14
votes
1answer
895 views

Should I use the first or last bits from a SHA-256 hash?

I have the need for a hexadecimal token that is smaller than the normal length of the hexadecimal representation of a SHA-256 hash. Should I take the first bits or the last bits? Which of them ...
19
votes
2answers
4k views

Why does nobody use (or break) the Camellia Cipher?

If Camellia is of equivalent security and speed to AES, concerns arise. First of all, assuming the above, why is Camellia so rarely used in practice? Why aren't there any breaks in Camellia? Does ...
9
votes
1answer
2k views

Why is CAMELLIA suddenly so widely used?

When nowadays I point my browser to https sites, the cipher that is on most occasions used is Camellia. My browsers (Chrome and Firefox) seem to prefer it, even when AES is available. Is that not ...
2
votes
1answer
137 views

Camellia cipher - is it a symmetric or asymmetric cipher?

I read a wiki page about Camellia cipher and know that it is a block cipher that can use 128-bit, 192-bit or 256-bit keys. But I cant find any information about if its a symmetric cipher (uses one key ...
5
votes
2answers
409 views

Camellia or AES - which should I use?

Imagine I am a software developer. I want a block cipher, and I have the choice of using AES or Camellia. Which should I use? Which makes more sense from a technical perspective, and why? I need ...
2
votes
2answers
203 views

Is OTP useful in modern electronic communication?

I have seen from time to time questions about various OTP schemes and "OTP" (i.e. stream cipher) schemes. The most common OTP troubles appear to be: How to extend size of previously generated key or ...
3
votes
2answers
152 views

Can you really insert the text you want in one-time pad?

The Wikipedia article "One-time pad ~ Authentication" says : For example, an attacker who knows that the message contains "meet jane and me tomorrow at three thirty pm" at a particular point can ...
-2
votes
1answer
301 views

Is my one time pad cipher secure?

This is a program that I made for encrypting text files. It uses a one time pad cipher to encrypt the files, but I do not know if there is any holes in my program that could be a vulnerability. Is my ...
2
votes
1answer
111 views

Genuine encryption from only one source?

Is there any technique so that if I encrypt data, the ciphertext would be genuine and if someone else tries to encrypt the same data the ciphertext could be determined as fake or just invalid? I want ...
0
votes
2answers
209 views

What block cipher is used for CBC-MAC?

What block cipher is used for CBC-MAC? DES, AES, 3DES? Or it doesn't matter?
1
vote
1answer
182 views

Can Secp 256 K1 curves “map” to a value on FIPS 186-3 or P-256?

I'm looking at Secp 256K1 vs UProve's FIPS 186-3 or P-256 implementation. Is there any relationship between the curves such that I can consistently "map" or "project" values from one curve to ...
4
votes
1answer
146 views

MARS and RC6 in CUDA

I am working on a project where I am benchmarking the 5 AES finalists (Rijndael, Twofish, Serpent, MARS and RC6) on CUDA Hardware. My problem is that I am no experienced CUDA programmer. My approach ...
1
vote
2answers
225 views

Generate random number which should depend on key

i want to generate random number for example from [1..100]. The algorithm which would generates random number should work similarly like a "Linear congruential generator", like this: $X_{n+1}≡(aX_n ...
3
votes
1answer
242 views

Which PRNG security requirements do I need for key generation

Do I need to worry about "predictability" an an allegedly secure CSPRNG if I am just using it to generate key material from time to time? Suppose that for some odd reason, I am less trustful today ...
1
vote
3answers
232 views

One-Time Pad Distribution of Keys Between Two Parties

Consider the One-Time Pad (OTP). Suppose two parties, A and B, generate a completely random secret key in a real-life meeting, and they keep this secret. Now A wants to send some message to B using ...
2
votes
1answer
285 views

How to properly add ECDSA private keys?

I'm currently working on an application that requires me to add two ECDSA private keys in order to make a new private key. The result has to have the property, that its corresponding public key is the ...
2
votes
1answer
272 views

Is using Ed25519 parameters in ECDSA safe?

I recently discovered the Curve25519 key exchange lib and the Ed25519 signature lib. Due to the speculations about NIST-designed curves, there is a chance that I ditch them and use the curves above ...
0
votes
1answer
170 views

ECDSA - point order criterion

i am creating some primitive demostration for ECDSA over small curve (p < 229). But my implementation have some weird issues. Verify process return false even if the signature is correct. Because I ...
4
votes
2answers
253 views

How is the x coordinate of a “point at infinity” encoded in a Secp256k1 signature?

I'm testing an implementation of Bitcoin, which uses the curve Secp256k1 for ECDSA, and I want to see how it handles the point at infinity ($0$) if present in a signature. For example, r could be the ...
2
votes
1answer
190 views

Can A PRNG Be Used To Generate Multiple Private Keys for ECDSA?

Suppose Alice seeds a cryptographically strong Pseudo Random Number Generator with a truly random number, and keeps it secret. Alice then uses the output of the PRNG to generate several 256-bit ...
1
vote
2answers
312 views

Chosen Plaintext Attacks against an Affine Cipher

Assuming the ability to launch Chosen Plaintext Attacks (CPA), how many oracle calls an attacker needs to break the affine cipher? and how
3
votes
1answer
169 views

Salsa20 in Davies-Meyer mode with a fixed key (message): is it one-way?

Say I have a function that calculates a pseudo random permutation and this function is easy to invert. For example $P(i) = AES_k(i)$ where $k$ is a publicly known key. So anyone can compute $P(i)$ ...
5
votes
3answers
743 views

What is the PRG period of stream ciphers such as RC4 or Salsa20?

I am confused about how long a stream cipher can be used before you should change the key. To be concrete, let me use the stream cipher based on RC4 as an example. Let's say I want to encrypt a very ...
1
vote
1answer
240 views

Security of Salsa20 with some known plaintext?

Basic question- if I'm encoding a bunch of known filetypes with salsa20, will it still be secure if the plaintext header is known? Assume that a different IV and Key are generated before each file is ...
4
votes
1answer
177 views

Is it safe to assume Salsa20 to be a PRP?

Often in security proofs a certain block cipher is assumed to be a pseudorandom permutation or PRP. I wonder if this goes for stream ciphers as well, and specifically for Salsa20. If limit ourselves ...
4
votes
1answer
185 views

hardware and software optimizations for Salsa20

I'm using the Crypto++ implementation of Salsa20 for a software project we are working on and I'd like to know if there are any SSE or software optimizations that will increase the speed of the CSPNG? ...
4
votes
1answer
730 views

Fast Salsa20 in java?

For Java I'm only aware of the Bouncy Castle implementation of Salsa20. However, that version is significantly slower than AES-CTR and ARC4. There are plenty of libraries in C, but are there any good ...
2
votes
1answer
183 views

Security considerations on “expand 32-byte k”-magic number in the Salsa20 famility of stream ciphers?

I'm currently examining the NaCl library written by Daniel J. Bernstein and I noticed that the library hard codes the sigma: ...
7
votes
1answer
569 views

What's wrong with MARS?

Why did the MARS cipher receive such a high number of negative votes in the AES competition? As I understand only MARS and Serpent implement measures to counter future cryptoanalytic attacks, so it ...
2
votes
1answer
262 views

LFSR using words

If I've got an LFSR, let's say a 16-bit Fibonacci LFSR as shown in the corresponding wikipedia article, which generates maximum length sequences, could I use it to create word sequences instead of bit ...
2
votes
1answer
132 views

Security of cloud computational protocols in UC Framework?

The universal composability allows one to the analyze security of cryptographic protocols . But it does have some gaps when it comes to analyzing few protocols especially two party cases when there is ...
2
votes
2answers
215 views

How to verify a number encrypted with an unknown key

Alice and Bob are going to follow the protocol below. Are there any crypto-constructions to help Bob verify the correctness of the answer he gets?: Alice encrypts a set of numbers using some ...
2
votes
3answers
247 views

understanding pairing $e:G \times G \to G_T$ and ( Decision)BDH assumption

From DrLecter's comment, I know that DDH problem can be efficiently solved with this $$e(g^a,g^b)\stackrel{?}{=} e(g,g^z).$$ I have some trouble to understand this map $e:G \times G \to G_T$. Am I ...
2
votes
1answer
117 views

Verifying encrypted addition

Alice has two secret numbers, a and b. She publishes c1=E(a), c2=E(b) and c3=E(a+b). Is there an encryption system E such that anyone would be able to prove that the c3 as published by Alice is ...
4
votes
2answers
133 views

How can one calculate the estimated RSA key life based on Moore's law?

How can someone estimate the number of years needed to factor an RSA key based on the advancement of technology if followed Moore's law?
3
votes
2answers
385 views

Encoding information in packet lengths to actively sidestep encryption

If you've only got an encrypted data channel can you actively sidestep the encryption and communicate with an outside party who can see the data but cannot decrypt it? Yes - if you can find some ...
1
vote
3answers
164 views

Does MAC provide origin authentication? Why not just use symmetric encryption?

Let's assume Bob and Alice has a secret key for MAC. If MAC is constructed by the key and the message, that means that both Alice and Bob can construct the exact same message and MAC. So, Bob can ...
3
votes
2answers
224 views

Does encrypting a hash of the message yield a secure MAC?

Is $mac = enc(hash(x))$ a secure MAC for hash = SHA256 and enc AES in CBC mode or ECB mode. Normally AES-CBC is malleable. But in this case I use a hash function for the one-way property. For ...
4
votes
1answer
158 views

Proof that MACing a hash of the message is also a secure MAC

I found a theorem that says: Let $MAC = (S,V)$ be a MAC for short messages over $(K,M,T)$. Let $H: M^{big} → M$. Define $MAC^{big} = (S^{big},V^{big})$ over $(K,M^{big},T)$ as: $S^{big}(k,m) = ...
1
vote
1answer
65 views

Safety when disclosing hashes of secrets used to calculate other secrets

In my application, I am generating a big random number and publishing a SHA256 hash of it. After the hash it published (but not the secret), anyone can submit any number, and the system will calculate ...
4
votes
2answers
560 views

Questions about Key Derivation Functions

My understanding is that a KDF is a function that takes a master secret and generates multiple keys. It is secure as long as the keys are "independent". If this is true, the following definition would ...
4
votes
3answers
1k views

NIST temporarily closed — will that have a negative impact on the future of cryptography?

For those who didn't notice yet, in the USA, the National Institute of Standards and Technology(NIST) has been shut down temporarily because of the poor fiscal situation of the USA. The text at their ...
3
votes
3answers
318 views

Where can I find useful data for cryptography/coding theory?

When implementing cryptographic/coding theory algorithms one need to use data like big prime numbers, numbers in $Z_n$ and their inverses, irreducible polynomials in $Z_n[x]$ and so on... While ...
1
vote
3answers
393 views

two public keys with same passphrase insecure? | can two hashes be compared?

Suppose someone had generated a 2.048-Bit RSA GPG/PGP key pair and published the public key on the usual key servers. Then he withdraws this key and generates a new one in 4.096-Bit RSA using the same ...
6
votes
0answers
532 views

Bleichenbacher 1998 “Million message attack” on RSA

I have been reading Bleichenbacher's 1998 paper on a forged message attack on RSA. The paper assumes access to an Oracle that takes a ciphertext $c$ and will check the decrypted text for valid PKCS #1 ...
2
votes
1answer
207 views

Non-linearity of a boolean function

What's the definition of non-linearity of a boolean function? Roughly saying it is minimum number of times it equals any affine function. But I don't get it mathematically. For example, if $f = ...
4
votes
1answer
173 views

Why does only length prepending improve the security of CBC-MAC

I know that length prepending improves security of CBC-MAC. However, wouldn't inserting the length elsewhere (middle, end or any other part of message) be equally good? After all, even the length is ...
1
vote
1answer
304 views

Serpent block cipher : S0 to S7 functions unclear

I am presently implementing the serpent block cipher in C++ following the specifications. It's important to mention that I'm implementing the cipher in bitslice mode. You'll need the The full ...
2
votes
1answer
96 views

Verifying a Signature

Alice publishes $(n,e)$ where $n$ is exponential modulus and $e$ is the public key. We have $n=80$ digit number and $e=3$. Bob, in correspondence with Alice, asked Alice to prove she is really Alice, ...
1
vote
1answer
61 views

Secure MAC implies that probability of same tags on different messages is negligible

So let any secure MAC (message authentication code) be given. Intuitively, I think it is clear that the probability of getting the same tag on two different messages is very small, i.e. negligible. I ...

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