instruction stringlengths 15 30k |
|---|
What is an approximate length for a digital signature (SHA) given the following?
Using RFID tags, size ranging from 100 to 2000 bytes (8000 bits), our project is setting aside a portion for a digital signature against its fixed data, which includes an unique 8-byte serial number and with 32 to 64 bytes of factory da... |
i dont get the utility to chain and reduce in generating rainbow tables.
Hash function is made to not have collisions, or at least chain only 2 times so we dont return to the concept of the hash dictionary.
So why we bother th machine to hash and reduce thousand of times to only store the latest value? |
Why we must hash lot of times in rainbow tables? |
I actually have a patent on a technique that can be used here -- or rather, I put in an application and then left that employer, and I don't know whether the patent issued; it seems based on [this Google patents link](https://www.google.com/patents/WO2002088912A3?cl=en&dq=william+whyte&hl=en&sa=X&ei=wzPvUbzpGtbK4AOehYH... |
The polynomial $p$ of degree $k$ can be reconstructed if at least $k+1$ correct points
are known (that is, we know that $(x_i,y_i), 0 \leq i \leq k$ are correct meaning that
$y_i = p(x_i), 0 \leq i \leq k$), and in this
case, Lagrangian interpolation works. If the points are being made available one at
a time inst... |
According to the paper, it takes up to 53 seconds for enough data to be transferred for the attack to be attempted. Then it takes 1-3 seconds with 50% probability on a 1.7 GHz Pentium-M to perform the crack (presumably this actually means between 1 and 6 seconds for 100% probability?).
I would do a back of the napki... |
Apparently it is *not* possible to compute all MD5 values from `0x00000000000000000000000000000000` to `0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF`.
As far as I understand this operation is hard because cryptographic hash functions make it difficult to calculate a message for a given digest (*Pre-image resistance*).
St... |
Suppose you got a prime $p = 2\mathbb\Pi_{i=0}^{n-1}q_i+1$, where $2^{k-1} \lt q_i \lt 2^k$ for some $k$ and all $0 \le i \lt n$, and that you also got a generator $g$ of one of the prime order sub groups of $\mathbb Z_p^*$. Also assume that the factorization of $\frac{p-1}{2}$ is unknown.
**Question #1:**
Are th... |
According to [this Q&A-discussion][1] it is possible to remove all personal data (name and mail address) that is attached to a public key.
1. What steps do I have to follow in order to remove all personal data?
2. Will that modified public key still allow me to decrypt data with my regular passphrase?
[1]: ht... |
How can I remove my personal data from my PGP public key? |
Is it possible to determine a PGP public key from encrypted data? |
I have implemented the Fortuna random number generator as described in chapter 10 of "Practical Cryptography" (Ferguson and Schneier, Wiley, 2003), the result can be found at http://www.seehuhn.de/pages/fortuna .
My question: How can I test my implementation? Or more specifically:
- Are there any known-correct t... |
Testing the Fortuna random number generator? |
Apparently it is *not* possible to compute all MD5 values from `0x00000000000000000000000000000000` to `0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF`.
As far as I understand this operation is hard because cryptographic hash functions make it difficult to calculate a message for a given digest (*Pre-image resistance*).
St... |
The comments attempt to answer by looking at the details of how rainbow tables work; however if you examine the problem they're attempting to solve, you can see that they would require a large number of hash computations, no matter how they operate internally.
Now, rainbow tables are a generic way to find preimages ... |
The following answer to another question describes the different values chosen as private key and public key. What are the advantages/disadvantages of each?
http://crypto.stackexchange.com/a/3606/7770
|
Is it possible to send messages over TLS with out encryption?
<br>
If so which cipher suite is needed for this? |
The comments attempt to answer by looking at the details of how rainbow tables work; however if you examine the problem they're attempting to solve, you can see that they would require a large number of hash computations, no matter how they operate internally.
Now, rainbow tables are a generic way to find preimages ... |
Bcrypt uses Blowfish to encrypt a derived key from the passphrase, and Blowfish is a cryptographic algorithm, but [here it is said that][1]:
> Note that
Salsa20/8 Core is not a cryptographic hash function since it is not
collision-resistant.
so how this is useful in Scrypt?
[1]: http://tools.ietf.or... |
How does Scrypt use Salsa? |
The polynomial $p$ of degree $k$ can be reconstructed if at least $k+1$ correct points
are known (that is, we know that $(x_i,y_i), 0 \leq i \leq k$ are correct meaning that
$y_i = p(x_i), 0 \leq i \leq k$), and in this
case, Lagrangian interpolation works. If the points are being made available one at
a time inst... |
(Disclaimer: I am regular software engineer with only basic crypto knowledge, so helpful if can be explained for a layman.)
I am concerned about generating weak keys on a shared linux box with openssl. Some questions:
1. How low does entropy have to be for it to be dangerous? If I run `cat /proc/sys/kernel/ran... |
What to watch for with openssl generating weak keys? (low entropy) |
(Disclaimer: I am regular software engineer with only basic crypto knowledge, so helpful if can be explained for a layman.)
I am concerned about generating weak keys on a shared linux box with openssl. Some questions:
1. How low does entropy have to be for it to be dangerous? If I run `cat /proc/sys/kernel/ran... |
I have the following set of equations:
$$M_{1}=\frac{y_1-y_0}{x_1-x_0}$$
$$M_{2}=\frac{y_2-y_0}{x_2-x_0}$$
$M_1, M_2, x_1, y_1, x_2, y_2,$ are known and they are chosen from a $GF(2^m)$. I want to find $x_0,y_0$
Does the previous set of equations is solvable?
And more...
If I have the following set... |
[This answer to another question](http://crypto.stackexchange.com/a/3606/7770) describes the different values chosen as private key and public key by various Ed25516 implementations. What are the advantages / disadvantages of each?
, I would like to know how random the value `ZZ` [in the DH protocol](http://www.ietf.org/rfc/rfc2631.txt) is. Obviously it is distinguishable from a Random Oracle, as the first byte will... |
I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have `urandom`). If you need other OS, please, inform me.
Also I will answer using source code of Linux 3.3.3 Kernel, because it is one of best documentation of `/dev/random` mechanics. And the other is paper: http://epri... |
What is the difference of the CDH problem in different groups? In particular, given a group $\mathbb{G}_1$ of order $q$ that is a subgroup of $\mathbb{Z}_q^*$, $q$ prime, and another group $\mathbb{G}_2$ of order $p$ that is a subgroup of $\mathbb{Z}_p^*$, $p = 2^n$, is solving the discrete log problem easier or harder... |
I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have `urandom`). If you need other OS, please, inform me.
Also I will answer using source code of Linux 3.3.3 Kernel, because it is one of best documentation of `/dev/random` mechanics. And the other is paper: http://epri... |
I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have `urandom`). If you need other OS, please, inform me.
Also I will answer using source code of Linux 3.3.3 Kernel, because it is one of best documentation of `/dev/random` mechanics. And the other is paper: http://epri... |
I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have `urandom`). If you need other OS, please, inform me.
Also I will answer using source code of Linux 3.3.3 Kernel, because it is one of best documentation of `/dev/random` mechanics. And the other is paper: http://epri... |
I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have `urandom`). If you need other OS, please, inform me.
Also I will answer using source code of Linux 3.3.3 Kernel, because it is one of best documentation of `/dev/random` mechanics. And the other is paper: http://epri... |
I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have `urandom`). If you need other OS, please, inform me.
Also I will answer using [source code of random.c][1] driver from Linux 3.3.3 Kernel, because it is one of best documentation of `/dev/random` mechanics. And the o... |
I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have `urandom`). If you need other OS, please, inform me.
Also I will answer using [source code of random.c][1] driver from Linux 3.3.3 Kernel, because it is one of best documentation of `/dev/random` mechanics. And the o... |
I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have `urandom`). If you need other OS, please, inform me.
Also I will answer using [source code of random.c][1] driver from Linux 3.3.3 Kernel, because it is one of best documentation of `/dev/random` mechanics. And the o... |
I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have `urandom`). If you need other OS, please, inform me.
Also I will answer using [source code of random.c][1] driver from Linux 3.3.3 Kernel, because it is one of best documentation of `/dev/random` mechanics. And the o... |
I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have `urandom`). If you need other OS, please, inform me.
Also I will answer using [source code of random.c][1] driver from Linux 3.3.3 Kernel, because it is one of best documentation of `/dev/random` mechanics. And the o... |
- How long does it take to extract a key from a FIPS-140 Level 2 device?
- What records are there of successful extraction?
- How much did the first extraction from a given device cost?
- How much would a subsequent extraction cost?
Dr. Google is usually very helpful on questions like this but he isn't hel... |
If a prime $q$ is large enough discrete logs and CDH in $\mathbb{Z}_q$ are traditional hard problems in cryptography.
Your other example $\mathbb{Z}_{2^n}$ is typically easy to solve because it can be solved progressively modulo increasing powers of $2$.
May be you intended to consider the finite field with $2^... |
PRFs are closely related to cryptographic hash functions. However, usually PRFs are built from PRGs (GGM construction), which in return can be created from cryptographic hash functions. However, the reverse construction is not trivial.
The security notion of both primitives is similar: If the PRF is drawn randomly f... |
I will answer considering Linux OS, as being one of most popular Unix-like OS (between OSes which have `urandom`). If you need other OS, please, inform me.
Also I will answer using [source code of random.c][1] driver from Linux 3.3.3 Kernel, because it is one of best documentation of `/dev/random` mechanics. And the o... |
The shared secret generated by the [Diffie–Hellman key exchange](http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange) is a random element of the subgroup of the [multiplicative group modulo $p$](http://en.wikipedia.org/wiki/Multiplicative_group_of_integers_modulo_n) generated by $g$.
In particular, for ... |
I am from a web development background (I don't know an awful lot about cryptography or how the algorithms themselves work), so I am asking this question in simple terms.
Consider a hash of the word 'test' using SHA1:
a94a8fe5ccb19ba61c4c0873d391e987982fbbd3
If I were to use SHA512 instead and then simply ... |
Is truncating a SHA512 hash to the first 40 characters as secure as using SHA1? |
> Are there any security implications for hashing and storing sensitive data like this?
The only way to "reverse" a hash is to mount a preimage attack. All secure modern-day cryptographic hashes have preimage resistance, which makes finding the preimage of any given digest computationally infeasible. So, your sensit... |
As a general rule, you should avoid SHA1 for new applications and instead go with one of the hash functions from the SHA-2 family.
As far as truncating a hash goes, [that's fine](http://crypto.stackexchange.com/questions/161/should-i-use-the-first-or-last-bits-from-a-sha-256-hash/163#163). It's explicitly endorsed b... |
It has recently emerged that a paper that was scheduled to appear at Usenix Security 2013, titled "Dismantling Megamos Crypto: Wirelessly Lockpicking a Vehicle Immobiliser", [has been censored](http://www.guardian.co.uk/technology/2013/jul/26/scientist-banned-revealing-codes-cars): a UK court issued a temporary injunct... |
It has recently emerged that a paper that was scheduled to appear at Usenix Security 2013, titled "Dismantling Megamos Crypto: Wirelessly Lockpicking a Vehicle Immobiliser", [has been censored](http://www.guardian.co.uk/technology/2013/jul/26/scientist-banned-revealing-codes-cars): a UK court issued a temporary injunct... |
I grasp that having known plaintext encrypted with the public key doesn't provide a means of discovering the private key (that being the whole point...).
But if one were in a situation where they didn't have access to the private key, but *did* have access to decryption by the private key, would the actual key be vu... |
Is a private RSA key vulnerable to a chosen plaintext encrypted with that private key? |
Moving the conversation further from [here][1] (I couldn't post a comment there because I have no reputation...), I don't think that the question is really answered in depth. I don't really think that encrypting twice with RSA is a waste of time. I think it depends on what is placed in-between the two encryptions (as C... |
Is it really not safer to encrypt twice with RSA? |
Moving the conversation further from [here][1] (I couldn't post a comment there because I have no reputation...), I don't think that the question is really answered in depth. I don't really think that encrypting twice with RSA is a waste of time. I think it depends on what is placed in-between the two encryptions (as C... |
Moving the conversation further from [here][1] (I couldn't post a comment there because I have no reputation...), I don't think that the question is really answered in depth. I don't really think that encrypting twice with RSA is a waste of time. I think it depends on what is placed in-between the two encryptions (as C... |
I am trying to find attacks on a theoretical protocol or prove its security. An initiator wants to establish a shared key with a responder with help of a trusted server.
Say we have the following roles (**I**: Initiator, **R**: Responder, **S**: Server).<br/>
Further we have the nonces: **sid**, **ni** (generated b... |
I have just learned about using PGP/GPG for email encryption and one thing has always bugged me:
**How is it possible that a message encrypted with somebody's public key can be decrypted only with that persons privated key?**
This concept of asymetric encryption is not very intuitive for me. Simply said I would e... |
Is there an intuitive explanation as to why only the private key can decrypt a message encrypted with the public key? |
I studied some encryption and decryption and I have found some very interesting problem to solve on the internet. I hope I am writing to right site - there are so many in StackExchange otherwise I hope some moderator will move it.
So, how to choose the appropriate public lets say `(i, m)` and private `(j, m)` keys w... |
Moving the conversation further from [here][1] (I couldn't post a comment there because I have no reputation...), I don't think that the question is really answered in depth. I don't really think that encrypting twice with RSA is a waste of time. I think it depends on what is placed in-between the two encryptions (as C... |
I have to deside, whether a document, which I am given, was correctly signed by a private key.
my Ideas is to sign the document id using RSA with the private key and to print the following onto the document:
encrypt(privateKey, id)
id
- Which requirenments should be fullfilled by the id, for the `enc... |
I have to decide, whether a document, which I am given, was correctly signed by a private key.
my Ideas is to sign the document id using RSA with the private key and to print the following onto the document:
encrypt(privateKey, id)
id
- Which requirenments should be fullfilled by the id, for the `enc... |
I am just learning about the GCM mode of AES and I'd like to start using it in my software. However, I have a question about its use. As I understand it the output of a GCM encryption operation is the ciphertext plus an "authentication tag." I know that the key needs to be guarded from discovery, but does this authe... |
Does the GCM authentication tag need to be protected? |
Assuming in the future there was a functioning 1024 qubit quantum supercomputer and it could run [Shor's algorithm][1] or [Grover's algorithm][2] to crack encryption very quickly. I'm interested in how the number of qubits translates to performance improvement over a regular 2 bit computer. Does this mean it could fact... |
In [point 4 of the Algorithm 4.2 (Classic Merkle Tree Traversal) of *"Post_Quantum_Cryptography.pdf"*][1] shows *"Stack_h.update(2)"* each stack receives two updates. I am trying understand "two updates" but I don't. There is an example on [page 19 of *"Boris_Ederov.bachelor.Merkle-Tree-Traversal-Techniques.pdf"*][2]. ... |
In [point 4 of the Algorithm 4.2 (Classic Merkle Tree Traversal) of *"Post_Quantum_Cryptography.pdf"*][1] shows *"Stack_h.update(2)"* each stack receives two updates. I am trying understand to understand why there are "two updates" but I don't. There is an example on [page 19 of *"Boris_Ederov.bachelor.Merkle-Tree-Trav... |
[Section 2 (end of page 42)][1] of *"Post_Quantum_Cryptography.pdf"* says:
> Then he generates the one-time signature $\sigma_{\text{OTS}}$ of the digest using the $s$-th one-time signature key $X_s$, $s\in\{0,\dots,2^H-1\}$.
My question: Is $s$ chosen in random form? Is it possible to have $s$ collisions to th... |
Given a server that broadcasts all received data to all connected peers (except the sender), two participants can have an encrypted conversation using a self-synchronizing method like CFB and some error detection.
How secure is this given that anyone can not only listen but also paste in chosen text into the stream?... |
You never state what $F$ is and what you mean by "safer/stronger", so I'm going to make a few assumptions.
For safer/stronger lets just use the purported bit strength. [RSA (the company) says][1] 2048 bit RSA is equivalent to a 112 bit symmetric key. Let $F$ be AES-128 (which we will assume has a bit strength of 128... |
The [Lamport-Diffie signature scheme](http://en.wikipedia.org/wiki/Lamport_signature) is said to be quantum-resistant. Why is that? What would a quantum attempt to attack this signature scheme look like, and how does it fail?
|
You never state what $F$ is and what you mean by "safer/stronger", so I'm going to make a few assumptions.
For safer/stronger lets just use the purported bit strength. [RSA (the company) says][1] 2048 bit RSA is equivalent to a 112 bit symmetric key. Let $F$ be AES-128 (which we will assume has a bit strength of 128... |
For safer/stronger lets just use the purported bit strength. [RSA (the company) says][1] 2048 bit RSA is equivalent to a 112 bit symmetric key. $F$ is basically a substitution cipher. If substituting bytes, there are $256!$ different substitution maps. We could say this has a bit strength of about $1684$. Combined in y... |
I am trying to find attacks on a theoretical protocol or prove its security. An initiator wants to establish a shared key with a responder with help of a trusted server.
We have the following roles: $I$: Initiator, $R$: Responder, $S$: Server.
There are long-term symmetric keys between all pairs of involved entit... |
Every time when I read a paper that has digital signature, when it comes to prove the security of a digital signature scheme, many chances that the author will use the forking lemma.
The forking lemma says as follows:
>Let A be a probabilistic polynomial time turing machine, given only the public data as input. I... |
Assuming in the future there was a functioning 1024 qubit quantum supercomputer and it could run [Shor's algorithm][1] or [Grover's algorithm][2] to crack encryption very quickly. I'm interested in how the number of qubits translates to performance improvement over a regular 2 bit computer.
For example, if I used Sh... |
I came across [this article][1], which describes a method, developed by UCLA CS professor Amit Sahai, for using functional encryption in order to achieve software obfuscation. The paper that the article refers to doesn't seem to be available online yet, but does anyone here have more information to share? Are the resul... |
This is past exam question which I am struggling to solve. I am not security expert nor intending to move in that direction in career path. The module is part of my MSc course and has nothing to do with my career. :-) So please spare me.
**Question:**
A stream of cipher operates on a data stream of 6-bit characters... |
This is a past exam question which I am struggling to solve. I am not a security expert nor intending to move in that direction as a in career path. The module is part of my MSc course and has nothing to do with my career. :-) So please spare me.
**Question:**
A stream of cipher operates on a data stream of 6-bit c... |
This is a past exam question which I am struggling to solve. I am not a security expert nor intending to move in that direction as a in career path. The module is part of my MSc course and has nothing to do with my career. :-) So please spare me.
**Question:**
A stream of cipher operates on a data stream of 6-bit c... |
This is a past exam question which I am struggling to solve. I am not a security expert nor intending to move in that direction as a in career path. The module is part of my MSc course and has nothing to do with my career. :-) So please spare me.
**Question:**
A stream of cipher operates on a data stream of 6-bit c... |
>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... |
Users in my system will be given a device that contains a 128-bit AES user key. The user key will be derived from 64 bits of public data related to the user, which is unique but very predictable (i.e. a sequential user ID value). The value of the user key must be kept secret from the users. I assume users are fully awa... |
Users in my system will be given a device that contains a 128-bit AES user key. The user key will be derived from 64 bits of public data related to the user, which is unique but very predictable (i.e. a sequential user ID value). The value of the user key must be kept secret from the users. I assume users are fully awa... |
Users in my system will be given a device that contains a 128-bit AES user key. The user key will be derived from 64 bits of public data related to the user, which is unique but very predictable (i.e. a sequential user ID value). The value of the user key must be kept secret from the users. I assume users are fully awa... |
I came across [this article][1], which describes a method, developed by UCLA CS professor Amit Sahai et al, for using functional encryption in order to achieve software obfuscation. The paper that the article refers to is available [here][2]. Has anyone digested the paper and have more information to share? Are the res... |
I am confused.
As I understand, if two users have a shared key, it is possible to encrypt message using symmetric key encryption.
So when a secret key is shared through *DH* asymmetric key exchange, it means that the users have their shared key. So why can't *AES* keys be generated from shared keys and continue... |
As I understand, if two users have a shared key, it is possible to encrypt a message using symmetric key encryption.
So when a secret key is shared through Diffie-Hellman asymmetric key exchange, it means that the users have their shared key. So why can't AES keys be generated from shared keys, and why not use only... |
Why is elgamal needed after a DH key exchange? |
This is a past exam question which I am struggling to solve. I am not a security expert nor intending to move in that direction as a in career path. The module is part of my MSc course and has nothing to do with my career. :-) So please spare me.
**Question:**
A stream of cipher operates on a data stream of 6-bit c... |
As I understand, if two users have a shared key, it is possible to encrypt a message using symmetric key encryption.
So when a secret key is shared through Diffie-Hellman asymmetric key exchange, it means that the users have their shared key. So why can't AES keys be generated from shared keys, and why not use only... |
>So why can't AES keys be generated from shared keys, and why not use only AES for message encryption after this point?
That is exactly what is done.
>if there is a shared key from a DH key exchange, why are we still talking about ElGamal asymmetric message encryption
Remember, DH is just one way to exchange a... |
**SIGMA**
The SIGMA paper does not describe how a "response message" for SIGMA-I would be implemented. If it was implemented as (for example) $B$ sending $\:\operatorname{MAC}_{K_m}\hspace{-0.02 in}(\text{"ACK"})\:$ to $A$, then that would not actually provide the desired _peer awareness_ property in the case where ... |
>So why can't AES keys be generated from shared keys, and why not use only AES for message encryption after this point?
That is exactly what is done.
>if there is a shared key from a DH key exchange, why are we still talking about ElGamal asymmetric message encryption
Remember, DH is just one way to exchange a... |
There is nothing related to passwords in AES. AES uses 128-bit _keys_, i.e. sequences of 128 bits. How you come up with such a key is out of scope of AES. _In some contexts_, you want to generate these 128 bits in a deterministic way from a password (and possibly some publicly known contextual data, like a "salt"); thi... |
To give you some background, I have two systems which need to encrypt their communication by `AES-128`. The key for the AES encryption should be exchanged with the `Diffie-Hellman (DH)` algorithm.
Now, I've read about `DH` and got a lot of question marks over my head. These are my questions:
1. Typically with AES... |
Can you help me understand how to handle AES encryption using a Diffie-Hellman key exchange? |
Could you explain the '[Breaking 104 bit WEP in less than 60 seconds][1]' paper, without skipping too much technical details.
Could you an answer questions, like: how is '1.24/256' a significant difference compared to '1/256'? And how this seemingly small difference could lead a '104-bit WEP key' to a reduced requi... |
> **Question:**
A stream of cipher operates on a data stream of 6-bit characters using a simple mono-alphabetic substitution technique. Estimate and explain the number of different substitution alphabets possible. The key is effectively the substitution alphabet, which can be expressed as a $384$-bit number (i.e. $64×... |
Every time when I read a paper that has digital signature, when it comes to prove the security of a digital signature scheme, many chances that the author will use the forking lemma.
The forking lemma says as follows:
> Let $\mathcal A$ be a probabilistic polynomial time turing machine, given only the public data... |
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