3
votes
2answers
430 views

What is the best way to store sensitive information on the client?

I am very inexperienced with cryptography and would like some advice on how to store some sensitive information on client machines. I have some information stored in a central database on my server. ...
3
votes
1answer
185 views

Chosen ciphertext insecurity in an ElGamal variant

I'm trying to prove something and if I can show that there is a simple way to calculate $(g^a \bmod p)^k$ if I know both $g^k \bmod p$ and $g^a \bmod p$, then (I think) it will help me prove it, but ...
3
votes
2answers
309 views

DSA signatures with related k and unknown payloads

Suppose that we are given DSA parameters $p$, $q$, $g$, a public key $y = g^x$, and two signatures $(r_1,s_1)$ and $(r_2,s_2)$. We are told that $(r_1,s_1)$ and $(r_2,s_2)$ were produced by related ...
3
votes
1answer
357 views

Understanding a Blowfish cryptanalysis

I'm reading a cryptanalysis on Blowfish, and I've come across something that I don't quite get. Let's denote $$\delta = a \oplus a'$$ where a and a' are bytes that cause a collision in some S-box ...
3
votes
1answer
228 views

Anonymity in end-to-end encryption

Considering a scenario of anonymous data transfer between seller and buyer using proxies. If we consider that there are three proxies that are being selected by the buyer to obtain data from different ...
3
votes
1answer
411 views

Understanding Feldman's VSS with a simple example

I'm trying to understand Feldman's VSS Scheme. The basic idea of that scheme is that one uses Shamir secret sharing to share a secret and commitments of the coefficients of the polynomial to allow the ...
3
votes
2answers
376 views

While generating a random Elliptic curve what are the conditions i have to considerd?

I want to generate a random elliptic curve over a prime field. What are the conditions I should satisfy? For the NIST recommended standard ECC-224 bit curve with prime $p=2^{224}-2^{96}+1$, a ...
3
votes
1answer
82 views

Padding in PMAC

How does padding in PMAC work? I have been told two stories: Firstly, I have been told that it works similar to CMAC padding, and secondly, I have been told that it does not need padding. I find the ...
3
votes
2answers
355 views

CPA Secure Chosen plaintext scheme

The example for IND-CPA secure schemes given is generally: for a random r, Enc_k(m) =(r|| E_k(r) XOR m) where E is a PRF But does the role of r and k really matter--i.e. isn't this equally ...
3
votes
1answer
412 views

Can ElGamal encryption and ElGamal signatures be used together sharing the same key-pairs?

I'm working on a encryption system where each party can store exactly a single ElGamal private key in a device. This is a hardware limit. The system must be expanded to support signatures and ...
3
votes
1answer
205 views

Is it OK to use a data-encryption key for key wrapping, too?

Our industry (area of cheap networked devices) has a standard that defines the usage of keys for both authentication and encryption using EAX mode of AES. This standard does not define key management, ...
3
votes
3answers
2k views

How do ciphers change plaintext into numeric digits for computing?

For example, in RSA, we use this for encryption: $ciphertext = (m^e \mod n)$ and for decryption. If our message is "hello world", then what number do we have to ...
3
votes
1answer
312 views

Recovering state of modified RC4 key scheduling algorithm

Consider this algorithm: ...
3
votes
1answer
254 views

Does ShaCrypt have the properties which make BCrypt slow on GPUs?

Sha256Crypt / Sha512Crypt are designed based on ideas of BCrypt but use SHA instead of Blowfish because of political requirements. The Sha*Crypt algorithms have the advantage that they are used in ...
3
votes
2answers
358 views

Order of hashing concatenation

A = hash("blue" + X); B = hash("pink" + X); If A and the literals are known and X is unknown, are there attacks on B aside from the attacks on directly on hash()? ...
3
votes
3answers
234 views

Are asymptotic lower bounds relevant to cryptography?

An asymptotic lower bound such as exponential-hardness is generally thought to imply that a problem is "inherently difficult". Encryption that is "inherently difficult" to break is thought to be ...
3
votes
2answers
394 views

secure multiparty computation for multiplication

Suppose there are $N$ parties $p_j$, each with a binary $b_j\in{\{0,1\}}$. The problem needs to compute the multiplication of number of ones times that of zeros, that is, ...
3
votes
2answers
1k views

Proof of security for RSA signatures

I am new to here, I was having some problems with a problem I am working on: "For each of the following variants of the definition of security for signatures, state whether textbook RSA is secure ...
3
votes
1answer
712 views

Generalizing the conversion of Diffie-Hellman to El Gamal

How can we generalize the conversion of Diffie-Hellman to El Gamal public-key encryption scheme? The goals is to be eventually able to show that any 2-round key-exchange protocol can be converted into ...
3
votes
1answer
1k views

ANSI X9.9 Cryptography Standards

I'm trying to create a ISO8583 Rev93 message. What is the standard way of generating MAC key in ANSI X9.9 DES-CBC encryption algorithm is used to encrypt the ...
3
votes
2answers
711 views

Using an RSA private key simultanously as an AES encryption key to generate random numbers?

Currently I'm implementing a PRNG for an embedded system. Using the RFC 4086 I've decided to use the X9.17 - to be more specific the successor X9.31- standard to implement my PRNG. X9.17 uses DES, but ...
3
votes
1answer
83 views

Is it safe to derive two different keys with the same password and key derivation function using two different salts?

I would like to know if the following process would harm the security of a fictional system if the persisted data would be stolen: in this system a user has a single password. The password will be ...
3
votes
1answer
222 views

RSA key pair generation and FIPS 140-2, 186-4

I need to securely generate RSA key pair and I need to have access to private exponent in order to process it further. What's the difference in FIPS 140-2 and FIPS 186-4 in connection with RSA key ...
3
votes
2answers
174 views

Which multiplicatively homomorphic encryption scheme supports encryption of 0?

I want a multiplicatively homomorphic encryption scheme that supports encryption of 0 (e.g. Elgamal doesn't support). I also want the multiplication to be operated on the ciphertext of 0, i.e., if ...
3
votes
2answers
153 views

Can a secret message be securely transmitted within a MAC tag?

A sender wants to transmit an ultra secret code $M$ which could be either 'go', 'stop' or 'wait'. This could be any selection of code words really and adopted for any use such as transmitting short ...
3
votes
1answer
58 views

NIZK Proof of knowledge N of M discrete logarithms (threshold)

It is well known how to produce a NIZK that curvepoints $aG$ and $aP$ have the same discrete logarithm $a$ with respect to the curvepoints they are multiplied by. There is also a way to prove that a ...
3
votes
1answer
354 views

Efficient AES - Use of T-tables

I'm really in trouble! I'm trying to understand how the T-tables in AES encryption work. But I don't know if I get the point. What I understood is that they are used to reduce the whole computation of ...
3
votes
1answer
81 views

Can we use numbers as a pad in the Vernam cipher - why or why not?

I was playing with the Vernam cipher on some online converter. But when I tried to encrypt my message string with numbers, it remained unchanged. Moreover, it was ignoring numbers and was encrypting ...
3
votes
1answer
176 views

“Practical” operations supported by functional encryption?

I'm curious about what operations have been developed into functional encryption schemes. What I mean by that is: what operations can be performed over encrypted ciphertexts? Obviously homomorphic ...
3
votes
3answers
170 views

Asymmetric algorithm for one time signing of small cleartext

I want to generate a key pair such that the private key can be used once to sign a small message (1024 bytes) at some indeterminate point in the future and the public key can be used to verify that ...
3
votes
1answer
172 views

Repeatable crypto

Is there a safe way to do crypto that will always produce the same result for a given input? My use case is transferring deltas of huge files, to a backup server. The backup server has no knowledge ...
3
votes
1answer
230 views

Possible CPA or CCA attack when using AES-CBC mode under the following situation?

Both client and server know a shared key in advance. Both sides us EVP_BytesToKey() derive a pair of same key and same IV. Then, client and server start transferring data encrypted by AES-CBC mode ...
3
votes
1answer
109 views

Partial hash code protocol for security tokens providing signatures

I've read the following two sentences within a beta protocol specification, which references ISO/IEC 7816-8 for PSO:HASH which already contains notions of a "partial hash". The off-card entity is ...
3
votes
1answer
108 views

Convergent Encryption: integrity and authenticity

I have one question about convergent encryption. Suppose I use AES-256 in CBC mode. I need a 256 bits key and a 128 bits IV. If I use SHA-256 to hash one file, then I can use directly that hash as ...
3
votes
2answers
173 views

Hash or compression function in Merkle tree nodes?

Wikipedia says about a Merkle tree: Nodes further up in the tree are the hashes of their respective children. Usually, a cryptographic hash function such as SHA-2 or SHA-3 is used for the hashing. ...
3
votes
1answer
130 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?
3
votes
1answer
279 views

KDF based on HMAC-SHA-256

Is KDF based on HMAC-SHA-256(Hashed Message Authentication Code, Secure Hash Algorithm) algorithm a suitable option to generate symmetric key from the secret key? What is the basic funda, behind ...
3
votes
1answer
149 views

Attack on DSA modification with bad hash function

Given a (bad) modification of DSA where the hash function is the identity ($H(m)=m$), I am now supposed to design an attack. So $(m, (r,s))$ are given and I should be able to create a legit signature ...
3
votes
1answer
95 views

Client authentication on limited hardware

I'm developing a authentication and authorization protocol for a Bluetooth device. The device should communicate with an Android app and needs to be able to authenticate the app during the connection ...
3
votes
2answers
106 views

Could scrypt salts contribute to entropy if passwords & salts are only stored in human memory?

I'm unsure of how exactly the salt is manipulated by scrypt and its potential contribution if any to overall password entropy if passwords & salts are stored only in human memory for a ...
3
votes
1answer
173 views

AES-128-CTR message integrity: Construction of HMAC

(Note: This post is not about "rolling my own", but rather trying to deepen my understanding of the underlying structures.) I'm having difficulties finding clear answers to some questions about the ...
3
votes
1answer
101 views

Need to generate new DH keys for every message in OTR

Going through the OTR specification I don't quite get why a new DH keys (and hence new symmetric key and hmac key) are generated for each message to be sent. Why is this necessary? I don't think it ...
3
votes
1answer
111 views

Cryptographic pseudo-random generation of address subsets

Suppose I have devices which broadcast, on an insecure channel, 32-bit addresses. I want to make these devices untraceable - one way to do so is to allocate for each device a distinct subset of ...
3
votes
1answer
137 views

Skein state size advantages

Skein is defined for three different internal state sizes: 256 bits, 512 bits, and 1024 bits – with Skein-512 as the “primary proposal”. Is there any advantage – other than potential memory or speed ...
3
votes
1answer
288 views

Analyzing ECB's and CBC's resistence to a known-plaintext attack

I'm trying to analyze the strength of a block cipher with CBC mode or with ECB mode on the scenario of an exhaustive search attack with knowledge of pairs of plaintext – ciphertext (known plaintext ...
3
votes
1answer
208 views

Why must IV be sent with each packet?

I was reading about Encrypt-then-MAC and everywhere I go it seems people include the IV with the ciphertext generated for each message then MAC that and send all the information concatenated. Why do ...
3
votes
1answer
83 views

In Pedersen Key Distribution, can the public key be persistent?

I am implementing a key distribution protocol described by Torben Pedersen in A Threshold Cryptosystem without a Trusted Party (EUROCRYPT'91). In the protocol, the $n$ parties distribute a public key ...
3
votes
1answer
295 views

Can I use Shamir's secret sharing scheme for multiplicative homomorphism for secure multiparty computation?

I would like to perform a dot product operation among $m$ parties using Shamir's $(m,m)$ secret sharing that is used for Secure Multiparty Computation. I am aware that Shamir's $(m,m)$ scheme is ...
3
votes
2answers
208 views

Asymmetric key derivation – Who derives the new pub key can't know the new private key

I'm looking for a peer reviewed protocol or algorithm that can do this, preferentially I need something based on elliptic-curves cryptography. Alice know the main public key of Bob (B-PUB-1). Alice ...
3
votes
1answer
60 views

Where did Martin Hellman acknowledge Ralph Merkle's contribution to public-key cryptography?

I am trying to find the article to reference when discussing the Diffie-Hellman key exchange. According to Wikipedia Hellman said that The system...has since become known as Diffie–Hellman key ...

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