4
votes
2answers
142 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
1answer
523 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
1answer
669 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
48 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
329 views

For RSA keys, is the safety of a given key-length different for signing vs encryption?

I'm wondering if considerations of key-length are different when thinking about signing vs encryption. If for some reason, it was safter to use smaller keys for signing that would be interesting.
4
votes
1answer
517 views

Making my steganography code more hard to detect and crack

I'm doing a college project about digital image steganography on MATLAB. So far i've been able to get the help i needed from cool guys on stackoverflow but i now need to make my algorithm more hard to ...
4
votes
1answer
203 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
1answer
308 views

Protocol for Randomized Oblivious Transfer?

If we define Oblivious Transfer as following: Alice inputs $(x_0,x_1) \in F^2$, where $F$ is a field, and Bob inputs $b\in\{0,1\}$, then Alice gets a dummy output(for which she knows nothing about b),...
4
votes
1answer
581 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
2answers
212 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
195 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
3answers
326 views

Cryptographic data structure: sparse array without membership test

I would like a data structure supporting two operations: $\mathsf{set}(k,v)$ and $\mathsf{get}(k)$, with the following properties: $k$ can be any number between $0$ and something impressively large, ...
4
votes
2answers
131 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
3answers
207 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
135 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
2answers
211 views

Are all stream-ciphers IND-CPA?

A stream cipher usually generates a stream of pseudo-random bits which get XORed with the plaintext to form the ciphertext. The stream is generated using a given IV / nonce and a secret key. CTR-mode ...
4
votes
1answer
705 views

True Random Number Generator by milliseconds per keystroke (TRNG-Kms)

The simplest way to generate truly random numbers for OTP keys is to measure the time in milliseconds between each keystroke on a keyboard. The randomness depends on the user typing in various speeds. ...
4
votes
1answer
121 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
104 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
1answer
95 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
536 views

Can I construct a feasible stream cipher out of HMAC and a secure hash algorithm?

I have constructed a stream cipher from a secure hash algorithm and a HMAC. Here is a brief description of the algorithm: Let: (Actually Objective-C styled pseudo code) ...
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
1answer
1k views

What is “Blinding” used for in cryptography?

What does "blinding" mean in cryptography, and where do we usually use it? Can you describe a sample implementation?
4
votes
1answer
228 views

Securely generating passwords based on a secret and a public identifier

I'm trying to create a good scheme for generating passwords in my systems management scripts. What I'd like to do is have a secret key (let's say hello123), which I ...
4
votes
3answers
547 views

Why are elliptic curves better than cyclic groups?

The set of points of an elliptic curve over a finite field is isomorphic to the direct product of two cyclic groups (i.e. $E(F_{p^n}) \cong Z_{s} \times Z_{t})$. What is the advantage of representing ...
4
votes
1answer
130 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
298 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
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
1answer
155 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
690 views

Diffie-Hellman key agreement with both Server Authentication and Perfect Forward Secrecy

I am trying to find the protocol with the least overhead, which still meets the following requirements: Server Authentication of server identity to client. The client has an authentic copy of the ...
4
votes
1answer
350 views

How hard is it to generate a partial RSA fingerprint collision?

When I use SSH to log into a new system, I get asked to verify that the fingerprint (a 32 hex digits string) of the hosts's RSA key is correct. How much if it must I actually compare (by hand/eye) to ...
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
1answer
158 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
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 ...
4
votes
1answer
261 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
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
741 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
2answers
473 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
2answers
2k views

Would a “Triple AES” (in the sense of how Triple Des works) serve for a dramatic increase in safety?

The system requires to be as paranoid as possible regarding security. One of the few contemplated changes to the current design is to use multiple encryption. First proposal was to use Serpent on top ...
4
votes
2answers
932 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
56 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
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
251 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
190 views

Does EAX use the same keys for encryption and authentication?

I am actually checking use of EAX AEAD mode, and following EAX spec review (scheme definition for my part), my question is: what about the derivation of the authentication and encryption keys? In the ...
4
votes
3answers
225 views

Why are we not using multiple ciphers per message?

I am aware of at least rsa, elgamal-encryption, and variations of elliptic-curves relying on different problems and that those problems are considered hard. However, if someone figures out a way to ...
4
votes
1answer
176 views

Public key in fully homomorphic encryption over the integers

I'm reading “Fully Homomorphic Encryption over the Integers” by van Dijk et al. I wonder why $x_0$, which is a component of the public key, should be an odd number?
4
votes
1answer
88 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
2answers
146 views

Encrypt-then-HMAC implementation - am I doing it right?

I want to implement authenticated encryption using C#. There is a dot net class called encryptAndAuthenticate, but it is only supported on windows 8 or later, and I need the code to also work on ...
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
244 views

Will hashing over and over eventually give the same hash?

There are many cases in maths where doing the same thing over and over again will bring you closer to a value. Derivation of polynomials is the striking example: no matter what you start with, you end ...

15 30 50 per page