9
votes
1answer
756 views

Why does a broken hash function undermine an HMAC?

For instance, what makes MD4 a bad choice for an HMAC? In this case I am asking about MD4 because its less than ideal. I know that a preimage attack can be used to undermine the system, but why? ...
8
votes
2answers
9k views

How does a chosen plaintext attack on RSA work?

How can one run a chosen plaintext attack on RSA? If I can send some plaintexts and get the ciphertexts, how can I find a relation between them which helps me to crack another ciphertext?
5
votes
2answers
332 views

Is there a hash algorithm that is slow to calculate but relatively fast to check?

Or more generally, is there a function or algorithm that is slow to calculate/execute, has a reliable execution time, and has a result that can be tested much more quickly than the calculation took?
7
votes
4answers
2k views

What is the largest performed/possible bruteforce attack to date?

I've read that cracking 128-bit key is currently out of reach of all humanity. However, I can't seem to find any information on what scope of brute force attacks have been performed or are possible at ...
2
votes
0answers
378 views

Diffie hellman key exchange on elliptic curve over an extension field [closed]

I am attempting to do a final semester project where I implement Diffie-Hellman key exchange on an elliptic curve over an extension field (2^256). Can anybody help me to generate the extension field ...
6
votes
4answers
3k views

Does encrypting twice using the same block cipher produce a security weakness?

If I use the output of a cipher, for example a block cipher such as AES and encrypt it again with the same algorithm, I read that this introduces weaknesses into the overall security of the system. ...
3
votes
1answer
4k views

Is AES restricted to only 64 characters for the key/password?

I am wondering if AES only supports 64 character passwords? When using truecrypt, the maximum character limit on passwords is 64 characters; however, when using WinRAR, the limit is 128 characters. ...
24
votes
1answer
19k views

What is the difference between CBC and GCM mode?

I am trying to learn more about GCM mode and how it differs between CBC. I already know that GCM provides a MAC which is used for message authentication. From what I have read, and seen code ...
1
vote
2answers
215 views

CPA distinguisher for matrix multiplication in GF(256) with randomized padding

What is the best CPA distinguisher for the function $F_k:\{0,1\}^{8n}\to\{0,1\}^{16n}$ described below? Let $E_k$ be a $2n\times2n$ matrix with elements in $GF(2^8)$, selected by generating bit ...
7
votes
1answer
209 views

In RSA, rationale for prime $p$ with $p-1$ having prime factor $u$ with $u-1$ having large prime factor?

In the 1978 RSA paper, it is recommended, among other things, to choose primes $p$ such that $(p-1)$ has a large prime factor $u$. This was motivated by Pollard's p-1 algorithm. Further, the authors ...
1
vote
2answers
318 views

My custom handshake

I am a beginner in cryptography and I designed a custom handshake between 2 entities $\rm C$ (client) and $\rm S$ (server) which should meet the following requirements: Requirements: The handshake ...
6
votes
2answers
566 views

Can I use guids / uuids as counters for key derivation?

I have a 512 bit master key, and 128 bit unique identifiers. I'm going to derive 256 bit keys to for use with deterministic and block ciphers. For reference, The KDF I'm going to use is Hash( C | Z)....
1
vote
1answer
358 views

Twofish key length

I'm using twofish. Say you have a 120 bit key, is there any difference in using a 128 bit bit keylength or a 256 bit one? Since the most significant digits of the key will be allo zero in both cases, ...
2
votes
3answers
789 views

RSA finding the inverse of the public exponent

I have a very basic doubt in RSA key generation and its usage. In RSA key generation you choose two large prime numbers of a very large order. Then you multiply them.(eq $p \cdot q = N$) Now, $\phi(N)...
9
votes
4answers
1k views

Predicting PRNG given some of its previous output

I a have a question about PRNGs and this is my very first experience with them. I have the following generator that takes a 56-bit seed $p$ during initialization and then chooses both $X$ and $Y$ ...
4
votes
1answer
1k views

Efficient and stateless anti-forgery method

One use of an anti-forgery token is to prevent Cross-Site Request Forgery (CSRF) attacks. The attacker doesn't need to sniff the wire in order to carry out a CSRF attack. This attack relays on the ...
3
votes
1answer
639 views

How are the primes used to generate RSA keys?

I am confused about how keys in RSA asymmetric encryption are generated and what the implications for open communications are. Textbooks say the one-way function is merely two primes (with some ...
5
votes
5answers
2k views

Crypto-Compression Algorithms?

I was wondering, whether there exist such algorithms/enciphering procedures which both compress and encrypt the input data. That means, for starters, the output will be both smaller in size and ...
7
votes
1answer
311 views

Is there any known malleability of scrypt outputs?

The scenario is single-use passphrase-based non-interactive message authentication. The obvious try to do this is to chose a random salt of the appropriate length and send: ...
12
votes
4answers
3k views

Why is the IV passed in the clear when it can be easily encrypted?

The initialization vector (IV) is exclusive or'd against the plain text before encryption for the first block sent in order to prevent an attacker from learning that duplicate message blocks are being ...
5
votes
4answers
927 views

Encryption algorithm that produces dummy output on incorrect passwords

Background: I've been thinking about using encryption in the context of backing up files to untrusted locations (to the point of making the file publicly and widely distributed for practically ...
2
votes
2answers
2k views

Symmetric vs. Asymmetric cryptographic approaches to data security

I know the basic differences between Symmetric vs. Asymmetric cryptography, but I'd love to know more details: Exactly why is the asymmetric approach slower than the symmetric? Why does it make ...
15
votes
3answers
1k views

Can I use a one time pad key twice with random plaintext?

I understand the basics of OTP: $|\text{key space}| = |\text{plaintext space}|$ implies perfect security, key reuse destroys this. Cryptanalysis on the $N$-Time Pad for $N > 1$ involves finding ...
4
votes
2answers
1k views

ECC algorithm pollard's $\rho$ complexity

One of the methods to break a ECDLP is Pollard's rho algorithm. When ECDLP is defined over a finite field $F_p$, and given a relation $S=w.T$, where S and T are a member of $F_p$. Then ECDLP is to ...
2
votes
2answers
220 views

Can the premaster secret generated by SRP be used as a secure private key?

It seems like the pre-master secret generated during the SRP protocol would make a good source to generate a shared private key using a secure hash to compress it down into a 128/256 symmetric key. ...
5
votes
1answer
2k views

Trouble with diffie-hellman groups

Is anyone able to point me towards a known-to-be-good diffie-hellman group? I currently use group 24 (RFC5114, 2048-bit MODP Group with 256-bit Prime Order Subgroup). It came to me that this group is ...
2
votes
1answer
280 views

Cryptographically strong pseudo-random seq. generators

If a pseudo-random sequence generator is built so that it uses an n-bit seed and outputs a string that is of length x. Let's say one wants to generate a bit string of length y and uses the previous ...
28
votes
4answers
112k views

How secure is AES-256?

The cipher AES-256 is used among other places in SSL/TLS across the Internet. It's considered among the top ciphers. In theory it's not crackable since the combinations of keys are massive. Although ...
38
votes
3answers
35k views

How does one attack a two-time pad (i.e. one time pad with key reuse)?

My question might appear the same as the question Taking advantage of one-time pad key reuse?, but actually I did read all the answers and none of them helped me with the details I need. I am new to ...
0
votes
1answer
350 views

Implementing Paillier Signature Scheme in Delphi

I've been trying to implement the Paillier Signature Scheme in Delphi, but I can't get it to work and I don't know where the problem is. First of all, I got my info about the scheme from this paper. ...
8
votes
1answer
4k views

Cryptographic security of PHP mt_rand() function using Mersenne Twister algo

At stackoverflow this question has been asked. It uses additional random entropy and a hash method (among others) to try and create a cryptographically secure pseudo-random number generator for PHP. ...
4
votes
2answers
396 views

What challenge should I use in a challenge-response proof-of-work?

In order to guard against denial-of-service attacks, I want to require clients to do some work (more work than the server does fulfilling the request) before talking to them. Client connects Server ...
6
votes
1answer
1k views

Do Rabin Fingerprints have any advantages over CRC?

Background In both, bitstrings are interpreted as a polnomical over GF(2) and they each can be used to implement a hash over a sliding window. The definitions of each are as follows: Rabin ...
5
votes
2answers
1k views

Does AES specify error messages when using the wrong password to decrypt?

I am writing a small program which uses AES. In testing it with wrong passwords, I get error prompts from Microsoft C# component saying "the padding is bad"; whereas I expect wrongly decoded texts. Do ...
2
votes
1answer
1k views

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 ...
21
votes
2answers
20k views

What are the advantages of TOTP over HOTP?

HMAC-based One Time Password (HOTP) was published as an informational IETF RFC 4226 in December 2005. In May, 2011, Time-based One-time Password Algorithm (TOTP) officially became RFC 6238. What ...
2
votes
2answers
426 views

Does AES with whitening mode have the following property?

This mode is that each time a random number $K$ is generated, you XOR it with plaintext $M$ and then pass the result to AES. Since AES is a pseudo random permutation, does AES have the following ...
13
votes
5answers
1k views

Can you make a hash out of a stream cipher?

A comment on another question made me wonder about something: Assume you're on a rather constrained platform — say, a low-end embedded device — with no built-in crypto capabilities, ...
5
votes
1answer
505 views

Are there any Implementations of Secure Indexes? [closed]

There are several papers about Secure Indexes, for Example "Searchable Symmetric Encryption: Improved Definitions and Efficient Constructions" and "Secure Indexes". ...
2
votes
1answer
80 views

Rapidly changing passwords?

Suppose you need to authenticate yourself to a program with the password - but the program's source code is public, the program doesn't have access to any private information and all your ...
6
votes
1answer
459 views

Can a user of a password-protected Wi-Fi sniff on other user's communication?

Imagine a Wi-Fi network protected by WEP/WPA/WPA2. Users can join the network in case they know the pre-shared key of the access point. Is it possible for a user A (who has successfully logged into ...
2
votes
0answers
443 views

Openssl implementation of improved sha1 hashing [closed]

I want to use the improved sha1 hashing algorithm from the Intel blog described here Intel sha1 hashing. Currently I am using Openssl sha1 hashing in my application. I wanted to know if anybody has ...
10
votes
1answer
477 views

Making ECDSA public keys one bit shorter

In compressed form, ECDSA public keys are one bit larger than the curve size. There are some situations where this extra bit is extremely inelegant. So I had the following idea as a way to remove it. ...
7
votes
3answers
1k views

How to authenticate over open channel?

I am making an arduino project to open my garage door. I want to make it so users with the passphrase can open the door. Due to computational restrictions it is unlikely encryption is possible. ...
8
votes
3answers
1k views

Is RSA padding needed for single recipient, one-time, unique random message?

I want a way to encrypt files using this process: http://crypto.stackexchange.com/a/15 . That is: generate a random password, use that to AES-encrypt a file, and use an RSA public key to encrypt the ...
1
vote
1answer
378 views

ZKIP for Paillier public key correctness

I'm using the Paillier cryptosystem in a protocol similar to mental poker. In the beginning of the protocol, each player generates a Paillier public key $(n,g)$. Later in the protocol, a player may ...
1
vote
1answer
363 views

Which encodings have |encoding key| >> |decoding key|?

I'm looking for an encoding scheme that requires a very large encoding key E (>10MB) and suffices with a relatively small decoding key ...
8
votes
3answers
1k 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 ...
12
votes
4answers
2k views

Is compressing data prior to encryption necessary to reduce plaintext redundancy?

As explained in William Stallings' Book, in PGP encryption is done after compression, since it reduces redundancy. I couldn't relate encryption strength with redundancy. Could anyone explain more on ...
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 ...

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