instruction stringlengths 15 30k |
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Why we use 1 permutation table in the first step of DES algorithm and one in the end of algorithm? |
What is the benefits of 2 permutation table in DES? |
When initiating an [oblivious transfer][1], why would someone use a 1-2 transfer rather than going for 1-n? Perhaps a slight time overhead for the extra message encrypts, but with everything as fast as it is these days is that even a concern?
[1]: http://en.wikipedia.org/wiki/Oblivious_transfer#1-2_oblivious_tra... |
Why Use 1-2 Oblivious Transfer When 1-N Exists? |
As we know sha1 is irreversible then why we append length of message to it? |
Why in Sha1 pre processing we append length of message? |
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 f... |
What is the general justification for the hardness of finding preimages for cryptographic hash functions? |
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 message.
How can I do this? |
How can I use asymmetric encryption, such as RSA, to encrypt an arbitrary length of plaintext? |
As we know, SHA-1 is irreversible, so why do we append the length of the message to the preimage? |
Why do we append the length of the message in SHA-1 pre-processing? |
It seems like the S-boxes in DES have essentially random values. How were these chosen? |
How are the DES S-box values determined? |
[Bitcoin](http://www.bitcoin.org/) addresses are RIPEMD-160 hashes of the public portion of a public/private ECDSA keypair. They are generally base-58-encoded. See [Address - Bitcoin](https://en.bitcoin.it/wiki/Address)
In [*Vanity bitcoin addresses: a new way to keep your CPU busy*](http://forum.bitcoin.org/index... |
How to provide secure "vanity" bitcoin address service? |
I get the theory of salt, but anywhere I see it being implemented, the examples always generate the salt at that time and it appears to be different for every hash. When I think about how to implement salt in logging into a website, wouldn't the salt need to be the same at account creation time as the salt used when th... |
Implementation details about Salt in a hash? |
AES has several different modes, but why would I use one over another? |
What are the practical difference between 256-bit, 192-bit, and 128-bit AES encryption? |
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. But what cryptographic hash is best suited for hashing passwords and the like, i.e. secure enoug... |
What general-purpose cryptographic hash should I use? |
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.
How does it work? How would I approach applying it to AES? What in-the-wild scenarios will it help me exploit and what ... |
What is differential cryptanalysis? |
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... |
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 EmailAddress) is there any effective decrease in your security if you simply ... |
Necessity of Randomness of Salts? |
What are the main weaknesses, if any, of a playfair cypher? 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? |
What are the main weaknesses, if any, of a playfair cypher? |
Salts must be unique. Randomness (with a "good" random generator) is _sufficient_ to ensure uniqueness.
A per-user ID (e.g. the user login name) is not sufficient for uniqueness, because it does not capture some occurrences which do happen in practice:
- two users on two distinct systems (running the same softwa... |
> If your system already has some other unique user identifier (be it an ID, user-name, or an EmailAddress) is there any effective decrease in your security if you simply use that value as your salt?
The salt in a hash actually forms two different purposes. Generally, it is acceptable for a salt to be known, so let ... |
What are the main weaknesses, if any, of a [playfair cypher][1]?
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?
[1]: http://en.wikipedia.org/wiki/Playfair_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.
<strike>How does it work? How would I approach applying it to AES? What in-the-wild scenarios will it help me exploit a... |
How do I apply differential cryptanalysis to a block cipher? |
I was attending a database encryption session at a developers conference. They 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 that first I heard of salting in encryption. I asked for clari... |
Salting when encrypting? |
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 have a general idea that the application of differential cryptanalysis is to look at the difference between inputs; [... |
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? |
With sufficient randomness, is XOR an acceptable mechanism for encrypting? |
Is it feasible to build a "pluggable" cypher using the various cryptographic hashing functions available? |
Basically it's analysis of a cryptographic cypher by the means of finding a relationship between the difference in the input data and the output data.
Ideally, the slightest difference in input data (cleartext), even a single bit, should produce a completely different cypthertext. However, if the cypher is not well-de... |
Basically it's analysis of a cryptographic cypher by the means of finding a relationship between the difference in the input data and the output data.
Ideally, the slightest difference in input data (cleartext), even a single bit, should produce a completely different cypthertext. However, if the cypher is not well-de... |
There are several known methods for implementing AES in constant time using SIMD operations, mostly based around fast byte shuffling (for instance [Hamburg](http://crypto.stanford.edu/vpaes/) and [Kasper/Schwabe](http://homes.esat.kuleuven.be/~ekasper/papers/fast_aes_slides.pdf)). Are there any similar approaches that ... |
Known methods for constant time (table-free) AES implemenation using 'standard' operations? |
Why we use a permutation table in the first step of [DES algorithm][1] and one in the end of algorithm?
[1]: http://en.wikipedia.org/wiki/Data_Encryption_Standard |
What are the benefits of the two permutation tables in DES? |
As we know, [SHA-1][1] is irreversible, so why do we append the length of the message to the preimage?
[1]: http://en.wikipedia.org/wiki/SHA-1 |
It seems like the S-boxes in [DES][1] have essentially random values.
How were these chosen?
[1]: http://en.wikipedia.org/wiki/Data_Encryption_Standard |
I've got a feeling that I'm going to write this a lot on here: define "sufficient". The question you must answer is "what do you want to protect and how much is it worth to you"? In general, a plain XOR cypher with a key shorter than the *total* plaintext encrypted is pretty weak, and methods for decrypting are more ... |
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.
... |
Taking advantage of one-time pad key reuse? |
Prime theory is of great interest to me! It is currently used in many cryptosystems to protect data (in making public keys, for example). There are always a few obscure researchers studying how to make prime factorization easier (or stronger I suppose).
There are so many branches of math that we use in cryptography ... |
I think I once faced recommendation, that IV should always be random and never be used twice with the same key. How serious is this weakness? |
What's the details of the DES weakness of reusing the same IV with the same key? |
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 certi... |
Protocol to generate Client Certificates at the start of a SSL session automatically? |
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 relativily simply, should it not just be avoided for all applications? |
Should DES be avoided when developing new systems? |
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 algorith... |
Known methods for constant time (table-free) AES implementation using 'standard' operations? |
The Rijndael S-Box design generates a [permutation cycle](http://groupprops.subwiki.org/wiki/Cycle_type_of_a_permutation) 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? |
What password hash should I use? |
Replacing the Rijndael S-Box? |
[Bitcoin](http://www.bitcoin.org/) 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](https://en.bitcoin.it/wiki/Addres... |
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 $d$ mod $\varphi(n)$). Given a plaintext message $m$, we obtain the ciphertext $c = m^e$ (mod $n$); we subsequently d... |
RSA with small exponents? |
First I must state that a _secure_ RSA encryption _must_ use an appropriate padding, which includes some randomness. See [PKCS#1][1] for details.
That being said, $d$ is the "private exponent" and knowledge of $d$ and $n$ is sufficient to decrypt messages. $n$ is public (by construction) so $d$ must be kept private ... |
>Are there efficient attacks against such an implementation?
Yes. You need to keep d larger than the 4th root of n=pq. Otherwise [Wiener's Attack][1] can be used to compute d.
[1]: http://en.wikipedia.org/wiki/Wiener%27s_Attack |
What's the details of the DES weakness of reusing the same IV in CBC mode with the same key? |
[As we know][1], 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 consid... |
Is using slow password hashing on the client side easier attackable than on the server side? |
Cryptography is being a lot of places already and people might just know about it. For example, whenever you access an HTTPS page, that's cryptography protecting you.
For desktop applications, many people use the [Truecrypt][1] application to protect their files. You also see a similar application in Windows BitLock... |
The thing here is:
When you just XOR the cyphertexts with each other, what you get is in fact the XOR result of both cleartexts.
> f(a) ⊕ f(b) = a ⊕ b
And after that point, all that's left is to use statistical analysis, as ir01 has mentioned.
In fact, the early cell phones used to implement a somewhat simi... |
[As we know][1], 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 consid... |
The first step In [Algebraic-attack][1] is expressing the cipher operations as a system of equations. Is there a automated tool to do this?
[1]: http://en.wikipedia.org/wiki/Algebraic_stack |
The first step In [Algebraic-attack][1] is expressing the cipher operations as a system of equations. Is there a automated tool to do this?
[1]: http://en.citizendium.org/wiki/Algebraic_attack |
[As we know][1], 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 consid... |
Theoretically you _can_ do encryption of long messages with RSA, in the same way that you can encrypt a long message with a block cipher. This requires an appropriate chaining mode, e.g. CBC: each plaintext "block" is first XORed with (part of) the encrypted previous block.
With RSA and proper padding, there is a pe... |
Does anyone know of any sites where intermediate values of multiple crypto algorithms (not just the famous ones), including hashes, can be found? I dont just mean intermediate values of encrypting. Im also looking for intermediate values of the key generation algorithms as well. Hopefully the initial values can be chan... |
NIST removed "The Lempel-Ziv Compression" test from [Statistical Test Suit][2] in revision 2008 and above see revision [2010][1]. Why this test removed?
[1]: http://A%20Statistical%20Test%20Suite%20for%20the%20Validation%20of%20Random%20Number%20Generators%20and%20Pseudo%20Random%20Number%20Generators%20for%20Cr... |
I ask this question because 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 ([paper][1]).
However, in 2008, [new parameters][2] were proposed after th... |
Is key size the only barrier to the adoption of the McEliece cryptosystem, or is it considered broken? |
Is key size the only barrier to the adoption of the McEliece cryptosystem, or is it considered broken/potentially vulnerable? |
Does anyone know of any sites where intermediate values of multiple crypto algorithms (not just the famous ones), including hashes, can be found? I dont just mean intermediate values of encrypting. Im also looking for intermediate values of the key generation algorithms as well. Hopefully the initial values can be chan... |
The huge key is definitely an issue. Another is the lack of standardization or recommendations. Should you use OAEP with McEliece, or some other padding? What parameters are actually secure? And so on. Part of the problem is that, while it has been around since the 70s, it was not considered particularly interesting un... |
Wide adoption of an asymmetric encryption algorithm, or a digital signature algorithm, requires at least the following:
- There must exist a reasonably clear standard which unambiguously says where each byte goes. It must cover endianness and similar issues. [PKCS#1][1] is such a standard, for RSA.
- The algori... |
A [recent paper][2] showed that the [McEliece cryptosystem][1] 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][3] were proposed after the old ones were found to be vul... |
The huge key is definitely an issue. Another is the lack of standardization or recommendations. Should you use [OAEP][1] with McEliece, or some other padding? What parameters are actually secure? And so on.
Part of the problem is that, while it has been around since the 70s, it was not considered particularly intere... |
The huge key is definitely an issue. Another is the lack of standardization or recommendations. Should you use [OAEP][1] with McEliece, or some other padding? What parameters are actually secure? And so on.
Part of the problem is that, while it has been around since the 70s, it was not considered particularly intere... |
[The definition of PBKDF2][1] states that I obtain a derived key* 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, sa... |
Why does PBKDF2 xor the iterations of the hash function together? |
NIST removed "The Lempel-Ziv Compression" test from [Statistical Test Suite][2] in revision 2008 and above and has not incorporated it since - see revision [2010][1]. Why was it removed? Does it no longer provide sufficient testing of a PRNG or was it simply superseded by better tests?
[1]: http://csrc.nist.gov/gr... |
Why did NIST remove The Lempel-Ziv Compression test from the Statistical Test Suite? |
Is it possible to identify the encryption method or atleast rule out some, 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. |
[bcrypt][1] is an intentionally slow hash algorithm. In my [last protocol idea][2], 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<sup>12</su... |
bcrypt: can one efficiently iterate valid hash output values? |
AES has several different variants (AES-128, AES-192, AES-256), but why would I use one over another? |
[As we know][1], 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 consid... |
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