# All Questions

692 views

### Encrypt-then-MAC: Do I need to authenticate the IV?

In the setting of Encrypt-then-MAC, do I need to include the IV in what I'm HMACing, or is authenticating just the AES-encrypted data sufficient?
541 views

### Authenticated DH, what protocols are secure?

I read about STS + variants being insecure in the SIGMA paper, which then proposes SIGMA as a replacement. Are the SIGMA variants still considered secure or are there some other protocol that's ...
2k views

### Why does the PKCS1 RSA private key structure contain more than just exponent and modulus?

The ASN.1 spec for the PKCS1 RSA private key format is as follows: ...
5k views

### How does the index of coincidence work in the Kasiki test?

I'm starting to learn about cryptanalysis and I am having a bit of difficulty understanding the Kasiski test's index of coincidence. I have a book (Cryptography Theory And Practice by Douglas Stinson) ...
715 views

### Why does the crypto_box functionality in NaCl library exposes the nonce to the programmer

The idea of crypto_box API in NaCl library is to shield the programmer away from the technical details and provide an easy to use functions for encrypting and encrypting messages. Given what I've ...
684 views

### Can one implement AES on 4-bit microcontroller?

It is possible to implement AES on 4-bit microcontroller? As far as I know, AES on 8-bit microcontroller are common especially for light weight application such as the RFID. I have yet to come across ...
359 views

### Is there an algorithm to find the number of intersections of two sets?

Suppose both I and my friend have a set of integer numbers. We want to know the number of common elements in our two sets but without knowing elements of the sets of each other. So I don't want my ...
1k views

### Does collision resistance imply (or not) second-preimage resistance?

I've seen contradictory results. Sometimes hash functions are collision-resistant but not necessarily second-preimage resistant. I've seen this kind of things in papers from Bart Preneel: “Security ...
501 views

### Given a message and signature, find a public key that makes the signature valid

Given a message $M$ and a signature $S$, is it feasible to find a RSA public key $(n,e)$ such that $S$ verifies as a valid signature on $M$ (using this public key)? What if we're given one public key ...
684 views

### How to best obtain bit sequences from throwing normal dice?

Throwing normal dice, one can get sequences of digits in [0,5]. Which is the best procedure in practice to transform such sequences into bit sequences desired?
2k views

### Accelerating SHA-1

I have a program where computing SHA-1 is the bottleneck. This is using OpenSSL 1.0.0e on a 2.6Ghz 16-core Opteron where I get about 325MiB/s throughput. (SHA1 here is via Andy Polyakov's x86-64 ...
267 views

### The effect of truncated hash on entropy

Suppose i have a 128 bits random binary string (128 bits of entropy), then i hash it using SHA-256, then i take the first 128 bits of the output hash. Does the taken bit string still have (almost) 128 ...
258 views

### Understanding Twist Security with respect to short Weierstrass curves

I'm trying to understand the "Invalid-curve attacks against ladders" section of SafeCurves Twist Security page and I have difficulties to apply it to short Weierstrass curves. That section claims ...
966 views

### Does having a known plaintext prefix weaken AES256? [duplicate]

I plan on storing data, encrypted with AES256, where the first dozen bytes are known plaintext. ('Hello World!' for example.) Does having this known plaintext prefix weaken AES256? I do not care ...
239 views

### Does there exist a proof-of-retrievability scheme that is publicly-verifiable, limited-use, and does not use homomorphic encryption?

I find myself wanting to test out a practical implementation of a proof-of-retrievability scheme, simply out of curiosity. These schemes seem to be divided into two variations, publicly-verified and ...
374 views

### Is there a proof for showing any cryptogram is crackable?

I commonly hear statements along the lines of "all cryptograms are crackable - it's only a matter of time". Is there a proof to show that any cryptogram is "crackable"? The proof may be of a more ...
335 views

### Is there an encyption scheme that combines additive homomorphism with ability to proxy re-encrypt?

Is there an encyption scheme that combines additive homomorphism with ability to proxy re-encrypt? I've tried digging around on the Internet but haven't found anything conclusive on the topic.
422 views

### Why the symmetric key layer in PGP?

I've been familiarizing myself with the basics of PGP. If I understand correctly, PGP symmetrically encrypts the data of interest using a random single-use key, then encrypts the encrypted data and ...
5k views

### How much stronger is RSA-2048 compared to RSA-1024?

How much stronger is RSA-2048 compared to RSA-1024? It is hard to imagine very big numbers. So what would be your way to explain the difference to someone who doesn't know much about cryptography?
1k views

### What is the difference between a hash and a permutation?

As defined by Wikipedia a hash function is [...] any algorithm or subroutine that maps large data sets of variable length to smaller data sets of a fixed length. For example, a person's name, ...
3k views

### What is the recommended replacement for MD5?

Since MD5 is broken for purposes of security, what hash should I be using now for secure applications?
2k views

### 128 bit hash with least chance of collision

I'm building a storage system for JSON documents where they are looked up on a 128 bit key. These JSON documents have a timestamp within them, but apart from that are user-entered data. These JSON ...
234 views

### Does the position of the salt improve its effectiveness when hashing?

Seems most documentation I have read suggests the salt should prefix the value to be hashed. Is this just for consistency, or is the salt more effective when prefixed?
2k views

### How key materials are generated in SSL V3 from master secret

The generation of key materials is given by ...
228 views

### Changing algorithms during encryption

Inspired by "Guarding against cryptanalytic breakthroughs: combining multiple hash functions", I am curious if there is a cryptographic reason to use only one algorithm during encryption. For ...
2k views

### What alphanumeric string length can be used to guarantee no hash collisions from CRC-64?

If I'm hashing alphanumeric strings (chars in the set 0-9, a-...
249 views

### Secure way to derive separate encryption and MAC keys from a single master key?

I want to derive a 256 bit encryption key and a 256 bit MAC key from a single 256 bit master key for an authenticated encryption scheme. I was considering the following construction to derive the two ...
615 views

### What are Cryptographic Multi-linear Maps?

I've encountered this term many times in the fields of Fully-Homomorphic Encryption and Obfuscation. I want to learn those subject and Cryptographic Linear Maps seems to be an obstacle in the way. ...
387 views

### Why to try get key out of white box crypto? How can one protect WBC itself?

Help me understand the following. With WBC the question is mainly being stated as can one try to break WBC to identify the key. My question is: Why would one ever want to do that anyways? If one can ...
455 views

### Can we proxy-re-encrypt using homomorphic encryption schemes?

Homomorphic encryption schemes are PKE schemes with an additional special method Evaluate. The Evaluate method takes input any function (as boolean circuit) and encrypted inputs of the function and ...
673 views

### uniqueness of the RSA public modulus

What is the probability that two separate RSA public moduli are the same? For example, consider a 2048-bit modulus. The number seems to be huge, but the choice for prime factors p and q is much more ...
761 views

### Are there any secure commutative ciphers?

This answer lists two commutative cipher algorithms - Pohlig-Hellman and SRA. However, they don't appear to be too secure. My question is, here there any commutative ciphers out there that are secure ...
690 views

### Is there an authenticated encryption scheme where the recipient can attribute the message to a single sender?

With a standard authenticated encryption scheme (or MAC), Alice and Bob share a symmetric key. When Alice sends something to Bob, Bob can check that it is authentic. At that point, Bob can deduce ...
582 views

### Is there a practical zero-knowledge proof for this special discrete log equation?

We have a multiplicative cyclic group $G$ with generators $g$ and $h$, as in El Gamal. Assume $G$ is a subgroup of $(\mathbb{Z}/n\mathbb{Z})^*$. There are two parties, Alice and Bob: Alice knows: ...
718 views

### If the PSK is known, is it possible to decrypt traffic from other clients in a WPA2 wlan network?

If in a public WLAN WPA2-PSK is used, but the PSK is more or less publicly available, does this mean that an attacker with that PSK can easily decrypt wlan traffic from/to other clients of that WLAN? ...
4k views

### What are the details of the DES weakness of reusing the same IV in CBC mode with the same key?

I think I once faced the recommendation, that the initialization vector should always be random and never be used twice with the same key. How serious is this weakness? Also, is AES less effected ...
155 views

### Is the reduction from left-or-right IND-CPA to real-or-random IND-CPA tight?

A modern trend in cryptography consists of defining security as rigorously as possible, and then designing schemes which are secure according to those definitions. Proving security comes in the form ...
468 views

### RSA Encryption and Signature - Weak Padding

Assume that I have an plaintext $m$ and it is padded with $randompad||00||m$ and then it is encrypted with RSA and a public encryption key so we get the encrypted $Sm$. Then to assure its integrity ...
465 views

### Security of Pohlig-Hellman exponentation cipher?

I am looking into implementing Pohlig-Hellman exponentation cipher and I would like to know how secure that algorithm is? I am guessing it's security relates greatly to the prime number used in it. ...
332 views

### How is the x coordinate of a “point at infinity” encoded in a Secp256k1 signature?

I'm testing an implementation of Bitcoin, which uses the curve Secp256k1 for ECDSA, and I want to see how it handles the point at infinity ($0$) if present in a signature. For example, r could be the ...
363 views

### Can I use PGP to sign a message without providing cryptographic non-repudation?

The difference between a digital signature and a MAC is non-repudiation. A message with a digital signature proves that only the sender could have signed the message, whereas a message with a MAC ...
2k views

### Do I have to recompute all hashes if I change the work factor in bcrypt?

The well-known article about why we should use bcrypt for hashing passwords mentions the work factor - some parameter to the algorithm that determines how long one hashing should be in terms of number ...
557 views

### Is ECB mode secure if plaintexts guaranteed to be unique?

I've got a scenario where I need to encrypt many small (16-byte) plaintexts. I want to use AES-128 in ECB mode. Notably, each plaintext is guaranteed to be unique, though each may differ by only a few ...
916 views

### What algorithm would give the shortest ciphertext for very short plaintexts?

I have to encrypt some very short messages (approx. 5 characters) and I would like the ciphertext to be as short as possible. I am currently using AES but the initialization vector (IV) is making my ...
672 views

### are CFB and OFB really meant for streaming?

CFB, OFB and other modes are meant for streaming and don't require padding. Are there still limitations such as the text needs to be greater than key length?
472 views

### How does the wider cryptographic community view non-abelian group based cryptography?

Is there perhaps some neural expository article on crypto systems based on non-abelian groups? I've gleaned that Anshel–Anshel–Goldfeld key exchange is the most well-known cryptographic algorithm ...
2k views

### Brute forcing CRC-32

I'm working on a cryptosystem which uses IDEA. The designer made the mistake of including a CRC-32B hash of the password unencrypted in the header, so that the system can quickly reject bad ...
906 views

### Alphabetic Substitution with Symbols

I was reading on a site about the Zodiac Killer and how he used a basic substitution cipher, but instead of substituting english letters and characters he substituted symbols. I was wondering, if you ...
Let $G$ be a cyclic multiplicative group of order $n$. Let $g$ be a (public) generator of $G$. The Diffie-Hellman (DH) problem asks: Given $g^x, g^y\in G$ for $x, y\in \mathbb{Z}^*_n$, to compute ...