instruction stringlengths 15 30k |
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Informally, In [functional encryption][1] system , a decryption key allows a user to
learn a *function* of the encrypted data. But all i see is that the function acts as an access control over the data.
Can we have richer functions, say can a user add two cipher texts and get the result in plain text using decrypt... |
Is Functional Encryption about Access Control over encrypted data alone? |
To answer your specific questions:
> From reading around I understand that ... encrypting a counter and use that as the IV is OK,
> is that true?
For CBC mode, that is absolutely true, as long as the key that you use to encrypt the counter is secret (that is, not known to any possible adversary).
> Sh... |
Tl;dr: In theory, yes, if you do it carefully. But in practice, it is better to use SecRandomCopyBytes().
Breaking it down a bit first, we step into a definition. "True" random numbers of the kind you are discussing are those that are truly unpredictable, where the preceding data has absolutely no relationship to ... |
My area of expertise is reverse engineering, specifically embedded systems. I do attack cryptographic systems, but this largely involves key recovery or exploiting the implementation.
I was asked to reverse engineer a simple Windows executable used as part of a challenge-response protocol used for intruder alarm sys... |
Help me describe/identify this challenge-response protocol/algorithm? |
My area of expertise is reverse engineering, specifically embedded systems. I do attack cryptographic systems, but this largely involves key recovery or exploiting the implementation.
I was asked to reverse engineer a simple Windows executable used as part of a challenge-response protocol used for intruder alarm sys... |
> Is there any mathematical result that gives us the minimum number of
> 1's in a 160-bit SHA-1 hash output?
A good (secure) hash function has output that is uniformly and evenly distributed and shouldn't be distinguishable from random value.
![enter image description here][1]
*[Chi-squared tests][2] of several... |
I'm student and I'm currently working on hill cipher. I started by using 2x2 matrix and transposed charset in simplest form (A = 0; B = 1 ...). This is pretty simple to break in case of choosen plaintexts (as i do).
My question is now the following, I want to attribute a random order to my charset (for example A = 2... |
I wanted to know that using encryption algorithm like AES,etc is it possible to generate fixed length cipher text no matter how long the plain text becomes? |
My area of expertise is reverse engineering, specifically embedded systems. I do attack cryptographic systems, but this largely involves key recovery or exploiting the implementation.
I was asked to reverse engineer a simple Windows executable used as part of a challenge-response protocol used for intruder alarm sys... |
I'm in the process of designing a cryptographic protocol which will reduce the impact an attacker will have if they gained root access to a server storing data.
The basic crux of it is that keys which were used to encrypt data are not stored on the server and are only ever in memory for a short period of time.
T... |
Is this a sensible cryptographic protocol intending to reduce the impact of compromised security? |
To answer your specific questions:
> From reading around I understand that ... encrypting a counter and use that as the IV is OK,
> is that true?
For CBC mode, that is absolutely true, as long as the key that you use to encrypt the counter is secret (that is, not known to any possible adversary).
> Sh... |
Have you considered generating the IV in a cryptographically pseudorandom way? This approach makes it easiest to be sure that you're not introducing some subtle weakness in your IV selection.
I would start by benchmarking how fast it is to use RandomNumberGenerator. If that's fast enough, job done. I would not gi... |
My area of expertise is reverse engineering, specifically embedded systems. I do attack cryptographic systems, but this largely involves key recovery or exploiting the implementation.
I was asked to reverse engineer a simple Windows executable used as part of a challenge-response protocol used for intruder alarm sys... |
> Is there any mathematical result that gives us the minimum number of
> 1's in a 160-bit SHA-1 hash output?
A good (secure) hash function has output that is uniformly and evenly distributed and shouldn't be distinguishable from random value.
![enter image description here][1]
*[Chi-squared tests][2] of sever... |
I have a general question about Padding used in RSA.
The heart of OAEP algorithm are the cryptographic hash functions H and G.
Does everybody (so also an adversary) know these functions?
**If YES:** How does it help the security, if he just can decode the padding and read the message?
**If NO:** What... |
Well, yes, everyone (or, at least, everyone who can use the public key) knows the hash function H and G; so we can assume that an adversary knows them as well.
You ask:
If YES: How does it help the security, if he just can decode the padding and
read the message?
Well, he can't decode the padding; the... |
I am going to apply a simple substitution cipher to my input, then encrypt the result with a Hill cipher. How can this be broken, in a chosen-plaintext threat model?
In other words, instead of the standard mapping of letters to numbers (A=0, B=1, .., Z=25), I will randomly reorder them (e.g., A=22, B=12, ...), wher... |
I'm writing a TLS 1.2 implementation (in C#) using a custom protocol.
My question is how to generate the IV attached to each message, I don't want to use RandonNumberGenerator because I've heard it is slow. Instead, I would like to use some PRNG. I'm only going to suppport AES128/256.
From reading around I under... |
Using an encryption algorithm like AES, is it possible to generate a fixed length cipher text no matter how long the plain text becomes? |
This can be broken. The exact nature of the attack will depend what modulus you use for the Hill cipher: are you working modulo a prime number, or working modulo 26?
Working modulo a prime $p$
--------------------------
*A simple attack, with no fancy mathematics needed.* One simple attack is to start by reques... |
This can be broken. The exact nature of the attack will depend what modulus you use for the Hill cipher: are you working modulo a prime number, or working modulo 26?
Working modulo a prime $p$
--------------------------
*A simple attack, with no fancy mathematics needed.* One simple attack is to start by reques... |
This can be broken. The exact nature of the attack will depend what modulus you use for the Hill cipher: are you working modulo a prime number, or working modulo 26?
Working modulo a prime $p$
--------------------------
*A simple attack, with no fancy mathematics needed.* One simple attack is to start by reques... |
I'm in the process of designing a cryptographic protocol which will reduce the impact an attacker will have if they gained root access to a server storing data.
The basic crux of it is that keys which were used to encrypt data are not stored on the server and are only ever in memory for a short period of time.
T... |
I want to multiply and divide binary polynomials over a Galois Field of size (2^128) (multi precision multiplication and extended euclidean algorithm). Moreover, I want to use the previous algorithms on an 8-bit microcontroller (ATMega128). Where can I find relevant libraries in C or Assembly ? Ι d prefer C, if the imp... |
I'm taking an online class on cryptography at corsera.org / Stanford, and the professor is explaining that it's OK to truncate an AES MAC to `w` bits as long as `1/2^w` is still negligible (say `w > 63`)
* Where did the value 1/2 come from?
* How is 64 and above negligible?
* What is non-negligible today (5/17... |
When truncating an AES MAC value by "w" , how do I justify that "w" is still negligible? |
***What are the potential security impacts of using CRAM-MD5 vor Emails, when not using an SSL connection?***
Background: My current server-provider tells me it's no problem to store the passwords in plain-text in the database, saying he has to do so because they use CRAM-MD5 for Email authentication. But something ... |
What are the potential security impacts of using CRAM-MD5 for Emails, when not using an SSL connection? |
What are the potential security impacts of using CRAM-MD5 for Emails, when not using an SSL connection? |
I want to multiply and divide polynomials, and implement the extended Euclidean
algorithm for polynomial greatest common divisors, over a Galois Field of size (2^128). Moreover, I want to use the previous algorithms on an 8-bit microcontroller (ATMega128). Where can I find relevant libraries in C or Assembly ? Ι would... |
I want to multiply and divide polynomials, and implement the extended Euclidean
algorithm for polynomial greatest common divisors, over a Galois Field of size $2^{128}$. Moreover, I want to use the previous algorithms on an 8-bit microcontroller (ATMega128). Where can I find relevant libraries in C or Assembly ? Ι wou... |
I'm taking an online class on cryptography at corsera.org / Stanford, and the professor is explaining that it's OK to truncate an AES MAC to $w$ bits as long as $1/2^w$ is still negligible (say $w > 63$)
* Where did the value $1/2$ come from?
* How is 64 and above negligible?
* What is non-negligible today (5/... |
The heart of OAEP algorithm used for RSA encryption are the cryptographic hash functions $H$ and $G$.
Does everybody (so also an adversary) know these functions?
* **If YES:** How does it help the security, if he just can decode the padding and read the message?
* **If NO:** What knowledge is needed for t... |
How does OAEP improve the security of RSA? |
Well, yes, everyone (or, at least, everyone who can use the public key) knows the hash function H and G; so we can assume that an adversary knows them as well.
You ask:
> **If YES:** How does it help the security, if he just can decode
> the padding and read the message?
Well, he can't decode the padding; t... |
Basically what the title is; GUIDs are unique by design. If you run the GUID through SHA1 and then Base64 the hash, will the resulting string have the same guaranteed uniqueness as the GUID, or not? |
Is Base64(SHA1(GUID)) still unique like the original GUID? |
Assuming [no cosmetics](http://stackoverflow.com/questions/968175/what-is-the-string-length-of-a-guid), the length of a GUID is 32 bytes so better question would be *"What's the collision probability of SHA1 with 32 bytes of input?"*
I'm sure someone else will answer with the exact statistics but the answer to your ... |
Base64 provides a 1:1 transform from input to output (and back again if desired). So if you take a set of unique items and base64 encode all of them they will all be unique.
So the question becomes if you run a GUID through SHA1, will the resulting hash have the same uniqueness as the GUID?
The answer is practica... |
I'm looking at using HMAC + CBC. The combination looks like this:
ciphertext = AES256(text, k1)
data = HMAC-SHA256(iv | ciphertext, k2) | iv | ciphertext
Where:
k1 = 32 byte key - Randomly generated, once
k2 = 32 byte key - Randomly generated, once
iv = 16 bytes - Randomly generated, for... |
Alternatives to HMAC + CBC? |
Base64 provides a 1:1 transform from input to output (and back again if desired). So if you take a set of unique items and base64 encode all of them they will all be unique.
So the question becomes if you run a GUID through SHA1, will the resulting hash have the same uniqueness as the GUID?
The answer is practica... |
While reading some text on cryptography, I found that algorithm *`A`*, that the adversary (called Eve by convention) runs to break the cipher message, needs two parameters, candidate message (*`y`*) and length of message (*`k`*).
Now, this *`k`* is being supplied in unary.
The reasons for that are being put for... |
Why this k parameter is in unary in adversary PPT algorithm? |
While reading some text on cryptography, I found that algorithm $A$, that the adversary (called Eve by convention) runs to break the cipher message, needs two parameters, candidate message ($y$) and length of message ($k$).
Now, this $k$ is being supplied in unary.
The reasons for that are being put forth in on... |
While reading some text on cryptography, I found that algorithm $A$, that the adversary (called Eve by convention) runs to break the cipher message, needs two parameters, candidate message ($y$) and length of message ($k$).
Now, this $k$ is being supplied in unary.
The reasons for that are being put forth in on... |
I have this data:
processor clock frequency: 2,1 ghz
message length: 16 byte
Speed: 4,3 Mbytes/s
how can calculate cycles and cycles per byte?
|
Computerized cryptanalysis has obviously made formerly "secure" hand ciphers like Playfair, Four Square, and the Hill Cipher obsolete because they can be defeated in seconds. But is there a hand cipher where a modern computer attack does not have an advantage over a cryptanalysis done by hand? There are certainly tasks... |
Are there any hand ciphers not obsoleted by computer cryptanalysis? |
What types of algorithms that are capable of signing a message are out there that run on a physical level, e.g. lacking the infrastructure of a standard PC, no memory, processor or motherboard in the traditional sense?
I'm curious whether there are some strong really low level implementations (if these statements do... |
I explained this at [Why does key generation take an input $1^k$, and how do I represent it in practice?](http://crypto.stackexchange.com/a/8199/351). Take a look at that one. Basically, it's an artificial little detail that's only there to keep the theoretical details squared away. You can safely ignore it on first... |
I recently found [this site][1], proposing a hashing algorithm for passwords.
They describe the following:
1. pad the password on both sides with `SHA1(email)` to `4 * length(password)`
2. `HMAC(padded password, global salt)`
3. choose a random salt of 22 characters
4. `bcrypt(hmac, salt)`
Is this algor... |
Is this algorithm secure? |
There are [cryptography algorithms for hardware applications][1] with restricted resources such as limited storage, gate count, or power consumption. and as @rath said There are some ASICs that do specialized crypto such as SHA or AES. But:
What is the communication protocol?
Is the communication protocol impleme... |
I'm read the implies $Def\,\, 2.1\, \Rightarrow Def\, 2.4$ in [this slides][1]. I have a doubt in the slide 10. Here say: Construct an adversary $A$ who outputs $m_0$, $m_1$ in the first step of $\mathsf{PRIV_{EAV}}(\Pi,A,n)$. Then if $A$ receives any ciphertext $c' \neq \tilde{c}$ it outputs a random
bit $b_0$; if it ... |
I'm thinking about buying an USB TRNG. The question is: How do I evaluate it's randomness ? I'm sure some are better than others but which is which ? Are thermal-noise better than radio-noise TRNGs? |
I'm reading the proof of the implication "Def 2.1 $\Rightarrow$ Def 2.4" in [these slides about Adversarial Indistinguishability and Perfectly-Secret Encryption][1]. I have a doubt in the slide 10. Here it says:
> Construct an adversary $A$ who outputs $m_0$, $m_1$ in the first step of
> $\mathsf{PRIV_{EAV}}(\Pi,A,... |
In a [previous question][1], I described a particular keyed "hash" that mapped a 5-digit input code into a 5-digit output code. It used a 8-bit key which is very insecure - more than 99% of the time, you can infer the key given a single input/output code pair.
I put "hash" in inverted commas as although it has somet... |
What key length is required to keep simple keyed "hash" secure? |
In a [previous question][1], I described a particular keyed "hash" that mapped a 5-digit input code into a 5-digit output code. It used a 8-bit key which is very insecure - more than 99% of the time, you can infer the key given a single input/output code pair.
I put "hash" in inverted commas as although it has somet... |
It is well known that you can compute any combinatoric function (and hence, any encryption method) with a sufficient number of NAND gates, and so the answer to your question would appear to be "yes, any algorithm is able to run on a "physical level", without a fancy GUI, multitheaded operating system, memory, processor... |
It is well known that you can compute any combinatoric function (and hence, any encryption method) with a sufficient number of NAND gates, and so the answer to your question would appear to be "yes, any algorithm is able to run on a "physical level", without a fancy GUI, multitheaded operating system, memory, processor... |
I was looking a [Wikipedia article on SHA-2][1], and the ["Comparison of SHA functions"][2] table seems to indicate that SHA-2 is less secure than SHA-1.
![enter image description here][3]
Is this true, or is the table wrong / misleading?
If it is true: Is SHA-1 better to use for signatures (w/HMAC), or should... |
Is SHA-2 "broken"? |
I was looking a [Wikipedia article on SHA-2][1], and the ["Comparison of SHA functions"][2] table seems to indicate that SHA-2 is less secure than SHA-1.
![enter image description here][3]
Is this true, or is the table wrong / misleading?
[1]: http://en.wikipedia.org/wiki/SHA-2
[2]: http://en.wikipedia.or... |
Is this table about SHA-2 collisions correct? |
I was looking a [Wikipedia article on SHA-2][1], and the ["Comparison of SHA functions"][2] table seems to indicate that SHA-2 is less secure than SHA-1.
![enter image description here][3]
Is this true, or is the table wrong / misleading?
What does 2^28.5 mean for SHA-256 compared to 2^60 for SHA-1 ([table in ... |
I was looking a [Wikipedia article on SHA-2][1], and the ["Comparison of SHA functions"][2] table seems to indicate that SHA-2 is less secure than SHA-1.
![enter image description here][3]
Is this true, or is the table wrong / misleading?
What does 2^28.5 mean for SHA-256 compared to 2^60 for SHA-1 ([table in ... |
I was looking a [Wikipedia article on SHA-2][1], and the ["Comparison of SHA functions"][2] table seems to indicate that SHA-2 is less secure than SHA-1.
![enter image description here][3]
Is this true, or is the table wrong / misleading?
What does $2^{28.5}$ mean for SHA-256 compared to $2^{60}$ for SHA-1 ([t... |
Is Wikipedia's table about SHA-2 collisions correct? |
Assume the following structure:
version | iv | ciphertext | HMAC(version | iv | ciphertext | ..., key)
The `version` starts out to be a "random" 64-bit number. It identifies the key used to encrypt the ciphertext for the purpose of key rotation (`version` is not, nor does it contain any part of the key). For ... |
Assume the following structure:
version | iv | ciphertext | HMAC(version | iv | ciphertext | ..., key)
The `version` starts out to be a "random" 64-bit number. It identifies the key used to encrypt the ciphertext for the purpose of key rotation (`version` is not, nor does it contain any part of the key). For ... |
In my cryptography class, the instructor suggested that in order to give the attacker a minimal advantage of 1/2 ^32, which means that after 2^48 blocks are encrypted, we have to change the key.
It seems that the advantage of 1/2 ^32 is somewhat arbitrarily chosen, and I'm not sure where that comes from. Can anyone... |
How much data can I encrypt with AES before I need to change the key in CBC mode? |
In my cryptography class, the instructor suggested that in order to give the attacker a minimal advantage of 1/2 ^32, which means that after 2^48 blocks are encrypted, we have to change the key.
It seems that the advantage of 1/2 ^32 is somewhat arbitrarily chosen, and I'm not sure where that comes from. Can anyone... |
Assume the following structure:
version | iv | ciphertext | HMAC(version | iv | ciphertext | ..., key)
The `version` starts out to be a "random" 64-bit number. It identifies the key used to encrypt the ciphertext for the purpose of key rotation (`version` is not, nor does it contain any part of the key). For ... |
Do I need to keep a 64-bit version number secret? |
In my cryptography class, the instructor suggested that in order to give the attacker a minimal advantage of 1/2 ^32, which means that after 2^48 blocks are encrypted, we have to change the key.
It seems that the advantage of 1/2 ^32 is somewhat arbitrarily chosen, and I'm not sure where that comes from. Can anyone... |
In my cryptography class, the instructor suggested that in order to give the attacker a minimal advantage of $1/2^{32}$, we have to change the key after $2^{48}$ blocks are encrypted.
It seems that the advantage of $1/2 ^{32}$ is somewhat arbitrarily chosen, and I'm not sure where that comes from. Can anyone explai... |
In my cryptography class, the instructor suggested that in order to give the attacker a minimal advantage of $1/2^{32}$, we have to change the key after $2^{48}$ blocks are encrypted.
It seems that the advantage of $1/2 ^{32}$ is somewhat arbitrarily chosen, and I'm not sure where that comes from. Can anyone explai... |
I am going to apply a simple substitution cipher to my input, then encrypt the result with a Hill cipher. How can this be broken, in a chosen-plaintext threat model?
In other words, instead of the standard mapping of letters to numbers (A=0, B=1, .., Z=25), I will randomly reorder them (e.g., A=22, B=12, ...), wher... |
Antoine Joux announced [the computation of discrete logarithm over $\mathbb{F}_{2^{257 \times 24}}$][1], which is now pretty close to what was being used in pairing-based cryptography.
According to Joux, "a direct consequence of this record is that supersingular curves (of
genus 1 or 2) defined over GF(2^257) canno... |
In the AUTH and UF-CMA games, an adversary is required to forge a ciphertext or message/tag pair to win the game. Given an encryption scheme $E$ and a PRF $F$, let $\hat{E} = C || F_k(C)$ and $C = E(x)$ (encrypt-then-MAC with PRF as MAC), how is the birthday attack mounted? I understand it is sufficient to ask ~ $\sq... |
How does the birthday attack work in AUTH and UF-CMA games? |
Why is SRP not widely used? |
What is the fastest elliptic curve operation f(P) in affine coordinates such that f^n(P)=P only if n is large? |
First let me apologize if this is an ill posed question. Let me also note that I do not in any way seek a comprehensive answer, simply your thoughts on what makes for a valuable asset to a company like the NSA. I am a Maths student in the US, and I realize that a problem-solving mind is at minimum required for said que... |
It seems we have aligned interests. I'm also a university student (although I am a math/comp sci double major) looking to pursue a career in cryptography. To that end, I have been self-studying it for a while now. So, take what I say with a grain of salt. As a forewarning, this post focuses specifically on what topics ... |
I'm going to develop a system that will have two keys. One supplied and generated by the system that's specific to an individual user and one from the user.
Does it make sense to:
1. Concatenate the two keys to make a "master" key and then encrypt the data?
2. Encrypt with one key and the encrypt again with the ... |
Proper Way to Encrypt Data with Two Keys? |
I'm going to develop a system that will have two keys. One supplied and generated by the system that's specific to an individual user and one from the user.
I plan on using AES.
Does it make sense to:
1. Concatenate the two keys to make a "master" key and then encrypt the data?
2. Encrypt with one key and the... |
All too often I describe some process saying that "he" did something to "his" message,
and it makes sense in my own head, but no one else can figure out which of the several people involved that those pronouns refer to.
Or worse -- sometimes I say that "the message is encrypted", but I don't say who does it and with ... |
I've been toying with some encryption scenarios recently. One of the hard ones I came across is a multi-party system.
So we have
1. Bob -- The person who sends the message (and knows it's recipients)
2. Jim -- The person who supervises who is capable of decrypting the message(but can not decrypt the message him... |
In RSA, some security authorities and/or standards allow $e=3$, others require or recommend $e>2^{16}$ (or perhaps some other minimum). I gathered the following:
- [PKCS#1][1] allows $e=3$ for both RSA digital signature and encryption.
- [ISO/IEC 9796-2][2] allows $e=3$ (in the context if RSA digital signature).
-... |
What security authorities and standards reject e=3 in RSA, when, and with what rationale? |
In RSA, some security authorities and/or standards allow $e=3$, others require or recommend $e>2^{16}$ (or perhaps some other minimum). I gathered the following:
- [PKCS#1][1] allows $e=3$ for both RSA digital signature and encryption.
- [ISO/IEC 9796-2][2] allows $e=3$ (in the context if RSA digital signature).
-... |
What security authorities and standards reject $e=3$ in RSA, when, and with what rationale? |
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