2
votes
1answer
158 views

What's the advantage of using OFB/CFB/CTR modes over a stream cipher

As far as I understand: The primary disadvantage associated with stream ciphers is the need for a random and unique key for each run in order to protect against reused-key attacks. OFB/CFB/CTR block ...
2
votes
1answer
139 views

How do P2P clients like uTorrent maintain authenticity of files?

I am very new to concepts of cryptography so bear with me and feel free to fill in any information that may seem obvious but something you think someone may not know. At first I was under impression ...
2
votes
1answer
180 views

Simple way to extend AES to 256-bit block size

What is the best way to use standard AES with a 128-bit block size to act as a 256-bit block cipher? I am aware of CMC and EME which seem to serve this purpose, but they seem to be more complicated ...
2
votes
1answer
73 views

Is concatenated data hashed with scrypt vulunarable to a length extension attack?

scrypt takes a salt, and a password plus some cost paramters to generate a key. So say we define it as this: key = scrypt(password, salt, cost) I am interested ...
2
votes
1answer
106 views

Shamir's Secret Sharing of modular inverse

Given a secret $K \bmod q$ which is shared among entities $E_1,…,E_n$ using polynomial Shamir's Secret Sharing, how can the inverse of $k$ be shared without revealing $k$ and $k^{-1}$?
2
votes
1answer
31 views

Is the CONF key sharing Problem equivalent to discrete log problem?

If there a proof in the literature which says the CONF Problem is equivalent to solving the discrete log ? Let $g$ be a generator of a cyclic group $\mathbb{G}$ of prime order $q$ CONF problem: ...
2
votes
1answer
144 views

How can I validate my implementation of Ansi 9.19?

I implemented an Ansi 9.19 mac generator in java, but now I don't know how to validate its functionality. I couldn't find any sample of a {plain text,key,mac} on the internet, and I want to know if ...
2
votes
4answers
836 views

Pseudocode for constant time modular exponentiation

I'm looking to implement modular exponentiation (for RSA) in constant time, but most of the examples I've found are more mathematical descriptions of the operations. Are there any references with ...
2
votes
1answer
181 views

Schnorr Identification protocol sometimes wrong?

I'm implementing the Schnorr Identification protocol as part of a larger protocol. In a large percentage of cases, around 20%, my test fails. I don't think it's a code problem, although possible, but ...
2
votes
1answer
282 views

How to specify a NIST SP 800-108 KBKDF

Strange question maybe, but say I want to specify a protocol that complies with NIST SP 800-108. This lists a whole set of key based key derivation functions, with a very loose set of parameters. How ...
2
votes
1answer
71 views

Can I publish the accumulator trapdoor and still keep it secure?

I'm implementing the Camenish-Lysyanskaya dynamic accumulator. It seems to me that the accumulator is provably secure because the trapdoor is unknown to the attacker. The risks should be the fact ...
2
votes
1answer
92 views

difference between leveled FHE and normal FHE scheme

What is/are difference/s between leveled Fully Homomorphic Encryption and normal Fully Homomorphic Encryption?
2
votes
2answers
175 views

Is HMAC necessary if the AES ciphertext of a message includes the CRC of the message?

I've seen that encrypting an image with AES-CBC then calculating the HMAC-MD5 value for authentication is a somewhat common and sound way to secure data. Alternatively, can I just calculate the CRC of ...
2
votes
1answer
217 views

SSL-like protocol with public-key hard-coded in the client

Imagine a SSL-like protocol that instead of using certificates signed by a trusted CA, has the server's public key hard-coded in the client. My question is: what happens if the server's private key is ...
2
votes
1answer
384 views

Why does HMAC need a fixed length padding?

As everybody knows in order to calculate HMAC we have to concatenate padding to the message. I am just curious why the padding needs to be fixed-length. Why do we need the blocksize parameter here? ...
2
votes
1answer
228 views

Is there a simple zero knowledge proof of $x$ for $b=x^x\pmod p$?

We have a multiplicative cyclic group $G$ which is a subgroup of $(\mathbb{Z}/n\mathbb{Z})∗$. There are two parties, Alice and Bob: If: Alice knows: $b$ and $x$ such that $x^x = b$; Bob knows: $b$. ...
2
votes
1answer
52 views

ECDSA signature verifiable 1-way transformations

Alice signs a message $m$ with her private key, yielding a signature ($r$,$s$). I want to prove to someone else that I have this signature, but I don't want them to have the knowledge of what ...
2
votes
1answer
88 views

Implementing AugPAKE over ECC

The AugPAKE spec says it can be implemented over elliptic curves. This sounds very promising, but they don't actually back that claim. Can this really be achieved? If so, how would one go about ...
2
votes
2answers
104 views

Soundness idea of basic zero knowledge prood

I am having a hard time figuring out the soundness proof of the bellow zero knowledge protocol. As it is a typical proof, I would love to deeply understand it. zero knowledge argument that $h \in ...
2
votes
1answer
579 views

Mapping of message onto elliptic curve and reverse it

I would like to perform a variant of Elliptic Curve ElGamal in java using the BouncyCastle libraries. I currently face the difficulty of mapping a message $m$ onto the elliptic curve $E_p$. I have so ...
2
votes
1answer
97 views

What is the practical relevance of the wordsize when using a Hash?

I understand that the word size is the internal "working package size" below the block size of a cryptographic hash function (and a block cipher), as each block is broken into words an than processed ...
2
votes
1answer
810 views

Hill cipher cryptanalysis - known plaintext known key size

Hello I want to know how to go about this problem I know the plaintext "abcdef" and the ciphertext. The key size is 2. I really can't figure out how to find the key for decrypting and encrypting.
2
votes
2answers
764 views

Digital Signature using symmetric key cryptography

Generally digital signature is a public key cryptography concept.But it needs high overhead. So is there any publication or link available where 'digital signature using symmetric key' has been ...
2
votes
1answer
103 views

Practical usage of S/KEY

The One-Time-Password (OTP) topic became quite popular in the recent years. But unlike the well-known HOTP and TOTP standards I haven't heard about any applications/systems implementing and using the ...
2
votes
1answer
181 views

Is it necessary to use single hash function for signature generation and HMAC?

I am trying to figure out if we need to use single hash function across all the places in a ssh session. i.e – signature generation/verification and HMAC calculation/validation. Is it possible to use ...
2
votes
1answer
136 views

Proving that a function is not a OWF (One-way-function)

I was trying to prove that a given function is not a one way function and I was not sure how to do it because maybe I had unclear what a one way function was (OWF). The definition that I have for a ...
2
votes
1answer
157 views

ElGamal with elliptic curves I

It is very interesting to see @tylo's answer on ElGamal with elliptic curves. Instead of mapping the message to the elliptic curve point it just reduces an elliptic curve point to its $x$ coodrinate. ...
2
votes
1answer
157 views

Why is CBC based on AES malleable in blocks other than the first block?

Perhaps I don't understand the answers to Malleability attacks against encryption without authentication, so trying to be more concrete... please be patient. If I want to change the first block and I ...
2
votes
1answer
223 views

Homomorphic encryption for vector addition

Building on the question and answer from Addition-only PHE in F# which ponders homomorphic cryptosystems to navigate, relatively, a single dimension without revealing absolute position (an encrypted ...
2
votes
3answers
547 views

How do institutions like banks do RSA with big primes?

When encrypting with RSA it is often infeasible to decrypt by just doing c^d mod n, because for example when using the primes $(p,q)=(12553,1233)$, which are small ...
2
votes
1answer
216 views

Using KDF output for password validation

I am currently revising an AES data format. I would like to determine whether a provided password/key is incorrect. Previously this was done by validating the HMAC, but this makes it impossible to ...
2
votes
1answer
76 views

Keyed-hash of a lot of small numbers

Let's say I have a list of several million numbers, with only 6-9 digits of secret information per number (this means that an attacker might efficiently guess the other digits). I have to store the ...
2
votes
1answer
115 views

Ephemeral Diffie-Hellman generator selection

I need to generate a EDH group on the fly in one has not been specified in a config file. Diffie-Hellman is kind of expensive in terms of computation, so I'd like to reduce the cost if possible. Is ...
2
votes
2answers
528 views

Are the SHA family hash outputs practically random?

Say I hashed the output from a random number generator (with nonce), would the resulting SHA256 hash be as random as the inputted number? And If I used the first 5 hex characters, and then used the ...
2
votes
1answer
355 views

How Multiplication Table is generated for GF(2^2) field

I was unable to solve the multiplication table given in the book for $\mathrm{GF}(2^2)$.However, I have managed to solve the addition table. Acoording to the Book multiplication is the AND operation, ...
2
votes
1answer
201 views

Short length asymmetric encryption?

I want to send out short messages to the world which listeners could verify to be send from me. The way this is usually done is to encrypt the message with a private key where the readers decrypt it ...
2
votes
1answer
89 views

Usability of padding scheme in blinded RSA signature?

In Wikipedia's article on Blind signature, in the section Dangers of blind signing which describes RSA blinding attack one reads the following This attack works because in this blind signature ...
2
votes
2answers
230 views

How can A pass a message to B, while preventing a third party from sniffing infos?

$A$ has a message that he wants to send to $B$, without a third party being able to learn the contents of the original message. $A$ encrypts the message and sends it to a server. The server decrypts ...
2
votes
2answers
155 views

Can the passphrase of a PGP key be deduced from an old backup?

Let's assume that Bob, at some point in time, has created a PGP (key length and other parameters are not relevant at this point). For some reason, he has chosen a very poor passphrase, for instance ...
2
votes
1answer
115 views

With Encrypt-then-MAC, and flexible protocols, do I need to authenticate the authentication?

Sorry, the question is not easy to state (or wasn't for me, anyhow). So I'm writing a very flexible format for security. It allows for full choice of cipher, key size, and so on, along with key ...
2
votes
1answer
138 views

Elliptic Curve Verifiable Secret Sharing

I'm reading this paper, which on page 3(Section IV.C) presents a Jointly Random Verifiable Secret Sharing Scheme for Elliptic curves. The algorithm makes sense to me save for this part: "Each ...
2
votes
2answers
344 views

Graphically representing points on Elliptic Curve over finite field

I have taken elliptic curve $E\colon y^2=x^3-4x+20$, defined over $\mathbb{F}_{29}$. The number of points on the curve, $\left|E(\mathbb{F}_{29})\right|=37$. I took base point $P=(1,5)$, and got ...
2
votes
1answer
145 views

Reusing PGP key when generating SSL Certificate Authority?

Does it make sense to reuse some of the keys from my PGP (gnupg, gpg) keyblock when generating my own SSL Certificate Authority (a toy one for myself and a few web services of mine)? In an ideal ...
2
votes
1answer
63 views

How to pronounce RIPEMD 160?

Do I pronounce it as a word or do I make it sound like "R-I-P-E-M-D" like by pronouncing each individual letter.
2
votes
1answer
446 views

A key-derivation function that is as strong as the stronger of PBKDF2 and scrypt

I want a key stretching system that's as strong as the stronger of scrypt and PBKDF2. The consensus now is that scrypt is by far the better system, but that might change if in the future, a weakness ...
2
votes
1answer
1k views

Is it true for Java that the transformation mode and padding is ignored when using RSA?

Consider GregS's comment to this question (more specific to bouncycastle's provider implementation): http://stackoverflow.com/questions/11717646/bouncycastle-rsa-key-size-and-padding The mode, ...
2
votes
1answer
58 views

Weaker Notion of Target Collision Resistance

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: While it might be easy to find a collision $M,M'$ in $F_K$ by making both $M,M'$ depend on $K$ ...
2
votes
1answer
72 views

Dependence on Keyed Hash Function

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: With an ACR hash function $F$ the key $K$ is announced and the adversary wins if she manages to ...
2
votes
1answer
372 views

Forge CBC-MAC given the MAC of two messages and of their concatenation

I found this question about CBC-MAC forging and want to make sure I understand it: Let a and be be two strings of block length one. Suppose the sender sends $(a, \text{CBC-MAC}(a)), (b, ...
2
votes
1answer
90 views

secret sharing of product of 2 numbers

Does evaluating the products of shares $a_jb_j$ give a secret sharing of $ab$? I think it's a no, but I am not sure on how to prove it.

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