4
votes
3answers
268 views

SIM security for two messages

Here SIM means the simulation based security Consider a two message encryption scheme:$$Enc:K \times M \times M \rightarrow C \times C$$ and $Enc(K, m, m')=(K \oplus m, K \oplus m')$, In ...
11
votes
3answers
2k views

At the current time, is SHA256 the de facto standard for strong cryptographic hashes?

At the current time, is SHA256 the de facto standard for strong cryptographic hashes? From what I am seeing of more sites utilizing it, I would suppose the answer to this is yes, but would like to ...
2
votes
1answer
163 views

Signature scheme with two private keys, neither derivable from the other

Is there any scheme, ideally one widely used or at least widely available, where you can treat both the signing and verifying keys as secret? Basically, the functionality I'm looking for is this: ...
1
vote
2answers
2k views
3
votes
1answer
216 views

A set of key pairs and one hash to secure them

I have a simple problem: I have a set of users' ECDSA key pairs, and say I want to encrypt them with a simple algorithm. I have access to one variable that uniquely identifies the user, so I hash it ...
3
votes
2answers
304 views

If Bob steals Alice's private key, how exactly would he read her encrypted documents?

So Bob grabs Alice's secret key when she isn't looking and her encrypted files, doesn't he need to know her passphrase to read her files? What I am reading is that no he does not need it but as far ...
17
votes
3answers
2k views

Hashing or encrypting twice to increase security?

Over on the bitcoin forums I asked why the bitcoin client computes SHA-256(SHA-256(x)) as its cryptographic hash for a variety of purposes. The leading theory--since the bitcoin author has ...
7
votes
1answer
342 views

How exactly would someone attempt to analyse ciphertext produced by popular encryption products such as Truecrypt/PGP?

I am interested in understanding what the process would be if an attacker wished to attempt to decrypt data secured by common tools such as OpenPGP, Truecrypt or the like. Are there any documented ...
2
votes
3answers
1k views

How does one scale encryption strength upwards from 256-bit?

I have seen many examples of encryption up to about 256-bit. But how does one programmatically scale the logic upwards in a language such as PHP or Java to say 1024-bit or even 4096-bit and higher? ...
9
votes
1answer
332 views

How did the Koblitz/Menezes papers affect the cryptography community?

Two highly-critical papers by Koblitz and Menezes (two well-regarded mathematicians who've contributed to the crypto community) were published years ago: Another Look at “Provable Security” in 2004, ...
10
votes
3answers
334 views

Is it possible to create an asymmetric cryptosystem where the private keys are not easily verifiable as such?

Plaintext that consists of an RSA key is easily recognizable as such, because it satisfies certain mathematical properties, in particular (See the answer for Why can an encrypted private key be brute ...
11
votes
2answers
508 views

Why use a 1-2 Oblivious Transfer instead of a 1 out of n Oblivious Transfer?

When initiating an oblivious transfer, why would someone use a 1-2 oblivious transfer rather than going for an 1 out of n oblivious transfer? Perhaps a slight time overhead for the extra message ...
15
votes
4answers
1k views

Can an Enigma-style cipher of sufficient complexity be considered secure in today's world?

Regarding the German Enigma machines, if I recall correctly, the reason they were defeated was because the Allies were able to generate a massive database of possible rotor settings, and because the ...
4
votes
1answer
979 views

Need an introduction to SPKI (or “SPKI for dummies”)

I am familiar with concepts such as the following: PKI and CAs ID based cryptography Certificateless cryptography PGP web of trust more or less anything in basic Crypto that is taught in a college ...
6
votes
2answers
1k views

Can a computationally unbounded adversary break any public-key encryption scheme?

Assume there is a public-key encryption scheme $(KeyGen, Enc, Dec)$ with perfect correctness (i.e., for all messages M and valid key-pairs (PK,SK), we have $Dec_{SK}(Enc_{PK}(M))=M$). Will there ...
4
votes
4answers
1k views

Webapp password storage: Salting a hash vs multiple hashes?

For security's sake, of course it's blasphemous to store passwords in plain-text; using a hash function and then doing a re-hash and comparison is considered much better. But, if bad guys steal your ...
7
votes
1answer
730 views

Implementation of Tao Xie and Denguo Feng's MD5 attack

It is well known that MD5 is completely broken today - however, to understand the theory behind the attacks I am looking for an implementation of the collision attacks described in the 2009 paper A ...
5
votes
1answer
530 views

What is the best method to determine the language used in a monoalphabetic substitution cipher?

Working on a cipher (which I assume to be a mono-alphabetic substitution cipher due to the letter frequency) I struggle with the fact that I don't know which language the plain text is written in. ...
7
votes
3answers
858 views

Converting a stream cipher into a block cipher

The well-known Counter-Mode (CTR) mode of operation for a block cipher essentially converts any block cipher into a stream cipher. Is there a way to do the reverse? In other words, given a "good" ...
23
votes
2answers
2k views

How do I apply differential cryptanalysis to a block cipher?

I've read a lot of summaries of block ciphers particularly with regards to the NIST competitions stating that reduced-round block ciphers are, for example, vulnerable to differential cryptanalysis. I ...
3
votes
2answers
204 views

Background for modular arithmetic function

I'm investigating this function: $a := ((b\cdot c) \bmod k) - (b \cdot c)/k$ where $/$ indicates integer division. Two things I've noticed: It's equivalent to multiplying a·b, and then ...
6
votes
2answers
346 views

Analysis of Repeatedly Enciphered Plaintext using Same Algorithm / Key

Please forgive the impracticality of this question, but I'm curious about the behaviors of encryption algorithms applied to their own output. Suppose I have an encryption algorithm E and (using the ...
10
votes
3answers
811 views

UMAC: to what extent is it in use today?

Inspired slightly by the Encrypt-then-MAC question. The most obvious message authentication code is probably HMAC or RFC 2104 which is basically a hash of the input, an xor with a key... you get the ...
4
votes
1answer
172 views

Is a changing public truecrypt container secure?

I have a Truecrypt container which I want to synchronise between computers (i.e. different people, that I want to share the data with). If I used Dropbox for synchronisation and someone downloaded ...
4
votes
1answer
307 views

How is the MJH double-block-length hash function constructed?

I'm looking for information on the MJH double-block-length hash function, but the best free source I could find is the diagram on page 18 of Efficient hashing using the AES Instruction Set (submitted ...
11
votes
5answers
720 views

How do I construct a 256-bit hash function from 128-bit AES?

I would like to generate a 256-bit hash on a microcontroller that has a 128-bit (only) AES engine. How can I construct a 256-bit hash function from a 128-bit cipher?
21
votes
4answers
16k views

Basic explanation of Elliptic Curve Cryptography?

I have been studying Elliptic Curve Cryptography as part of a course based on the book Cryptography and Network Security. The text for provides an excellent theoretical definition of the algorithm but ...
3
votes
1answer
953 views

Digital Signature Algorithm signature creation

I was studying DSS from "Cryptography and Network Security" by William Stallings. What puzzled me was the DSS approach figure described in the text. It says it uses Public and Private Keys for ...
6
votes
3answers
832 views

ANSI X9.31: the purpose of the date/time vector in the PRNG?

What is the exact purpose of the date/time vector $dt$ in the ANSI X9.31 PRNG? $$ I := E_K(dt) $$ $$ R := E_K(I \oplus V_{old}) $$ $$ V_{new} := E_K(R \oplus I) $$ Specifically, the document seems ...
6
votes
3answers
3k views

SHA1 usage for passwords, alternatives and advantages?

My application can authenticate via openid and oauth (facebook, twitter, etc) and also with its own authentication system. I previously switched hashing from MD5 to SHA1 and during migration I had to ...
3
votes
1answer
671 views

KeeLoq showing that decryption is indeed the inverse of encryption

In some text I am reading, there is an exercise asking to show that KeeLoq decryption function is the inverse of the encryption function. Details about KeeLoq are given in the Wikipedia article. As I ...
2
votes
1answer
142 views

How to collect, process, and transmit data securely?

In my question "Authenticating data generated by a particular build of an open source program", Dave Cary requested that I post a question stating my real problem on a high level rather than the ...
5
votes
1answer
275 views

If a cryptanalytic breakthrough is made, what process should be followed?

If a researcher manages to make a cryptanalytic breakthrough on a cryptographic algorithm or protocol that is in use, what should they do? Has this ever happened before? What are the implications for ...
6
votes
4answers
515 views

Does MD5 generate 128 independent bits?

I heard that there are 128 stochastically independent bits in an MD5 output. Is that true? If so, are there any citations or proofs for that?
5
votes
2answers
210 views

How are constructs with data-dependent swaps and rotations cryptanalyzed?

Linear and differential cryptanalysis seem well suited for constructs with a (relatively) simple fixed structure of boolean expressions. But some ciphers incorporate swaps of array elements where the ...
2
votes
3answers
1k views

Dictionary attack on pass-phrases on common algorithms

I don't yet perfectly understand the difference between brute-force and dictionary attack since this differentiates one attacking the key and another attacking password: apparently attacking passwords ...
15
votes
2answers
1k views

Is HTTPS secure if someone snoops the initial handshake?

Let's say I'm on an open wireless network that's being actively sniffed and I connect to an HTTPS site. Even though my subsequent traffic is encrypted, couldn't the sniffer use the data from the ...
13
votes
3answers
2k views

Known methods for constant time (table-free) AES implementation using 'standard' operations?

There are several known methods for implementing AES in constant time using SIMD operations, mostly based around fast byte shuffling (for instance Hamburg and Kasper/Schwabe). Are there any similar ...
5
votes
1answer
2k views

Any good file format alternative to PGP for encrypting data at rest?

I'd like to create an encrypted file that: Can be decrypted on a variety of platforms (mainly Windows, OS X, and Linux) with knowledge of the key. Can be decrypted with existing tools (perhaps ...
5
votes
1answer
153 views

Proof that Alternating Step Generator and modifed ASG' have equivalent security?

The Alternating Step Generator (ASG) is a PRNG combining 3 LFSRs. Output of the ASG is the XOR of the output of two clock-gated LFSRs. At each step, a single one of these LFSRs is clocked, according ...
8
votes
3answers
1k views

How can my application make sure the right symmetric key is used for decryption?

I am developing an Encryption/Decryption Utility using Adobe AIR. I have chosen the AES algorithm and currently use the as3crypto library for this. My query is in ...
14
votes
1answer
4k views

What is a white-box implementation of a cryptographic algorithm?

What is a white-box implementation? Does a white-box implementation have specific properties?
8
votes
2answers
1k views

How should I store passwords that need to be available in plain text?

Suppose I need to store login information for a third-party website for a few users, how would I go about doing it? Since I am logging into a third party website, I need the password in plain-text, ...
12
votes
3answers
678 views

Where do I securely store the key for a system where the source is visible?

I have a customer with an Access database (ugh!) in which credit cards are stored in plaintext (yikes!), so amongst other changes I'm doing in the app, I'm applying some encryption in there. I've ...
10
votes
4answers
1k views

Properties of PRNG / Hashes

There are a lot of quite elaborate PRNG's out there (e.g. Mersenne Twister et.al.), and they have some important properties, especially when it comes to crypto applications. So, I was wondering how ...
17
votes
3answers
3k views

Reverse engineering a hash?

I understand this may not be the best place to ask a question like this, but I believe that this community may be the best/only place I can ask such a question. I have inputs and outputs from an ...
8
votes
5answers
317 views

Reduction from signatures to encryption?

Is it possible to construct an (asymmetric) encryption scheme from a signature scheme? If the signature scheme is deterministic and allows existential forgery (e.g. RSA), then the answer is yes ...
1
vote
1answer
721 views

iSeries (AS/400) Database File: password encryption

I am helping with a project in which an old software system on an iSeries is having a brand new .NET UI applied to it. It's going well... except... In order to allow users to login and maintain ...
10
votes
2answers
535 views

Why is there an enormous difference between SAT solvers?

SAT solvers are very important in algebraic attacks, for example walksat and minisat. However, when solving the benchmark problems available here there is an enormous performance difference between ...
6
votes
1answer
158 views

Are there any tools for expressing the cipher operations as a system of equations?

The first step In Algebraic-attack is expressing the cipher operations as a system of equations. Is there an automated tool to do this?

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