5
votes
1answer
288 views

What is the “compression function” in Merkle-Damgård?

Is the "compression function" in Merkle-Damgård just a collision-resistant, one-way hash function but one that operates only on fixed size inputs? If so, is MD just a way to extend it to work on ...
5
votes
1answer
423 views

Small subgroup confinement attack on Diffie-Hellman

I am trying to understand the small subgroup confinement attack on the Diffie-Hellman algorithm. I will present the attack and try to explain why it works. Small subgroup confinement attack on the ...
5
votes
0answers
281 views

Given a 'good' basis for a lattice, how can we solve the CVP?

I'm doing a little bit of reading about lattices. I read that if we can find a 'short' basis for our given lattice, we can solve CVP and SVP very efficiently. However, the paper didn't describe an ...
5
votes
1answer
271 views

HMAC construction based on the combination of two hash functions

I would like an $\operatorname{HMAC}$ that's based on two different hash functions ($H_1$ and $H_2$), so that a break of the combined $\operatorname{HMAC}$ would imply a break of $\operatorname{HMAC}(...
5
votes
3answers
575 views

Secret sharing scheme with possibility to change the secret

Here's a scenario I want to handle with a secret sharing scheme: Alice wants to share the secret $S$ with Bob, Charlie and Dave. Alice generates private shares based on $S$ for Bob, Charlie and Dave,...
5
votes
1answer
137 views

How can I figure out a hash algorithm?

This may be trivial, and may be impossible... this is outside my field but willing to learn. I have a hardware device that accepts a network MAC address (6 bytes of data) like this: 38-60-77-EC-1D-33 ...
5
votes
1answer
215 views

What functions allow for practical indistinguishability obfuscation?

Recently, there has been a major theoretical breakthrough in program obfuscation, in the area of indistinguishability obfuscation. Background: Indistinguishability obfuscation is a form of program ...
5
votes
1answer
565 views

Computing IV for CBC from PBKDF2 + HKDF

Note that this question is somewhat similar to Can I use my random IV (for AES) as a salt for PBKDF2? My current encryption format computes two random PBKDF2 salts (encryption and HMAC, 8 bytes each) ...
5
votes
4answers
1k views

Security proofs for CBC mode

I'm looking for different approaches to proofs for the security of CBC mode encryption. What are the best sources of information about this subject?
5
votes
1answer
161 views

How can I find the order of the group that an elliptic curve is defined over?

I have a Weierstrass elliptic curve ($y^2=x^3+a \times x+b \mod p $) How can I find the order of the group itself? I have seen Mathematica has a GroupOrder[] ...
5
votes
2answers
946 views

ElGamal signature without calculating the inverse

I stumbled upon this question in some textbook. Propose a variant of ElGamal signature scheme such that there is no need to calculate the inverse $k^{-1}$ as it is usually done using the EEA. ...
4
votes
2answers
438 views

Provable Encryption

Is it possible to encrypt data in a way that it can be proven that the data is encrypted, without revealing the key? Alice chooses some plaintext, then she encrypts it with a certain scheme. She ...
4
votes
1answer
204 views

RSA time function

I am doing a paper on textbook RSA (highschool-level). I am explaining and proving the significance of the public key exponent in time and security. So far, I have encrypted a constant message with a ...
4
votes
3answers
216 views

paradox on fully homomorphic equality checking

Imagine, a client encrypts a corpus of data (say documents of text) with the public key of a Fully Homomorphic Encryption scheme (FHE) and outsources the data to an untrusted server.Now the client ...
4
votes
2answers
303 views

How to Distribute the Shares using Secret Sharing for arbitrary Monotone Access Structures

Consider 4 people, $A$,$B$,$C$,$D$, & a secret $s\in\{0,1\}^k$. Construct a scheme which enables the following subsets of people to retrieve the secret $\{A,B\}$, $\{A,C\}$, $\{B,C,D\}$. I know ...
4
votes
3answers
281 views

Can insecure algorithms be combined to form a secure algorithm?

This is hypothetical as I can't think of any reason to do this, but out of curiosity... Could I, for example, take the MD5 digest of a message and concatenate it with the SHA-1 digest (not quite ...
4
votes
1answer
124 views

Are there comprehensive alternatives to RSA?

If we wished to, is there a comprehensive alternative to RSA? I say comprehensive as I wonder if there is one which does both encryption and digital signature like RSA? If not, simply what ...
4
votes
1answer
97 views

Changing encryption key without revealing the original key

Say that Alice encrypts a $\text{plaintext}$ with $\text{key}_{m}$ and gives the $\text{ciphertext}_m$ to Bob. Alice wants to send several gigabytes of $\text{plaintext}$ to Eve but she is on a mobile ...
4
votes
2answers
966 views

Is this a correctly formatted PGP session key packet?

RFC 4880 may be full of information, but it can be incredibly vague at times, so im looking for someone who actually knows the answer to this. Given this public key: ...
4
votes
2answers
343 views

Brute-force attack given small search-space and hash prefix

Suppose we have a value $secret which is a random 56-bit (7-byte) string. Suppose an adversary wishes to discover $secret. The ...
4
votes
1answer
132 views

Is it ever unsafe to compress an EC point?

I am working with a library that outputs EC points in uncompressed form. To save space, I'm considering modifying said library to use compressed EC points. Assuming that I keep track of the sign bit ...
4
votes
2answers
62 views

Yao's Garbled Circuit- Why does Alice need to rearrange the circuit?

My understanding of Yao's Garbled Circuit (based mostly on this summary) is as follows: Alice creates a garbled circuit based on the function f to be computed. She then hard-codes her input into the ...
4
votes
2answers
144 views

Turning a 64 bit block cipher into a 128 bit block cipher

There are quite a few block cipher modes of operation that require 128 bits. There are also modes of operation where a higher block size than 128, e.g. a block size of 256 bit would even be practical. ...
4
votes
2answers
213 views

Addition step in MD5 implementation

In MD5, there are four rounds. After every round, why do we need to add the computed $Q$ values to the initial values and then take this value as input to the next round. For example after the first ...
4
votes
1answer
673 views

Differential cryptanalysis - breaking the last round of FEAL4?

I've been trying to learn cryptanalysis. I've come across this resource which proved very helpful: http://theamazingking.com/crypto-feal.php So far I've been almost successful in breaking FEAL4 ...
4
votes
1answer
91 views

Can I store the initialisation vector (IV) in the filename?

I'm encrypting a file and I'm using an initialisation vector. Given that the IV doesn't need to be a secret, is it secure if I add it to the encrypted file's name?
4
votes
3answers
382 views

How feasible is word-level frequency analysis over English (or any language)?

Say I have some black box which, given any English word, deterministically outputs a token for that word. Assume our black box is implemented using strong cryptography, i.e. the hardness of reversing ...
4
votes
1answer
526 views

Blind signature with openssl

I'm trying to understand (to reproduce in practice) how does blinded tokens work. Currently i'm lacking examples. According to Wiki the blinded signature protocol is the same as ordinary signature ...
4
votes
2answers
137 views

Hash functions vs Stream Ciphers in terms of Speed

We know that it's possible to create a hash using a stream cipher (for instance, RC4 or AES in Counter Mode). As stated here: "A Pseudo Random Generator (PRG) and a hash function are both Pseudo ...
4
votes
1answer
164 views

CBC-MAC just to verify integrity

I have a message and I want to verify its integrity and authenticity. I have a shared secret (key) that is not used for other encryption. Is it safe to send a plaintext message containing all the ...
4
votes
1answer
592 views

Using a derived key for CMAC

Consider the following authenticate-and-encrypt scheme that uses AES-128 in CBC mode for encryption and AES-128 - based CMAC for authentication: Two keys are derived from the master key k (16 byte): ...
4
votes
1answer
745 views

Can I use my random IV (for AES) as a salt for PBKDF2?

Also, where do I store my salt (can I just store it at the beginning of the encrypted file)?
4
votes
1answer
2k views

What is the 'Version Incompatibility" Problem in TLS 1.1/1.2 and 1.0?

It's unclear to me if there are multiple problems, or a single one, (and just what that problem is) with servers incorrectly negotiating down from a client offering 1.1 or 1.2 to something they ...
4
votes
1answer
49 views

Could a strong round function be immune to slide attacks

An excerpt from the wikipedia article on slide attacks states: ...The only requirements for a slide attack to work on a cipher is that it can be broken down into multiple rounds of an identical F ...
4
votes
1answer
252 views

Encryption-Decryption-Encryption

I have learned from Applied Cryptography that triple encryption with three independent keys in encryption-decryption-encryption (EDE) process gives security not higher then $2^{2n}$ when it comes to ...
4
votes
1answer
197 views

Keyed digest function with odds of collision below the birthday bound?

I wonder if it is possible to devise a function $F(K,S,R_S)\mapsto D$ where: $K$ is some key (I have freedom on $K$, it could even be generated by a trusted party); $S$ is in $\{0,1\}^s$, say $s=32$;...
4
votes
1answer
128 views

What is Deterministic Authenticated Encryption?

I came across something known as deterministic authenticated encryption in my studies and a lot of people were associating it with Synthetic IV mode. I am having trouble what exactly DAE is because I ...
4
votes
1answer
265 views

Solving Vignere Encryption

I'm currently trying to crack a cypher that I believe is Vignere Encrypted and I'm currently stuck. I calculated the key length by finding repeated sequences in the cypher and calculating the the ...
4
votes
1answer
850 views

ECC vs RSA: how to compare key sizes?

I know and I have understood the details of RSA, elliptic curve cryptography, (EC)DH and (EC)DSA. I keep reading everywhere that (if we don't consider non-deterministic computers) "ECC can achieve ...
4
votes
1answer
157 views

low-exponent RSA

I have questions from http://link.springer.com/chapter/10.1007%2F3-540-68339-9_1 Suppose we have 2 messages $m_1$ and $m_2$ related by a known relation $m_2=m_1+1$. Suppose further the messages are ...
4
votes
2answers
146 views

Is there an asymmetric encryption algorithm where the public key cannot be derived from the private key

Is there an encryption algorithm that will produce a public key and a private key such that the public key cannot be generated using the private key. For example, if the private key is compromised ...
4
votes
1answer
169 views

Reason for difference in assumptions for practical private-key and public-key crypto

Theoretical cryptography tells us that everything in the world of private-key cryptography (CCA-secure symmetric encryption, message authentication codes, etc.) can be built from one-way functions and ...
4
votes
1answer
109 views

Using HMAC as a nonce with AES-CTR encrypt-and-MAC

Suppose I use the following encrypt-and-MAC construction: $E(k_1, k_2, n, m) = E_\text{AES256-CTR}(k_1, n, m) \| \text{HMAC-SHA256}(k_2, m)$, where: $k_1$ and $k_2$ are 256-bit keys $n$ ...
4
votes
2answers
330 views

Advantages of combined PRNGs

This question is related to this one in intention, and this one in scope. Would I be guarding myself against future attacks on a single PRNG if I combine two (or more) together? I am thinking of ...
4
votes
5answers
234 views

How does generating random numbers “remove entropy from your system”?

In a previous question, I quoted the --gen-random entry in the GPG Man Pages. At the end it says: PLEASE, don’t use this command unless you know what you are ...
4
votes
1answer
1k views

Secure order preserving hash function

Is there a construction of an order preserving hash function that keeps the preimage property of a crypto hash function? By order preserving hash function (OPHF) i mean for $x<y$ then $OPHF(x) < ...
4
votes
3answers
5k views

Export from US of crypto software with key-size > 56 bits still needs permission?

Elsewhere someone stated that he couldn't publish his crypto software on the Internet, because US Export Regulations require approval, if the key size is greater than 56 bits. I know that several ...
4
votes
2answers
1k views

Why is the discrete logarithm problem assumed to be hard?

This might be a quite stupid question: since a naive brute force algorithm to solve the discrete logarithm problem will only take O(n) time for a group G with order ...
4
votes
2answers
479 views

Can I combine two PRNGs to make use of more seed data?

I have 320 bits of seed data (actually 512 bits of data with 320 bits of entropy, derived from a Diffie-Hellman shared secret and nonces). The PRNG I am using at the moment is the android version of ...
4
votes
1answer
2k views

HMAC-SHA1 input size

I know that the HMAC is a message authentication code that uses a cryptographic key in conjunction with a hash function (SHA1 , MD5, etc.). The HMAC output is 160 bits for HMAC-SHA160 and 256 bits for ...

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