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Why do we use a permutation table in the first step of DES algorithm and one at the end of the algorithm?
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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 encrypts, but with everything as fast as it is these days is that a concern?
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As we know, SHA-1 is irreversible, so why do we append the length of the message to the preimage?
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Since most cryptographic hash functions are simple, compact constructions does this simplicity impose a limit on the complexity and the size of a function that can generate preimages? That is, given a cryptographic hash function, H of some length and complexity can we lower or upper bound the complexity/size of a funct... |
RSA is not designed to be used on long blocks of plaintext like a block cipher, but I need to use it to send a large (encrypted) message.
How can I do this?
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It seems like the S-boxes in DES have essentially random values.
How were these chosen?
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Bitcoin addresses are RIPEMD-160 hashes of the public portion of a public/private ECDSA keypair (along with an abbreviated hash of the hash to provide a check code, as @pulpspy notes in a comment). They are generally base-58-encoded. See Address - Bitcoin
In Vanity bitcoin addresses: a new way to keep your CPU busy, ... |
I understand the theory behind the use salts in hash functions, but when I see it implemented, the implementations always generate the salt on the fly and the salt appears to be different for every hash.
When implementing a salt for users signing into a website, doesn't the salt need to be the same at account creati... |
AES has several different variants:
AES-128
AES-192
AES-256
But why would someone prefer use one over another?
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Using a cryptographic hash to store e.g. passwords in a database is considered good practice (as opposed to storing them plaintext), but is subject to attacks on said cryptographic hash, assuming the database is compromised.
Which properties of a cryptographic hash function make it suitable for hashing passwords and th... |
I have 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 have a general idea that the application of differential cryptanalysis is to look at the difference between inputs; ... |
What are the mainstream techniques borrowed from algebraic geometry (or some other branch of mathematics) which are currently used in cryptography/coding theory? I've only heard about a small subset of elliptic curves and hermitian curves. I've also heard about research of hyperelliptic curves but I don't know if some ... |
As I understand it, SSL involved the use of a public-private key pair. How does this enable two-way communication?
Suppose I have some server with which I wish to communicate securely. I connect to it, and it supplies me a certificate, which contains its public key (assume the certificate is valid). I then take my r... |
Given the desire to have unique salts for each user of your system, is it actually necessary to create a cryptographically-random salt for each user?
If your system already has some other unique user identifier (be it an ID, user-name, or an Email address) is there any effective decrease in your security if you simply ... |
What are the main weaknesses of a Playfair cipher, if any?
I know that they depend on none of the letters missing, but that is an easy fix if a letter gets dropped. Besides that, are there any other problems with it?
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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 database, but it does support salting with encryption. This was the first I heard of salting in encryption. I asked for clarifi... |
I have heard criticism of various cryptosystems saying that "at their heart, they were just XOR."
Is this just ignorance, or is there something inherently wrong with XOR based ciphers?
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A few years ago I devised a symmetric-key system that worked like so:
H() = some hashing function
h = the number of bits in the result of H()
key = bits to be used as a key
^ = the XOR operation
+ = concatenation
len() = the length function (in bits)
max = ceiling(len(cipertext) / h)
pad = pad_0 + ... ... |
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 approaches that allow a constant-time AES to be implemented using only standard C operations? The closest thing I've seen ... |
Suppose Alice wants to send encryptions (under a one-time pad) of $m_1$ and $m_2$ to Bob over a public channel. Alice and Bob have a shared key $k$; however, both messages are the same length as the key $k$. Since Alice is extraordinary lazy (and doesn't know about stream ciphers), she decides to just reuse the key.
... |
I think I once faced the recommendation, that the initialization vector should always be random and never be used twice with the same key.
How serious is this weakness?
Also, is AES less effected than DES?
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A more secure form of 'cookie' could be created for SSL communications through the following method.
The client generates and requests the server to sign a certificate.
Then the client authenticates to the https server using the client certificate.
Additional basic auth is used to tie the client certificate to the sit... |
What is the currently industry-standard algorithm used to generate large prime numbers to be used in RSA encryption?
I'm aware that I can find any number of articles on the Internet that explain how the RSA algorithm works to encrypt and decrypt messages, but I can't seem to find any article that explains the algorithm... |
According to wikipedia…
DES is now considered to be insecure for many applications
However, since it can be broken relatively easy, should it not just be avoided for all applications?
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I'm wondering if there have been any recent advances (say, the past 5-10 years) in human usability for cryptography and/or authentication?
By that I mean something that makes it easier for an average person to make use of the benefits of cryptography. It strikes me that although we have all these great algorithms for ... |
The Rijndael S-Box design generates a permutation cycle of type $2+27+59+81+87$. What effect would replacing that permutation with a cycle of type $256$ have on the security of AES?
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Just to establish notation with respect to the RSA protocol, let $n = pq$ be the product of two large primes and let $e$ and $d$ be the public and private exponents, respectively ($e$ is the inverse of $d \bmod \varphi(n)$). Given a plaintext message $m$, we obtain the ciphertext $c = m^e \bmod n$; we subsequently dec... |
As we know, one should use a slow password hashing algorithm instead of a fast one for storing passwords, to hinder brute force attacks when the database is compromised. The problem with this is that for every login we also need to run this slow password hashing algorithm, which for remote logins could be a considerate... |
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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Does anyone know of any sites where intermediate values of multiple crypto algorithms (not just the famous ones), including hashes, can be found? I don't just mean intermediate values of encrypting. I'm also looking for intermediate values of the key generation algorithms as well. Hopefully, the initial values can be c... |
NIST removed "The Lempel-Ziv Compression" test from the Statistical Test Suite in revision 2008 and above and has not incorporated it since – see revision 2010.
Why was it removed? Does it no longer provide sufficient testing of a PRNG or was it simply superseded by better tests?
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A recent paper showed that the McEliece cryptosystem is not, unlike RSA and other cryptosystems, weakened as drastically by quantum computing because strong Fourier sampling cannot solve the hidden subgroup problem.
However, in 2008, new parameters were proposed after the old ones were found to be vulnerable.
So my que... |
The definition of PBKDF2 states that I obtain a derived key (1) by calling a pseudorandom function a bunch of times recursively:
$U_1 = PRF(password, salt)$
$U_2 = PRF(password, U_1)$
…
$U_n = PRF(password, U_n-1)$
The standard then defines the actual key material K as
$K = F(password, salt, n)$
where F is $U_1 \op... |
Is it possible to identify the encryption method, or at least rule out some of them, by looking at the encrypted text?
For example, if you have 3 encrypted strings where the first 10 characters are the same on each of the strings.
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bcrypt is an intentionally slow hash algorithm. In my last protocol idea, I wanted to use it to expand a password and then only transfer the bcrypt-hashed password.
An efficient attack on this would be an ability to iterate all bcrypt hashes (or only these from passwords in a dictionary, maybe $2^{12}$ ones) without ac... |
I have created an application that will be able to read any file and encrypt it using AES Encryption. For efficiency, I am reading a block of data, encrypting it and so on. So for decrypting, I just have to read each block and decrypt using the same key to get the data back. This was what I had in mind.
But it turns ou... |
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 (create a forgery (m, sig); encrypt plaintexts using AES with sig as the key; send m in clear to the signer along with ciphert... |
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 the two - Walksat is much faster than minisat for these problems. Why is this?
This implementation of walksat appears to h... |
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 forced?):
$$d = e^{-1} \mod (p-1)(q-1).$$
Is it impossible to design a public key system where it was harder to verify ... |
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 contain the most entropy?
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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 system can be accepted as secure until proven otherwise. Please correct me if I'm wrong.
That aside, are there any set ... |
In the computer security class (in which cryptography is a big chapter) that I took, I remembered the professor said about current asymmetric cryptography algorithms are based on integer factorization (i.e. prime numbers) and discrete logarithm.
So my question is, are there any asymmetric cryptographic algorithms that ... |
Most of the time, when some data must be encrypted, it must also be protected with a MAC, because encryption protects only against passive attackers. There are some nifty encryption modes which include a MAC (EAX, GCM...) but let's assume that we are doing old-style crypto, so we have a standalone encryption method (e.... |
Related to "Is it possible to derive the encryption method from encrypted text?".
Given ciphertexts generated by any of the major asymmetric ciphers (RSA, ElGamal, ECC, etc..) can these ciphertexts be distinguished from random noise? Justify why, why not?
That is do the major asymmetric ciphers have any statical bias... |
Can someone tell me which mode out of ECB and CBC is better, and how to decide which mode to use? Are there any other modes which are better?
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"Normal" discrete logarithm based cryptosystems (DSA, Diffie-Hellman, ElGamal) work in the finite field of integers modulo a big prime $p$. However, there exist other finite fields out there, in particular binary fields $GF(2^n)$. There is a specific attack described by Coppersmith for discrete logarithm in a binary fi... |
I am aware that MD5 has a known collision vulnerability and should not be relied upon when uniqueness is required, but in the environment I am working on I only have access to MD5 hash function. Background detail below, but in general how much and how bad is the change in the probability of hash collision if I removed ... |
What is a white-box implementation?
Does a white-box implementation have specific properties?
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Generally, algorithms based on discrete logarithm specify that private keys are chosen as scalars between 1 and the order of the group (denoted $q$ here). For instance IEEE P1363 and FIPS 186-3 both specify this range for DL private keys. Is it a problem if the key is not uniformly chosen from this range?
One possibili... |
So according to Wikipedia unbalanced feistel ciphers provide greater provable security. Specifically, they state:
The Thorp shuffle is an extreme case of an unbalanced Feistel cipher in which one side is a single bit. This has better provable security than a balanced Feistel cipher but requires more rounds.
Which is... |
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 idea.
However, I've also discovered UMAC which sounds very much like a hash function/lookup table to select an appropriat... |
I understand my group theory (allegedly), so I can make partial sense of The Hidden Subgroup problem:
Given a group $G$, a subgroup $H \leq G$, and a set $X$, we say a function $f : G \Rightarrow X$ separates cosets of $H$ if for all $g_1, g_2 \in G$, $f(g_1) = f(g_2)$ if and only if $g_1H = g_2H$.
Hidden subgroup pr... |
Could you give me some reference to start on Hermitian Curves. Some papers or textbooks would be perfect, and please mention if it's math inclined or comp.sci. inclined.
I've only seen hermitian curves in coding theory but if you know of applications of these curves to cryptography please let me know.
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So we've already had a question on replacing the Rijndael S-Box. My question is - can we use a different finite field other than the one given by $x^8 + x^4 + x^3 + x + 1$ in $GF(2^8)$. In other words, would any irreducible polynomial over this field do the trick or are the special considerations and properties of that... |
So I've often looked at serpent and thought it was a very strong contender in AES. Not so long ago I was looking for evidence as to why it didn't beat rijndael. So far, the closest I've got answering that is this:
The 32 rounds means that Serpent has a higher security margin than Rijndael; however, Rijndael with 10 ro... |
Is there a point where increasing the complexity of an encryption algorithm will make it easier to break using a timing attack?
Or is there no connection here at all?
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About X.509 certificates serial numbers the RFC 5280 says:
The serial number MUST be a positive integer assigned by the CA to each certificate. It MUST be unique for each certificate issued by a given CA (i.e., the issuer name and serial number identify a unique certificate). CAs MUST force the serialNumber to be a ... |
Suppose I had a message in Chinese (or another non-phonetic language) and I wanted to encipher it.
Some of the simplest encryptions in English are substitution ciphers, but such ciphers don't seem to be a viable option for a language such as chinese (it's more similar to a code...). What would be a simple cipher for s... |
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 what practical ways can I detect if the password provided at the time of decryption valid or not.
As of now I thought of encrypting a constant string with th... |
Lets say that I am using 128 bit HMAC. How many operations are needed to find a "non secure" message. Is a birthday attack possible?
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Assume I want to design a protocol (or data format or similar) including some cryptographic hash, and want it to be as future-proof as possible, i.e. I want to avoid that breakthroughs in cryptography make my old data insecure.
If I use only one hash algorithm, I can have bad luck to pick just the one which will be bro... |
I googled it, but got no result. There is a binary executable of a white-box implementation of DES (scroll down), but no source code is provided.
The same page also links various papers about white-box cryptography, but these don't contain the implementation.
I want to study such an implementation to learn about the t... |
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 hash functions like SHA1 or MD5 would perform in such a scenario / compare to actual PRNG's. For instance, one could use t... |
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 day key was encoded twice in the beginning of each message.
Given that computers of today can simulate an Enigma-ish mach... |
If I'm hashing alphanumeric strings (chars in the set 0-9, a-z, and A-Z) with a CRC-64 hashing function, how long can my strings be while guaranteeing no hash collisions?
Stated differently: If I have a set of all CRC-64 checksums computed from alphanumeric strings $0$ to $N$ bytes, how large can $N$ be before I see a... |
I have an ID-Secret scheme and I'd like to hear if there are any vulnerabilities present.
Party 1 and Party 2 hold some credentials, an ID and a Secret.
Party 1 Creates this message:
HMAC(ID, SECRET)
IV + ENCRYPT(MESSAGE, SECRET, IV)
Using: SHA-256 and AES-CBC-256
Party 2 can then verify the user and decrypt the messa... |
I've always been interested in encryption but I have never found a good explanation (beginners explanation) of how encryption with public key and decryption with private key works.
How does it encrypt something with one key and decipher it with another key?
|
The message format includes a datetime field in the clear. Is it okay to also use this field (or some hash thereof) as the initialization vector?
In this case, CBC is the mode being used.
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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 in-house hash developed years ago and the developer is no longer working with us any more. I need to determine the process t... |
It's been said that CRC-64 is bijective for a 64-bit block.
It the corresponding statement true for typical cryptographic hashes, like MD5, SHA-1, SHA-2 or SHA-3?
For example, would SHA-512 be bijective when hashing a single 512 bit block?
|
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 initial (non-secure) handshake to decrypt my subsequent data?
|
My primary question is:
Is there an easy way to create a bijective mapping from points on an elliptic curve E (over a finite field) to the integers (desirably to $\mathbb{Z}^*_q$ where $q$ is the order of E)?
To phrase it a second way, given a point on the curve that is chosen with uniform randomness, can you transla... |
I know that there is a formula to calculate the result of any input but is the logic of how the machine actually worked documented?
I have seen schematics for the circuitry and even a how to make your own at home site. But has the logic been broken down? Is there source for a virtua Enigma Machine that works by repli... |
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 compatibility with the existing software installation, we need to figure out what encryption/hashing method the previous vend... |
Are there any evidence (other than not being cracked so far) that the Bitcoin protocol is secure? "How secure" is it?
(I realize that this might not qualify as a meaningful question - feel free to edit if you can improve it. Also, if you liked this question, you might like the Bitcoin Proposal.)
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I know that the keys are somehow mathematically related, and I know that they're somehow generated via prime numbers, but how exactly are they related? I realize that it'd probably be best for me to go read about it inside of the book, if it has it, but this is something that I've been wondering about for a little whil... |
I'm starting to learn about cryptanalysis and I am having a bit of difficulty understanding the Kasiski test's index of coincidence. I have a book (Cryptography Theory And Practice by Douglas Stinson) about it that I'm going through, but it seems to skip something, which is causing my confusion. More likely it's me who... |
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 used Rijndael as the algorithm of choice, but I'm struggling to find the correct approach to storing the encryption/decrypti... |
According to the Wikipedia entry for the Alternating Step pseudorandom number generator, there is no public cryptanalysis for this device since it was invented back in 1987 by C.G. Gunther. I have several questions about this:
If it hasn't been cracked since 1987, why is it not considered to be a candidate for a "one ... |
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, not hashed. And no, the website in question does not provide any API.
In addition, the server I am using is accessible to ... |
I found the below code online, but in its original state, the encryption was not using any padding (Padding.None), and was encrypting in blocks of 8. I wasn't able to get this code to properly encrypt / decrypt data so I was forced to have it encrypt the full length of the text. Is there something fundamentally wrong w... |
The RP in ECC would be to find $a_1,\ldots,a_n$ (integers) given $P$ and $Q_1,\ldots,Q_n$ (points in the EC) such that $P = a_1 \cdot Q_1 + \ldots + a_n \cdot Q_n$.
Is it hard when DH-like assumption holds in the EC?
Is is hard in secp256k1 or Curve25519?
|
I'm looking to incorporate more crib usage in breaking ciphers in unknown enciphering schemes, or at least to gleam what information I may. This seems to be a big hurdle to me, and I'm looking for references on incorporating cribs into decryption schemes.
Now allow me to clarify. By "crib" I mean a word or a set of wor... |
I would like to improve the performance of hashing large files, say for example in the tens of gigabytes in size.
Normally, you sequentially hash the bytes of the files using a hash function (say, for example SHA-256, although I will most likely use Skein, so hashing will be slower when compared to the time it takes to... |
I currently use the following method to generate a different password on every website I have to login:
password = SHA1 ( mainPassword . domainName . number )
where . stands for concatenation.
Here are my questions:
Is this secure enough?
I mean, if somebody knows my algorithm and one or many passwords, could he find... |
If part of the password is a whole regular English word, does the entropy of that part depend on the number of English words in existence, the number of English words known by the choosing algorithm, the number of English words assumed by the attacker?
Does the language matter, is the average entropy per word in German... |
Even if RSA decided to cancel the Factoring Challenge, it seems that some teams keep working on it. According to Wikipedia, RSA-768 has been factored in late 2009.
What are the current large integer factorization algorithms and what the mathematical principles behind them? What are the ways for improvement (faster algo... |
We discuss a lot of topics and use measures of entropy to determine how difficult it is for an attacker to be successful. What does entropy mean in the context of cryptography? How is entropy calculated in the general case?
|
I've been reading a paper [1], and I've ran across something called a "Group Cipher", which is similar to homomorphic encryption, with an important difference.
In homomorphic encryption we have an encryption operation $E_k: P \rightarrow C$, where $P$ is the set of plain texts and $C$ is the set of cipher texts. We als... |
I have been reading the question "What is a white-box implementation of a cryptographic algorithm?" and it led to this short article / Q&A which states in question 2:
Q2: What is the difference with code obfuscation?
Related and
complementary techniques for protecting software implementations but
with different securi... |
I came across this while refreshing my cryptography brain cells.
From the RSA algorithm I understand that it somehow depends on the fact that, given a large number (A) it is computationally impossible to derive at the two prime numbers (B & C) whose product it is.
But that isn't the Fermat Theorem, is it?
It's somethin... |
In the past I have used the Chi-squared test to check the statistical randomness of my generator. Is this a good test to use? Are there other tests?
|
I had heard that although it's easy to implement message digest functions like MD5, SHA-1, SHA-256 etc. in CUDA (or any other GPU platform), it is impossible to implement bcrypt there.
bcrypt is different from these hash functions, in that Blowfish is a block cipher, and in order to produce a one way hash a "feed back... |
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 sometimes it is easy to write a simple program to get simple examples of these it's handy to have a list with some non-trivia... |
Situation:
Several participants contribute encrypted random numbers. These numbers will be used to generate community-agreed random (by simple multiplication).
Question:
Is there any way to detect same products values without revealing multipliers? Maybe it is possible to use some homomorphic encryption?
What I origina... |
Currently, a magnet link containing a 40-digits long SHA-hash value, is assigned to every torrent which is created. Therefore, this hash should be unique to identify a torrent and send the right bytes (packages) to the right people. So therefore, there are $16^{40}$ (1461501637330902918203684832716283019655932542976) p... |
In the face of non-quantum attacker, Keccak[r=1088,c=512] with 512 bits of output provides:
Collision resistance up to $2^{256}$ operations
Preimage resistance up to $2^{256}$ operations
Second preimage resistance up to $2^{256}$ operations
In general however, using quantum computing, specifically Grover's Algorithm,... |
I have heard that in enterprises it is common that the IT admin has a master passphrase that can unlock any content that an employee encrypts.
It would be temping to think, that the IT admin's passphrase is encrypted with the employees passphrase, but then the IT can't change his passphrase.
How does such encryption ac... |
Recentally, an attack on AES was discovered which reduces its computationally complexity, by a very slight amount.
The first key recovery attack on the full AES-128 with computational
complexity $2^{126.1}$.
The first key recovery attack on the full AES-192 with computational
complexity $2^{189.7}$.
The first key rec... |
D-wave systems has released a commercially viable quantum computer. This means in theory, that all asymmetric encryption algorithms — such as RSA — are now useless due to the speed at which quantum computers can factor.
Has RSA been cracked yet? If not, why hasn't it?
The reason for our company's concern is an upcoming... |
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