11
votes
4answers
332 views

Do parts of a hash carry the properties of the entire hash?

When I need to generate unique id's based on some information hashing is typical choice. However, sometimes that id needs to be of a particular size. I've seen a lot of schemes (HMAC-MD5-96 in SSH, ...
11
votes
2answers
1k views

How does the MOV attack work?

What exactly is the MOV attack, how does it actually work, and what is it used for? It's explained briefly here and I'd like to know what it is more / what is it fully used for.
10
votes
1answer
8k views

HMAC vs MAC functions

I've read definitions of MAC and HMAC, but can't say I've completely grasped the differences. What are principle differences? When to use one and when the other?(Typical Use Cases)
10
votes
2answers
792 views

Which blind signature schemes exist, and how do they compare?

I'm looking into blind signature schemes for use as digital cash. I have come across blinded RSA, and Lucre(DH based). Are there other schemes available, and how do they compare? I suspect there ...
10
votes
5answers
349 views

Tactics available to help prove security of a new system?

I believe that the accepted tactic to "prove" a system as secure is to allow the crypto-community to review it and if no vulnerabilities are found over a long period of time (5 or 6 years), then a new ...
9
votes
2answers
266 views

Definition of “pepper” in hash functions

I am confused about the notion of "pepper" in the context of storing hashes of users' passwords. Definition 1: A pepper is a secret key Looking around the Internet, for example here or here, a ...
9
votes
1answer
550 views

How to build an electro-mechanical public key cipher machine?

It is generally assumed that asymmetric encryption schemes were invented in 1973 at GCHQ in Britain and, independently, in 1976 at the MIT. Imagine, if the abstract idea of having a public key and a ...
9
votes
1answer
556 views

What does “running in polynomial time” really mean?

I'm currently learning private-key cryptography. I've been able to see that perfect secrecy is achievable if no assumption is made about the computational power of the attacker. However, perfect ...
9
votes
2answers
5k views

Are there any known collisions for the SHA-2 family of hash functions?

Are there any known collisions for the hash functions SHA-1, SHA-224, SHA-256, SHA-384, and SHA-512? By that, I mean are there known values of $a$ and $b$ where $F(a) = F(b)$ and $a ≠ b$?
9
votes
2answers
1k views

Salting when encrypting?

I was attending a database encryption session at a developers conference. The presenter (who was a published author on the subject) said that MS SQL Server did not support salted hashes in the ...
8
votes
1answer
870 views

Homomorphic (encrypted) comparison to an integer

When working with an additive homomorphic encryption scheme (say Pallier's), is there an efficient way to get the encrypted value of a comparison test to an integer value (I realise that an ...
8
votes
3answers
369 views

Encrypting files with known headers

If I encrypt a file of a known format that has a lengthy header (e.g. an XML Excel file), does that render the encrypted file susceptible to a "known plain text" attack? In other words, if the first ...
8
votes
2answers
1k views

Can one reduce the size of ECDSA-like signatures?

Using $n$-bit ECDSA, a signature has a size of $2·n$. It is possible to recover the public key from this signature, which shows that there is a publicly visible redundancy in the signature. Is ...
8
votes
4answers
827 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 $Y$ ...
8
votes
1answer
921 views

AES and perfect ciphers

I'm taking a crypto class this semester and after learning the definition of a perfect cipher. I started wondering how this definition applies to AES. Obviously AES isn't a perfect cipher, since the ...
8
votes
3answers
2k views

How to attack a classical cipher using known partial plaintext?

I have a ciphertext generated by a classical cipher. I do not know what was cipher used to generate it. I do however have the beginning of the plaintext. What are the cryptanalysis approaches for ...
7
votes
2answers
498 views

Would RSA make sense if we used no computers?

I was recently wondering - would RSA be useful if we brought it to, say, medieval times? Could you choose the keys so that you could encrypt/decrypt messages quite easily, but factoring the private ...
7
votes
1answer
449 views

Why are cryptography algorithms not exported to certain countries?

There have been strict laws about the export of crypto software to certain countries. I can understand the intent, but never gauged the fact that there is nothing stopping these countries from ...
7
votes
3answers
397 views

Testing the Fortuna random number generator?

I have implemented the Fortuna random number generator as described in chapter 10 of "Practical Cryptography" (Ferguson and Schneier, Wiley, 2003), the result can be found at ...
7
votes
2answers
741 views

Is FIPS 140-2's “Continuous random number generator test” practical?

Section 4.9.2 of FIPS PUB 140-2 specifies, amongst other things, a "Continuous random number generator test." Here are the relavant bits: If each call to a[n] RNG produces blocks of n bits ...
7
votes
1answer
2k views

Generating Random Primes

Although this has been extensively discussed around here, I'm curious whether my approach makes sense, or I should just stick to "the standard version". I'm implementing some homomorphic encryption ...
7
votes
2answers
704 views

Is ISAAC considered secure as a cryptographic random number generator?

The ISAAC site claims it is secure, but I wasn't aware of any trusted 3rd party sources that verified its security.
7
votes
3answers
5k views

Is Blowfish strong enough for VPN encryption?

I'm looking at an OpenVPN connection between two sites configured to use 128 bit Blowfish in CBC mode, and trying to figure out how to assess the strength, but I just don't know enough of the maths. ...
6
votes
1answer
948 views

Does collision resistance imply (or not) second-preimage resistance?

I've seen contradictory results. Sometimes hash functions are collision-resistant but not necessarily second-preimage resistant. I've seen this kind of things in papers from Bart Preneel: “Security ...
6
votes
2answers
226 views

Fast hashing into elliptic curve

Is there a fast algorithm for mapping $n$-bit numbers $s$ (for fixed $n$) into a cyclic subgroup of an elliptic curve (over a finite field) in which the Discrete Logarithm Problem is hard? By fast, I ...
6
votes
2answers
198 views

Does there exist a proof-of-retrievability scheme that is publicly-verifiable, limited-use, and does not use homomorphic encryption?

I find myself wanting to test out a practical implementation of a proof-of-retrievability scheme, simply out of curiosity. These schemes seem to be divided into two variations, publicly-verified and ...
6
votes
3answers
335 views

Deterministically combine more than one source of entropy

What is the proper/canonical way to do this? For example, $0 < r1 < 1$ and $0 < r2 < 1$. Presuming uniformly distributed probabilities for the two, combining/averaging them is going to ...
6
votes
3answers
435 views

Slow one-way pseudo-random permutation?

I'm looking for a slow one-way pseudo-random permutation; or in other words a block cipher $E_K: P\in\{0,1\}^b\mapsto C\in\{0,1\}^b$ with moderate block size $b\approx 64$ bits, wide key $K$, ...
6
votes
1answer
596 views

Why does the crypto_box functionality in NaCl library exposes the nonce to the programmer

The idea of crypto_box API in NaCl library is to shield the programmer away from the technical details and provide an easy to use functions for encrypting and encrypting messages. Given what I've ...
6
votes
2answers
2k views

Will our app be FIPS 140-2 compliant if we use our own AES algorithm implementation?

We are in the processing to understand if our software applications is FIPS 140-2 compliant or not. Currently in our application, we are using our own implementation of AES algorithm. AES is a FIPS ...
6
votes
2answers
2k views

Is there a hash function which has no collisions?

Is there a hash function which has no collisions? To clarify: it would be some function which would produce variable-length output, and never produce the same output for differing input. It would ...
6
votes
2answers
6k views

What is the difference between MAC and HMAC?

In reference to this question, what are the "stronger security properties" that HMAC provides over MAC. I got that MAC requires an IV whereas HMAC doesn't. I also understood that MAC may reveal ...
6
votes
1answer
350 views

Why doesn't CTR mode require blocking?

I've been reading a bit about block cipher modes and I have a relatively straightforward question regarding CTR. In essence, I was hoping you guys would be kind enough to validate my understanding of ...
6
votes
1answer
241 views

Finding where I am in a linear recurrence relation

Suppose I have a linear recurrence relation $$a(n) = c_1 a(n-1) + \dots + c_k a(n-k) + d,$$ where the constants $c_1,\dots,c_k,d$ are given and the initial values $a(0),\dots,a(k-1)$ are given as ...
6
votes
1answer
392 views

Does TLS use RC4-drop[n]?

According to many sources, the first few (n) bytes of the RC4 keystream are strongly biased, and therefore should be discarded before using the keystream to encrypt anything; this precaution is ...
6
votes
3answers
610 views

Security equivalence proofs for breaking RSA

It is my understanding that while a practical solution to the factoring problem will definitely break RSA, it has never been proven that the security of RSA is equivalent to factoring. In otherwords, ...
6
votes
3answers
462 views

Is there some way to generate a non-predictable random number in a decentralised network?

Is there a way to generate a random number with given restrictions: It will be used in a decentralised network with a big number of peers (no central authority to generate it) Its generation should ...
6
votes
1answer
375 views

Threshold Signatures for RSA and DSA

I have a few questions relating to threshold signatures: a scheme where $n$ participants hold a key share and any $t$ of them can conduct a protocol using their shares that results in a valid RSA or ...
6
votes
2answers
1k views

New PRG from old

Is the following statement true: If $G: \{0,1\}^k \to \{0,1\}^n$ is a PRG, then so is $G':\{0,1\}^{k+l} \to \{0,1\}^{n+l}$ defined by $$G'(x||x')=G(x)||x'$$ where $x \in \{0,1\}^k$ and $x' \in ...
6
votes
4answers
189 views

Authenticating data generated by a particular build of an open source program

[I was torn between posting here or security.stackexchange.com. In the end, I felt that this was more of a design question, rather than an implementation question and so chose this forum.] My ...
5
votes
1answer
720 views

SHA1 no longer considered secure for SSL Certificates — what about Cipher Suites?

Many browsers and Internet companies have recently claimed that SSL Certificates with a signature algorithm of SHA1 will imminently no longer be considered secure. Most notably, Google and Google ...
5
votes
1answer
406 views

Is MD5 second-preimage resistant when used only on FIXED length messages?

I fully realize that MD5 should not be used in any new project, but in my particular situation I have severe CPU performance issues, so MD5 is convenient. I have read a lot about MD5 security for this ...
5
votes
1answer
667 views

Diffie-Hellman Parameter Check (when g = 2, must p mod 24 == 11?)

I'm adding some Diffie-Hellman groups to a program as specified in RFC 3526, More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE). When I test some of the group ...
5
votes
2answers
453 views

KCV and compatibility with block cipher modes of operation

There has been lately a question on KCV (key check value), value provided by many CRYPTOKI (PKCS#11) implementations. I don't particularly like KCV, but I decided to ask about proper use of KCV. ...
5
votes
1answer
231 views

Can two people with different one time pads securely exchange a message like this?

Alice has a message, generates a one time pad, encrypts her message and sends it to Bob. Bob generates his own one time pad, encrypts the message again, and sends it back to Alice. Alice then decrypts ...
5
votes
1answer
187 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) = ...
5
votes
1answer
480 views

Where could I submit my algorithm?

I have come up with an encryption algorithm and I am looking for a place to possibly publish it. Any ideas?
5
votes
1answer
776 views

What are the constraints for an IV using AES in CBC mode?

I'm designing a protocol for use into a VPN software. The VPN frames are encapsulated into AES-256 CBC encrypted frames. I understand that IVs must be uniquely used for each message encrypted with ...
5
votes
1answer
741 views

Should the secret key of Shamir's secret-sharing algorithm be interpreted byte by byte?

Should the secret message of Shamir's secret-sharing algorithm be interpreted and processed byte by byte? Interpreting it byte by byte makes it easier to process, but in case one of the shareholders ...
5
votes
2answers
342 views

Is SHA-1 still practical secure under specific scenarios?

It is conjectured that SHA-1 has been broken from the "research" perspective but no in real world. That is that there is an algebraic attack that explores weaknesses on its algrebraic construction. ...

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