instruction stringlengths 15 30k |
|---|
> Are the results as ground breaking as the article suggests?
This result will prove to be a very important one for theoretical crypto. The analogy to fully homomorphic encryption (mentioned above by Samuel) is useful, since that is another well-known result that was hugely groundbreaking from a theoretical point of... |
>What is known about the security of [identity-based encryption](http://en.wikipedia.org/wiki/Identity-based_encryption) schemes
<br>
against attacks that involve seeing multiple ciphertexts and then
<br>
receiving the private keys corresponding to _some_ of those ciphertexts?
<br>
For example, is it known that... |
Don't believe every claim ever made in any paper ever written, particularly when the paper provides little or no justification for the claim; not everything you read reflects the cryptographic consensus. This is particularly true for a paper written in 2002, which is a time our understanding of authenticated encryptio... |
[Homomorphic encryption schemes][1] are PKE schemes with an additional special method Evaluate. $Evaluate$ method takes input any function (as boolean circuit) and encrypted inputs of the function and evaluates the circuit. The results of such function are still encrypted under the public key $PK$ .
Now if the user ... |
[Homomorphic encryption schemes][1] are PKE schemes with an additional special method Evaluate. $Evaluate$ method takes input any function (as boolean circuit) and encrypted inputs of the function and evaluates the circuit. The results of such function are still encrypted under the public key $PK$ .
Now if the user ... |
can we proxy reencrypt using homomorphic encryption schemes? |
You can probably prove the security against your game from the security in the IND-ID-CCA game of Boneh and Franklin (see http://courses.cs.vt.edu/cs6204/Privacy-Security/Papers/Crypto/IBE-Weil-Pairing.pdf). The idea is to create an adversary $\mathcal{B}$ against IND-ID-CCA from your adversary $\mathcal{A}$. Essential... |
On page 14 of ["Keccak and the SHA-3 Standardization" (February 6, 2013)](http://csrc.nist.gov/groups/ST/hash/sha-3/documents/Keccak-slides-at-NIST.pdf) it says:
> - Instantiation of a sponge function
- the permutation KECCAK-f
- 7 permutations: b → {25,50,100,200,400,800,1600}
- Security-speed trade-of... |
What exactly is the base for the KECCAK claim that a security strength of 256 bits is "post-quantum sufficient"? |
As we can know that the best two-pass AKE protocols with DH message can achieve is the weak form of perfect forward security (wPFS) which guarantees security against the passive adversary. But this [paper][1] claims that the AKE it proposed can achieve perfect
forward security against active adversary in one round.
... |
if there exist an two-pass AKE protocol that is secure in eCK model and also has PFS? |
Well, cryptographers have been contemplating a post-quantum world for some time now.
Quantum computing, although in its infancy as far as real-life computers go, has been studied in a theoretical sense for a quite a while. [Shor's algorithm](http://en.wikipedia.org/wiki/Shor%27s_algorithm) was published 19 years ago... |
As we can know that the best two-pass AKE protocols with DH message can achieve is the weak form of perfect forward security (wPFS) which guarantees security against the passive adversary. But [*"Strongly Secure One Round Authenticated Key Exchange Protocol with Perfect Forward Security"* (Hai Huang)][1] claims that t... |
It is possible to achieve PFS against active adversaries in two messages.
The "as we know" that you mention is incorrect; this misconception seems to stem from over-interpreting Krawczyk's result in his <a href="http://eprint.iacr.org/2005/176.pdf">HMQV paper</a> from 2005. At best, the argument seems to hold from pro... |
As we can know that the best two-pass AKE protocols with DH message can achieve is the weak form of perfect forward security (wPFS) which guarantees security against the passive adversary. But [*"Strongly Secure One Round Authenticated Key Exchange Protocol with Perfect Forward Security"* (Hai Huang)][1] claims that th... |
Does there exist a two-pass AKE protocol that is secure in eCK model and also has PFS? |
It is possible to achieve PFS against active adversaries in two messages.
The "as we know" that you mention is incorrect; this misconception seems to stem from over-interpreting Krawczyk's result in his <a href="http://eprint.iacr.org/2005/176.pdf">HMQV paper</a> from 2005. At best, the argument seems to hold from pro... |
Well, cryptographers have been contemplating a post-quantum world for some time now.
Quantum computing, although in its infancy as far as real-life computers go, has been studied in a theoretical sense for a quite a while. [Shor's algorithm](http://en.wikipedia.org/wiki/Shor%27s_algorithm) was published 19 years ago... |
Can we access control functionality in Fully homomorphic encryption schemes? |
[Homomorphic encryption schemes][1] are PKE schemes with an additional special method Evaluate. Evaluate method takes input any function (as boolean circuit) and encrypted inputs of the function and evaluates the circuit. The results of such function are still encrypted under the public key PK.
Sometimes, it would b... |
I was reading this article and had a question....
http://arstechnica.com/security/2013/08/gone-in-30-seconds-new-attack-plucks-secrets-from-https-protected-pages/
As I understand it TLS/SSL work in such a way that even the Requested URL is encrypted. So, my question is...would it be possible to create an SSL pro... |
Could BREACH be used to determine an HTTPS URL? |
Assume an ElGamal Cryptosystem. Assume a set of three players, *P1*, *P2* and *P3*. The private key **x** is shared among the players. The player *P1* has a piece of the private key **x1**, *P2* has the piece **x2** and *P3* has **x3**.
Assume the following messages, **m1** and **m2**.
Let E(m1) and E(m2) be two ... |
How oblivious test of plaintext equality work? |
Assume an ElGamal Cryptosystem. Assume a set of three players, *P1*, *P2* and *P3*. The private key **x** is shared among the players. The player *P1* has a piece of the private key **x1**, *P2* has the piece **x2** and *P3* has **x3**.
Assume the following messages, **m1** and **m2**.
Let E(m1) and E(m2) be two ... |
How does an oblivious test of plaintext equality work? |
In the zero knowledge proof using QR, why do we even bother with the server sending us b = 0 back? As I understand it, the server selects b = 0 or b = 1. If b == 0, then the client (alice) sends back random number (r) squared mod x, while if it's b == 1, then alice sends back r^2 * y mod x. How is b = 0 of any value th... |
Zero Knowledge Proof - Quadratic Residue, why two options? |
Can we give access to controlled functionality in Fully homomorphic encryption schemes? |
What "zero knowledge proof using QR" are you talking about?
In [this one](http://www.cs.princeton.edu/courses/archive/fall07/cos433/lec15.pdf), "we bother with" Bob sending Alice $\:b=0\:$ so that if
<br>
for both possible values of $b$, there exists a response which would cause Bob to accept
<br>
then $x$ is a ... |
What "zero knowledge proof using QR" are you talking about?
In [this one](http://www.cs.princeton.edu/courses/archive/fall07/cos433/lec15.pdf), "we bother with" Bob sending Alice $\:b=0\:$ so that if $\:\: y\not\equiv 0 \;$ and
<br>
for both possible values of $b$, there exists a response which would cause Bob to ... |
"OAEP was proved IND-CCA-secure by Fujisaki et al. [28] under the assumption that the underlying trapdoor permutation is partial-domain one-way. This is in general a
stronger assumption than just one-wayness, but fortunately both assumptions are equivalent in particular for RSA."
My question is what is exactly the ... |
The above discussion indicates how knowing the factors of n allows us to find the roots efficiently. If the modulus n has been factored into prime then its solution was discussed above.
However, the converse statement is also true, that is quadratic residuosity problem could reduce to integer factorization, meanin... |
Assume an ElGamal Cryptosystem. Assume a set of three players, $P_1$, $P_2$ and $P_3$. The private key $x$ is shared among the players. The player $P_1$ has a piece of the private key $x_1$, $P_2$ has the piece $x_2$ and $P_3$ has $x_3$.
Assume the following messages, $m_1$ and $m_2$.
Let $E(m_1)$ and $E(m_2)$ be... |
In the zero knowledge proof using QR, why do we even bother with the server sending us $b = 0$ back? As I understand it, the server selects $b = 0$ or $b = 1$. If $b = 0$, then the client (Alice) generates a random number $r$ and sends back $r^2 \bmod x$, while if $b = 1$, then Alice sends back $r^2 \, y \bmod x$. How ... |
Zero-knowledge proof using quadratic residue: why two options? |
E. g. block ciphers have GCM or OCB, that take single cryptographic primitive (block cipher) and provide both authentication and encryption.
Are there similar constructs for stream ciphers, which provide both authentication and encryption using only one cryptographic primitive - actual stream cipher (or keystream)? |
Authenticated encryption mode for stream cipher? |
I'm not realy experienced with programming and such but I am wondering if the following could be implemented in, lets say, python
we have the following ciphertext sequince AO AP AL EE IL OP QR IV OD DD GH DK OL FK PO AD GG PO FK OL DK GH OL OT RI DH SK
and I know part of the plaintext says:risetovotesir
Than f... |
Assume an ElGamal Cryptosystem. Assume a set of three players, $P_1$, $P_2$ and $P_3$. The private key $x$ is shared among the players. The player $P_1$ has a piece of the private key $x_1$, $P_2$ has the piece $x_2$ and $P_3$ has $x_3$.
Assume the following messages, $m_1$ and $m_2$.
Let $E(m_1)$ and $E(m_2)$ be... |
At Blackhat this week there was a talk saying RSA is (essentially) doomed in the near future.
http://www.networkworld.com/news/2013/080213-black-hat-elliptical-curve-cryptography-272476.html?page=1
What "two devastating attacks in the past 6 months" is Mr Ritter referring to, and real is the threat to RSA? |
At Blackhat this week there was a talk saying RSA is (essentially) doomed in the near future.
http://www.networkworld.com/news/2013/080213-black-hat-elliptical-curve-cryptography-272476.html?page=1
What "two devastating attacks in the past 6 months" is Mr Ritter referring to, and how real is the threat to RSA? |
I'm not realy experienced with programming and such but I am wondering if the following could be implemented in, lets say, python
we have the following ciphertext sequence
AO AP AL EE IL OP QR IV OD DD GH DK OL FK PO AD GG PO FK OL DK GH OL OT RI DH SK
and I know part of the plaintext says:
risetov... |
How can we construct a (pseudo) random permutation from a (pseudo) random function ?
I've seen a method from Luby-Rackoff which allows to construct a PRP of length $2N$ from a PRF of input/output length $N$. Is there a method to keep the same length $N$ ?
I'm looking for a method that has better security bounds t... |
Given a TOTALLY HYPOTHETICAL scenario in which passwords for over a million accounts in over 200 databases have been stored using 3DES, a key derived from a (single) MD5 hash of a customer-controlled (rarely changed from the default) pass-phrase, a hard-coded IV (same for every account in every database), and a cipherm... |
This question is related to [this one](http://crypto.stackexchange.com/q/270/5231) in intention, and [this one](http://crypto.stackexchange.com/q/6531/5231) in scope.
**Would I be guarding myself against future attacks on a single PRNG if I combine two (or more) together?**
I am thinking of using the Fortuna PRNG... |
RSA and ElGamal can be implemented using the technique of Elliptic curves. I am confused on why the it seems that Elliptic curves are not so popular in cryptographic applications since they provide the same security with smaller key sizes?
|
Why are you afraid of "overseeding" the PRNG? The Java `SecureRandom` class specifies the [`setSeed`](http://docs.oracle.com/javase/7/docs/api/java/security/SecureRandom.html#setSeed(byte[])) function as:
public void setSeed(byte[] seed)
So just feed the 320 bits (40 bytes) data to the `setSeed` function and ... |
In mix Columns we have:
C(x) = {03}X^3 + {01}X^2 + {01}X^1 + {02}
In Viktor Fischer's [Paper][1] on MixColumn andInvMixColumn Resource Sharing, in page 2, Equation 8, it's been said that C^2(X) can be computed as follows:
C^2(x) = {04}X^2 + {05}
Can anybody explain why is that?
Thank you
[1]: http://www.ke... |
How to Compute C^2 in AES MixColumns Matrix? |
I am interested in the theoretical consideration of the bit strength of an encryption key and its precursor.
Assume a given environment as follows:<br>
a. My encryption algorithm is AES-256<br>
b. My key derivation function is PBKDF2 with HMAC-SHA1 with<br>
b1. Salt strength of 128 bit (32 chars of a truely ra... |
Since 18th of December 2010 Germany makes use of an identity card that should allow electronic authentication:
![enter image description here][1]
(image source: [commons.wikipedia.org][2])
From a users perspective, the authentication seems to work as follows:
1. A program requests permission to read inform... |
How does identification currently work with the German identity card? |
I am interested in the theoretical consideration of the bit strength of an encryption key and its precursor.
Assume a given environment as follows:<br>
a. My encryption algorithm is AES-256<br>
b. My key derivation function is PBKDF2 with HMAC-SHA1 with<br>
b1. Salt strength of 128 bit (32 chars of a truely ra... |
It is possible to achieve PFS against active adversaries in two messages.
The "as we know" that you mention is incorrect; this misconception seems to stem from over-interpreting Krawczyk's result in his <a href="http://eprint.iacr.org/2005/176.pdf">HMQV paper</a> from 2005. At best, the argument seems to hold from pro... |
In mix Columns we have:
$$C(x) = \{03\}X^3 + \{01\}X^2 + \{01\}X^1 + \{02\}$$
In Viktor Fischer's [Paper][1] on `MixColumn` and `InvMixColumn` Resource Sharing, in page 2, Equation 8, it's been said that $C^2(X)$ can be computed as follows:
$$C^2(x) = \{04\}X^2 + \{05\}$$
Can anybody explain why is that?
... |
Sysadmins have switched from one password hash algorithm to another many times.
There is a standard process for this switch.
The Modular Crypt Format (MCF) is the standard scheme for formatting encrypted passwords, as used by passwd, crypt, etc.
(see http://crypto.stackexchange.com/questions/2968/why-are-there-sig... |
In AES algorithm, in the key schedule,
Why does the expansion of a 256 bit key need an extra application of the S-box, unlike the expansion of 128 bit and 192 bit keys ?
(The obvious answer would be "because it is more secure" ... but why ? And if it is more secure, why don't use it on 128 and 192 ?) |
There is a paper from Peter W. Shor from 1994: http://www.csee.wvu.edu/~xinl/library/papers/comp/shor_focs1994.pdf "*Algorithms for Quantum Computation: Discrete Logarithms and Factoring*", and I have a question about the paper and algorithms presented.
For integer factoring problem, Shor's algorithm is working as f... |
There is a paper from Peter W. Shor from 1994: http://www.csee.wvu.edu/~xinl/library/papers/comp/shor_focs1994.pdf "*Algorithms for Quantum Computation: Discrete Logarithms and Factoring*", and I have a question about the paper and algorithms presented.
For integer factoring problem, Shor's algorithm is working as f... |
> Every time a file on the data partition is created, Data Protection creates a new 256-bit
key (the “per-file” key) and gives it to the hardware AES engine, which uses the key to
encrypt the file as it is written to flash memory using AES CBC mode. The initialization
vector (IV) is the output of a linear feedback s... |
iOS Data Protection: What is the Input of the LFSR? |
To give some general intuition:
Longer keys give the attacker more "degrees of freedom" in a related-key attack. Therefore, defending against related-key attacks likely requires more complex key schedule if the key is long, than if the key is short. That might explain, at a conceptual level, why 256-bit keys requi... |
> Every time a file on the data partition is created, Data Protection creates a new 256-bit
key (the “per-file” key) and gives it to the hardware AES engine, which uses the key to
encrypt the file as it is written to flash memory using AES CBC mode. The initialization
vector (IV) is the output of a linear feedback s... |
In 2012 a group of researchers collected a large amount of RSA moduli and calculated their greatest common divisor in order to find common factors between them. By finding a common factor they could the divide and completely factorize the key. They concluded that the vulnerable moduli where generated by PRNGs with inad... |
Does it necessarily mean that an RSA moduli generated with poor randomness is not random? |
There is a paper from Peter W. Shor from 1994: http://www.csee.wvu.edu/~xinl/library/papers/comp/shor_focs1994.pdf "*Algorithms for Quantum Computation: Discrete Logarithms and Factoring*", and I have a question about the paper and algorithms presented.
For integer factoring problem, Shor's algorithm is working as f... |
I have an elliptic curve defined over finite field where S1=a.P . Is it valid to say that S1.P can also be computed. P is the generator of the group.
What my real question is that. Should 'a' be always an integer or can it be a group element itself?? |
I have an elliptic curve defined over finite field where $S_1=aP$ . Is it valid to say that $S_1P$ can also be computed. $P$ is the generator of the group.
What my real question is that. Should '$a$' be always an integer or can it be a group element itself?? |
I am a bit confused on the hardness of the discrete logarithm problem. Does it become intractale only when it is mod n, where n is a large composite number (Like RSA key). What about if it is mod a prime number p? what is the complexity and method for solving it? Can we reduce the problem of integer factorization to d... |
> Should '$a$' be always an integer or can it be a group element itself??
Well, $aP$ is defined to be:
$aP \equiv \underbrace{P + P + \ldots + P}_\text{a times}$
From that definition, we see that this makes sense only if $a$ is an integer; it needs to be a count of the number of $P$'s to add together. |
In Section 7.2 "Validation of HOTP Values" of the HOTP spec (RFC 4226) it says, in part,
> The HOTP client (hardware or software token) increments its counter
> and then calculates the next HOTP value HOTP client. If the value
> received by the authentication server matches the value calculated by
> the client,... |
What is the exact difference between CKM_RSA_PKCS & CKM_RSA_X_509 mechanisms is PKCS#11 v2.20 specification (Please explain with an example)?<br/>
|
would something like the following (php code) improve security (against rainbow attacks, not brute force)?
$password='your password';
$salt='some salt';
$step1=md5($password.$salt);
$step2=md5($step1.$password).$step1;
$hash=md5($step2);
the idea is to increase cyphertext length and md5 it... |
salting with password hash to improve security? |
A system is used to securely store account numbers.
When an account number is added to the system it is securely encrypted and stored in a database along with the IV and a reference to the encrypted record is returned (so that the encrypted record can be retrieved at a later date).
If a second request for an ide... |
I was wondering if changing the cipher mode to AES-XTS in EncFS [1] would be a sound idea. EncFS uses AES in CBC mode until the last 1KB block, and CFB mode for the last block if len(block) < 1KB. This ensures that len(plaintext) == len(ciphertext).
The design seems somewhat convoluted, but understandable consideri... |
Is AES-XTS considered safe to encrypt multiple files with the same keys? |
I was wondering if changing the cipher mode to [AES-XTS in EncFS](http://www.arg0.net/encfs) would be a sound idea. EncFS uses AES in CBC mode until the last 1KB block, and CFB mode for the last block if `len(block) < 1KB`. This ensures that `len(plaintext) == len(ciphertext)`.
The design seems somewhat convoluted,... |
I have an application where I want to be able to send an encrypted file, and then mete out "keys" that allow the receipient to decrypt the file from a certain point to the end of the file. Actually, it's from a certain point to the beginning, and I'm doing the above by reversing the file.
Looking at AES (and thus Ri... |
Swapping Key and IV in AES? Safe? |
In order to setup an authenticated shared secret key between two clients, I am faced with the choice between two possibilities:
- A: Let users set a (low-entropy) shared password and then perform some password-authenticated key exchange that is resilient to low-entropy secrets (as described [here](http://tools.ietf.... |
Instead of using a hash function, use a pseudo-random function (PRF). Unlike hash functions, which are publicly computable, PRFs use a secret key. HMAC-SHA256 is a widely supported PRF (although usually it's used as a MAC, which is a bit different). Use a different key for the PRF then you use for encryption.
Anothe... |
How secure would this code be against cryptanalysis? |
As I understand it, the less people know about the internals of my protocol or cipher, the more secure the protocol is. However [Kerckhoffs's principle](https://en.wikipedia.org/wiki/Kerckhoffs%27s_principle) states that
> A cryptosystem should be secure even if everything about the system, except the key, is public... |
Why should I make my cipher public? |
When publishing the algorithm other people can review your algorithm and design.
With this they can find flaws in your design and therefore improve this.
The principle behind this is simply that many eyes find more errors than two eyes.
Why do you think does the NIST review new candidates for cipher suites (AE... |
Security against attacks using rainbow tables requires using a salt. In your first step $A_1 = MD5(P || S)$, you're already mixing in a salt. Provided that the salt is indeed globally unique (and randomly generating the salt is a correct way of achieving that), you already have protection against rainbow tables.
In ... |
In Section 7.2 "Validation of HOTP Values" of the HOTP spec ([RFC 4226](http://tools.ietf.org/html/rfc4226#section-7.2)) it says, in part,
> The HOTP client (hardware or software token) increments its counter
> and then calculates the next HOTP value HOTP client. If the value
> received by the authentication ser... |
Would something like the following improve security (against rainbow attacks, not brute force)? Assume that $P$ is a user-chosen password, and the objective is to obtain a hash $H$ for password checking.
* Let $S$ be a random salt.
* Let $A_1 = MD5(P || S)$
* Let $A_2 = MD5(A_1 || P) || A_1$
* Let $H = MD5(A_... |
Actually, that wikipedia article already answers your question:
> It is moderately common for companies and sometimes even standards bodies as in the case of the CSS encryption on DVDs – to keep the inner workings of a system secret. Some argue this "security by obscurity" makes the product safer and less vulnerabl... |
Actually, that wikipedia article you mention in your question already answers your question:
> It is moderately common for companies and sometimes even standards bodies as in the case of the CSS encryption on DVDs – to keep the inner workings of a system secret. Some argue this "security by obscurity" makes the pro... |
Actually, that wikipedia article you mention in your question already answers your question:
> It is moderately common for companies and sometimes even standards bodies as in the case of the CSS encryption on DVDs – to keep the inner workings of a system secret. Some argue this "security by obscurity" makes the pro... |
Actually, that wikipedia article you mention in your question already answers your question:
> It is moderately common for companies and sometimes even standards bodies as in the case of the CSS encryption on DVDs – to keep the inner workings of a system secret. Some argue this "security by obscurity" makes the pro... |
Wanted to add some to Mikero's answer.
There are three main contributions of the research
1. A proposed indistinguishability obfuscation for NC1 circuits where the security is based on the so called Multilinear Jigsaw Puzzles (a simplified variant of multilinear maps).
2. Pair the contribution in 1 with Fully ... |
I'm writing a client application that wants to store some secret information with a storage service. The client has to authenticate the user with the service and the service should not be able to decode the information that it stores for the user.
I want to minimize the information the user has to memorize or store ... |
Yes, you should be able to handle this situation readily. There are many optimizations available.
One key observation is that if you're going to go to disk, then you might as well read lots of data: it takes just as long to read an entire block of data as to read 1 byte. So, I suggest you store the data on disk in... |
Your proposal involves the server storing $H(P)$, and the client authenticating by sending $H(P)$ to the server in cleartext. This has multiple problems:
- For authentication purposes, $H(P)$ essentially becomes the user's credential. The server stores this credential in cleartext. That's less than ideal, because... |
I'm writing a client application that wants to store some secret information with a storage service. The client has to authenticate the user with the service and the service should not be able to decode the information that it stores for the user. Authentication against the service has to be based on username - passwor... |
I agree that Gilles' interpretation in the comments is the only one that makes sense; the RFC clearly contains an editorial error, and should read either (_emphasis_ indicates corrections):
> "If the value _calculated_ by the authentication server matches the value calculated by the client, then the HOTP value is va... |
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