8
votes
2answers
903 views

Will repeated rounds of SHA-512 provide random numbers?

If I hash a keyword with SHA-512 and then feed the output as the key for the next round ....and keep repeating this process, will I gather a stream of random numbers?
3
votes
1answer
152 views

Zero-knowledge proof for committing a choice?

Let's say there are 100 choices (which are publicly known), each represented as a different string, and today you have to choose one of them. You need not reveal what that choice is right now, though. ...
3
votes
1answer
78 views

Does displaying HMAC hashes of user inputs compromise the HMAC validation?

Here is the scenario: "SECRET" is a secret key that is kept private and only used in this one application. The application allows users to enter in any "VALUE" (within various open-ended limits such ...
1
vote
1answer
70 views

Secret Sharing 1 Required

Devise a scheme so that a message M can be shared among X, Y and Z in such a say that the only way of recovering the message is when X is present with either Y or Z. When X isn't present or each is ...
2
votes
1answer
320 views

SHA256-based stream cipher

Can anyone comment if the stream cipher described here is safe? The author claims it to be unbreakable, but does not provide any evidence or proof to support this. For completeness, I have reproduced ...
0
votes
0answers
38 views

Locally Dedcodable Codes and Private Information Retrieval

Does anyone know about Locally Decodable Codes (LDCs)? It seems the private information retrieval schemes that based on LDCs are computationally efficent (answer a query without processing the whole ...
1
vote
1answer
89 views

Is it possible to deduce the IV from CBC ciphered data, without knowing the key?

The question came up here, which left me thinking: Is it possible to deduce the IV from CBC ciphered data, without knowing the key? And if not, why is it considered a bad idea to create an IV by, for ...
-1
votes
1answer
64 views

What's wrong with this “order-preserving MAC” function?

Please note: this is purely a thought experiment and not intended for any real-world usage! I came up with a simple function $\mathrm{OPF}$ to map the integers $[0,C)$ (where $C$ is the "ChunkSize") ...
4
votes
2answers
401 views

Is Chaocipher a secure cipher under ciphertext-only attack?

Chaocipher was invented by John F. Byrne in 1919. The algorithm was recently revealed – see Moshe Rubin's Chaocipher Revealed, the Algorithm (PDF). While a known plaintext attack successfully finds ...
0
votes
1answer
137 views

IV = Filename XOR CipherKey?

i want to encrypt transparently using Rijndael. So this is, what I thought of and I would like to have an opinion from "Experts" whether this will harm encryption strenght. I am using chunks of 1MB ...
0
votes
1answer
87 views

Multi cipher CTR

What is your opinion on this scheme: 1)get IV from CSPRNG 2)set 3 encryption keys k1,k2,k3 (3*key_lenght) (KDF/PBKDF) 3)Use 3 ciphers in CTR mode like this: The initial IV is encrypted using ...
2
votes
2answers
632 views

RSA Key Generation Parameters - public exponent, certainty, string-to-key count

I want to know what values are appropriate for the public exponent and certainty when generating an RSA Key (for example using Bouncy Castle RSAKeyGenerationParameters function). What is the ...
1
vote
0answers
89 views

Is there a malleable pubkey digital signature scheme?

I'm trying to find a special kind of Digital Signature Scheme. The scheme should allow me to transform a signature $s=\text{sign}(sk,m)$ (for a private key $sk$ and a message $m$) into a signature ...
3
votes
1answer
277 views

Convert m-Sequence into a de Bruijn Sequence

In his paper Alternating Step Generator Controlled by de Bruijn Sequence, C.G. Günther states on page three that a de Bruijn sequence (..) can easily be obtained from an m-sequence (maximal length ...
0
votes
2answers
153 views

Convert SpookyHash to semi-secure 192bit hash [closed]

What do you think of converting SpookyHash into semi-secure 192bit hash function by this way: increase security by using 8 * 8byte = 512bit blocks instead of 8 * 12byte blocks, but remaining 8 * ...
3
votes
1answer
200 views

XOR with constant key in CBC mode

I have a hypothetical encryption scheme where somebody uses the one-time pad in CBC mode. That is, the block cipher is $E(k, m) :=k⊕m$, and that block cipher is used in CBC mode. Now, I am assuming ...
2
votes
2answers
274 views

How could Fully Homomorphic Encryption support power operations?

Fully Homomorphic Encryption (FHE) enables arbitrary functions computed on encrypted data, because it supports both addition and multiplication. But I wonder if FHE supports power operations. For ...
0
votes
2answers
217 views

Why are Fortuna entropy pools reset after a reseed?

I've implemented LibTomCrypt's version of fortuna and am wondering about the pool resetting after a reseed. In fortuna.c in libtomcrypt we have the following (which is that part that adds the entropy ...
7
votes
2answers
284 views

RSA: Letting $p$ and $q$ have different bit-size

I am aware that there are concerns if $p$ and $q$ are close i.e. $\Delta=|p-q|$ can't be too small. But I would like to know if there are any known attacks for cases where $p$ and $q$ take on ...
5
votes
1answer
155 views

Is it possible to determine or estimate the period for Blum-Micali PRG?

The Blum-Micali is a cryptographically secure pseudorandom number generator. The construction (from wikipedia): Let $p$ be an odd prime, and let $g$ be a primitive root modulo $p$. Let $x_0$ be a ...
5
votes
1answer
498 views

Secure AES Key Generation via Salsa20?

I am not creating my own cryptography or anything, but to learn about the Dos and Donts of cryptography, I am looking into AES encryption and ways to generate keys for that. As far as I've ...
1
vote
0answers
75 views

Definition of a Statistical Test

In Professor Boneh's online Cryptography course at Coursera, I am a little puzzled by his definition of a statistical test where he writes: A(x) = iff |#0(x) - #1(x)| <= 10.√n Now, if – as ...
2
votes
0answers
69 views

Separate TLS Client Write and Server Write Keys in AES-CCM Mode

I am designing a system that uses TLS-like handshaking on a resource-constrained embedded system. I have settled on AES-CCM mode for symmetric session encryption and authentication, allowing ...
3
votes
1answer
85 views

Compression step of PGP

We do compression in PGP. Does this step increase/decrease or either have any effect regarding the authentication of a message?
1
vote
0answers
57 views

Is timestamping possible without publishing?

The Wikipedia article Trusted Timestamping states that we can timestamp data by publishing it's hash. Publishing can be done either independently or via a third-party publisher, as explained by ...
1
vote
2answers
279 views

Wrong Test Vector for HKDF with HMAC-SHA256 [closed]

I have implemented HKDF by myself and it seems to work good. I managed to replicate all the test vectors listed here but... except this one: ...
1
vote
1answer
87 views

Prefixing data before encryption for passphrase confirmation?

I'm encrypting data with AES, using CryptoJS. CryptoJS returns an empty string in case the data couldn't be encrypted with the given passphrase. I'm not sure if this is intended, or happens in all ...
4
votes
4answers
275 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 ...
0
votes
1answer
74 views

encrypting data signals using the time variations between two independent coupling functions [closed]

There was a recent article about a new encryption technique that encrypts "using the time variations between two independent coupling functions". What is this technique they are referring to?
4
votes
1answer
390 views

For a one-time pad, which MAC method is information-theoretically secure?

In the the main post about MAC methods it mentions a few methods: Authenticate And Encrypt: The sender computes a MAC of the plaintext, encrypts the plaintext, and then appends the MAC to the ...
3
votes
1answer
357 views

Can I use Shamir's secret sharing scheme for multiplicative homomorphism for secure multiparty computation?

I would like to perform a dot product operation among $m$ parties using Shamir's $(m,m)$ secret sharing that is used for Secure Multiparty Computation. I am aware that Shamir's $(m,m)$ scheme is ...
0
votes
1answer
129 views

What are alternatives to number theory based crypto? [closed]

Quantum crypto,lattice based crypto, Neurocryptography and cellular automata based cryptography are alternatives to number theory based crypto. I need to know what are the other hard problems like ...
0
votes
1answer
185 views

Thoughts on secure hashing and the “mds” algorithm

Almost a decade ago Raymond Chen blogged [1] about the "hash reset attack" cautioning developers to record the content length in addition to the message digest. Recently I came up with a variation on ...
3
votes
2answers
269 views

Is that possible to use encryption higher than 256-bit?

When browsing websites with Chrome sometimes I can see a green lock and encryption info inside. It says the connection with this website is 128-bit or 256-bit encrypted with TLS1.2. So I'm wondering ...
10
votes
2answers
11k views

HMAC-SHA1 vs HMAC-SHA256

I have three questions: Would you use HMAC-SHA1 or HMAC-SHA256 for message authentication? How much HMAC-SHA256 is slower than HMAC-SHA1? Are the security improvements of SHA256 (over SHA1) enough ...
2
votes
0answers
45 views

Delinearization in whitebox techniques

I'm reading through Chow et al.'s paper and Link et al.'s paper. There's a part I don't understand about Delinearization. They convert some steps of AES/DES into a Matrix, then divide the matrix in ...
3
votes
1answer
136 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 ...
5
votes
3answers
504 views

Is it possible to create a “proof-of-upload” system for BitTorrent ratio tracking?

One issue that private BitTorrent trackers that track users' share ratios often run into is how to keep track of people who are screwing with their upload statistics, something known as ...
3
votes
1answer
116 views

Key-dependent encryption in TAHOE-LAFS

TAHOE-LAFS uses a form of "key-dependent encryption", in that a private key SK is encrypted with the truncated hash of the private key: ...
2
votes
1answer
162 views

What is the difference between PRF and a Random Oracle?

What is the difference between Pseudo Random Functions and Random Oracles? Is the difference only about the domain of PRFs and Random Oracles, former having a fixed domain and latter can act on any ...
3
votes
1answer
815 views

TLS Key Block calculation - What is a PRF?

In Section 6.3 of the RFC for TLS 1.2, it is written that the key_block is derived from the following formula: ...
2
votes
2answers
373 views

Private RSA key for HMAC key

I am creating software tokens for future request authentication, and I want to use an HMAC for the token to make them tamper-resistant. To ensure I can check the HMAC later I need a secret, persistent ...
1
vote
0answers
94 views

How to compare different signature scheme's performance?

I am interested in comparing the computational performance of two signature schemes. However, I am unsure how to do that. My candidates are RSA and Shamir's ID Based Signature scheme. To get a fair ...
1
vote
2answers
729 views

How do I create a short signature? (e.g. less than 100 bytes)

I want to put information into a QR code and have it signed so that the authenticity of the information can be verified. But QR codes have limited storage capacity and when I used e.g. gpg I get ...
1
vote
3answers
317 views

What is h in this RSA variant?

I am trying to implement a proposed improved algorithm of RSA . Here the author has increased the number of exponents. However I am unable to understand what $h$ is in the Key generation step. Can ...
0
votes
1answer
757 views

Decrypt AES-128 with key file but missing IV? [duplicate]

I want to decrypt a file that has been encrypted using AES-128 in CBC mode using OpenSSL. I got the “.key” file – which is 32 digits – but when I try to decrypt with OpenSSL, the program asks me for ...
1
vote
1answer
168 views

Encrypting the same message using different schemes

$E_1$ and $E_2$ are IND-CPA secure encryption schemes. $E$ is defined as: $k_1,k_2 \leftarrow K_1 \times K_2$ . $E_{k_1,k_2}(m) \leftarrow E_{1,k_1}(m)||E_{2,k_2}(m)$. Hope the notations are in an ...
4
votes
0answers
148 views

Can a commutative block cipher be indistinguishable from a random one, assuming a random permutation of keyspace?

Define a commutative block cipher with keyspace the finite set $K$, and message space the finite set $S$, to be an application $$\begin{align} E:K\times S&\mapsto S\\ (k,x)&\mapsto ...
0
votes
1answer
202 views

Demonstrating Diffie-Hellman key exchange using only p, A, B;

I'm trying to figure out how to demonstrate DH key exchange using what is given to me. I'm unsure of how to tackle the problem. Question: Demonstrate Diffie-Hellman key exchange. Given p=104933, ...
0
votes
3answers
737 views

Generating a number using SHA512 that users can later verify (once I publish secret seed)

I've wrote following C# method that basically takes 3 seeds and gives you number in 0-100 range. I then use that number for variety of purposes but for this question the most important one is the ...

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