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You can make brute forcing the password as difficult as you need to. You didn't define the overall problem very well. It's not clear why or how public-key cryptography would even be an alternative. In the usual case, you have nothing to 'bootstrap' with. Your keys or credentials are stored encrypted, and you need th...
When a user wants to communicate with another user (let's say alice and bob) What is sent over the network during the initial communication ? Alice uses her kerberos client and enters her password and username. Something which is not clear to me : Will he send a message containing : - time - use...
What I'll describe works with any homomorphic scheme, whether multiplicative (Elgamal) or additive (Paillier; maybe exponential Elgamal or BGN depending), but I'll describe it with multiplicative. I assume what you mean is something like this: you have, say, five people. They all generate a random value r_i, and pos...
Two parties: a client and a server are to a agree on a symmetric key. Both the client and the server are aware of a `master password`. The way this is currently done is: 1. Both parties hash the `master password` using SHA-256 to create a `password hash`. 2. The `password hash` is used by the client to encrypt a ...
The goal of the [Tahoe-LAFS 100 Year Cryptography project][1] is to "enhance Tahoe-LAFS's cryptographic system so that Tahoe shipped today/next year might remain safe from cryptographic attacks for a 100 years." Its developers are openly collaborating knowledgeable experts in the field, with at least all the informati...
How long would the 100 Year Cryptography Project have secured its data had it been started 100 years ago?
In 1911, there was no computer either. With much more modest goals of security for the next few months, the German army came up with the [Enigma][1], which was broken. Before the 1970s, cryptography was essentially a military-only domain, so there were no "openly collaborating knowledgeable experts". The situation was ...
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...
> password = sha1 ( mainPassword . domainName . number ) > Is this secure enough? Answer is no. Let me explain. The only part that is really unknown in the above is `mainPassword`. The rest can easily be guessed by a hacker. If your original password is weak (not many characters, no digits, no specials characters),...
RSA has not been cracked. No one has demonstrated practically viable computing that's anywhere in the realm of breaking RSA. There is no reason to change any of your practices. The first thing to understand is that D-Wave has a long history of [repeatedly making bogus claims to the popular press](https://www.tech...
[Discrete logarithms in GF(p)][1] share the same asymptotic complexity as integer factorization for general numbers --- Lp[1/3,1.923] for general integers, Lp[1/3,1.587] for [special integers][2]. [Discrete logarithms in GF(p^n)][3] have the same asymptotic complexity as factoring special integers , i.e. L(p^n)[1/3, 1....
Currently I'm implementing a PRNG for an embedded system. Using the RFC4086 I've decided to use X9.17 - to be more specific the succesor X9.31- standard to implement my PRNG. X9.17 uses DES but X9.31 uses AES!!! Now I need to specify the value of the AES key for X9.31. I know that at least my AES key must be unpredic...
Usind a RSA private key simultanously as a AES encryption key to generate random numbers?
Currently I'm implementing a PRNG for an embedded system. Using the [RFC 4086][1] I've decided to use the X9.17 - to be more specific the succesor X9.31- standard to implement my PRNG. X9.17 uses DES, but X9.31 uses AES!!! Now I need to specify the value of the AES key for X9.31. I know that at least my AES key must ...
Using an RSA private key simultanously as an AES encryption key to generate random numbers?
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 writing a shell script piping together common tools). - Is in such a simple file format such that it'd be relativ...
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 writing a shell script piping together common tools). - Is in such a simple file format such that it'd be relativ...
When a user wants to communicate with another user (let's say Alice and Bob) What is sent over the network during the initial communication? Alice uses her Kerberos client and enters her password and username. Something which is not clear to me: Will she send a message containing - time - username ...
What is sent on the initial communication with Kerberos?
Just to complement Thomas's reply, here are a couple of papers that do not rely on SIMD registers to implement bitsliced AES: - [How Far Can We Go on the x64 Processors?][1] (source in appendix) - [A Fast and Cache-Timing Resistant Implementation of the AES][2] ([source code][3]) [1]: http://www.iacr.org/a...
If Camellia is of equivalent security and speed to AES, concerns arise. First of all, assuming the above, why is Camellia so rarely used in practice? Why aren't there any breaks in Camellia? Does that mean that Camellia is currently more secure than AES, or is this a result of the lack of research on the crypta...
If [Camellia][1] is of equivalent security and speed to AES, concerns arise. First of all, assuming the above, why is Camellia so rarely used in practice? Why aren't there any breaks in Camellia? Does that mean that Camellia is currently more secure than AES, or is this a result of the lack of research on the c...
I'm reading that AES uses 4x4 bytes (448 = 256 bits key) matrix for performances matters (since it's a requirement for common standard encryption algorithms), but are there implementations with choosable key size for this particular algorithm, so I can make brute-force attacks harder with a good password ? Also, i...
Are there public slow-but-strong algorithms out there that resist brute-force attacks better?
Why does nobody use (or break) the Camellia Cipher?
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 can take longer, right ? But it's not necessarily true depending how unaware users can be, and that's where dictiona...
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 can take longer, right? But it's not necessarily true depending how unaware users can be, and that's where dictionar...
What happens if the computer processing the torrent file gets struck by a cosmic ray and processes incorrectly? What happens if an asteroid strikes the Earth and destroys every computer before the torrent file can finish downloading? If the probability of a failure occurring is many orders of magnitude lower than th...
Is combining digests (created using a hash function) using arithmetic addition, and then hashing and publishing the result, less secure than publishing the set of digests? Does the answer change if the outer hash function is different from the inner function? I am working on a proposal for Bitcoin to enable m-of-...
Is combining digests (created using a hash function) using arithmetic addition, and then hashing and publishing the result, less secure than publishing the set of digests? Does the answer change if the outer hash function is different from the inner function? I am working on a proposal for Bitcoin to enable m-of-...
Is digest=HASH(HASH(a)+HASH(b)) equivalent to publishing two digests?
I have a scenario similar to the one described in [Wikipedia: hash list][1], but with a twist. I'm looking for a cryptographically secure hash function that would create the same root hash for the same file, no matter how the the file is chopped up for the individual hashes in the list. E.g. case 1: File is divided...
[Post-quantum cryptography](http://pqcrypto.org/) concentrates on cryptographic algorithms that remain secure in the face of large scale quantum computers. In general, the main focus seems to be on public-key encryption algorithms and public-key signature algorithms - but there are dozens of other constructs like hashe...
What is the post-quantum cryptography alternative to Diffie-Hellman?
I have a scenario similar to the one described in [Wikipedia: hash list][1], but with a twist. I'm looking for a cryptographically secure hash function that would create the same root hash for the same file, no matter how the the file is chopped up for the individual hashes in the list. E.g. case 1: File is divided...
If I have a message M and a public key P, and I encrypt them alongside one another: CIPHER = AES(M + P, aes_key) Is CIPHER now weakened if someone knows what P is? It seems like if I encrypted only P: CIPHER = AES(P, aes_key) that it should be possible to recover aes_key from CIPHER. I realize that ...
Is it safe to encrypt a public key alongside a message with AES?
Probably not in this case, although not for the reason you gave. As @Sadeq points out in the comments, AES is resistant to known-plaintext attacks. This means that by simply knowing M+P or P (the entire plaintext you've encrypted) it should not be possible to recover the AES key. However, key recovery isn't the o...
This problem (i.e. generating a second preimage) is analyzed in the paper ["A generalized birthday problem"][1], by David Wagner, Crypto 2002. If you add 8 public keys then the attack requires somewhere around 2^64 hash computations, if you add 16 public keys before hashing them then the complexity is somewhere aro...
@D.W. is probably closest to the real reason (this was fifteen years ago, so things get a bit hazy), there was some concern about short cycles, and it was effectively free - you're already iterating the hashing deliberately to slow things down so speed isn't an issue - so why not do it? You've also got to remember the...
Can you create a strong blockcipher with small blocksize, given a strong blockcipher of conventional blocksize?
This problem (i.e. generating a second preimage) is analyzed in the paper ["A generalized birthday problem"][1][[Citeseer]][2][[Slides]][3], by David Wagner, Crypto 2002. If you add 8 public keys then the attack requires somewhere around 2^64 hash computations, if you add 16 public keys before hashing them then the...
A recent approach to creating a decentralized online currency, called [Bitcoin](http://www.bitcoin.org/bitcoin.pdf), has been generating some interest. The goal is to have a way to transfer currency without a central authority and without double spending or counterfeiting. Their approach is to have all the nodes in the...
[Post-quantum cryptography](http://pqcrypto.org/) concentrates on cryptographic algorithms that remain secure in the face of large scale quantum computers. In general, the main focus seems to be on public-key encryption algorithms and public-key signature algorithms - but there are dozens of other constructs like hashe...
There are several uses of cryptographic algorithms in the protocol. ## Accounts/Transactions To move money from one account to another, you need to collect some data (the previous transaction(s) which got you the money, the target account, the amount you want to transfer) and sign them, using the private key wh...
A recent approach to creating a decentralized online currency, called [Bitcoin][2], has been generating some interest. The goal is to have a way to transfer currency without a central authority and without double spending or counterfeiting. Their approach is to have all the nodes in the network try to verify a transact...
Could one prevent double spending in decentralized digital currencies (like Bitcoin) without all transactions being public?
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 indices to be swapped are dependent on dynamic cipher state and others may include rotations by a variable number of bit...
How are constructs with data-dependent swaps and rotations cryptanalyzed?
If AES is ever cracked, who should be notified?
[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 question is: How do I assure myself that the results data submitted to me was generated by my official build of an open sourc...
The best solution is to have one source file that you keep secret. Have a flag that marks whether it's the official build or the open build. The open build will not use that source file. Inside that source file, have an algorithm that is obfuscated that generates a checksum that you can verify. Keep the algorithm secre...
The best solution is to have one source file that you keep secret. Have a flag that marks whether it's the official build or the open build. The open build will not use that source file. Inside that source file, have an algorithm that is obfuscated that generates a checksum that you can verify. Keep the algorithm secre...
The best solution is to have one source file that you keep secret. Have a flag that marks whether it's the official build or the open build. The open build will not use that source file. Inside that source file, have an algorithm that is obfuscated that generates a checksum that you can verify. Keep the algorithm secre...
I heard that there are 128 stochastically independent bits in an MD5. Is that true? If so, are there any citations or proofs for that? Thank you!
Does MD5 generates 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?
Does MD5 generate 128 independent bits?
The best solution is to have one source file that you keep secret. Have a flag that marks whether it's the official build or the open build. The open build will not use that source file. Inside that source file, have an algorithm that is obfuscated that generates a checksum that you can verify. Keep the algorithm secre...
A lot of answers have said "a mathematical structure" which is absolutely right, but I still see there might be a question: how on earth can one exist? I'll try and fill that gap at a simpler level. So a simple caeser-shift cipher might look like this: `x+7`. In our really simplistic, dangerous example, the "key" he...
If a researcher manages to make a cryptographic 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 release and how do those relying on such systems ensure they are not caught in a situation where the crypto-system on wh...
My understanding of hashing and encryption is rather limited. I certainly do not understand the mathematical formulas at play in these algorithms. With that said, what part do prime numbers play in cryptography? When I was in college, one of my professors told me that the fact that there is no formula to predict a p...
What place do prime numbers have in cryptography?
If a cyptanalytic breakthrough is made, what process should be followed?
Yes, that's correct. Take a look at this question / answer : http://crypto.stackexchange.com/questions/292/how-does-asymmetric-encryption-work about the related math, in a simple way. When you deal with prime numbers, and have a very large prime number, there's no easy way to know it, since you have to test all...
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 release and how do those relying on such systems ensure they are not caught in a situation where the crypto-system on...
If a cryptanalytic breakthrough is made, what process should be followed?
No, there are expressions relating input values which demonstrate extremely strong correlations between output bits. The output bits of of MD5 are not independent, particularly if the party supplying the input to MD5 chooses for them not to be. These correlations are sufficiently powerful to enable the generation of...
Well, first off, the question doesn't arise that often in practice. People do find cryptographical weaknesses; however, generally they are purely of theoretical interest, or if they could be used in a real attack, it may take quite a while before someone figures out how to use it. As an example of the second case, Ms...
[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 question is: How do I assure myself that the results data submitted to me was generated by my official build of an open sourc...
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 algori...
I am interested in exploring ECC implicit certificates, specifically using the ECQV protocol. While the actual implementation would not difficult to perform using building blocks provided by most ECC capable libraries, I really would like to compare my application with a reference implementation. Does anyone know of...
"What is the simple cipher for such an alphabet?" I'm assuming you have some message written by paper in a written language with thousands of "characters", and you want to write an encrypted message on paper using more-or-less the same set of characters. One could use a substitution cipher, as you suggested, usin...
How to collect, process, and transmit data securely?
I am interested in exploring ECC [implicit certificates][1], specifically using the [ECQV protocol][2]. While the actual implementation would not difficult to perform using building blocks provided by most ECC capable libraries, I really would like to compare my application with a reference implementation. Does anyo...
Are there reference implementations of ECQV implicit certificates?
is it possible to find discrete logarithm with 256 bit long modulus and 160 long exponent in adequate time? If yes,i'm searching for working software for it. Do you know some?
Are there any ongoing or current practical attempts to solve instances of the discrete logarithm problem of the order of magnitude used in cryptographic applications, for example with a 256 bit modulus and 160 bit exponent? I am interested in efforts similar to the work of members of the [Mersenne Prime forum][1] and [...
How to practically find solutions to a discrete logarithm?
Are there any ongoing or current practical attempts to solve instances of the discrete logarithm problem of the order of magnitude used in cryptographic applications, for example with a 256 bit modulus and 160 bit exponent? I am interested in efforts similar to the work of members of the [Mersenne Prime forum][1] and [...
Are there any ongoing or current practical attempts to solve instances of the discrete logarithm problem of the order of magnitude used in cryptographic applications, for example with a 256 bit modulus and 160 bit exponent? I am interested in efforts similar to the work of members of the Mersenne Prime forum www.mersen...
This question comes up often enough in the context of cryptography that it probably is relevant in a practical sense. I suspect we'll hear even more about it if homomorphic encryption raises interest in "computation in an adversarial setting". It's not just theoretically unsolvable. A great many software development...
This question comes up often enough in the context of cryptography that it probably is relevant in a practical sense. I suspect we'll hear even more about it if homomorphic encryption raises interest in "computation in an adversarial setting". It's not just theoretically unsolvable. A great many software development...
Not an answer to your question, but a security point. Revealing hashes of unencrypted content is a security vulnerability. For example, consider the hash of a configuration file, which is mostly known, but differs only for a 8 character password. It becomes really easy to iterate through all passwords and check whic...
To walk you through RSA from start to end, here's how it works. 1. Choose two large distinct primes $p$, $q$. Calculate $n=pq$. 2. Calculate $\phi(pq)$. This happens to be $(p-1)(q-1)$. 3. Choose $e$ such that $gcd(e, \phi(p,q)) = 1$ and $e < \phi(p,q)$. 4. Compute $de = 1 \mod \phi(p,q)$. 5. Do some crypto; $c ...
Not an answer to your question, but a security point. Revealing simple hashes of unencrypted content is a security vulnerability. (Revealing HMACs or anything with a secret component do not pose the same vulnerability, even if they are calculated from the unencrypted content.) For example, consider the hash of a con...
A lot of answers have said "a mathematical structure" which is absolutely right, but I still see there might be a question: how on earth can one exist? I'll try and fill that gap at a simpler level. So a simple caeser-shift cipher might look like this: $x+7$. In our really simplistic, dangerous example, the "key" he...
256-bit discrete logarithms on a prime field are definitely not of the order of magnitude used in cryptographic applications. Secure sizes for this problem are in the thousands of bits, very much like integer factorization. To break that example discrete logarithm, you probably want to use [Index Calculus][1], m...
What I'll describe works with any homomorphic scheme, whether multiplicative (Elgamal) or additive (Paillier; maybe exponential Elgamal or BGN depending), but I'll describe it with multiplicative. I assume what you mean is something like this: you have, say, five people. They all generate a random value $r_i$, and p...
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 encrypt...
Homework? Try using ElGamal encryption. I.e., given an ElGamal ciphertext that encrypts an integer $m$ and your own factor $f$, try finding a way to generate another ElGamal ciphertext that encrypts $f·m$. One difficulty is doing this knowing only the public key. Another difficulty is to avoid that an attacker k...
Is there a cumulative commitment scheme?
When I read about a known plaintext attack, for example on AES the block size is always 128 bits, does it mean the attacker will only supply 128 bits data words as "plain text" ?
What is the smallest plaintext/ciphertext size for an algorithm like?
What I'll describe works with any homomorphic scheme, whether multiplicative (Elgamal) or additive (Paillier; maybe exponential Elgamal or BGN depending), but I'll describe it with multiplicative. I assume what you mean is something like this: you have, say, five people. They all generate a random value $r_i$, and p...
When I read about a choosen plaintext attack, for example on AES the block size is always 128 bits, does it mean the attacker will only supply 128 bits data words as "plain text" ?
I'm looking for a scheme where signing may be costly (slow) but batch verification with different signers is the fastest possible. Modified ECDSA allows batch verification for different signers with a 4X speedup [1]. Is there any signature scheme than can do better? [1] Fast Batch Verification of Multiple Sign...
What is the signature scheme with the fastest batch verification protocol for multiple signers?
[Rabin signatures][1] have a very fast verification algorithm: a simple squaring modulo some integer. RSA signature verification (with a public exponent equal to 3) is also very fast. These signature algorithms are simple to implement and will beat ECDSA for verification speed, even if batch verification is used for EC...

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