instruction stringlengths 15 30k |
|---|
Since MD5 is broken for purposes of security, what hash should I be using now for secure applications? |
What is the recommended replacement for MD5? |
How can I find two strings $m_1$ and $m_2$, knowing that I know $m_1 \oplus m_2$? |
Can anyone explain me in details why does the following statement hold true in Chaum and van Antwerpen scheme for undeniable signature?
> The probability that a dishonest signer is able to successfully
> mislead the verifier in either verification or disavowal is 1/q where
> q is the prime number in the signer's... |
Ok, lets try again from scratch. First, some group theory definitions:
- A group is a set of elements and an operation $\times$ which satisfies a specific set of properties (closure, associativity, an identity element, inverses)
- A subgroup is a subset of elements of a group which, along with the group operatio... |
Chaum undeniable signature - justification for probability of misleading the verifier? |
The [well-known article about why we should use bcrypt for hashing passwords][1] mentions the *work factor* - some parameter to the algorithm that determines how long one hashing should be in terms of number of computations. This as the article says allows for keeping up with Moore Law - when we have faster processors ... |
Do I have to recompute all hashes if I change the work factor in bcrypt? |
Can anyone explain me in details why does the following statement hold true in Chaum and van Antwerpen scheme for undeniable signature?
> The probability that a dishonest signer is able to successfully
> mislead the verifier in either verification or disavowal is 1/q where
> q is the prime number in the signer's... |
Can anyone explain me in details why does the following statement hold true in Chaum and van Antwerpen scheme for undeniable signature?
> The probability that a dishonest signer is able to successfully
> mislead the verifier in either verification or disavowal is 1/q where
> q is the prime number in the signer's... |
Can anyone explain me in details why does the following statement hold true in Chaum and van Antwerpen scheme for undeniable signature?
> The probability that a dishonest signer is able to successfully
> mislead the verifier in either verification or disavowal is 1/q where
> q is the prime number in the signer's... |
Can anyone explain me in details why does the following statement hold true in Chaum and van Antwerpen scheme for undeniable signature?
> The probability that a dishonest signer is able to successfully
> mislead the verifier in either verification or disavowal is 1/q where
> q is the prime number in the signer's... |
Wikipedia lists the following pseudocode for HMAC:
function hmac (key, message)
if (length(key) > blocksize) then
key = hash(key) // keys longer than blocksize are shortened
end if
if (length(key) < blocksize) then
// keys shorter than blocksize are zero-pad... |
What do the magic numbers 0x5c and 0x36 in the opad/ipad calc in HMAC do? |
Can you please explain me what is the reason of using nonces in the [Needham-Schroeder](http://en.wikipedia.org/wiki/Needham%E2%80%93Schroeder_protocol) and in the [Kerberos](http://en.wikipedia.org/wiki/Kerberos_%28protocol%29) protocols for Key Transport.
In particular :
- Can you explain me the use of $N_A$?... |
Can you please explain me what is the reason of using nonces in the [Needham-Schroeder](http://en.wikipedia.org/wiki/Needham%E2%80%93Schroeder_protocol) and in the [Kerberos](http://en.wikipedia.org/wiki/Kerberos_%28protocol%29) protocols for Key Transport.
In particular :
- Can you explain me the use of $N_A$?... |
Would it be difficult to find $\:$ workfactor,salt,password0,password1 $\:$ such that
bcrypt(workfactor,salt,password0) = bcrypt(workfactor,salt,password1)
<br>and $\:$ password0 != password1 $\:$ and $\:$ workfactor is not too small
?
<br><br><br>
(If no, then the combination of bcrypt with PAKE described in ... |
bcrypt - collision-resistance against chosen salt and work factor? |
For Needham-Schroeder, $N_A$ is used to prevent a replay attack. Look at the message from $S\to A$. Without a nonce, a malicious server could replay an old but valid response. The message would still decrypt, etc. Furthermore, the key $K_{AB}$ would be the same as a previously used key which could open up the system to... |
I am no expert at bcrypt, but looking at the [pseudocode on Wikipedia][1] makes be believe that you would. The reason being is that the *cost* value is used to calculate the state in *EksBlowfishSetup*. That state is never saved, so to recover it, you would have to recompute from the beginning. If the state were saved,... |
I've see many articles, most of them basically the same, praising Fujitsu for cracking what is referred to as a 923 bit pairing based encryption. I understand that in comparing RSA to AES you've got something like a power of 10 for the key lengths to be equally strong. From what I read it seems that this pairing base... |
Would it be difficult to find $\:$ cost,salt,key,password0,password1 $\:$ such that
bcrypt(cost,salt,key,password0) = bcrypt(workfactor,salt,key,password1)
<br>and $\:$ password0 != password1 $\:$ and $\:$ cost is not too small
?
<br><br><br>
(If no, then the combination of bcrypt with PAKE described in [this ... |
Would it be difficult to find $\:$ cost,salt,input,password0,password1 $\:$ such that
bcrypt(cost,salt,password0,input) = bcrypt(workfactor,salt,password1,input)
<br>and $\:$ password0 != password1 $\:$ and $\:$ cost is not too small
?
<br><br><br>
(If no, then the combination of bcrypt with PAKE described in ... |
The overall idea is a sound migration strategy (the original `salt` and `new_secure_salt` shall be stored, or perhaps for some portion derivable exactly from material keyed-in by the user, e.g. user name lowercased).
Using `bcrypt(MD5(salt + password), new_secure_salt)` is a huge improvement over `MD5(salt + passwor... |
The overall idea is a sound migration strategy. The nice thing is that security for all users is upgraded in a one-shot operation, rather than at the hypothetic next login of each user. Of course, original `salt` and `new_secure_salt` shall be stored, or perhaps for some portion derivable exactly from material keyed-in... |
To increase the work factor, there are three approaches you could take:
**Rehash all passwords immediately, continue the same hash**
Take the hash currently stored in the password file, and hash it a few more times.
We may (see below) be able to do this immediately with *every* entry in the password file.
Exa... |
I am using AES encryption (Rijindael) with Base-64 encoding in Obj-C and VB. I am currently using the following two projects to achieve this:
Obj-C: https://github.com/dev5tec/FBEncryptor/tree/3c739f18c94ea257aba5cb7b6a97f49af2589977
Visual Basic:
http://www.codeproject.com/Articles/15280/Cryptography-101-for-th... |
I know that when you use a stream cipher you are just XOR'ing the plain text with the PRNG stream, such that you know if a specific byte is in position 3 of the plain text it will still be in position 3 of the cipher text. I know that this is not true for block ciphers in ECB/CBC/other modes except OFB. But why? W... |
> But why?
It is because the cipher is designed with the [avalanche effect][1] property. Namely, changing a single bit in the plaintext (or key) should result in a significant change in the ciphertext. So really, it happens because ciphers are designed with that property.
>What is happening behind the scenes?
... |
For storing passwords, it seems that the use of intentionally slow hash functions (like PBKDF2 and bcrypt) is now recommended. This is because most hashes are designed to be fast, which also makes certain attacks on them parallelizable on GPUs.
Now I'm wondering if it would make sense to use slow hash functions for ... |
Does it make sense to use slow hashes in digital signatures? |
List questions are really off topic here, but since the question hasn't been closed yet:
1. [Dan Boneh's Free Online Course][1]
2. [Introduction to Mathematical Cryptography][2]
3. This website when you have questions (see the [FAQ][3] before posting)
[1]: https://www.coursera.org/course/crypto
[2]: http... |
Can Anybody tell me why the expectation value of the Index of Coincidence is the following?
For a cipher text string of length $L$, where $n$ is the number of
alphabet characters, $κ_r = 1/n$, and $κp = \sum_{i=1}^n p_i^2$
$$ E(IC) = \frac{1}{t}*\frac{L - t}{L - 1}*k_p + \frac{t - 1}{t}*\frac{L}{L - 1}*k_r$$ |
Can Anybody tell me why the expectation value of the Index of Coincidence is the following?
For a cipher text string of length $L$, where $n$ is the number of
alphabet characters, $k_r = 1/n$, and $k_p = \sum_{i=1}^n p_i^2$
$$ E(IC) = \frac{1}{t}*\frac{L - t}{L - 1}*k_p + \frac{t - 1}{t}*\frac{L}{L - 1}*k_r$$ |
It's been over 30 years since Rivest, Shamir and Adleman first [publicly](http://people.csail.mit.edu/rivest/Rsapaper.pdf) described their algorithm for public-key cryptography; and the intelligence community is thought to have known about it for around 40 years—possibly longer.
It's fair to assume that, durin... |
How much computing resource is required to brute-force RSA? |
The answer to the original question would have been: **Yes**, it would be impossibly difficult to exhibit `workfactor`, `salt`, `password0`, `password1` such that `bcrypt(workfactor, salt, password0) = bcrypt(workfactor, salt, password1)`; but even if that was feasible, it would <strike>not matter much, because in norm... |
The answer to the original question would have been: **Yes**, it would be impossibly difficult to exhibit `workfactor`, `salt`, `password0`, `password1` such that `bcrypt(workfactor, salt, password0) = bcrypt(workfactor, salt, password1)`; but even if that was feasible, it would <strike>not matter much, because in norm... |
The answer to the original question would have been: **Yes**, it would be impossibly difficult to exhibit `workfactor`, `salt`, `password0`, `password1` such that `bcrypt(workfactor, salt, password0) = bcrypt(workfactor, salt, password1)`; but even if that was feasible, it would <strike>not matter much, because in norm... |
The brute force technique described in the question is hopeless, as pointed in this [other answer][1].
However there are much better techniques to attack RSA keys, including [GNFS][2]. Therefore 1024-bit RSA keys, even though they offer sizable security, can no longer be considered entirely safe from predictable aca... |
The brute force technique described in the question is hopeless, as pointed in this [other answer][1].
However there are much better techniques to attack RSA keys, including [GNFS][2]. Therefore 1024-bit RSA keys, even though they offer sizable security, can no longer be considered entirely safe from predictable aca... |
The brute force technique described in the question is hopeless, as pointed in this [other answer][1].
However there are much better techniques to attack RSA keys, including [GNFS][2]. Therefore 1024-bit RSA keys, even though they offer sizable security, can no longer be considered entirely safe from predictable aca... |
Are there any known collisions for the hash functions SHA-128, 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) = x` and `a ≠ b`? |
Are there any known collisions for the SHA-2 family of hash functions? |
In short, no.
So, what is the current state of cryptanalysis with SHA-1 (for reference only as this question relates to SHA-2) and SHA-2? Bruce Schneier has declared [SHA-1 broken][1]. That is because researchers found a way to break full SHA-1 in $2^{69}$ operations. Much less than the $2^{80}$ operations it should... |
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`? |
This is by no means a scientific observation, but it seems to me that stream ciphers receive a lot less attention than block ciphers. Is there any reason for this? (Is it because block ciphers are accepted as superior?)
Additionally, since there is no "AES" of stream ciphers, is there any reference of currently secu... |
Are Stream Ciphers Less Secure? |
I am trying to get the idea of cyclic attacks againts assymetric RSA encryption.
Taken from _Handbook of applied cryptography_ .
Let $k$ be a positive integer such that $c^{e^{k}} = c\space mod(n) $ (1).
There for
$k-1$ it holds that $c^{e^{k-1}} = m\space mod(n) $ (2) where $m$ is the message for encryption $n$ ... |
How cyclic attack on RSA works? |
what is the capacity of AES in terms of File Encryption?
is it really good to encrypt a large files in AES? ex. I am encrypting a 8GB of File... is it still good to used AES?
is it still good to used AES-OFB in encryption of large files?
|
I am trying to get the idea of cyclic attacks againts assymetric RSA encryption.
Taken from _Handbook of applied cryptography_ .
Let $k$ be a positive integer such that $$c^{e^{k}} = c\mod n \tag{1}.$$
There for
$k-1$ it holds that $$c^{e^{k-1}} = m \mod n \tag 2$$ where $m$ is the message for encryption $n$ is... |
How does the cyclic attack on RSA work? |
I'm not sure if what I'm asking is even a valid question but here goes.
Would it be possible to add a mechanism to an encryption algorithm that would mean it had to be a certain time of the day or a certain day of the year for the encryption to be able to be decrypted.
Obviously this would require encrypted acces... |
Is it possible to make time-locked encrytion algorithm? |
I'm not sure if what I'm asking is even a valid question but here goes.
Would it be possible to add a mechanism to an encryption algorithm that would mean it had to be a certain time of the day or a certain day of the year for the encryption to be able to be decrypted.
To Clarify, It should only be decryptable at... |
I am trying to get the idea of cyclic attacks againts assymetric RSA encryption.
Taken from _Handbook of applied cryptography_ .
Let $k$ be a positive integer such that $$c^{(e^{k})} = c\mod n \tag{1}.$$
There for
$k-1$ it holds that $$c^{(e^{k-1})} = m \mod n \tag 2$$ where $m$ is the message for encryption $n... |
The basic principle of encryption (Kerchoff's principle) is that the only thing necessary to decrypt data is the key. So really your question is, is "How can I make the full key available only during a certain time?" The two obvious answers are to either a) Only reveal the key during that time, or b) make the key depen... |
When a hardware system is protected by by a 4digit password what is the advantage of the attacker into breaking that system?Isn't it $2^{10}*2^{10}*2^{10}*2^{10}=2^{40}$. If this is the $\textit{Adv}$ of the attacker then is this reasonable cost? In my eyes this seems like a simple brute force attack for 4 numbers. I m... |
What is the advantage of an attacker over breaking 4digits PIN? |
When a hardware system is protected by by a 4digit password what is the advantage of the attacker into breaking that system?Isn't it $2^{10}*2^{10}*2^{10}*2^{10}=2^{40}$. If $\frac{1}{2^{40}}$ is the $\textit{Adv}$ of the attacker then is this reasonable computation for the attacker? In my eyes this seems like a simple... |
When a hardware system is protected by a 4 digits password, what is the advantage of the attacker into breaking that system?
Isn't it $2^{10}*2^{10}*2^{10}*2^{10}=2^{40}$? If $\frac{1}{2^{40}}$ is the $\textit{Adv}$ of the attacker then is this reasonable computation for the attacker? In my eyes this seems like a si... |
What is the advantage of an attacker over breaking a 4 digits PIN? |
When a hardware system is protected by a 4 digits password, what is the advantage of the attacker into breaking that system?
Isn't it $10*10*10*10=10^{4}$? If $\frac{1}{10^{4}}$ is the cost of such an attack then is this reasonable computation for the attacker? In my eyes this seems like a simple brute force attack ... |
I am developing a symmetric crypto library and have reached a roadblock. Looking at block ciphers, it is quite obvious that all block ciphers are trivially abstractable as a simple primitive consisting of:
- a key schedule
- a permutation function (which takes as an input a key, possibly a "tweak" and a data inpu... |
what is the capacity of AES in terms of File Encryption?
is it really good to encrypt a large files in AES? ex. I am encrypting a 8GB of File... is it still good to used AES?
is it still good to used AES-OFB in encryption of large files?
What also will be the issue if I used AES-ECB in file encryption? is the onl... |
what is the capacity of AES in terms of File Encryption?
is it really good to encrypt a large files in AES? ex. I am encrypting a 8GB of File... is it still good to used AES?
is it still good to used AES-OFB in encryption of large files?
What also will be the issue if I used AES-ECB in file encryption? is the onl... |
I have a cryptographic problem with the following characteristics:
1. I need to generate a set of relatively short messages; say 20 bytes in length
2. The contents of the messages themselves is not important to me as long as they are unique within a certain set.
3. These messages will be sent to a consumer.
4. Th... |
##The Scenario
I have a client-side web application that bounces requests against a server-side API. For the sake of simplicity, every request must pass a username and password. This is similar to old school XML-RPC where the username and password are passed as parameters in every request.
However, as this is a... |
How can I validate a hashed password if all I have is another hash? |
Something like this should work, right?
Server Side
--
To store the password, store the result of: sha2(sha2(client_salt + password) + server_salt), as well as client and server salts
Client Side
--
1. Have the server transmit client_salt to the client (since salts should always be unique, this will likely re... |
The capacity of AES in terms of file encryption is practically unlimited for the time being, especially in [OFB][1] mode.
An 8 [GB][2] file comprises short of $2^{29}$ 128-bit AES blocks. If one uses [CBC][3] or <strike>OFB</strike> [CFB][4] mode</strike>, odds of a collision (that is, the same block appearing in ci... |
Something like this should work, right?
There's no reason you have to use sha2, you can use any cryptographic hash function. I just used sha2 everywhere for simplicity.
Server Side
--
To store the password, store the result of: sha2(sha2(client_salt + password) + server_salt), as well as client and server salt... |
The capacity of AES in terms of file encryption is practically unlimited for the time being, especially in [OFB][1] mode.
An 8 [GB][2] file comprises short of $2^{29}$ 128-bit AES blocks. If one uses [CBC][3] or <strike>OFB</strike> [CFB][4] mode</strike>, odds of a collision (that is, the same block appearing in ci... |
The capacity of AES in terms of file encryption is practically unlimited for the time being, especially in [OFB][1] mode.
An 8 [GB][2] file comprises short of $2^{29}$ 128-bit AES blocks. If one uses [CBC][3] or <strike>OFB</strike> [CFB][4] mode</strike>, odds of a collision (that is, the same block appearing in ci... |
The capacity of AES in terms of file encryption is practically unlimited for the time being, especially in [OFB][1] mode.
An 8 [GB][2] file comprises short of $2^{29}$ 128-bit AES blocks. If one uses [CBC][3] or <strike>OFB</strike> [CFB][4] mode</strike>, odds of a collision (that is, the same block appearing in ci... |
Can Anybody tell me why the expectation value of the Index of Coincidence is the following?
For a cipher text string of length $L$, where $n$ is the number of
alphabet characters, $k_r = 1/n$, and $k_p = \sum_{i=1}^n p_i^2$
$$ E(IC) = \frac{1}{t}*\frac{L - t}{L - 1}*k_p + \frac{t - 1}{t}*\frac{L}{L - 1}*k_r$$
... |
Would TOR-like onion routing work for this problem?
* On Monday, each person posts a "package" -- Alice's address (Alice's public encryption key) (one of the original 4 keys), a blob of data encrypted by Alice's public encryption key, the whole thing signed by his own private signing key (one of the original 4 keys)... |
I have finally managed to verify some simple PGP signed message blocks. However, I discovered that for some reason, my implementation limits me to verifying data that is 9-16 bytes long. no less. no more.
is there some instruction somewhere ([RFC4880][1] or elsewhere) that specifies how to deal with plaintext data ... |
The capacity of AES in terms of file encryption is practically unlimited for the time being, especially in [OFB][1] or [CTR][2] mode.
An 8 [GB][3] file comprises short of $2^{29}$ 128-bit AES blocks. If one uses [CBC][4] or <strike>OFB</strike> [CFB][5] mode</strike>, odds of a collision (that is, the same block app... |
There is a profusion of articles proposing signature schemes _without random oracles_ ([see for yourself][1]). What does that _mean_, and why does it _matter_?
[1]: http://www.google.com/search?q=signature%20scheme%20without%20random%20oracles |
Why does it matter for a signature scheme to be without random oracles? |
How can I determine if I am generating a unique and strong Initialization Vector? How can I generate it? If my mode is generating Keystream? Is there a scientific explanation in generating a unique and strong Initialization Vector? |
How can I determine if I am generating a unique and strong Initialization Vector? If my mode is generating Keystream? Is there any scientific explanation in generating a unique and strong Initialization Vector? |
Suppose Alice chooses a random Prime $p$ and a random private Key $a \in \mathbb{Z}^*_p$. She also chooses a random generator $g \in \mathbb{Z}^*_p$, which is **not** a generator of $\mathbb{Z}^*_p$ and therefore $\langle g\rangle \subset \mathbb{Z}^*_p$ (as opposed to $\langle g\rangle = \mathbb{Z}^*_p$). Alice then c... |
Suppose Alice chooses a random Prime $p$ and a random private Key $a \in \mathbb{Z}^*_p$. She also chooses a random generator $g \in \mathbb{Z}^*_p$, which is **not** a generator of $\mathbb{Z}^*_p$ and therefore $\langle g\rangle \subset \mathbb{Z}^*_p$ (as opposed to $\langle g\rangle = \mathbb{Z}^*_p$). Alice then c... |
"Unique" is relatively straight forward. It's something you haven't used before. Unique usually only applies to the individual party, you only need to verify that the IV is unique for your usage with the current key.
"Strong" relates to the requirements that your mode of operation has for the IV. Different modes req... |
Suppose Alice chooses a random Prime $p$ and a random private Key $a \in \mathbb{Z}^*_p$. She also chooses a random generator $g \in \mathbb{Z}^*_p$, which is **not** a generator of $\mathbb{Z}^*_p$ and therefore $\langle g\rangle \subset \mathbb{Z}^*_p$ (as opposed to $\langle g\rangle = \mathbb{Z}^*_p$). Alice then c... |
Suppose Alice chooses a random Prime $p$ and a random private Key $a \in \mathbb{Z}^*_p$. She also chooses a random number $g \in \mathbb{Z}^*_p$, which is **not** a generator of $\mathbb{Z}^*_p$ and therefore
$$\langle g\rangle \subset \mathbb{Z}^*_p$$
as opposed to $\langle g\rangle = \mathbb{Z}^*_p$, which wou... |
Suppose Alice chooses a random Prime $p$ and a random private Key $a \in \mathbb{Z}^*_p$. By accident, she also chooses a random number $g \in \mathbb{Z}^*_p$, which is **not** a generator of $\mathbb{Z}^*_p$ and therefore
$$\langle g\rangle \subset \mathbb{Z}^*_p$$
as opposed to $\langle g\rangle = \mathbb{Z}^*_... |
Suppose Alice chooses a random Prime $p$ and a random private Key $a \in \mathbb{Z}^*_p$. By accident, she also chooses a random number $g \in \mathbb{Z}^*_p$, which is **not** a generator of $\mathbb{Z}^*_p$ and therefore
$$\langle g\rangle \subset \mathbb{Z}^*_p$$
as opposed to $\langle g\rangle = \mathbb{Z}^*_... |
"Unique" is relatively straight forward. It's something you haven't used before. Unique usually only applies to the individual party, you only need to verify that the IV is unique for your usage with the current key.
"Strong" relates to the requirements that your mode of operation has for the IV. Different modes req... |
Are there any hashing functions that if two are used in conjunction (with the same salts) will return the same response regardless of ordering?
i.e.
hash1(salt1 + hash2(salt2 + password)) == hash2(salt2 + hash1(salt1 + password))
|
Are there any hashing functions that if two are used in conjunction (with the same salts) will return the same response regardless of ordering?
i.e.
hash1(salt1 + hash2(salt2 + password)) == hash2(salt2 + hash1(salt1 + password))
|
"Unique" is relatively straight forward. It's something you haven't used before. Unique usually only applies to the individual party, you only need to verify that the IV is unique for your usage with the current key.
"Strong" relates to the requirements that your mode of operation has for the IV. Different modes req... |
I have finally managed to verify some simple PGP signed message blocks. However, I discovered that for some reason, my implementation limits me to verifying data that is 9-16 bytes long. no less. no more.
is there some instruction somewhere ([RFC4880][1] or elsewhere) that specifies how to deal with plaintext data ... |
According to the <a href="http://www.uscourts.gov/uscourts/Statistics/WiretapReports/2011/2011WireTap.pdf">US 2011 Wiretap Report</a>, encryption — on the off chance that it is encountered — has been no hurdle to retrieving the content of a conversation.
> Public Law 106-197 amended 18 U.S.C. § 2519(2)(b) in 2001 to... |
How are state wiretaps obtaining plaintext from encrypted transmissions? |
Background
------------
One issue with modern security proofs is that they are usually asymptotic. In other words, such proofs are usually formulated as follows: For any polynomial-time adversary $\mathcal A$, we can select a **large enough** security parameter $n$, such that the probability that $\mathcal A$ break... |
Are there any hashing functions that, if two are used in conjunction (with the same salts) will return the same response regardless of ordering?
I.e. are there hash-functions $H_1$, $H_2$ such that for all suitable salts $s_1$, $s_2$ and all passwords $x$ we have
$$ H_1(s_1, H_2(s_2, x)) = H_2(s_2, H_1(s_1, x)... |
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