0
votes
1answer
75 views

What is the proper way to generate a fixed length key from a variable length password?

The password is entered by the user and the key is processed locally. Neither the password nor the key will be stored, only used and forgotten (but hopefully not by the user). Is SHA256 by itself ...
0
votes
1answer
76 views

How much of Probability do I need to know to study Cryptography?

I have just started MSc in Computer Security. I didn't study probability before. Now, I have some short time to complete my MSc semester. What are the minimum topics from 'Probability' do I need to ...
0
votes
1answer
63 views

How to encrypt a vector $e\in Z^{m}$ using LWE

I have a vector $e \in Z^{m}$, now I want to encrypt it by using LWE (Learning With Errors). Because in LWE, we have to encrypt of a bit $\{0,1\}$. How can I encrypt a vector $e \in Z^{m}$? Thank you ...
0
votes
1answer
61 views

Randomness in generation of Virtual Machines from pre-built Image [closed]

I am studying Multi-Tenancy vs. Virtualization, and I got thinking about randomness, true randomness, virtualization etc, and I couldn't find anything on it. From what I gather, ...
0
votes
1answer
101 views

Affine transformation in AES: Matrix representation

I know that the affine transformation of the AES can be represented both as a polynomial evaluation over $\operatorname{GF}(2^8)$ and as a matrix-vector multiplication (see, e.g., p.212 C.4 of The ...
0
votes
1answer
60 views

New to cryptology, studying key exchange

I'm just getting an overview of some of the concepts in key exchanging at the moment, and I'm a bit disoriented on a few topics, namely key stretching, key derivation functions and perfect forward ...
0
votes
1answer
67 views

uniform vs. non-uniform PPT

I'm trying to understand PPT and in particular what the differences are in uniform and non-uniform PPT's. First, this is how I see it: A Probabilistic Polynomial-Time algorithm A is an algorithm that ...
0
votes
1answer
71 views

RSA weak padding

Suppose to have a function for numbers expressed in 8-bit $\in [0,2^8-1]$ defined as: $$f(x)=x||x||x||x$$ where $|f(x)|$ is exactly 32 bits. e.g., suppose x=2 (00000010) so ...
0
votes
1answer
56 views

chaining multiple key derivation functions together

I was looking at PBKDF2, bcrypt and scrypt as options for key derivation; and would like to try using them all together in order to get the cryptographic strength of the strongest one (which seems to ...
0
votes
1answer
175 views

RC4 , Is it possible to find the key if we know the plaintext and ciphertext?

Is it possible to find the key if we know the plaintext and ciphertext with RC4. How should I write the algorithm?
0
votes
1answer
58 views

HMAC tag to verify the key (verification in the reverse direction)

A quick question. Are there any problems with the following HMAC use-case: We have two entities ($A$ and $B$), both of them have a secret ($A$ has $S_0$ and $B$ has $S_1$) $A$ generates a random ...
0
votes
1answer
75 views

What practical uses can random hash collisions be put to?

Using magic, I've built a machine that instantly generates hash collisions. To try it out, I set the dial to SHA256 and typed in "e7cf3ef4f17c3999a94f2c6f612e8a888e5b1026878e4e19398b23bd38ec221a". I ...
0
votes
1answer
59 views

RSA Decryption from samples

I am working on a project for a client which involves decryption of data which was encrypted using RSA. The system in question retains three independent copies of each data-set, but each copy is ...
0
votes
1answer
121 views

Rijndael S-boxes: Where do the $\mu$ and $\nu$ polynomial ring elements come from?

I've asked some other questions before about Rijndael's S-boxes, and step by step I'm coming to an understanding; but those steps often guide me to new questions. I did some lines of code to ...
0
votes
1answer
94 views

How do you break encryption on a message? [closed]

This may seem like a simple question, but I have just started studying cryptography and I can't seem to find any information on this at all. I am trying to write a "simple" encrypted messenger using ...
0
votes
1answer
24 views

Reordering of multiple signed blocks

I would like to refer you to page 430 of the Handbook of Applied Crypto, where the authors said "... reordering of multiple signed blocks presents a security risk..." This statement was made in ...
0
votes
1answer
47 views

Workaround to implementing Forward Secrecy

In the case where I am self-signing my digital certificates, if I just change my certificate (generate a new one) every week or month, will this have (sort of similar to) the same effect as ...
0
votes
2answers
150 views

CTR HMAC SHA256 implementation

After trying to invent my own AES mode, I decided to just implement something that's known to work, namely CTR with HMAC SHA256. From what I understand, I can use the IV as a counter, and the IV ...
0
votes
1answer
103 views

Keeping the key and IV safe when AES256 encrypting publicly known data

I am encrypting some publicly known data, and some private data using the same key, using AES256 with CBC. An attacker does not have the ability to choose what the public data is. Is this safe? (It ...
0
votes
2answers
77 views

Find the prime factorization in the DLP

Suppose we have $g^x \equiv h \pmod N$ Where $N = pq$ and $p$, $q$ are distinct primes. Also $(p-1)/2$ and $(q-1)/2$ are primes. If we know what $x$, $g$ and $h$ are, is it possible for us to know ...
0
votes
1answer
152 views

How to compare between two cryptographic algorithms in terms of security? [closed]

How to compare between two cryptographic algorithms (e.g. SHA-1 and SipHash) in terms of security? That is, how could one prove that algorithm X is more secure than algorithm Y?
0
votes
1answer
77 views

One-one correspondance complete function

I am reading the paper by John B. Kam and Georges I. Davida (1979) titled Structured Design of Substitution-Permutation Encryption Networks. On page 749 it reads ...
0
votes
1answer
67 views

Are there cryptographic schemes that mask as unencrypted traffic?

Are there cryptographic schemes that make encrypted traffic look like unencrypted traffic?
0
votes
2answers
72 views

Would the Fiat Shamir identification scheme be more secure if I design it with an exponents higher than 2?

Let's say both Prover and Verifier would use nth power instead of 2nd when creating public keys and when verifying. I know it would slow it down but would that cause the protocol to be more secure?
0
votes
1answer
109 views

Drawback of ElGamal encryption

I am trying to find out some disadvantages of ElGamal cryptography but I'm not able to figure out what's wrong with the algorithm. The only one I found is that a known-plain text attack is possible in ...
0
votes
2answers
76 views

Solve the congruence using Pohlig-Hellman algorithm

Use the Pohlig-Hellman algorithm to compute a solution to: $3^x\equiv 2 \pmod {65537}$ My attempt: $p-1 = 65537-1 = 65536= 2^{16}$ $x= 2^0x_0+2^1x_1+2^2x_2+...+2^{15}x_{15}$ For $x_0$: ...
0
votes
2answers
76 views

Schnorr's identication protocol

Schnorr's identification protocol is said to be a honest verifier zero knowledge protocol. Does it imply it will not work if the verifier is dishonest? I'm wondering if this is the case, it would ...
0
votes
1answer
59 views

Linear Recurrence, keystreams, and periods

I am trying to understand the following problem: Consider the following linear recurrence over $Z_2$ of degree four: $z_{i+4} = (z_{i+3} + z_{i+2} + z_{i+1} + z_{i}) \bmod 2$ i >= 0. ...
0
votes
1answer
113 views

Convert numeric message from RSA ciphertext to alphabetic

I have a set of numeric values that are supposed to represent a message encoded via textbook RSA. For example: 60889 37946 13359 61629 35960 13747 I've decrypted ...
0
votes
1answer
100 views

Why would using a random seed with other variables be bad for ecrypting if you can't guess the key?

I asked this on stack exchange with it being code based, but someone suggested to ask here. Basically, I've done a script in python and wanted an option to encrypt the input, but I couldn't get any ...
0
votes
1answer
46 views

Implications of reversing CBC encryption

What would be the security implications if we reverse CBC encyption? Here is a picture of CBC encryption: Here is a picture of CBC decryption: My guess is that the encryption would work if we ran ...
0
votes
1answer
48 views

Using hashed password as shared secrets. Good or bad idea?

I built a system that allowed users to generate an unlimited number of email addresses. Instead of storing each one in a db, I created addresses dynamically and made the address an hmac signed ...
0
votes
2answers
153 views

How to show that a function is not a PRP?

So I have to answer these questions, but I honestly have no idea what I am doing or where to begin. Can I get some advice on how to tackle these? Consider a PRP candidate function $f$ on $K = X = ...
0
votes
1answer
218 views

How many attempts does it take to crack a 32-bit password hash with this scenario?

How many attempts does it take to crack (match) a 32-bit password hash from a database of 4 million password hashes? Correct me if I'm wrong, but to crack a 32-bit password hash would take roughly ...
0
votes
1answer
82 views

Is it necessary to encrypt nonce in cryptographic communication?

Server and client have a pre-shared key K. Assuming that that pre-shared key is safe, so nobody other than server/client can encrypt/decrypt. Client sends a nonce N to server, server then sends back ...
0
votes
1answer
52 views

Requirement for the exponent generation during DH protocol [duplicate]

The setting is classic DH protocol. Alice computes $A = g^a \bmod p$. Alice then sends $A$ to Bob. Then Bob will send $B = g^b \bmod p$ to Alice. Are there any requirements for the values $a$ and ...
0
votes
1answer
164 views

Symmetric mutual authentication with client using a derived secret

I'm attempting to find a client/server authentication protocol that allows the client and server to authenticate each other when the client doesn't know the server secret but does have a sensitive key ...
0
votes
1answer
124 views

Java RSA Padding: no libraries used

I'm trying to implement RSA without using any java libraries using the algorithm from wikipedia HERE So, anyone can clearly see that it is a raw RSA, and can directly be attacked. The question is ...
0
votes
1answer
83 views

Prime modulus for RSA and sharing a secret?

According to this paper entitled "Using Commutative Encryption to Share a Secret" they define their modulus to be a large prime p, which is public. Both exponents are private in this case. According ...
0
votes
1answer
50 views

Stream cipher key length to send

I was being argued that stream cipher's key that is the length of the message must be sent to the destination for them to be able to decrypt the message. My point is don't you only have to send the ...
0
votes
1answer
70 views

Weak Boneh-Boyen Signature in Composite order group

In the paper Ring Signatures of Sub-linear Size Without Random Oracles, authors have remarked that the scheme is instantiable in composite order setting, too. I am attaching the following reference ...
0
votes
1answer
41 views

Using Permutation polynomial to compute a MAC

Is the following MAC secure? For a block $y_i$ in a file, we defined a MAC as follows: $Mac_i:PRF(k,i) \cdot g^{y_i \cdot r_i} \bmod p$. Where $p$ is a prime number, $g \in \mathbb{G}$,$PRF(k,i)$ is ...
0
votes
1answer
51 views

How do you derive the lambda and beta values for endomorphism on the secp256k1 curve?

You can see a little background about this on this bitcointalk post by the late Hal Finney. $\beta$ and $\lambda$ are the values on the secp256k1 curve such that: $$\begin{align} \lambda^3 &= 1 ...
0
votes
1answer
176 views

What is the difference between a hash function and a pseudorandom function? [duplicate]

Read the title. I've seen in RFCs that some MAC functions are called "pseudorandom functions". What are those? How are they different than hash functions? Why can't a hash function be used instead?
0
votes
2answers
123 views

Multiple encryption of part of block

Is there any security risk in encrypting parts of blocks twice? I'd like to get around block sizes and make encrypted messages just as long as the unencrypted ones: Block 1: Byte 0 - 7 Block 2: Byte ...
0
votes
1answer
89 views

XOR cipher with three different ciphertexts and repeated key, key length known. How do I find the plaintexts?

Let us say we have three different plaintexts (all alphabets, A-Z): $x$, $y$ and $z$, each of length $21$. Let the key, $a$, be also of length $21$. Now, what we have is $x \oplus a$, $y \oplus a$ ...
0
votes
1answer
88 views

One time pad probability

I'm trying to solve a question about one time pads but I'm not sure if my assumptions are correct. I am to assume that I have a one time pad with perfect secrecy. This one time pad is used to ...
0
votes
1answer
55 views

Is there a reference that prove that the AES Key Schedule generate random looking round keys?

Starting from uniformly random generated AES master key, is there a reference that prove that an specific roundkey can be considered as uniformly random generated as well ?
0
votes
1answer
87 views

How do I communicate the value of the initialization vector to the end user? Should it be part of the encrypted message?

I am taking a cryptography class and our first homework is to implement a 32-bit block cipher. I implemented a simple block-cipher that uses CBC. Currently, my implementation reads both the 32-bit IV ...
0
votes
1answer
79 views

Proving that an encryption scheme is susceptible to certain attacks

I'm currently trying to prove the following: Where p is a prime number of cryptographic size, prove that: e(m) = am + b (mod p) where a and b are private is open to a known plaintext attack e(m) = ...

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