-1
votes
0answers
17 views

Substitution cipher with hexidecimal key and encrypted plaintext? [duplicate]

This is a challenge for a past CTF that I was unable to find a writeup for. The challenge gives two ciphers (http://pastebin.com/raw.php?i=Fq98trFw). The "key" has a length of 32, and the flag has a ...
4
votes
1answer
51 views

Do asymmetric signatures require constant-time verification?

To avoid a timing attack, HMAC signatures are usually compared in constant-time (every byte is compared, and the results aggregated). Is the same necessary for asymmetric signature algorithms such ...
1
vote
1answer
21 views

Determining nonce between unsynced systems

It's my first venture into cryptography, and I'm struggling with how to use a nonce in an encryption standard between two digital radios. The two radios support AES encryption, only requiring a ...
0
votes
1answer
30 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
72 views

ECDSA for encryption

ECDSA keys are much shorter than RSA keys, but typically only used for signing, not encryption. Is there an accepted way of doing this? (https://bitcointalk.org/index.php?topic=196378.0 ...
1
vote
1answer
103 views

Deterministically generate a RSA public/private key pair from a passphrase (with sufficient entropy)?

Is it possible to deterministically generate public/private RSA key pairs from passphrases? Would giving the (key generating) algorithm data made from key-stretching the passphrase (instead of a ...
-1
votes
2answers
45 views

Improving on a clocked PRNG

I recently developed a PRNG from scratch with a little inspiration from the initialization function used in the HC stream cipher. The state ($S$) of the basic version is an array of 5 bytes and uses ...
1
vote
3answers
62 views

Deciphering text encrypted with a changing cipher

I know there are many ways to crack basic ciphers were each letter is mapped to some other letter, but what ways are there to decode something that was encrypted using a cipher that changed after ...
0
votes
0answers
32 views

how to calculate nonlinearity?

I am struggling to understand how to calculate non-linearty of an s-box, could someone show me a detailed example? I found this question which helped a little, but I still didn't get it, if someone ...
0
votes
0answers
97 views

Naive implementation of Rainbow Table and/or Hellman's trade-off

Is there any naive implementation of Hellman's cryptanalytic time memory tradeoff in C and/or a naive implementation of Oechslin's rainbow table algorithm in C as well? I have seen some ...
0
votes
1answer
66 views

MAC in SSH packet encryption, benefits to not including it?

What are the benefits to not including MAC in a SSH packet encryption? I understand what the MAC is there for, but if it was not included would there be an advantage? Is the MAC somewhat redundant ...
0
votes
0answers
45 views

Composition of non-perfect encryption keys

Definition 1: A key $k$ is called perfect if it is uniformly distributed from the adversary's point of view; a key $k$ is called $ε$-perfect, if its distribution has an $ε$ trace (statistical) ...
1
vote
2answers
66 views

Applying Trapdoor Function directly to plaintext

Suppose we have a secure trapdoor function defined on (G, F, F^-1) where G is a randomized algorithm outputs a key pair (pk, sk). Why would this trapdoor function become insecure if we apply F ...
1
vote
1answer
123 views

Concatenation of two strong hashes may have striking weakness

For any hash functions $H_0$ and $H_1$, it is easily proved that their concatenation $H_0\|H_1$, defined by $(H_0\|H_1)(X)=H_0(X)\|H_1(X)$, is at least as resistant as the strongest of $H_0$ and $H_1$ ...
5
votes
2answers
166 views

What happens when a RC4 stream gets corrupted?

I want to encrypt a large file using RC4. But what happens if the encrypted file gets corrupted (bytes modified or lost)? Can I still decrypt the rest of the file correctly? If not, what is the best ...
0
votes
0answers
47 views

Making PRFs out of PRGs

Is it possible that we take a $PRG$ $G(k)$ of stretch $n\cdot2^n$, and read its output as the table of a $PRF$ $F(K)$ with input and output size of $n$? Intuitively it sounds possible however I read ...
0
votes
1answer
66 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
0answers
30 views

Weaknesses of truncated hash value [duplicate]

I want to apply one of SHA-2 functions to a plain text and then truncate the hash value into parts and store them into different areas so are there any security implications for truncating the hash ...
0
votes
0answers
35 views

I was wondering if RSA is an homomorphic crypto system? [duplicate]

I was wondering if RSA is homomorphic and how secure is it. I have read there is a variable of this but i don't fully understand the mathematics.
1
vote
0answers
28 views

Is RSA-OAEP secure when e=3? [duplicate]

Is RSA-OAEP secure when the exponent is equal to 3?
0
votes
0answers
31 views

Practical strength of non-2^n RSA key lengths [duplicate]

It has for many years been popular to use RSA keys that have lengths that are powers of two. E.g. 1024-bit, 2048-bit and 4096-bit key lengths are all popular for use with OpenPGP implementations such ...
0
votes
1answer
83 views

Why not send the identity in the encrypted message?

I've recently started reading the book "Applied Cryptography" written by Bruce Schneier. Quite a few times I stumbled across protocols who send the identity outside of the encrypted message (see step ...
0
votes
0answers
30 views

Resynchronizing brute-force attack against stream ciphers

There is a stream cipher without any known cryptographic weaknesses. The attacker knows parts of the key so she can reduce the key-space to a size that can be practically brute-forced. However the ...
1
vote
0answers
49 views

Given only RSA public key parameters, what security issues might I be able to detect?

I'm attempting to automate some checks against a large list of .NET assemblies, and want to include a test to see if the RSA parameters used to sign the assemblies are sensible. However, since I don't ...
0
votes
1answer
51 views

CCA-attack is possible in RSA, but how the decryption key $d$ is known to anyone

CCA-attack is possible in text-book RSA, but how the decryption key $d$ is known to anyone, as it is kept secret with receiver.
0
votes
1answer
198 views

What is the most light-weight symmetric cipher thats still usefull?

I need a very light encryption scheme which costs almost no performance during encryption (while decryption may be slow). I found 5 algorithms: RC4, DES, LED, PRESENT, and Piccolo. But how do I ...
0
votes
0answers
29 views

Signature constructor using Self-signed certificate

I've the following scenario: One application signs JWT access/id tokens using the private key contained in a self-signed certificate, and then passes its public key to other parties to validate that ...
0
votes
0answers
24 views

Where I can find a list of certified software / hardware RNGs compliant to NIST SP 800-22?

I need to use NIST SP 800-22 approved software / hardware to generate RSA key pair. Where I can find a list of certified software / hardware RNGs compliant to NIST SP 800-22? And how is the SP ...
0
votes
2answers
111 views

Reusing a one-time pad?

I have an embedded device. It produces larges files (200 MB). I want the device to encrypt the file before writing to disk, so I gave them all a 16-byte random key. How can I transform the large ...
0
votes
1answer
65 views

Steganography Decryption From Completely Black Image

I am trying to decrypt a completely black gif that I know has been encrypted by some sort of steganography. I've tried subtracting the bytes of the images with each other, and so far I have gotten a ...
0
votes
1answer
71 views

why do files downloaded with http and https have the same size?

while working with Encryption mostly cipher text is always larger the plain text, why isn't that the case when downloading a file using https ? even that compression isn't much used anymore because if ...
0
votes
0answers
17 views

Block size of 128 bit in (quantum) future [duplicate]

Is a block size of 128 bit enough to ward off expected attacks in the future (something like 30 years ahead) with the possible risk of quantum computers? Other formulation: If we were to create a ...
0
votes
0answers
33 views

Requested to encrypt data with AES256, digital certification, timestamped

at my job we have been tasked by gov to encrypt some data we send to them. The "law" says we have to do the title above, but they only need us to do the AES256 thing because the data is not really ...
2
votes
1answer
115 views

Differential Fault Analysis of AES

Let's say: an error is induced into an implementation of the AES-128 to produce a faulty ciphertext. The error is a one-byte eror and is always induced on the last state byte, during round9, before ...
0
votes
1answer
41 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
53 views

decrypt a saml 2.0 response having aes 128 and rsa [closed]

I need to decrypt a saml 2.0 response. But, the response object has reference to aes 128 and rsa algorithms, and I am having hard time in finding a way to decrypt. Below is the structure of the ...
0
votes
0answers
26 views

How to check if a sequence is generated by a linear congruential generator? [duplicate]

Assume a linear congruential generator is defined by $x_i=(x_{i-1}\cdot a+c)\bmod m$ and I am given the values $a$, $c$, $m$ and a sequence of numbers starting at the $n$th bit. Can I tell whether ...
0
votes
1answer
102 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 ...
-1
votes
0answers
30 views

Does “cryptography rather [try] to minimize the expansion of ciphertext”?

In a question's comment on crypto.SE (Is there an efficient way to hide the encrypted plaintext length with a block cipher?) it was stated cryptography rather tries to minimize the expansion of ...
1
vote
0answers
47 views

Statistical saturation attack on block ciphers

I was wondering if anyone around here could give me some explanation on this type of attacks. Pretty much the only thing that I could find is this What I really need is basic explanation of the ...
3
votes
2answers
106 views

Why does consecutive permutations or consecutive substitutions not enhance security?

My professor stated that permutations which are done repeatedly does not further enhance security than just one permutation. He also stated that fact this applies to consecutive substitutions as well. ...
1
vote
0answers
40 views

Encryption with a known plaintext

I'm building an application where users can be a part of groups, and can obtain and distribute URLs from the group page that allow anyone who clicks on them to join said group. Groups should not ...
0
votes
1answer
25 views

What is the benefit of applying the tweak a second time using XTS?

I tried to understand the inner working of XTS. What I understood is that it is applying the tweak two times: once on the plaintext and once on the output. What is the benefit of applying the tweak ...
2
votes
0answers
49 views

Statistical saturation attack on block ciphers

I was wondering if anyone around here could give me some explanation on this type of attacks. Pretty much the only thing that I could find is this What I really need is basic explanation of the ...
0
votes
1answer
57 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
2answers
49 views

What does variable-base point/scalar multiplication mean in ECC?

I am a bit confuse about the term, variable-base point/scalar multiplication, in Elliptic Curve Cryptography. What I have understood so far. It means that the base or point on EC is variable/unknown. ...
0
votes
0answers
39 views

Is there any task better and less complex than oblivious transfer? [closed]

I wanna know whether there can be any algorithm less complex than oblivious transfer because OT consumes O(n) time complexity where n is the number of messages involved. So can you please help me out ...
2
votes
4answers
135 views

Can we reverse a hash when we know part of the input?

While I read that hashes are not meant to be reversible, but assuming we have part/half of the message/input of a hash? Is there any method that we can use to recover the remaining message via this ...
0
votes
0answers
23 views

Weakness in double hashing? [duplicate]

I remember reading an account of how double hashing (i.e. sha256(sha256(x))) can make some form of communication protocol between Alice and Bob trivially insecure, ...
0
votes
0answers
54 views

Key exchange using hash function

Consider two types of attackers on a key exchange (KE) protocol and subsequent communication between the parties which used this KE instance to establish a secure communication key. Attacker Eve ...

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