1
vote
2answers
140 views

Verifying Without Knowing Key

Say user Bob sends an encrypted message to a server. People can download the message from this server and later get the key directly from Bob. Is it possible for the server to somehow verify that ...
2
votes
1answer
37 views

On the security definition of password-authenticated key exchange

I found in all PAKE papers, the security is defined as something like this: Let $Succ(A)$ be the probability that an attacker $A$ successfully distinguished the session key from a random string. Then ...
-3
votes
0answers
18 views

Man-in-the-middle-Attack three Parties [duplicate]

How does this work in Diffie Hellman ? I'm working for days on this but i'm a little bit Confused. If Oscar changes only the last key (for example Kabc ) is this enough ?
2
votes
1answer
23 views

Using a round function intended for an SP network in a Feistel network?

If I use the round function from a secure SP network (such as AES) and use it in a Feistel netwok, is this a good starting point for the second cipher? My thought is "yes" because: it already has ...
2
votes
1answer
34 views

Why are {0,1}-matrices almost-MDS only when n is 2, 3, or 4?

In this paper authors claim that {0,1}-matrices are almost-MDS (have branch number n - 1) on when n is 2, 3, or 4. For example, how can this two matrices have the same branch number? ...
1
vote
0answers
273 views

What are the problems of IKEv1 aggressive mode (compared to IKEv1 main mode or IKEv2)?

I would like to seek confirmation/clarification of the following statements about IKE: IKEv1 aggressive mode is supposed to be “insecure” if used with PSK. But as far as I can see, correct (or more ...
9
votes
1answer
186 views

State recovery of Micalli-Schnorr random number generator modulo a prime

Let $p$ be a public prime of $n$ bits. Define the sequence $x_i=z_{i-1}^e \bmod p$, where $z_i$ is the $k$ high-order bits of $x_i$, and $r_i$ the remaining $n-k$ bits of $x_i$ (thus ...
0
votes
1answer
15 views

Chosen Ciphertext Attack on the Public Key in PGP

In Secrets & Lies (2000), Schneier describes a chosen ciphertext attack on the public key under PGP: "Since RSA and ElGamal are malleable, known changes can be made to the symmetric key that ...
5
votes
1answer
311 views

What does 0…0 and 1…1 mean

I am very new to cryptography but can someone explain what exactly does 0...0 and 1...1 mean in general term. Is it that 1st and ...
1
vote
1answer
19 views

Can a compressed checksum be considered unique

I'm kind of guessing that the answer may be no. Since the compression is trying to reduce the output size using various optimization methods, this may cancel the properties of a hash algorithm. But ...
1
vote
1answer
51 views

How to create and validate potential grids when attacking a Playfair cipher?

First up: I am not asking you to decypher anything. This question is about attacking a Playfair cipher; more specifically – how to create potential grids when doing so. I'm currently working on a ...
14
votes
2answers
1k views

NIST Diffie-Hellman prime: how was it picked? Where did it come from?

According to this Matasano Crypto challenge, the NIST "likes" the following prime modulus, which appears to be expressed in hexadecimal: ...
7
votes
1answer
209 views

Is the “New Hope” Lattice Key Exchange vulnerable to a lattice analog of the Bernstein BADA55 Attack?

In the paper, "Post Quantum Key Exhange - A New Hope," the authors present a lattice-based key exchange based on the work of Chris Peikert. In this "New Hope" key exchange the authors try to gain ...
3
votes
0answers
81 views

XTS and 256-bit data blocks

I am developer of an application that uses XTS with ciphers which use 128-bit blocks (AES, Serpent and Twofish). I now want to use XTS with a cipher which uses 256-bit blocks (Shacal-2). AFAIK, I ...
3
votes
2answers
262 views

Which elliptic curves are quantum resistant? [closed]

If I want to learn about quantum resistant crytography what are the best resources? Which type of elliptic curves should I be studying?
3
votes
2answers
63 views

Can I use a block cipher in CTR mode to generate keys from a master key?

Suppose I have an AEAD mode based on a block cipher that requires two or more keys. In some cases, there is a complex proof that one key suffices. But finding such a proof is hard. An alternative ...
-2
votes
0answers
24 views

How to know the size of an encrypted key that is encrypted using public key

I encrypted a key with a size of 32 bytes. I encrypted it using a public key. How will I know how big is the encrypted key(ciph)? My code: ...
5
votes
1answer
57 views

Is ElGamal encryption still secure if the randomness is known to be even?

In ElGamal encryption $(g^r, g^mg^{kr})$, if the randomness $r$ is always chosen from even numbers, and the attacker knows about this, is it still provable secure?
4
votes
2answers
43 views

How long should a HMAC cryptographic key be?

In the top answer to this question the answerer says 'with a sufficently long key'. I am using a SHA1-HMAC (may switch to SHA256-HMAC) algorithm to verify cookies sent to clients as authentication ...
3
votes
0answers
41 views

Can the attack in https://eprint.iacr.org/2008/469.pdf be parallelized?

Can this attack be parallelized? It is a $2^{57}$ complexity collision attack on SHA-1 based on disturbance vectors. If it is practical it is easy on a GPU ($\approx1.1$ GPU-days). If it is ...
3
votes
1answer
28 views

How does Feistel cipher create diffusion between blocks?

From what I've read about Feistel ciphers, I can understand that within each block, there is both confusion and diffusion. But it is unclear to me how a Feistel cipher permutates among blocks of data. ...
0
votes
1answer
38 views

Creating ECDH using OpenSSL

For academic reasons, I'm playing around with OpennSSL 1.0.2g. I tested RSA encryption/decryption. I created key exchange with DHKE. But I'm struggling to find a way, to create ECDH, using only ...
3
votes
3answers
94 views

Can an adversary distinguish a private key from a pseudo-random string of the same length?

Apologies if this is a dumb question but allow me to describe the dilemma I have: Suppose that I am protecting a private key on a device using a password & PBKDF2. The obvious attack would be an ...
1
vote
1answer
208 views

How are calculated numbers in AES S-box?

I have a problem calculating the 0x02 value of the AES S-Box. I followed the steps descibed in “How are the AES S-Boxes calculated?”, but my answer is not correct. ...
-2
votes
0answers
30 views

How do Galios fields work in AES? [duplicate]

I am trying to learn how AES functions but I keep hanging at the subject of Galios fields. I am reading a lot about them but I just can not find an understandable explaination in how they work. Could ...
4
votes
2answers
642 views

Calculate the RSA private exponent from the CRT parameters

I am trying to make the RSA structure of Openssl manually, knowing the public key ($n$, $e$) and the CRT parameters $p$, $q$, $d_P$, $d_Q$, and $u = q^{-1} \mod p$. That is, I want to get the $d$ ...
1
vote
1answer
40 views

tweakable block cipher and ideal block cipher

Can the set of block ciphers obtained from a tweakable block cipher (using distinct tweaks) be considered as the set of ideal block ciphers ?
1
vote
1answer
45 views

Insecurity of CBC-MAC

Can anyone explain why CBC-MAC is not secure for variable length message? I read many books, but there are still some confusion. Such as the highlighted parts in previous pictures ...
3
votes
1answer
39 views

Is a chosen ciphertext attack on PGP still possible?

In a paper from 2002 (pdf), Jallad et al describe a chosen ciphertext attack on PGP by taking advantage of the CFB mode that the block ciphers use under PGP. It's quite a devastating attack. Is this ...
19
votes
3answers
13k views

How cryptographically secure was the original WW2 Enigma machine, from a modern viewpoint?

If cryptanalysts today were to crack the original Enigma machine, “how fast” or “how easily” could they do it? What methods would they use? The original cracking was significantly helped by operator ...
3
votes
1answer
47 views

The role of the dealer in visual cryptography

In visual cryptography schemes (VCS), there is special person called dealer. The dealer sets up the system, and the only one who knows about secret image. After encryption, the dealer distributes the ...
0
votes
1answer
42 views

Appropriate scrypt parameters when generating an scrypt hash

What values for CPU, memory and parallel difficulty should be used when generating an scrypt hash? i.e. ...
-3
votes
0answers
32 views

how a MITM attack works against tripartite in DH? [closed]

Must Oscar change only the Key $K_{abc}$ (Alice, Bob and Charlie) or does he have one key for everyone of them ?
3
votes
2answers
593 views

How does blocksize affect security?

I have recently read about how blockciphers and how stream ciphers work, and although I think I finally got everything right, I'm still wondering how increasing blocksize increases security.
40
votes
1answer
4k views

What is a “freestart collision”?

In their work on SHA-1 collisions (cf. the EUROCRYPT-2016 paper “Freestart collision on full SHA-1” by Stevens, Karpman, and Peyrin) Stevens et al show that they are able to generate "freestart ...
4
votes
2answers
99 views

RSA 3072 is 65,536 more difficult to factor than 2048?

If RSA 3072 is equal to 128 bit symmetric and 2048 is equal to 112 bit symmetric then 128-112= 16 and 2^16= 65,536. In that case wouldn't this make 3072 more future proof against quantum computers or ...
-2
votes
1answer
50 views

Randomness of a Key in a One Time Pad

The keys used in an One Time Pad are required to be randomly generated from a keyspace $\mathcal{K}$, we require 256 bit random keys, i.e. $\mathcal{K}=\{0,1\}^{256}.$ Suppose we randomly select an ...
0
votes
1answer
53 views

Key Hierarchy and key Encryption

If i have two 56-bit DES keys, and I use these two key to encrypt a 256-bit AES key. Like this $K_{AT}$ encrypts $K_{BT}$ and $K_{BT}$ encrypts $K$ What is the effective security of key $K$? Is it ...
5
votes
2answers
199 views

Finding the subgroup in isogeny-based cryptography

Isogeny-based cryptography is one of the newest post-quantum cryptography. Hardness of this system is based on finding isogeny between two elliptic curves. Also this is theorem: Elliptic curves ...
1
vote
1answer
37 views

McEliece variants that support signatures

Besides the Niederreiter cryptosystem, are there any other variants that support digital signatures? Can any of them sign arbitrary signatures unlike the Niederreiter cryptosystem? What are the key ...
1
vote
1answer
47 views

Expire signature

I am looking for a way to have a 'did it expired' key. There's a similar question here, but I have to prevent reusing of the 'did it expired' signature . A possible solution would be a two one-way ...
1
vote
2answers
76 views

How to break a 32 bit block cipher without key?

From what I know Skip32 is the only 32 bit block cipher and I heard its not secure. What do I do or which tool do I use to break a 32-bit block cipher? I did search for this but all I find is attacks ...
8
votes
3answers
11k views

What makes RSA secure by using prime numbers?

I am just learning about the RSA algorithm. Looking at the first two steps: Choose two distinct prime numbers $p$ and $q$. Compute $n = pq$. I have some probably stupid questions: Why do $p$ ...
1
vote
2answers
55 views

What does $\langle A, B\rangle$ mean?

I'm reading about CL-PKC and in Al-Riyami's FullIndent, I find some notations I must be sure to understand, to well explain it ...
14
votes
2answers
231 views

Formal verification in cryptography

I have seen in some places that people use formal verification and/or computer-aided verification for cryptography (tools like ProVerif, CryptoVerif, etc.). How do these approaches work?
4
votes
2answers
49 views

Motivation for the definition of statistical distance

Statistical distance is a widely used measure in cryptography for comparing two distributions. One can define various other measures for capturing the differences between two distributions, but why do ...
2
votes
3answers
1k views

Comparing Files: Is it better to use multiple hash algorithms or just one?

I'm looking to compare (for byte-for-byte equality) two binary files. I already have the process down: Check for existence etc. Compare file lengths Use a hash or CRC of a sort (right now I use ...
2
votes
1answer
32 views

What is the diffrence between Feistel networks and SPN?

I recently read about the concept of Feistel Networks and Substitution Permutation Networks but what is exactly the difference between the two ?
3
votes
1answer
40 views

Order of multiple encryption algorithms

as you can see on image above, in VeraCrypt you can select either Serpent -> Twofish -> AES or ...
0
votes
0answers
39 views

Is this Commitment Scheme binding?

Commitment scheme is given as follows: Alice chooses a random r the same size as x and calculates $y = h(x||r)$ where $||$ represents concatenation. Open: Alice sends x to Bob, and Bob calculates $y ...

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