instruction stringlengths 15 30k |
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I have eleven ciphertexts that were encrypted with the same key (which I don't know). I want to decrypt the last ciphertext. I read similar question like http://crypto.stackexchange.com/questions/6020/many-time-pad-attack but I can't solve my problem with their answer.
I'm new in Crypto can anyone tell what can I do... |
Say that we have a secure random number generation that outputs 32 bit random numbers, so it's output is a true random number between 0 and a MAX.
What is the best way to map this random number to a defined range (say I want something random between X and Y) without loosing too much entropy? |
Correct way to map random number to defined range? |
How to prove that if a cryptosystem has perfect secrecy for a given plaintext probability distribution then it will have perfect secrecy for all other possible plaintext probability distributions?
Do we have to make any additional assumptions on probability distributions (probability of each plaintext is not zero?) ... |
Topic. Does key authentication in SSH(2) protect from MitM by providing authentication and integrity?
----
No tag for mitm? Hm. |
Does key authentication in SSH protect from MitM? |
Often in security proofs a certain block cipher is assumed to be a pseudorandom permutation or PRP. I wonder if this goes for stream ciphers as well, and specifically for Salsa20.
If limit ourselves to the scope of one Salsa20 block (64 bytes), being fed 64 bytes of plaintext data, is it safe to assume $Salsa20_{key... |
Is it safe to assume Salsa20 to be a PRP? |
Topic. Does key authentication in SSH(2) protect from MitM by providing authentication and integrity? |
Is it possible to deduce the plaintext block or the key, given only a Simplified-DES ciphertext block (e.g. c=01110110)? I'm reading Cryptography And Network Security, by William Stallings and I'm bit confused. |
I've not been able to understand exactly the reason behind Blowfish being faster than AES. Is it dependent on the block size? Or is it processor dependent? (if Yes, then lets assume that AES accelerators are not used) I'd like to know the exact reason behind it. Thanks. |
Why exactly is Blowfish faster than AES? |
Topic. Does key authentication in SSH(2) protect from MitM by providing authentication and integrity and why? |
I've recently read some articles about the Interlock Protocol, [Article by Ron Rivest and Adi Shamir](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.159.1673&rep=rep1&type=pdf).
I'm wondering if it is possible to protect Key-exchange under Public-Key cryptography from the MITM attack by using this protocol... |
I am reading an article on cryptography at following location:
http://www.entrust.com/resources/pdf/cryptointro.pdf
> Historically, encryption systems used what is known as symmetric cryptography. Symmetric cryptography uses the same key for both encryption and decryption. Using symmetric cryptography, it is safe... |
Assume RSA with a public modulus $N$ of $n$ bits, a small odd public exponent $e$, plaintext $M$ a random non-negative integer less than $2^m$ for some integer parameter $m$, with $M\mapsto C=M^e\bmod N$ (textbook RSA) used for encryption. Inasmuch as it matters, assume there a $2^r$ ciphertexts available to the attack... |
In textbook RSA with low public exponent, how big does a random message needs to be? |
[Simplified-DES][1] is a toy cipher with <strike>16-bit</strike> 8-bit block and 10-bit key, intended for educational purposes. Here in an [alleged implementation][2]; [another][3]; yet [another][4].
If one knows one block of ciphertext, but nothing about the plaintext and key, the plaintext can not be guessed entir... |
[Simplified-DES][1] is a toy cipher with <strike>16-bit</strike> 8-bit block and 10-bit key, intended for educational purposes. Here in an [alleged implementation][2]; [another][3]; yet [another][4].
If one knows one block of ciphertext, but nothing about the plaintext and key, the plaintext can not be guessed entir... |
[Simplified-DES][1] is a toy cipher with <strike>16-bit</strike> 8-bit block and 10-bit key, intended for educational purposes. Here in an [alleged implementation][2]; [another][3]; yet [another][4].
If one knows one block of ciphertext, but nothing about the plaintext and key, the plaintext can not be guessed entir... |
[Simplified-DES][1] is a toy cipher with <strike>16-bit</strike> 8-bit block and 10-bit key, intended for educational purposes. Here in an [alleged implementation][2]; [another][3]; yet [another][4].
If one knows one block of ciphertext, but nothing about the plaintext and key, the plaintext can not be guessed entir... |
[Simplified DES][1] is a toy [Feistel cipher][2] with <strike>16-bit</strike> 8-bit block and 10-bit key, and only two rounds, intended for educational purposes. Here is a [preview of the original paper][3]), and [an implementation][4]; [another][5]; yet [another][6].
If one knows one block of ciphertext, but nothin... |
[Simplified DES][1] is a toy [Feistel cipher][2] with <strike>16-bit</strike> 8-bit block and 10-bit key, and only two rounds, intended for educational purposes. Here is a [preview of the original paper][3], and [an implementation][4]; [another][5]; yet [another][6].
If one knows one block of ciphertext, but nothing... |
I am taking cryptography class on Coursera, I learned that compression function `h(H, m) = E(m, H) ⨁ m` is insecure, while other variants like `h(H, m) = E(m, H) ⨁ m ⨁ H` (Davies-Meyer) or `h(H, m) = E(m, H) ⨁ H` (Miyaguchi-Preneel) is secure, why? |
Why h(H, m) = E(m, H) ⨁ m insecure? |
I am taking cryptography class on Coursera, I learned that compression function $h(H, m) = E(m, H) ⨁ m$ is insecure, while other variants like $h(H, m) = E(m, H) ⨁ m ⨁ H$ (Davies-Meyer) or $h(H, m) = E(m, H) ⨁ H$ (Miyaguchi-Preneel) is secure, why? |
Why $h(H, m) = E(m, H) ⨁ m$ insecure? |
I am trying to understand how CBC-mode in SSL/TLS can be attacked.
I have been looking around online but all examples and explanations are very hard to understand and follow. Can you give a simple explanation for how such attacks happen? |
How can Cipher Block Chaining (CBC) in SSL be attacked? |
The Davies-Meyer compression function $h(H, m) = E_m(H) \oplus H$ is said to be secure. So too is the Miyaguchi-Preneel compression function $h(H, m) = E_m(H) \oplus m \oplus H$. Why are these secure? How do we know that they are secure?
(Here $E$ is a block cipher with $n$-bit key and $n$-bit plaintext; $E_k(x)$... |
Why are the Davies-Meyer and Miyaguchi-Preneel constructions secure? |
I am taking a cryptography class on Coursera. I learned that the compression function $h(H, m) = E_m(H) \oplus m$ is insecure (even though other variants like Davies-Meyer or Miyaguchi-Preneel are secure). Why is this compression function insecure? What is the attack?
(See also http://crypto.stackexchange.com/q/... |
Why $h(H, m) = E(m, H) \oplus m$ insecure? |
Why is $h(H, m) = E(m, H) \oplus m$ insecure? |
I'd like to use Python's random.choice seeded with a HMAC-SHA1 tag to generate a MAC encoded in a variable set of chars.
import random, hmac, hashlib, string
SECRET = 'something secret'
MESSAGE = 'some message'
MAC_CHARS = string.ascii_letters + string.digits
MAC_LEN = 10
random.seed... |
Is it secure to pass a SHA1 HMAC tag to Python's random.seed before using random.choice to generate a MAC? |
I'd like to use Python's random.choice seeded with a HMAC-SHA1 tag to generate a MAC encoded in a variable set of chars.
import random, hmac, hashlib, string
SECRET = 'something secret'
MESSAGE = 'some message'
MAC_CHARS = string.ascii_letters + string.digits
MAC_LEN = 10
random.seed... |
How secure is passing a MAC to Python's random.seed before using random.choice to generate a MAC? |
I know scrypt was designed to lessen the GPU/ASIC advantage.
We now have [litecoin](http://www.litecoin.org) as a real-world example of this. However, it hasn't worked out perfectly. Most coins are mined by GPUs, although the advantage is probably smaller than it would have been with SHA-256.
I wonder how the fu... |
I have the following scenario:
* Archives compressed with 7z, hundreds of MiB in size
* GPG to encrypt the archives (binary, without ASCII armor)
* [PAR2][1] to create error correction data
### Question
If I encrypt the archive, will it compromise the security of the GPG-encrypted data if the error correctio... |
GPG and PAR2 error correction data from the plain archive, will it compromise security? |
I have the following scenario:
* Archives compressed with 7z, hundreds of MiB in size
* GPG to encrypt the archives (binary, without ASCII armor)
* [PAR2][1] to create error correction data
### Question
If I encrypt the archive, will it compromise the security of the GPG-encrypted data if the error correctio... |
Assume RSA with a public modulus $N$ of $n$ bits, a small odd public exponent $e$, plaintext $M$ a random non-negative integer less than $2^m$ for some integer parameter $m$, with $M\mapsto C=M^e\bmod N$ (textbook RSA) used for encryption. Inasmuch as it matters, assume there a $2^r$ ciphertexts available to the attack... |
Assume RSA with a public modulus $N$ of $n$ bits, a small odd public exponent $e$, plaintext $M$ a random non-negative integer less than $2^m$ for some integer parameter $m$, with $M\mapsto C=M^e\bmod N$ (textbook RSA) used for encryption. Inasmuch as it matters, assume there are $2^r$ ciphertexts available to the atta... |
We can choose random $(H,m,m′)$ and construct $H′$ as follows:
$$H' = D(m', E(m, H) \oplus m \oplus m')$$
Why is $E(m, H) \oplus H$ secure?
Because you can't find a random $H'$ where $$H' = D(H', E(m, H) \oplus H \oplus H')$$ |
Assume RSA with a public modulus $N$ of $n$ bits, a small odd public exponent $e$, plaintext $M$ a random non-negative integer less than $2^m$ for some integer parameter $m$, with $M\mapsto C=M^e\bmod N$ (textbook RSA) used for encryption. Inasmuch as it matters, assume there are $2^r$ ciphertexts available to the atta... |
I have the following scenario:
* Archives compressed with 7z, hundreds of MiB in size
* GPG to encrypt the archives (binary, without ASCII armor)
* [PAR2][1] to create error correction data
### Question
**1. Encrypted archive + plain archive error correction data**
If I encrypt the archive, will it compro... |
Topic. Does key authentication in SSH(2) protect from MitM by providing authentication and integrity and why?
|
I am trying implement the [McEliece crytosystem][1] in SAGE. My question is How I will be able to choose the appropriate matrix $S$ and $P$?. I ask this because when I trying obtain the vector $\hat{m}=mS$ in $\hat{G}\hat{m} = Decoding(c)$ I get: matrix equation has no solutions, even after verify that $G$ is full ran... |
We are encrypting a small positive integer (1-1000) with a constant key using AES256 encryption. We are considering two approaches to make this secure; use an initial vector (which we then need to store as well), or just prepend some junk to the integer before encryption (possibly 6 characters, this is the plaintext w... |
Can prepending "junk" be equivalent to an IV when encrypting using CBC? |
I am very inexperienced with cryptography and would like some advice on how to store some sensitive information on client machines.
I have some information stored in a central database on my server. A snapshot of this database is distributed along with my application. Every time the application is opened on the clie... |
What is the best way to store sensitive information on the client? |
I am very inexperienced with cryptography and would like some advice on how to store some sensitive information on client machines.
I have some information stored in a central database on my server. A snapshot of this database is distributed along with my application. Every time the application is opened on the clie... |
I am very inexperienced with cryptography and would like some advice on how to store some sensitive information on client machines.
I have some information stored in a central database on my server. A snapshot of this database is distributed along with my application. Every time the application is opened on the clie... |
How do cryptography in SSH(2) work?
Does key authentication in SSH(2) protect from MitM by providing authentication and integrity and why?
|
> How do cryptography in SSH(2) work?
This is a very broad question. If you're interested in the details, then read the relevant RFCs that describe the SSH protocol. In particular:
- [RFC 4253 - The Secure Shell (SSH) Transport Protocol](http://tools.ietf.org/html/rfc4253)
- RFC 4252 - The Secure Shell (SSH) A... |
> How do cryptography in SSH(2) work?
This is a very broad question. If you're interested in the details, then read the relevant RFCs that describe the SSH protocol. In particular:
- [RFC 4253 - The Secure Shell (SSH) Transport Protocol](http://tools.ietf.org/html/rfc4253)
- RFC 4252 - The Secure Shell (SSH) A... |
Looking at the first step of AES encryption I see that we XOR the key with the plaintext block. This means that if we encrypt a null block of all zeros, we are effectively encrypting the key under itself, and the output is basically a hash of the key. I'd be interested to know if this insight make sense.
But my actu... |
Consider the following version of padded RSA encryption, where encryption of m is done by setting m′ = (0^k||r||00000000||m) for a random r (of length 8 btyes = 64 bits) and then computing the ciphertext c = (m′)^e (mod N). For concrete, further assume that N is a 1024-bit RSA modulus, and that the scheme only handles ... |
I assume that this is a homework problem, and so I won't give you the answer. However, here are a few hints:
- What does 'it is a valid ciphertext' mean?
- If $c$ decrypts to the value $m'$ (not counting the depad operation; that is, if $m' = c^{1/e} \bmod N\ $), what does $2^ec \bmod N\ $ decrypt to? What is t... |
I want to encrypt a small piece of data that is less that 16 bytes in size (think SSN), and I'll be using a 256bit encryption key. The typical suggestion is to never use ECB, but if there is just a single block being encrypted, is it a concern?
The reason I want to use ECB is because I want the encrypted value of t... |
Consider the following version of padded RSA encryption, where encryption of $m$ is done by setting $m′ = (0^k~||~r~||~00000000~||~m)$ for a random $r$ (of length 8 btyes = 64 bits) and then computing the ciphertext $c = (m′)^e$ (mod $N$). For concrete, further assume that $N$ is a 1024-bit RSA modulus, and that the sc... |
Consider the following version of padded RSA encryption, where encryption of $m$ is done by setting $m′ = (0^k~||~r~||~00000000~||~m)$ for a random $r$ (of length 8 btyes = 64 bits) and then computing the ciphertext $c = (m′)^e$ (mod $N$). For concreteness, further assume that $N$ is a 1024-bit RSA modulus, and that th... |
16 bytes is 128 bits, which matches the block size of AES-256, but not "256 bit block" in the title. Hence the question is ambiguous: was it meant 16, or 32 bytes?
For 16 bytes: ECB reduces to single-block encryption, and **yes** ECB is safe, for a definition of safe that let one test identity of plaintexts by testi... |
In this scenario, you don't need to do any padding (except for the first encryption).
If what you're encrypting with RSA is a random number between 0 and the modulus size, then you don't need any padding. Padding is there to prevent the attacker from using the multiplicative properties of RSA to derive one plaintex... |
I want to encrypt a small piece of data that is less that 16 bytes in size (think SSN), and I'll be using a 256bit encryption key. The typical suggestion is to never use ECB, but if there is just a single block being encrypted, is it a concern?
The reason I want to use ECB is because I want the encrypted value of t... |
16 bytes is 128 bits, which matches the block size of AES-256, but not "256 bit block" in the (original) title. Hence the question is ambiguous: was it meant 16, or 32 bytes?
For 16 bytes: ECB reduces to single-block encryption, and **yes** ECB is safe, for a definition of safe that let one test identity of plaintex... |
CipherCloud claims to support , among other things, searchable encryption. A bunch of speculation seems to suggest they did this via some breathtakingly incompetent means( unfortunately such speculation "appears" to be copyrighted)
Regardless of their actual methodology, if we assume their encrypted data is searcha... |
A lower bound on the insecurity of CipherCloud? |
CipherCloud claims to support , among other things, searchable encryption. A bunch of speculation seems to suggest they did this via some breathtakingly incompetent means( unfortunately such speculation "appears" to be copyrighted)
Regardless of their actual methodology, if we assume their encrypted data is searcha... |
That paper refers to the numerical ordering, while what would be relevant for searchable encryption without changes to the providers is the substring ordering. $\:$ (Thus, their bound does not apply.)
<br>
What do you mean by "existing queries in a database"?
If your assumption holds, then
<br>{<br>
Encryption m... |
[Shannon's paper][1] states:
> A system is pure if all keys are equally likely and if for any three
> transformations, $T_i$, $T_j$, $T_k$ in the set, the product
> $T_iT_j^{-1}T_k$ is also a transformation in the set.
But this is exactly the construction of Tripple-DES! So, using this terminology, Tripple-DES ... |
[Shannon's paper][1] states:
> A system is pure if all keys are equally likely and if for any three
> transformations, $T_i$, $T_j$, $T_k$ in the set, the product
> $T_iT_j^{-1}T_k$ is also a transformation in the set.
But this is exactly the construction of Triple-DES! So, using this terminology, it is not a *... |
CipherCloud claims to support , among other things, searchable encryption. A bunch of speculation seems to suggest they did this via some breathtakingly incompetent means( unfortunately such speculation "appears" to be copyrighted)
Regardless of their actual methodology, if we assume their encrypted data is searcha... |
I also watched the video (thanks Sid, for the link) and after looking at it, it reveals some of the other methods that Ciphercloud appears to be using to preserve search. Nothing appears to be an implementation of any sort of homomorphic encryption.
I snapped a copy of one screen after the response from John is ente... |
They are not using any exotic encryption. In fact I don't even think they are doing any encryption, just 1:1 mapping (tokenization) after lowering the case on plain text data. For details, I did some basic crypt-analysis on a still from their [publicly visible demo video][1].
[images removed due to DMCA request]
... |
When performing Shamir secret sharing I'm trying to find z<sub>i</sub>, such that z = x + y. Where n = 6 and t = 3.
I believe this would be the correct solution (correct me if I'm wrong):
1.) Each party computes x<sub>i</sub> + y<sub>i</sub> = z<sub>i</sub><br>
2.) Each party shares z<sub>i</sub> with the other ... |
Given a ciphertext $y$, describe how to choose a ciphertext $\hat{y} \neq y$, such that knowledge of the plaintext $\hat{x}=d_K(\hat{y})$ allows $x=d_k(y)$ to be computed.
So I use the fact that the decryption function is multiplicative: $d_K(y_1)d_K(y_2)= d_K(y_1y_2)$.
Thus we have $d_k(y\hat{y})=x\hat{x}$.
N... |
When performing Shamir secret sharing I'm trying to find $z_i$, such that $z = x + y$. Where $n = 6$ and $t = 3$.
I believe this would be the correct solution (correct me if I'm wrong):
1. Each party computes $x_i + y_i = z_i$
2. Each party shares $z_i$ with the other 2 parties
3. Each party uses Lagrange ba... |
Looking at the first step of AES encryption I see that we XOR the key with the plaintext block. Why is the actual key involved at all, why not just use the round keys derived from the key schedule? |
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... |
**Update to answer new question**
Technically the question is a little confused. The key schedule is defines the first round key as the encryption key. So when you say "why not use the round keys derived from the key schedule", the answer is that you are using the round keys derived from the key schedule. The first ... |
I could not find any evidence pointing towards homomorphic encryption. What I could find were different combinations of deterministic and format preserving encryption. There is probably also a variant that preserves order, but I couldn't find any material depicting it.
This post is based on material published on the... |
I could not find any evidence pointing towards homomorphic encryption. What I could find were different combinations of deterministic and format preserving encryption. There is probably also a variant that preserves order, but I couldn't find any material depicting it.
This post is based on material published on the... |
I could not find any evidence pointing towards homomorphic encryption. What I could find were different combinations of deterministic and format preserving encryption. There is probably also a variant that preserves order, but I couldn't find any material depicting it.
This post is based on material published on the... |
Yes.
For DES, it looks like the composition $T_i\circ T_j^{-1}\circ T_k$ is normally not a $T_l$ with another key $l$.
I do not have a proof of this fact.
So DES is not a pure system, and otherwise triple-DES would not more secure than DES itself.
In a comment, fgrieu linked a proof for a weaker fact, that ... |
I could not find any evidence pointing towards homomorphic encryption. What I could find were different combinations of deterministic and format preserving encryption. There is probably also a variant that preserves order, but I couldn't find any material depicting it.
This post is based on material published on the... |
I could not find any evidence pointing towards homomorphic encryption. What I could find were different combinations of deterministic and format preserving encryption. There is probably also a variant that preserves order, but I couldn't find any material depicting it.
This post is based on material published on the... |
What you want is basically impossible in this way. The crux of the matter is that if you want your application to access the encrypted data, your application needs to have the key in some shape or form.
Due to the limitations you mentioned, this key needs to be on the client machine, in the application's code or da... |
Another approach taken by researchers for carrying out *computations over encrypted data* is **Instance Hiding**.
In brief, If a user wants to outsource the computation of a function for a particular input x (instance). She transforms the input x to an encrypted input y (thus hides it) in such a way that the server ... |
What is the difference between Instance hiding and Encryption? |
Another approach taken by researchers for carrying out *computations over encrypted data* is **Instance Hiding**.
In brief, If a user wants to outsource the computation of a function for a particular input x (instance). She transforms the input x to an encrypted input y (thus hides it) in such a way that the server ... |
Suppose that the only public key cryptography schemes that we knew were [Diffie Hellman][1], [RSA][2] and [ElGamal][3].
How much would this set technology and civilization back? A
re there important applications of elliptic curve cryptography [specifically] which lack viable substitutes?
|
Suppose that the only public key cryptography schemes that we knew were [Diffie Hellman][1], [RSA][2] and [ElGamal][3]. How much would this set civilization back? Are there important applications of elliptic curve cryptography [specifically] without viable substitutes?
[Edit: I came across [this question][4] which g... |
Another approach taken by researchers for carrying out *computations over encrypted data* is **Instance Hiding**.
In brief, If a user wants to outsource the computation of a function for a particular input x (instance). She transforms the input x to an encrypted input y (thus hides it) in such a way that the server ... |
Another approach taken by researchers for carrying out *computations over encrypted data* is **Instance Hiding**.
In brief, If a user wants to outsource the computation of a function for a particular input x (instance). She transforms the input x to an encrypted input y (thus hides it) in such a way that the server ... |
Much of the literature and latest papers suggest that [homomorphic encryption][1] is still not practical yet.
How is [CipherCloud][2] able to achieve this? Does anyone have an idea? Their website does not provide much information about how their system works.
[1]: http://en.wikipedia.org/wiki/Homomorphic_encr... |
I'll take the question as: how can one make a chain of RSA encryptions with _distinct_ public keys $(N_j,e_j)$, that is $D_{j+1}=\operatorname{RSA-encrypt}((N_j,e_j),D_j)$, without having the ciphertext size growing at each encryption?
I'll use two facts:
1. When plaintext $x$ behaves to an adversary about as a u... |
I'll take the question as: how can one make a chain of RSA encryptions with _distinct_ public keys $(N_j,e_j)$, that is $D_{j+1}=\operatorname{RSA-encrypt}((N_j,e_j),D_j)$, without having the ciphertext size growing at each encryption?
I'll use two facts:
1. When plaintext $x$ behaves to an adversary about as a u... |
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