# All Questions

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### What are the safe ways to derive HMAC key using block cipher?

Suppose we have a state of block cipher initialized with some key unknown to us, that is, we have the state after running key schedule, but we have no access to actual key or subkeys, all we can do is ...
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### Self-expiring symmetric keys, or: cryptography in absence of secure deletion

I can encrypt some data D using a random symmetric key K, obtaining a ciphertext C, and then encrypt K with my public key Pub and obtain H. So far so good: I can only decrypt C if I have H and my ...
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### Why can't Homomorphic encryption schemes support algorithms with conditions/branching?

If it isn't already apparent from the title of my question, i should make clear that I have only a very basic understanding of homomorphic encryption. I would like to know why homomorphic encryption ...
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### How is the EAX-mode used in EAP-PSK (RFC 4764)?

According to the specification of RFC 4764, contents in EAP's third message and fourth message are required to be encrypted by AES-128 in EAX mode (also called protected channel). In RFC 4764, the ...
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### Stream cipher malleability

Considering a stream cipher that produces a ciphertext $c$ from a message $m$ and a key $k$ is it possible to apply operations (multiplication and/or addition) directly to $c$ without knowing the key ?...
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### How many different keystreams can I produce with AES in Counter Mode?

I'm using a 64-bit nonce (incremented between the streams) and a 64-bit counter (incremented within a stream) for AES in CTR mode. How many different key streams can I produce with this setup?
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### Using “Additional Authenticated Data” as a secondary key

In implementing a cipher in GCM or CCM mode, you are provided the option to add "Additional Authenticated Data" (AAD). This AAD is required for decrypting the cipher text, and seems to be used when ...
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### Generating a strong unique Initialization Vector

How can I determine if I am generating a unique and strong Initialization Vector? If my mode is generating Keystream? Is there any scientific explanation in generating a unique and strong ...
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### Byte location of plaintext vs ciphertext when using a block cipher or stream cipher

I know that when you use a stream cipher you are just XOR'ing the plain text with the PRNG stream, such that you know if a specific byte is in position 3 of the plain text it will still be in ...
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### Use of nonces in Kerberos and Needham–Schroeder protocols

Can you please explain me what is the reason of using nonces in the Needham-Schroeder and in the Kerberos protocols for Key Transport. In particular : Can you explain me the use of $N_A$? And ...
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### Regarding matsui's Paper on Linear Cryptanalysis of DES

I's going throught the the Paper by Matsui on Linear Cryptanalysis of DES . In that he says $NS_{5}(16,15)=12$ And then in the next paragraph he says considering the expansion and permutation phases ...
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### Separate Read and Write Keys in TLS Key Material

Why does the TLS protocol use different symmetric keys for receiving and sending data? Isn't it enough to have a single key used for both reading and writing?
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### Transparent cipher

I'm trying to implement this protocol: Alice has her permanent key $K_1$. She computes $E(K_1, P)$. She wants to share $P$ with Bob, Carol and Dave. For each of them she: 2.1. Generates $K_2$. 2.2....
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### Recovering the key in an ElGamal Signature Scheme variant

From page 318 in Stinson's "Cryptography: Theory and Practice", question 7.3: Suppose that Alice is using ElGamal Signature Scheme. In order to save time in generating >the random numbers k that ...
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### Public Key Encryption with forward secrecy

Is there a known non-trivial system with plausibly secure public key encryption where: 1. $\;\;$ the $\:$ time $\:$ function is efficiently computable 2. $\;\;$ [pubkey,privkey] $\:$ pairs are ...
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### Finding the LFSR and connection polynomial for binary sequence. [closed]

I have written a C implementation of the Berlekamp-Massey algorithm to work on finite fields of size any prime. It works on most input, except for the following binary GF(2) sequence: $0110010101101$ ...
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### Distributed knowledge problem

I need a way to distribute knowledge among multiple parties - lets start with two. The idea is while the knowledge is originally created by one party I need to transform it into a situation where no ...
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### RC4 : Keystream reconstruction?

First up: it's a step in some challenge found on the Internet, where we get an application and a file. The application must be feed with that file and a password (the password is the flag for the ...
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### Secure Remote Protocol - Are these 2 equations equivalent?

I'm implementing the Secure Remote Protocol that is specified to have this equation for the parameter S, the premaster secret. ...
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### Shannon entropy calculation: is $H(A|R·A) = H(A)$?

Suppose I generate a random $m×m$ matrix $R$, where each of its elements belongs to $\mathbb Z_n$. I ensure that $R$ is invertible in $\mathbb Z_n^{m×m}$. Now I take a non-random $m×m$ ...
288 views

### synchronization of counters in HOTP

How is synchronization of counter values achieved in the HOTP protocol? As I understand it, the server increments its counter value only if a match (of the OTP value) is found. What happens at ...
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### AESManaged Trade Compliance Issue in United States

I am currently using AESManaged encryption for my silverlight application. Does AESManaged encryption have a severe trade compliance issue in Bureau of Industry and Security in terms of shipping the ...
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### Is there an effective way for public key generation in homomorphic encryption schemes?

Is there any way to effectively generate valid distributed public key in schemes like ElGamal or Benaloh for a number of participants? There's no need in private keys since there's no intent to ...
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### When is AES chosen instead of a stream Cipher (e.g., RC4) during an SSL connection?

During an SSL handshake, the browser sends a list of cipher suites to the server, from which the server selects one option. For data encryption, two of the several possibilities for the ciphers in ...
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### One-wayness proof of Elgamal

Here in the proof of Elgemal one-wayness: Consider an adversary V that can invert random Elgamal encryptions with $ε$ probability. $V *$ : Adversary to compute the Diffie-Hellman function: ...
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### In what sense addition modulo $n$ ($n>2$) isn't linear in the field $\mathbb{F}_2$?

I've been reading the Reason why “XOR” is a linear operation, but ordinary “addition” isn’t? question, in which one of the answers states that addition modulo $n$ ($n>2$) is linear in $\mathbb{Z}_n$...
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### Attacks from releasing decrypted CCM payload before verifying tag

CCM specifies (Section 1.7) that no plaintext data should be released before the MAC is verified. Why is this recommendation given and what attacks are possible against CCM if data is treated like a ...
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### Older key as PGP subkey?

Currently I have a 2048-bit PGP key. Suppose I create a new 4096-bit key. Can I add that old key as a subkey of this new one?
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### Last Sbox of a block cipher for Linear Cryptanalysis

I figured I should try to learn about linear cryptanalysis with a super simple cipher with only one Sbox. The problem is that with that approach the last subkey bits seem to be involved in the linear ...
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### How does BB84 fare against an attacker who can entangle qubits?

Consider the following attack on the BB84 quantum key exchange protocol (as far as I can tell, it should work similarly for E91): After Alice sends her key bits encoded in qubits with randomly chosen ...
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### Are transposition ciphers insecure?

Given plaintext: P[i], i = 1..N Given stream of cryptographic numbers S[i, j], i = 1..N, S[i, j] in [1, j] generated by RC5 for example We encrypt using Fisher-Yates shuffle algorithm: ...
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### Probability of hash collision in exhaustive mapping from input to output

Suppose I had a hash function which produced a 256-bits output for any given input (such as SHA-3-256). Now suppose i did an exhaustive mapping from every possible 512-bits input to a given 256-bits ...
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### Am I correctly understanding TLS 1.2 PRF (Pseudorandom Function)?

According to RFC 5246 Section 5 PRF(secret, label, seed) = P_<hash>(secret, label + seed) and ...
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### How to find the keyword of the Playfair text knowing/not knowing the plaintext and having decrypted text?

I know how the Playfair cipher works. I would like to know how can I find the keyword of the play fair cipher in two cases: 1) knowing the plaintext 2) not knowing the plaintext? Some examples: <...
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### If I encrypt data with two different ECC Private keys, how secure is the result?

I'm wondering whether encrypting data with two different 256-bit ECC keys will result in a more secure encryption (minimum 384 bit equivalent) or will result in data that's effectively encrypted with ...
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### 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 ...
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### Multiplying integers modulo $2^{255}-19$ using the Curve25519 polynomial reduction algorithm

I'm reading the Curve25519 paper and I see that in page 11, under the title "Multiplying integers modulo $2^{255}-19$", it says: The coefficients of $x^{10}$; $x^{11}$; ...; $x^{18}$ in $uv$ are ...
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### How can I show RSA OAEP IND-CPA secure

How can I show, that RSA with OAEP is IND-CPA secure by using G,H one way function?
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### Understanding the Hidden Subgroup Problem specific to Integer Factorization

I've been reading about the Hidden Subgroup Problem (HSP), specifically trying to understand how it is related to the integer factorization problem. I've read What exactly is the impact of the hidden ...
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### Can (EC)DH keys be used as the signed content in X.509 certificates?

From Wikipedia's article on ECDH: The public keys are either static (and trusted, say via a certificate)… Also see this question on this very site. These indicate to me that a certificate (...
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### reconstructing data on a noisy channel?

I have an interesting situation where I have a block cipher and a noisy channel. The interesting thing is that I happen to know much about the data. The cipher is a balanced Feistel network, and for ...
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### Client Puzzles and HMAC

I'm learning about client puzzles for DoS (Denial of Services) Protection, and I came across this question. For each request, the server sends the client a freshly generated random challenge r and ...
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### Is it OK to reseed a Deterministic Random Bit Generator from itself?

I want a deterministic PRNG for simulation purposes, and I don't want to have to worry about spurious correlations like those the DMCT is designed to protect the Mersenne Twister against. So even ...
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### Distinguishing two sets of pseudorandom values when their keys differ by one

Suppose we use a pseudorandom function $PRF$ and a random key $k$ to generate a set of pseudorandom values: $\forall i, 1\leq i \leq n: w_i=PRF(k,i)$ Now, consider instead of picking a fresh key, ...
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### Are timing attacks exploitable if each key is used only once?

Suppose that one performs (for example) AES-GCM encryption and decryption using an algorithm that is vulnerable to timing attacks, but each key is used only once. Is it still possible to perform a ...