely generate an asymmetric key pair from a short passphrase?

Background info: I am planning on making a filehost with which one can encrypt and upload files. To protect the data against any form of hacking, I'd like not to know the encryption key ($K$) used for ...
2k views

What is the disadvantage 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 ...
2k views

### Why is Pearson hash not used as a cryptographic hash?

The original algorithm produces 1 byte long hash and is (of course) not suitable for cryptography use. But according to wikipedia, it is possible and easy to produce Pearson hash of any length, ...
2k views

### Is HTTPS secure if someone snoops the initial handshake?

Let's say I'm on an open wireless network that's being actively sniffed and I connect to an HTTPS site. Even though my subsequent traffic is encrypted, couldn't the sniffer use the data from the ...
13k views

### HMAC vs MAC functions

I've read definitions of MAC and HMAC, but can't say I've completely grasped the differences. What are principle differences? When to use one and when the other?(Typical Use Cases)
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### Is Wiener's attack on RSA extendable to larger keys with low hamming weight?

Using small private exponents with RSA improves performance. However, it has been shown (Wiener, 1990) that if $\log d \leq \frac14 \log N$, the private exponent $d$ can be reconstructed from the ...
13k views

### How long does it take to crack DES and AES?

Suppose that a single evaluation of a block-cipher (DES or AES) takes 10 operations, and the computer can do $10^{15}$ such operations per second. How long would it take for to recover a DES key, ...
9k views

### What are preimage resistance and collision 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?
2k 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 ...
1k views

### How robust is discrete logarithm in $GF(2^n)$?

"Normal" discrete logarithm based cryptosystems (DSA, Diffie-Hellman, ElGamal) work in the finite field of integers modulo a big prime $p$. However, there exist other finite fields out there, in ...
16k views

### When to use RSA and when ElGamal asymmetric encryption

If i am not wrong in cryptography there are 2 basic cryptographic schemes for public key cryptography. RSA encryption whose security is based on the infeasibility of solving the factoring of big ...
7k views

### How does order-preserving encryption work?

Order-preserving encryption (OPE) is, apparently, a method of encrypting data so that it's possible to make efficient inequality comparisons on the encrypted items without decrypting them. I've been ...
5k views

### Specification of the Megamos crypto algorithm

It has recently emerged that a paper that was scheduled to appear at Usenix Security 2013, titled "Dismantling Megamos Crypto: Wirelessly Lockpicking a Vehicle Immobiliser", has been censored ...
267 views

### Is it possible for Alice and Bob to both sign a message “simultaneously”?

Let's say that there is a message which is considered valid if signed by both Alice and Bob. Alice could sign the message and then give it to Bob, so that he can sign it and give it back to her. But,...
22k 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?
1k views

### Mapping points between elliptic curves and the integers

My primary question is: Is there an easy way to create a bijective mapping from points on an elliptic curve E (over a finite field) to the integers (desirably to $\mathbb{Z}^*_q$ where $q$ is the ...
1k 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: ...
686 views

### Is there an AES identity key?

The following questions are of pure theoretical nature. I don't have an application in mind. Is there a key that makes AES the identity function? Is there a key that makes AES the identity function ...
1k views

### What security do Cryptographic Sponges offer against generic quantum attacks?

In the face of non-quantum attacker, Keccak[r=1088,c=512] with 512 bits of output provides: Collision resistance up to $2^{256}$ operations Preimage resistance up to $2^{256}$ operations Second ...
3k views

### Is 80 bits of key size considered safe against brute force attacks?

I came across KATAN Family of Ciphers for small domain input blocks . They cipher arbitrary block lengths 32,48,64 but their key size 80 bits only. Is 80 bits of key size considered safe with ...
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 ...
6k views

### Using the same secret key for encryption and authentication in a Encrypt-then-MAC scheme

Is it a weakness to use a single shared secret for protecting messages using a Encrypt-then-MAC scheme? Assuming a system is using AES-256-CBC and a SHA1-HMAC and the same secret key for both ...
1k views

### Can I use a one time pad key twice with random plaintext?

I understand the basics of OTP: $|\text{key space}| = |\text{plaintext space}|$ implies perfect security, key reuse destroys this. Cryptanalysis on the $N$-Time Pad for $N > 1$ involves finding ...
4k views

### What is the difference between a stream cipher and a one-time-pad?

A (synchronous) stream cipher is an algorithm which maps some fixed-length key to an arbitrary-length key-stream (i.e. a sequence of bits): $C : \{0,1\}^k \to \{0,1\}^{\infty}$. This key-stream is ...
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### How long would the 100 Year Cryptography Project have secured its data had it been started 100 years ago?

The goal of the Tahoe-LAFS 100 Year Cryptography project is to "enhance Tahoe-LAFS's cryptographic system so that Tahoe shipped today/next year might remain safe from cryptographic attacks for a 100 ...
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?
1k views

### Random oracle model proofs and programmability

Proving the security of a scheme with the random oracle model (ROM) involves two steps: first you prove that the scheme is secure in an idealized world where a random oracle exists, and then you ...
1k views

### Is the CBC weakness in XML Encryption a new discovery? Are other applications vulnerable?

The RUB in Germany reports that XML encryption is broken. This is essentially the W3C standard for protecting XML documents from prying eyes. Does this mean that an attacker can only see a single ...
3k views

### What is the most practical fully homomorphic cryptosystem?

Craig Gentry recently gave the first fully homomorphic cryptosystem. Quite a bit of work has been done since extending his work. It seems, however, that no system is practical for real world use. ...
1k views

### Can an Enigma-style cipher of sufficient complexity be considered secure in today's world?

Regarding the German Enigma machines, if I recall correctly, the reason they were defeated was because the Allies were able to generate a massive database of possible rotor settings, and because the ...
10k views

### Why hash the message before signing it with RSA?

The diagram below illustrates the process of digitally signing a message with RSA: As diagram shows, the message is first hashed, and the signature is then computed on the hash, rather than on the ...
11k 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?
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. http://www.daemonology.net/blog/...
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### 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 ...
1k views

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

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 ...
3k views

### What is a tweakable block cipher?

Pretty simple question - but I can't seem to find much information about it. What exactly is a tweakable block cipher? How do they differ from traditional block ciphers? What is the 'tweak'? Is it ...
577 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 ...
3k 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 ...
2k 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, ...
504 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 ...
244 views

### Formal verification in cryptography

I have seen in some places that people use formal verification and/or computer-aided verification for cryptography (tools like ProVerif, CryptoVerif, etc.). How do these approaches work?
17k 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 ...
9k views

### How strong is the ECDSA algorithm?

Some cryptographic algorithms are as strong as the size of their key is, while other have some weaknesses that limit their strength (such as SHA-1). How strong is the ECDSA algorithm, and does that ...
21k 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 ...
955 views

### What NIST protocol was allegedly backdoored by NSA in 2006?

From a recent NY Times article: Cryptographers have long suspected that the agency planted vulnerabilities in a standard adopted in 2006 by the National Institute of Standards and Technology ...