3
votes
1answer
192 views

Feasibility of using a base 26 LFSR for cryptography by hand

I have been playing with base 26 LFSRs (i.e. using the alphabet) and noticed that the XOR operation for base 26 is just the tabula recta and so can be done very quickly. This made me wonder whether a ...
3
votes
1answer
2k views

RSA square and multiply

If an RSA encryption uses a modulus $n$ of 15000 bits and $e = 3$ or $e = 2^{16} + 1$, how many squarings and multiplications will we need for encryption, and for decryption? Could you please explain ...
3
votes
3answers
280 views

understanding pairing $e:G \times G \to G_T$ and ( Decision)BDH assumption

From DrLecter's comment, I know that DDH problem can be efficiently solved with this $$e(g^a,g^b)\stackrel{?}{=} e(g,g^z).$$ I have some trouble to understand this map $e:G \times G \to G_T$. Am I ...
3
votes
1answer
162 views

Stream vs Block cipher: Synchronising and the receiver

I am learning cryptography at school and I was wondering: How are the blocks/streams after decryption synchronized at the receiver to form one long meaningful message? Am I right by saying that ...
3
votes
1answer
77 views

Should I use distinct MACs for each user, or should I use a *direction* flag when communicating?

Alice and Bob want to communicate using a stream cipher. At the beggining, they create a session key $K_s$ and exchange it via some secure channel (using some asymmetric algorithm). After every ...
3
votes
1answer
933 views

RSA key pair generation using PRNG with same seed

I see a lot of Q/A where persons are trying to generate a specific key pair using static data such as a password. Now say we use a known PRNG (dangerous assumption), seeded with a the data as a static ...
3
votes
1answer
362 views

Hamming Distance vs. Index of Coincidence

When analyzing a repeating-key xor cipher to find the key length, I've read about two key methods (assuming there aren't just repeating chunks of ciphertext for Kasiski's method), for some assumed key ...
3
votes
1answer
601 views

Generating an IV for ESP 3DES-CBC

I have an application written in C, running in Linux. It uses IPsec (ESP) (manually putting the IPSec packet together) to send certain packets. The algorithm used ...
3
votes
2answers
698 views

Is there any implementation of scrypt that allows a specific limit on memory?

The answerer has commented that scrypt's memory use is "only a function of r". $\:$ However, he has not addressed my argument that it also depends on N, in one of my comments from over 5 days ago. I ...
3
votes
2answers
220 views

Does impersonating an SRP server give you enough information for an off-line dictionary attack?

In a comment to an answer I wrote to another question, CodesInChaos wrote that: "Problem with SRP is that an attacker who impersonates a server learns the password hash, enabling offline search." ...
3
votes
2answers
235 views

Parallel-resistant proof-of-work scheme?

I am looking for a proof-of-work scheme which cannot be effectively parallelized. For example, in hashcash (and by extension bitcoin) you have some collision-resistant hash function $f()$, a target ...
3
votes
1answer
262 views

Quantum resistance of Lamport signatures

The Lamport-Diffie signature scheme is said to be quantum-resistant. Why is that? What would a quantum attempt to attack this signature scheme look like, and how does it fail?
3
votes
1answer
662 views

Formal definition of (perfect) forward security/secrecy

In recent weeks the concept of (perfect) forward security/secrecy has been mentioned a lot, primarily in the context of the shocking revelations about NSA eavesdropping. As far as I'm aware, this ...
3
votes
1answer
804 views

Security of authenticated encryption modes gcm & ccm

I have two questions for Clarification for AE mode choice criteria GCM : it appears to be actually the most popular and widely used AE mode of operation. however it is also well-known to be highly ...
3
votes
4answers
329 views

Is it possible to anonymize web traffic so that the IP Address cannot be determined while still being able to determine distinct IPs?

I'd like to find a way to anonymize IP addresses in web logs to ensure user privacy given the following requirements: If given an IP address, there is no way to look up which requests came from the ...
3
votes
2answers
534 views

What does “message schedule” mean in SHA-256?

I am trying to understand the sha-256 algorithm from FIPS 180-2. I understood the padding and parsing of the message string. However after that it states (page 15): For $i = 1$ to $N$: { ...
3
votes
2answers
187 views

Is this OTP scheme safe?

I have a message $m_1$ and I want to encrypt it by XORing it with two keys $k_1$ and $k_2$: $$c_1 = m_1 \oplus k_1 \oplus k_2$$ So far so good. Now I was wondering if I could create a "fake" result ...
3
votes
1answer
281 views

In ECDSA, how many field operations are used for signature verification?

I am wondering about the computational cost of ECDSA signature verification, in term of multiplications in the base field; and, as an aside, in term of (much cheaper) additions. To make things ...
3
votes
1answer
157 views

implementing long term archive encryption

Let's say I want to create private archives for the long term (e.g. more than 30 ). The archives' sizes could be anywhere from 1 GB to 30 GB. As far as I understand I could go down two ways: The ...
3
votes
1answer
307 views

Addition with Shamir secret sharing

When performing Shamir secret sharing I'm trying to find $z_i$, such that $z = x + y$. Where $n = 6$ and $t = 3$. I believe this would be the correct solution (correct me if I'm wrong): Each party ...
3
votes
1answer
1k views

Triple-DES Brute Force

Shannon's paper states: A system is pure if all keys are equally likely and if for any three transformations, $T_i$, $T_j$, $T_k$ in the set, the product $T_iT_j^{-1}T_k$ is also a ...
3
votes
2answers
446 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
194 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
328 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
232 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
2answers
416 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
84 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
1answer
9k views

AES vs Blowfish taking key-length into account

I understand that Blowfish is getting old, but is still a secure algorithm, and that AES is very popular, and is recommended by most people. My question is, how do the two compare when a 256-bit key ...
3
votes
2answers
367 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
211 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
3k 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
318 views

Recovering state of modified RC4 key scheduling algorithm

Consider this algorithm: ...
3
votes
1answer
255 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
378 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
239 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
410 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
725 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
729 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
2answers
71 views

Why must we generate the signature before any compression being done at PGP?

I have done some research on PGP and I found out there are of two reasons why we must generate the signature before doing any compression. Yet, I am not sure I really understand them both. One ...
3
votes
1answer
74 views

Secure method of encrypting data within a database

I'm designing a program which makes queries to a database stored locally which could potentially contain a lot of sensitive information. As such the information must be kept secure as possible from ...
3
votes
1answer
71 views

Standard format to encode AES cipher and random IV

As far as I understood, a secret key (same length as CBC block size) and an initialization vector (IV) are needed to decrypt a AES-256-CBC encrypted string. As the IV should better be random, it has ...
3
votes
1answer
99 views

Cryptographic random numbers for key generation

I am trying to understand how a cryptographic library works (for example, one that provides assymetric encryption such as RSA), but I'm running into a few problems about the key-generation. There are ...
3
votes
1answer
83 views

Why would you use a weaker hash function than cipher strength?

Some cryptographic libraries (like “Bouncy Castle”) support many modes where the maximum number of bits which can be produced by the hash are significantly fewer than the cipher strength. For ...
3
votes
1answer
49 views

Implementing forward secrecy in OpenPGP with public key authentication

I am trying to consider the feasibility of implementing an encrypted file sharing system, which both provides forward secrecy but also allows public key verification. I am stuck on the marriage ...
3
votes
1answer
47 views

EAX cipher mode with nonce equal header

I am using EAX AEAD with an 8-byte message header which carries a 32-bit sequence number, a message length, and some other bits that I don't mind exposing. Assuming the sequence number never repeats ...
3
votes
2answers
57 views

Does AES(x || x) provide secure message authentication when message fits in a single block?

Suppose I have x = <64 bits of data>. I build a 128 bit block $P = x || x$, and transmit a message $M = AES(K, P)$. The receiver has the same $K$. The ...
3
votes
1answer
54 views

Can I reduce the MAC size in TLS?

The MAC size is reduced to 80 bits in truncated_hmac extension , which says "forging of MAC values cannot be done off-line: in TLS, a single failed MAC guess will cause the immediate termination of ...
3
votes
1answer
39 views

Software interface for KDF and PBKDF?

I'm working on an implementation of Krawczyk and Eronen HKDF from RFC 5869. From Krawczyk's original paper, he identifies four inputs to a KDF in Cryptographic Extraction and Key Derivation: The HKDF ...

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