instruction stringlengths 15 30k |
|---|
Suppose that in order to avoid loss in transmission, a private key scheme $(Gen, Enc, Dec)$ is modified to be $(Gen, Enc', Dec')$, such that the message m is encrypted independently three times as $Enc'(m)=(c_1,c_2,c_3)$, each $c_i=Enc(m)$.
For decryption, the 3 transmitted ciphertexts are decrypted using $Dec$. If ... |
Why isn't the IND-CCA property inherited when sending redundant ciphertext? |
Suppose that in order to avoid loss in transmission, a secret key encryption scheme $(Gen, Enc, Dec)$ is modified to be $(Gen, Enc', Dec')$, such that the message m is encrypted independently three times as $Enc'(m)=(c_1,c_2,c_3)$, each $c_i=Enc(m)$.
For decryption, the 3 transmitted ciphertexts are decrypted using ... |
IND-CCA2 summary:
1. get encryption key.
2. make decryption queries on chosen ciphertexts.
3. obtain challenge ciphertext on m_L, m_R, two equal-length messages chosen by you, the attacker. The challenger is suppsed to randomply pick one, encrypt it and return the ciphertext.
4. make decryption queries as in 2. ... |
[EDIT: I am assuming a public key system here]
IND-CCA2 summary:
1. get encryption key.
2. make decryption queries on chosen ciphertexts.
3. obtain challenge ciphertext on m_L, m_R, two equal-length messages chosen by you, the attacker. The challenger is suppsed to randomply pick one, encrypt it and return th... |
Why do we use a permutation table in the first step of [DES algorithm][1] and one at the end of algorithm?
[1]: http://en.wikipedia.org/wiki/Data_Encryption_Standard |
I understand the theory behind the use salts in hash functions, but when I see it implemented, the implementations always generate the salt on the fly and the salt appears to be different for every hash.
* When implementing a salt for users signing into a website, doesn't the salt need to be the same at account cr... |
If an attacker got cipher text c how would they crack
1) openssl encrypted file
2) GPG encrypted email
3) truecrypt file
assuming they do not have the private key or the symmetric password?
I know the crypto for dummies answer in some of these examples is "well they would brute force it duh" but how then... |
How exactly would someone attempt to crack encryption? |
In the Diffie-Hellman key exchange, one of the steps involves calculating a primitive root of a prime number $p$. How would one go about doing so, considering that $p$ could be very large?
Is there some sort of algorithm or equation?
<sup>(if this question belongs on Math.SE, please move it there)</sup> |
How does one calculate a primitive root for Diffie-Hellman? |
For the [Diffie-Hellman][1] protocol I've heard that the [generator][2] 3 is as safe as any other generator. Yet, 32-bit or 256-bit exponents are sometimes used as generators. What is the benefit of using these very large generators if they are just as safe as the generator 3?
If 3 is not the safest generator, how ... |
When initiating an [oblivious transfer][1], why would someone use a [1-2 oblivious transfer][2] rather than going for an [1 out of n oblivious transfer][3]? Perhaps a slight time overhead for the extra message encrypts, but with everything as fast as it is these days is that a concern?
[1]: http://en.wikipedia.o... |
Why use a 1-2 Oblivious Transfer instead of a 1 out of n Oblivious Transfer? |
I am interested in understanding what the process would be if an attacker wished to attempt to decrypt data secured by common tools such as OpenPGP, Truecrypt or the like. Are there any documented records of such undertakings, or papers I can read to understand how a cryptanalyst approaches breaking an implementation? ... |
How exactly would someone attempt to analyse ciphertext produced by popular encryption products such as Truecrypt/PGP? |
I developed a p2p-app in C# which sends and receives encrypted text messages (50KB). For encryption my app uses AES 128 bit in CBC cipher mode. For each message it uses a new randomly-generated IV.
Whatever, after reading the following two publications I have some concerns about my solution:
[http://www.h-online... |
Is CBC really dead? |
Due to a number of recently asked questions about Diffie-Hellman, I was thinking this morning: must $g$ in Diffie-Hellman be a generator?
Recall the mathematics of Diffie-Hellman:
Given public parameters $p$ (a large prime) and $g$ (always referred to as a generator of $\mathbb{Z}_p$).
1. Alice chooses $a$ a... |
For Diffie-Hellman, must g be a generator? |
"As far as security goes, clearly having g be a generator is best as $|g|$ (or the order of $g$) will be greatest when $g$ is indeed a generator. Is that the only reason for choosing $g$ to be a generator?"
Yes that is the only reason to choose a generator, to keep the group large enough. |
Due to a number of recently asked questions about Diffie-Hellman, I was thinking this morning: must $g$ in Diffie-Hellman be a generator?
Recall the mathematics of Diffie-Hellman:
Given public parameters $p$ (a large prime) and $g$ (always referred to as a generator of $\mathbb{Z}^*_p$).
1. Alice chooses $a$... |
I developed a p2p-app in C# which sends and receives encrypted text messages (50KB). For encryption, my app uses AES 128 bit in CBC cipher mode. For each message it uses a new randomly-generated IV.
Whatever, after reading the following two publications I have some concerns about my solution:
* [First solutions ... |
I developed a p2p-app in C# which sends and receives encrypted text messages (50KB). For encryption, my app uses [AES][1] 128 bit in [CBC cipher mode][2]. For each message it uses a new randomly-generated IV.
Whatever, after reading the following two publications I have some concerns about my solution:
* [First ... |
> As far as security goes, clearly having g be a generator is best as $|g|$ (or the
> order of $g$) will be greatest when $g$ is indeed a generator. Is that the only
> reason for choosing $g$ to be a generator?
Yes that is the only reason to choose a generator, to keep the group large enough. |
Plaintext that consists of an RSA key is easily recognizable as such, because it satisfies certain mathematical properties, in particular (See [answer][1] / [question][2])
$$d = e^{-1} \text{ mod } (p-1)(q-1)$$
Is it impossible to design a key system where it was harder to verify that the decryption of the key wa... |
Suppose I had a message in Chinese (or another non-phonetic language) and I wanted to encipher it.
Some of the simplest encryptions in English are substitution ciphers, but such ciphers don't seem to be a viable option for a language such as chinese (it's more similar to a code...). What would be a simple cipher fo... |
I understand that signing is often a case of hashing data and then encrypting the hash with the private key. What properties keep Diffie-Hellman from being useful for this? |
Why can't Diffie-Hellman be used for signing? |
I'm using PHP to encrypt a string and Java to decrypt it on a remote server.
To perform the encryption, I'm using the following strategy:
http://www.logikdev.com/2010/11/01/encrypt-with-php-decrypt-with-java/
My problem is that the strings I encrypt are pretty small and very similar each other. Nevertheless, I... |
How to enhance randomness of AES? |
I'm using PHP to encrypt a string and Java to decrypt it on a remote server.
To perform the encryption, I'm using the strategy that can be found [here][1]:
My problem is that the strings I encrypt are pretty small and very similar each other. Nevertheless, I'd like a high degree of randomness in the encrypted val... |
I'm using PHP to encrypt a string and Java to decrypt it on a remote server. To perform the encryption, I'm using the strategy that can be found [here][1].
My problem is that the strings I encrypt are pretty small and very similar each other. Nevertheless, I'd like a high degree of randomness in the encrypted values... |
I have read in Applied Cryptography that the NSA is the largest hardware buyer and the largest mathematician employer in the world.
* How can we reason about the symmetric ciphers cryptanalysis capabilities of code-breaking agencies like the [NSA][1] or [GCHQ][2] given that they have performed first class unpublish... |
I have a ciphertext generated by a [classical cipher.][1] I do not know what was cipher used to generate it. I do however have the beginning of the plaintext.
* What are the cryptanalysis approaches for classical ciphers?
* How do you suggest trying to recover the plaintext in this instance?
* How can I leverage... |
How to attack a classical cipher using known partial plaintext? |
My understanding of HOTP is that each password is unique and based on a counter.
PASSWORD = HOTP(K,C)
Where C is an incremental counter.
What I wish to know, is how you keep the client and server in sync? Looking at current systems (google authenticator), there appears to be no indication if the actu... |
How does HOTP keep in sync? |
My understanding of HOTP is that each password is unique and based on a counter.
$$PASSWORD = HOTP_1(K,C)$$
Where $C$ is an incremental counter.
What I wish to know, is how you keep the client and server in sync? Looking at current systems (google authenticator), there appears to be no indication if the a... |
My understanding of [HOTP][1] is that each password is unique and based on a counter.
$$PASSWORD = HOTP_1(K,C)$$
Where $C$ is an incremental counter.
What I wish to know, is how you keep the client and server in sync? Looking at current systems (google authenticator), there appears to be no indication if ... |
I, myself, do not plan on getting into a situation where I would be unable to use a computer in order to communicate securely. However, I can think of many practical situations in which mental cryptography would be useful.
Is there a secure cryptosystem that is simple enough to be performed mentally?
Clearly, on... |
Is there a secure cryptosystem that can be performed mentally? |
For the purposes of sighing and verifying signatures, what is the value of the Hash function? Why would it matter if SHA1 is later determined to be easy to break? Since a Public/Private key process is still used, is SHA1 only a utility to create smaller data to sign? The data itself is also sent in the clear most of th... |
What is wrong with using SHA1 in digital signatures? Why is a robust hash function needed? |
For the purposes of signing and verifying signatures, what is the value of the hash function?
Why would it matter if SHA1 is later determined to be easy to break? Since a Public/Private key process is still used, is SHA1 only a utility to create smaller data to sign?
The data itself is also sent in the clear most... |
I have read in Applied Cryptography that the NSA is the largest hardware buyer and the largest mathematician employer in the world.
* How can we reason about the symmetric ciphers cryptanalysis capabilities of code-breaking agencies like the [NSA][1] or [GCHQ][2] given that they have performed first class unpublish... |
How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ? |
Plaintext that consists of an RSA key is easily recognizable as such, because it satisfies certain mathematical properties, in particular (See the [answer][1] for [Why can an encrypted private key be brute forced?][2]):
$$d = e^{-1} \mod (p-1)(q-1).$$
Is it impossible to design a public key system where it was h... |
Is it possible to create an asymmetric cryptosystem where the private keys are not easily verifiable as such? |
Two highly-critical papers by Koblitz and Menezes (two well-regarded mathematicians who've contributed to the crypto community) were published years ago: [one in 2004][1], [another in 2006][2].
Have these papers had any noticeable impact on how research is conducted in the provable-security community? Or have these... |
How did the Koblitz/Menezes papers affect the cryptography community? |
Using the terminology of [the ECDSA wikipedia page, ][1]ECDSA (and DSA) signatures require a random k value for each signature which ensures that the signature is different each time even if the message and key are the same. For some applications a "constant" signature may be desirable.
It seems to me that there wou... |
Can ECDSA signatures be safely made "deterministic"? |
Why is Google using RC4 for their HTTPS/ssl?
`$ openssl s_client -connect www.google.com:443|grep "Cipher is"
New, TLSv1/SSLv3, Cipher is RC4-SHA
`
Isn't RC4 unsafe to use? |
Why is Google using RC4 for their HTTPS/SSL?
$ openssl s_client -connect www.google.com:443 | grep "Cipher is"
New, TLSv1/SSLv3, Cipher is RC4-SHA
Isn't RC4 unsafe to use? |
Google is using RC4, but isn't RC4 considered unsafe? |
Two highly-critical papers by Koblitz and Menezes (two well-regarded mathematicians who've contributed to the crypto community) were published years ago: [Another Look at “Provable Security”][1] in 2004, [Another Look at “Provable Security”. II][2] in 2006.
Have these papers had any noticeable impact on how research... |
[EDIT: I am assuming a public key system here]
IND-CCA2 summary:
1. get encryption key.
2. make decryption queries on chosen ciphertexts.
3. obtain challenge ciphertext on $m_L$, $m_R$, two equal-length messages chosen by you, the attacker. The challenger is suppsed to randomply pick one, encrypt it and retur... |
I know there are 100's of AES programs and websites out there but I can't seem to find a site that give me the detail I think I would/should need to write my own java program.
I was to write AES form scratch without using the built in libraries of java.
I know that AES has S boxes like DES has were would I look f... |
Where can i get the info to implement AES? |
I have seen many examples of encryption up to about 256-bit. But how does one programmatically scale the logic upwards in a language such as PHP or Java to say 1024-bit or even 4096-bit and higher?
Are there any websites that explain the workings of a very large encryption on a modern 64-bit computer? For example wh... |
How do one scale encryption strength upwards from 256-bit? |
How does one scale encryption strength upwards from 256-bit? |
It's very unlikely to yield any more security. The security of an encryption algorithm is directly tied to how well it has been studied. There really are no well-studied symmetric algorithms with keyspaces over 256-bits. So the short answer is -- nobody knows yet.
The question is kind of like "what kind of cars will... |
It's very unlikely to yield any more security. The security of an encryption algorithm is directly tied to how well it has been studied. There really are no well-studied symmetric algorithms with keyspaces over 256-bits. So the short answer is -- nobody knows yet.
The question is kind of like "what kind of cars will... |
As @David Schwartz said, using something like 3AES should work to scale up the security (but see my comment on the meet-in-the-middle attack). But seriously, why would you even need 512-bits of security in a symmetric algorithm? According to the [analysis on wikipedia][1] it would take 50,955,671,114,250,100,000,000,00... |
For **asymmetric encryption** such as embodied by [RSA][1], the "key length" is conventionally taken to be the length of one of the public key elements. Such a key is _highly structured_ (only a very few sequences of bits of that length are valid keys) and security relies on robustness of this structure with regards to... |
I am thinking quite a lot lately abut the problem of secure, privacy-preserving social networking. Distributing the network among trusted, preferably self-hosted servers (like Diaspora, GNU Social etc. attempt to do) is obviously not a good solution for the the large majority of non-technical users, so my thoughts curr... |
Encryption scheme for social-network-like data sharing data via untrusted server? |
What is the "Random Oracle Model"? Is it an "assumption" akin to the hardness of factoring and discrete log? Or something else?
And why do some researchers have a strong distrust of this model? |
What is the "Random Oracle Model" and why is it controversial? |
[Discrete logarithms in $\mathbb{F}_{p}$ ][1] share the same asymptotic complexity as integer factorization for general numbers: $L_p[1/3,1.923]$ for general integers, $L_p[1/3,1.587]$ for [special integers][2]. [Discrete logarithms in $\mathbb{F}_{p^n}$][3] have the same asymptotic complexity as factoring special inte... |
I do not fully understand how this takes place.... so Bob grabs Alice's secret key when she isn't looking and her encrypted files, doesn't he need to know her passphrase to read her files? What I am reading is that no he does not need it but as far as I understand gpg it would still ask for a passphrase to decrypt a d... |
If Bob steals Alice's private key, how exactly would he read her encrypted documents? |
SHA-256(SHA-256(x)) was proposed by Ferguson and Schneier in their excellent book "Practical Cryptography" (later updated by Ferguson, Schneier, and Kohno and renamed "Cryptography Engineering") as a way to make SHA-256 invulnerable to "length-extension" attack. They called it "SHA-256d". We started using SHA-256d for ... |
So Bob grabs Alice's secret key when she isn't looking and her encrypted files, doesn't he need to know her passphrase to read her files?
What I am reading is that no he does not need it but as far as I understand GPG, it would still ask for a passphrase to decrypt a document. Please explain why Bob would not need ... |
How exactly would someone crack a private key passphrase? |
<p>
Crypto - Stack Exchange is for <span class="revision-comment">software developers, mathematicians and others interested in cryptography</span>.
</p>
Cryptography Stack Exchange is for asking questions about the mathematics and properties of cryptographic systems, their analysis ("cryptanalysis") and subsidiary... |
The OpenPGP standard (RFC 4880) defines in [section 3.7][1] a number of String-to-Key (S2K) conversion functions, which are used for password-encrypted messages (i.e. ones without using a public key scheme), as well as for (the secret part) in private keyrings.
The format of the private key rings itself is not docum... |
The OpenPGP standard (RFC 4880) defines in [section 3.7][1] a number of String-to-Key (S2K) conversion functions, which are used for password-encrypted messages (i.e. ones without using a public key scheme), as well as for (the secret part) in private keyrings.
Most probably the *Iterated and Salted S2K* will be use... |
How much harder is it to determine the secret key for a mono-alphabetic substitution cipher if each word is translated into a different language before the cipher is applied? |
How much harder is it to determine the secret key for a mono-alphabetic substitution cipher, if each word is translated into a different language before the cipher is applied? |
At the current time, is SHA256 the de facto standard for strong encrypted hashes? From what I am seeing of more sites utilizing it I would suppose the answer to this is yes but would like to hear what the community here has to say about this and why it is happening. |
At the current time, is SHA256 the de facto standard for strong encrypted hashes? |
At the current time, is SHA256 the de facto standard for strong cryptographic hashes?
From what I am seeing of more sites utilizing it, I would suppose the answer to this is yes, but would like to hear what the community here has to say about this and why it is happening. |
At the current time, is SHA256 the de facto standard for strong cryptographic hashes? |
Here SIM means the simulation based security
Consider a two message encryption scheme:$$Enc:K \times M \times M \rightarrow C \times C$$
and $Enc(K, m, m')=(K \oplus m, K \oplus m')$,
In the real world experiment, the adversary $Eve$ gets $(K \oplus m, K \oplus m')$ as ciphertext, and thus, the information leak... |
Is there any scheme, ideally one widely used or at least widely available, where you can treat both the signing and verifying keys as secret?
Basically, the functionality I'm looking for is this:
1. You derive two keys, $S$ and $V$.
1. $S$ cannot be derived from $V$, nor can $V$ be derived from $S$.
1. A si... |
Is it possible to deduce the original function that used to generate those pre-paid cards number that are used for charging your mobile phone credits?
For Example, If I've collected about 1000 of those cards, how do |
Is it possible to deduce the original function that used to generate those pre-paid cards number that are used for charging your mobile phone credits?
For Example, If I've collected about 1000 of those cards, how to analyse those numbers to that I can re-generate the generated numbers again using my own software? |
This is a lossy algorithm. You will lose information during the translation and reverse-translation steps. Introducing loss into any algorithm obviously increases the difficulty in pulling the clean plaintext out, since it's potentially impossible to pull the clean plaintext out even with the key.
Even so, fairly no... |
In which case , does repeated encodings gives back the plain text ?
any idea ? |
In RSA Algo , small number of repeated encodings give back the plain text , whats the cause? |
I'm assuming you are using "primitive RSA" (i.e. without padding), and encryption of a message $x$ is simply $x^{e}\, \bmod n$, with $(e, n)$ being the public key.
If for all messages we have $(...((x^e)^e)...)^e \bmod n = x = x^1$, with $k$ repetitions, we have actually $x^{e^k} \bmod n = x^1$, and this is equivale... |
This is a lossy algorithm. You will lose information during the translation and reverse-translation steps. Introducing loss into any algorithm obviously increases the difficulty in pulling the clean plaintext out, since it's potentially impossible to pull the clean plaintext out even with the key.
Even so, fairly no... |
How much harder is it to determine the secret key for a mono-alphabetic substitution cipher, if each word is translated into a different language before the cipher is applied?
EDIT
If somehow computers could create their own language or set of languages such that there is a one-to-one correspondence between words... |
The question is about the "cycling attack" against RSA. The attack needs to be considered, in principle, for any deterministic asymmetric cipher with the same input and output domain, including RSA (without padding): the attacker repeatedly encrypts the ciphertext, until she obtains the ciphertext again; the last value... |
Sometimes the easiest way to describe security of a type of cryptography is to say that "the time it takes to solve for an x-bit key would be y years". How would one go about doing such a calculation for RSA, DH, and ElGamal. In other words, given x, how to solve for y?
<sub>If this fits better on Math.SE, please mo... |
How to calculate the time it'll take to crack RSA or DH? |
Antoine Joux very kindly sent me the following on the topic:
> People worry that [logarithms over fields with composite exponent] might be easier,
> this is why they use prime exponent. For some factorization of the exponent, viewing
> the finite field as a tower of extensions $(p^{n_1})^{n_2}$ indeed makes things... |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.