-1
votes
2answers
180 views

Python KDF - Master Key and Derivatives

The encrypted file can be opened by following combinations (either any 2 DK, or 1 MK): DK1 + DK2 DK1 + DK3 DK2 + DK3 DK1 + DK2 + DK3 MK Any heads-up with Python ...
2
votes
2answers
208 views

Is a collision attack possible on MD5 if message is known partially

Is it possible to find $m'$ such that $\operatorname{MD5}(m'||m_1)=\operatorname{MD5}(m_1||m_2)$ if I know $m_2$ and $\operatorname{MD5}(m_1||m_2)$? Is this a collision or a preimage attack? Also in ...
5
votes
1answer
213 views

What functions allow for practical indistinguishability obfuscation?

Recently, there has been a major theoretical breakthrough in program obfuscation, in the area of indistinguishability obfuscation. Background: Indistinguishability obfuscation is a form of program ...
5
votes
1answer
441 views

FHE - Brakerski's “Scale Invariant” Scheme

I thought the current state of the art for fully homomorphic encryption was Brakerski, Gentry and Vaikuntanathan's scheme (BGV) based on standard/ring LWE employing modulus switching for noise ...
8
votes
1answer
888 views

How realistic is a dictionary attack on a secure remote password protocol (SRP) verifier?

I'm deploying a secure remote password protocol implementation and I'm wondering what the consequences are when the client generated verifier gets leaked to an attacker. I've read Thomas Wu's paper ...
4
votes
1answer
174 views

Homomorphic Encryption - Smart Vercauteren Batching

I'm going through Smart and Vercauteren's paper "Fully Homomorphic SIMD operations" and had a question about some notation used in the paper. In section 2 of the above it is stated that for each ...
1
vote
1answer
172 views

Brute force RSA cracking

Suppose one had a complete list of primes up to $2^{n+1}-1$. Then wouldn't one be able to crack an $n$-bit RSA public key in time $O(\pi(2^{n+1}-1))$, making RSA insecure? Thanks, René
3
votes
1answer
242 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 ...
4
votes
2answers
154 views

Is ”generating transcripts that are indistinguishable from actual messages in a signature scheme” not universal forgery?

In the Pointcheval paper “Security arguments for digital signatures and blind signatures”, they have proved the security of Schnorr signature for existential forgery under adaptively chosen-message ...
2
votes
2answers
220 views

Comparing two values without revealing them

Suppose Alice wants to prove to Bob that t1 is less than t2. t1 and ...
1
vote
1answer
324 views

BigInteger in Java and Randomness

I'm looking for some PRNGs and – as I have been using Java for a long period of time – I thought that the BitInteger class, together with the ...
5
votes
1answer
220 views

Universal Circuits in Indistinguishability Obfuscation Candidate Construction

I am currently working on an implementation of the candidate construction for an indistinguishability obfuscator that was recently proposed by Garg et al. The relevant paper can be found here. ...
2
votes
1answer
198 views

Does this protocol provide Perfect Forward Secrecy / are there potential security flaws?

We have a HMAC key (128 bit/16 bytes) an AES key (128 bit/16 bytes) an AES IV (128 bit/16 bytes) a random salt (128 bit/16 bytes) Each packet is signed with a HMAC (SHA256) After each packet is ...
1
vote
1answer
1k views

Value of “counter” when decrypting AES in CTR mode?

If – when using AES in CTR mode – you decide to use the setup where you combine the IV and the block count value into a single value, as suggested here, the first 12 bytes are your IV and the last 4 ...
2
votes
1answer
560 views

Message lengths with AES CTR mode?

Im working on my own AES implementation as a personal educational exercise (not for production use), and a book im using as a resource states: In the case of CTR, we are merely XORing the output ...
4
votes
4answers
404 views

What is the difference between authenticating and signing a document?

Authenticating a document means to verify it is from the sender. But it seems like signing a document also verifies it’s from the sender. What exactly is the relationship or difference between “...
3
votes
2answers
423 views

What are the standard procedures in cryptanalysis to analyze unknown ciphertext?

What are the "standard procedures in cryptanalysis" to analyze unknown ciphertext? In other words: Are there any protocols, officially acknowledged checklists or something like that which represent a ...
0
votes
1answer
319 views

ECDSA: Retrieving non-random k [duplicate]

I have a question regarding the random $k$ number of ECDSA encryption. As far as I know, it is possible to retrieve $k$ (and thus the private key) from two signed messages if both used the same $k$. ...
5
votes
1answer
729 views

Mixing Entropy Sources by XOR?

Assume that I have two sources of entropy (say, a hardware RNG and a CSPRNG) and that the two sources are independent (do not know anything about the internal state of the other RNG). Can I mix them ...
1
vote
1answer
110 views

How can a digital signature algorithm incorporate a random data if it is to be authenticated on a separate system

I am trying to understand how RSA digital signatures can incorporate any data from a PRNG into the final signature. Does this mean that separate implementations might fail to verify each others ...
1
vote
1answer
97 views

Fiat-Shamir security problem question

I am trying to understand the Fiat-Shamir identification protocol, but have a problem with understanding how this protocol is supposed to be save, even if you repeat it several times. I am using ...
2
votes
1answer
180 views

Pseudo random permutation for arbitrary size domains

Popular block ciphers like AES or Twofish are keyed pseudo random permutations on the domain $\{0,1,\dots,2^{k}-1\}$ with $k\in\{128,192,256\}$ or similar. I'm interested in pseudo random ...
3
votes
1answer
253 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 ...
1
vote
1answer
141 views

Are those RSA keys flawed?

I used rsa-json.native to generate RSA keys for a node.js application that will use secure-peer later to connect two clients with each other. Now I have 2 questions: In secure-peer/index.js I've ...
9
votes
4answers
5k views

What are the requirements of a nonce?

Sometimes I read that a nonce has to be a random number but I disagree. A nonce just can't repeat itself. You could increase in by 1 every time if you are sure it would never repeat.
6
votes
1answer
162 views

How can one detect if a hash function doesn't have a full image?

Suppose we have a hash function $H: \{0,1\}^* \mapsto \{0,1\}^n$, that doesn't get all the $n$ length bitstrings as a value for any input, but is otherwise secure. Can that fact be detected? For ...
6
votes
1answer
392 views

Significance of rotation constants in SHA-512?

In a single round of SHA-512, in the operations $\Sigma_0(A)$, $\Sigma_1(E)$ why are the constants $28$, $34$, and $39$ used for the number of rotations? What significance do these numbers have? What ...
2
votes
1answer
90 views

Familiar key signing over video call

Say that a friend of mine and I have both generated new PGP keys, and we want to use a video call to verify each other's new keys because we live on opposite sides of the planet. While we could both ...
1
vote
2answers
384 views

Attack on a key-exchange,symmetric-key cryptography protocol

This is an exam question in Oxford 's Computer Security course: Here is the start of a protocol, based on a long-term secret key $k_{AB}$ previously shared between Alice and Bob, designed to offer ...
3
votes
2answers
879 views

Is there a length-preserving encryption scheme?

Is there a length-preserving encryption scheme, that preserves the lengths of input sizes such that the length of the input plain text is same as length of the output cipher text ?
5
votes
2answers
225 views

Obfuscate an “I'll reveal if you do” function

Is there any cryptographic obfuscation scheme that will let me obfuscate a program that reveals my secret, if you reveal yours? Formalization. Let $C(\cdot),V(\cdot)$ be a secure commitment scheme. ...
3
votes
2answers
177 views

How do I construct variable length input (VIL) in a block cipher mode?

I've just been skimming NIST's "Five Confidentiality Modes". I understand those block cipher modes are used to securely encrypt input longer that the block cipher length. However, the common block ...
1
vote
2answers
142 views

Sender and receiver having different moduli conflicts with encryption and signing in RSA

I should implement a security protocol as a part of which I need to: Encrypt the message with receiver public key. Sign it with my private key. Send it to receiver. Suppose that the system uses a ...
3
votes
1answer
272 views

ECDSA: How to retrieve a non-random k

I have a question regarding the random $k$ number of ECDSA encryption. As far as I know, it is possible to retrieve $k$ (and thus the private key) from two signed messages if both used the same $k$. ...
21
votes
3answers
41k views

RSA encryption with private key and decryption with a public key

When using the RSA cryptosystem, does it still work if you instead encrypt with the private key and decrypt with the public key? What about in the case of using RSA for sender authentication?
2
votes
0answers
182 views

RSA encryption and signature padding flaw

This is Oxford's computer security exam question: Suppose that Bob has published an RSA encryption key $ke$ (retaining in secret the corresponding decryption key $kd$), and Alice has published a ...
8
votes
3answers
430 views

Security of a security protocol for key exchange, using symmetric-key cryptography

This is an exam question: $A \to B$: Alice, $nonce_A$ $B \to A$: $nonce_B$, $E_{k_{AB}}(nonce_A \| k_1)$ $A \to B$: $E_{k_{AB}}(nonce_B \| k_2)$ Alice and Bob both compute $k_s$ = $k_1$ xor $k_2$ $...
3
votes
1answer
457 views

What differences between Menezes–Vanstone ECC and ElGamal ECC?

After researching ECC encryption, I found that we can use ElGamal cryptosystem with elliptic curve and can we use Menezes-Vanstone cryptosystem with elliptic curve. What is the essential difference ...
1
vote
1answer
416 views

Prime factorization of RSA modulus

Consider RSA public-key encryption with public modulus $N=3953$. Suppose we know that the public keys $e_1=337$ and $e_2=23$ correspond with the decryption information $d_1=3385$ and $d_2=2663$. ...
6
votes
2answers
2k views

New PRG from old

Is the following statement true: If $G: \{0,1\}^k \to \{0,1\}^n$ is a PRG, then so is $G':\{0,1\}^{k+l} \to \{0,1\}^{n+l}$ defined by $$G'(x||x')=G(x)||x'$$ where $x \in \{0,1\}^k$ and $x' \in ...
2
votes
1answer
407 views

Break double encryption

Let $E_k$ : {0,1}$^l$ be a block cipher encryption function with block-size $l$ and key-length $n$. In class, we saw that a double encryption with two independent keys $E{}'_{k_1k_2}(x)$ = $E_{k_1}(...
6
votes
1answer
531 views

Prove that textbook RSA is susceptible to a chosen ciphertext attack

Given a ciphertext $y$, describe how to choose a ciphertext $\hat{y} \neq y$, such that knowledge of the plaintext $\hat{x}=d_K(\hat{y})$ allows $x=d_k(y)$ to be computed. So I use the fact that the ...
1
vote
0answers
118 views

Is OAEP reversible?

Given nothing more than some integer $m =$ OAEP($M$), is it possible to recover the original plaintext $M$? In other words, without being given the hash functions or the random string used for ...
-1
votes
1answer
142 views

Which areas in CS will be (or have been) most affected by fully homomorphic cryptography? [closed]

I'm in the middle of planning a 5000ish word essay on fully homomorphic cryptography, the current practical implementations and their limitations. Which areas of CS as a subject have been (or will be)...
2
votes
1answer
158 views

Probability for collisions of a one-way compression function

Given a one-way compression function $h:\{0,1\}^n \rightarrow \{0,1\}^m$ and an attacker that picks $x_1 \ldots x_t \in \{0,1\}^n$ (uniformly distributed), I have to show that the probability to find ...
1
vote
2answers
8k views

How to decode an OTP message?

First off, I'm extremely sorry if this comes off as a "homework dump", but to clarify, it isn't. I don't want the answer, I want to know how to get to the answer. My cryptography professor gave us an ...
2
votes
1answer
264 views

How to prove this LFSR equation?

I am having difficulty solving part (ii) of Excercise 5.17 from Becker's Cipher Systems. Exercise 5.17 $\quad$If $f(x),g(x)$ are any two polynomials over $GF(2)$ with $f(0)=g(0)=1$ show that ...
1
vote
1answer
145 views

Weak Stream Cipher cryptanalysis

Hi I have a stream cipher which uses a keystream of: F(r) || F(r+1) || F(r+2) || ... Suppose F(r) is the RSA encryption algorithm: ...
1
vote
1answer
515 views

Can AES decrypt with a wrong key?

The title is asking the question in layman terms, but what I really mean to ask is this: Let $x \in \lbrace 0,1 \rbrace^{128}$ (an arbitrary input block) and let $K \in \lbrace 0, 1 \rbrace^{128}$ (...
0
votes
0answers
268 views

Blum-Blum-Shub Cryptosystem

I am given $N$, which is Bob's encryption key. I was able to successfully factor $N$ into $N=p*q$. Alice encrypts a messages for bob and sends $a = [1,1,1,0,0,0,1,0,1,1,0,...]$ $b = 1545164156...$ ...

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