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If there are $n$ servers for broadcasting a symmetric key in such a manner that following holds:
1. No server alone can find the key $K$ alone.
2. Any two servers can pass $k_i$ and $k_j$ such that $K=f(k_i, k_j)$ where $i\neq j$
How can a symmetric key be distributed between $n$ entities? |
Assumes that we have 3 signature algorithms, $S^A$ with key pair $(sk^A,pk^A)$, $S^B$ with key pair $(sk^B,pk^B)$,$S^C$ with key pair $(sk^C,pk^C)$. We denote by $\epsilon$, $\epsilon'$ and $\epsilon''$ the advantages for breaking $S^A$, $S^B$ and $S^C$ respectively, in the sense of weak unforgeability.
I have a com... |
I am currently reading about PBKDF2, and understand that the salt is used only once, while the password is used multiple times in the computation of the final key (see [this](http://crypto.stackexchange.com/questions/3725/why-is-the-salt-used-only-once-in-pbkdf2-while-the-password-is-used-often) question). How would th... |
I've read the following two sentences within a beta protocol specification, which references ISO/IEC 7816-8 for PSO:HASH which already contains notions of a "partial hash".
> The off-card entity is responsible of computing the intermediate hash over the first part of the data to be signed. The intermediate hash-code... |
On the surface, SHA1 and MD5 look pretty similar. Their diagrams include chunks of bits, bit rotation, xor and special functions. Their implementations are roughly the same length (at least the ones I've seen). Yet it's widely known that MD5 is broken, but currently SHA1 isn't.
Is the security difference from the in... |
Assumes that we have 3 signature algorithms, $S^A$ with key pair $(sk^A,pk^A)$, $S^B$ with key pair $(sk^B,pk^B)$,$S^C$ with key pair $(sk^C,pk^C)$. We denote by $\epsilon$, $\epsilon'$ and $\epsilon''$ the advantages for breaking $S^A$, $S^B$ and $S^C$ respectively, in the sense of weak unforgeability.
I have a com... |
I just recently learned that the SHA-3 finalist [Keccak][1] allows for variable length output. As the only answer to [this question][2] states, "[it] need to have an output length at least equal to the input length," which I understand just fine. However, I'm curious if this guarantees against collisions.
* Is ther... |
Are variable-length crypto hash functions still susceptible to collisions? |
Note that the above discussion may be out of date after the Snowden revelations. For example, see the "Hardening Internet Infrastructure" panel at IETF88 where Schneier speculates that the NSA expects to be able to break RC4 and is recruiting staff to do so.
In the black budget that was released, DNI, James Clappe... |
Note that the above discussion may be out of date after the Snowden revelations. For example, see the ["Hardening Internet Infrastructure" panel at IETF88](http://www.ietf.org/live/ietf88/text.html) where Schneier speculates that the NSA expects to be able to break RC4 and is recruiting staff to do so.
> In the bl... |
> Is there any work done to show or prove collision resistance gained by increasing digest length?
Actually, as CodesInChaos has mentioned, the variable length versions of Keccak ("SHAKE128" and "SHAKE256") are known not to have any collision resistance beyond their security level, independent of how long we make th... |
I recently came across an interesting [paper](http://www.cl.cam.ac.uk/~sjm217/papers/protocols08cookies.pdf) detailing the use of hardened session cookies. Each cookie includes a preimage of the password hash, and the preimage is hashed once more and compared to the stored password hash as part of the cookie authentica... |
I have been studying the Diff. Cryptanalysis and finding it hard to do a problem. The problem is ![enter image description here][1]
[1]: https://i.stack.imgur.com/kDGdN.png
I have calculated the tables programmatically and found that the second table has a max value of 6 where as the first has a max value of ... |
I recently came across an interesting [paper](http://www.cl.cam.ac.uk/~sjm217/papers/protocols08cookies.pdf) detailing the use of hardened session cookies. Each cookie includes a preimage of the password hash, and the preimage is hashed once more and compared to the stored password hash as part of the cookie authentica... |
Your calculations are correct. The 2nd table has 2 entries of 6 in its DDT, and 18 entries of 4. The Hamsi s-box has 24 entries of 4.
What can you infer from these tables? First off, CryptWizard001 is a liar. Second, the larger the max value in the DDT, the more vulnerable the s-box is to differential cryptanalysis,... |
As stated in the HMAC RFC (https://www.ietf.org/rfc/rfc2104.txt) : "The strongest attack known against HMAC is based on the frequency of collisions for the hash function H".
How can a collision benefit an attacker?
I would have thought that the strongest attack would be a brute force attack aimed at discovering the... |
Computing Hash of Hashes Unsafe? |
I've read the following two sentences within a beta protocol specification, which references ISO/IEC 7816-8 for PSO:HASH which already contains notions of a "partial hash".
> The off-card entity is responsible of computing the intermediate hash over the first part of the data to be signed. The intermediate hash-code... |
What's the point of a Meet In The Middle attack while using, for example, a double AES encryption and using one time keys?
You can recover the keys for a secret message already known and you can't use those keys to retrieve other secret messages.
So why would an attacker use it? |
What's the point of a Meet In The Middle attack using i.e. double AES with throw-away keys? |
As mikeazo points out, a meet-in-the-middle (or any other known plaintext attack) against AES is believed to be infeasible; I'll generalize your question to "if we never reuse a key, why do we care about known plaintext attacks? If the attacker already knew the message, why do we care if he could decrypt it?"
The r... |
Both client and server know a shared key in advance. Both sides us EVP_BytesToKey() derive a pair of same key and same IV. Then, client and server start transferring data encrypted by AES-CBC mode using this pair of key and iv.
Is there any possible CCA or CPA? Should I use randomized IV every time? Is it safe to tr... |
Possible CPA or CCA attack when using AES-CBC mode under the following situation? |
Is it true that first time RSA algorithm was rejected by a journal? |
RSA was rejected by which journal? |
I am currently trying to implement a very fast and simple encryption algorithm in Lua for the purpose of using it with ComputerCraft.
The main problem is that bitwise operations in Lua (5.2 at least) are very slow and the implementation ComputerCraft can only handle 32 bit integers in their bit-library, so implement... |
Security of very simple XOR with random? |
I asked this at bitcoin.stackexchange.com first, but it seems that this is more of a crypto-question anyway.
I'm interested in using a [Hierarchical Deterministic][1] Bitcoin wallet branch as a "shared secret" between two parties. Basically one party could send an index (in plain text) to the other party, along with... |
Can Bitcoin HD public keys be used for symmetric encryption? |
I'm pretty sure that by now folks might have come across [this research][1] from Genkin-Pipman-Tromer (**GPT**) on extracting the RSA key used by GnuPG (**GPG**) just by measuring the ground potential. I'm going to call it the `GPT attack on GPG` for short. I have done work on extracting informational content of a proc... |
Rather than [LuaCrypto](http://mkottman.github.io/luacrypto/) or some other Lua binding to some (blazing fast) low-level cryptographic library in some other language;
I'm assuming that you want a "pure Lua" implementation.
Would one of the following work for you?
* ARCFOUR in pure Lua [(a)](http://www.computercraf... |
Rather than [LuaCrypto](http://mkottman.github.io/luacrypto/) or some other Lua binding to some (blazing fast) low-level cryptographic library in some other language;
I'm assuming that you want a "pure Lua" implementation.
Would one of the following work for you?
* ARCFOUR in pure Lua [(a)](http://www.computercraf... |
Is hashing a list of hashes safe? |
> Are variable-length crypto hash functions still susceptible to collisions?
Yes. Even if you choose an output length equal to the input length, you expect some collisions from even an ideal PRF. E.g., if my test code works: $\operatorname{SHAKE256}(\text{'6'}, 8) = \operatorname{SHAKE256}(\text{'8'}, 8)$.
> Is t... |
There are several well-regarded block cipher modes for authenticated encryption which have made their way into standards and protocols: [CCM](http://en.wikipedia.org/wiki/CCM_mode), [EAX](http://en.wikipedia.org/wiki/EAX_mode), [GCM](http://en.wikipedia.org/wiki/GCM_mode), [OCB](http://en.wikipedia.org/wiki/OCB_mode), ... |
There are several well-regarded block cipher modes for authenticated encryption which have made their way into standards and protocols: [CCM](http://en.wikipedia.org/wiki/CCM_mode), [EAX](http://en.wikipedia.org/wiki/EAX_mode), [GCM](http://en.wikipedia.org/wiki/GCM_mode), [OCB](http://en.wikipedia.org/wiki/OCB_mode), ... |
I'm trying to understand why some key stretching are better than other.
The [wikipedia article][1] presents 3 different key stretching method.
> 1) A collision prone simple key stretching method:
>
key = hash(password)<br /> for 1 to 65536 do<br /> key = hash(key)
>
> 2) A better simple key stretching... |
Why are some key stretching better than other? |
I'm trying to understand why some key stretching methods are better than others.
The [wikipedia article][1] presents 3 different key stretching methods:
> 1. A collision prone simple key stretching method:
> key = hash(password)
for 1 to 65536 do
key = hash(key)
> 2. A better si... |
Why are some key stretching methods better than others? |
> How did they jump from the spectrogram showing the RSA exponentiation timings straight to the secret bits?
Actually, they're jumping to secret bits; however those aren't the secret bits you're thinking of.
The bits displayed above are not the actual bits of $p$ and $q$. Instead, those are the bits from the sec... |
> How did they jump from the spectrogram showing the RSA exponentiation timings straight to the secret bits?
Actually, they're jumping to secret bits; however those aren't the secret bits you're thinking of.
The bits displayed above are not the actual bits of $p$ and $q$. Instead, those are the bits from the sec... |
Is there a safe way to do crypto that will always produce the same result for a given input?
My use case is transferring deltas of huge files, to a backup server. The backup server has no knowledge of what's in the byte stream (and it shouldn't).
In order to transfer only the changed parts of the file, any block ... |
I am new to Crypto field. Many papers are boasting of not using Random Oracle model. Instead, those prove security in Standard Model. I am surprised how do these models differ. Can anyone please clarify me? Does using Hash function mean that the paper makes use of Random Oracle? |
How does Random Oracle and Standard Model differ? |
As stated in the HMAC RFC ([RFC 2104](https://www.ietf.org/rfc/rfc2104.txt)):
> The strongest attack known against HMAC is based on the frequency of collisions for the hash function H.
How can a collision benefit an attacker? I would have thought that the strongest attack would be a brute force attack aimed at di... |
Could someone point to results or proofs about the probability of $$HMAC(k, m_1) = HMAC (k, m_2)$$ assuming the underlying hash function is SHA-256? Would those probabilities be higher/lower if $m_1$ and $m_2$ are not independent?
Please don't point to birthday attack results. I am only interested in the probabilit... |
Proving HMAC collision probability bounds? |
Perhaps you could do something with Visual Cryptography.
Maybe something like:
* Gather a few low-resolution images (symbols or short text phrases), perhaps a few more images than you have kids
* Use visual cryptography to split each image into 2 random-looking images, and print each random-looking image on its ow... |
I have been looking at both the paper as well as the sourcecode of [Spritz — a spongy RC4-like stream cipher and hash function](http://people.csail.mit.edu/rivest/pubs/RS14.pdf) by Ronald L. Rivest and
Jacob Schuldt.
The paper states
> We have also performed extensive simulations to look for detectable statisti... |
How should I treat a new cipher release like Spritz? |
I have been looking at both the paper as well as the sourcecode of [Spritz — a spongy RC4-like stream cipher and hash function](http://people.csail.mit.edu/rivest/pubs/RS14.pdf) by Ronald L. Rivest and
Jacob Schuldt.
The paper states
> We have also performed extensive simulations to look for detectable statisti... |
I would pick **EAX** as it is by far the simplest to implement and therefore to understand and audit. It is reasonably fast if based on AES.
**GCM** seems quite popular, but I personally see a number of issues with it:
* it is very difficult to implement in software (which is not surprising, since it was develope... |
I would pick **EAX** as it is by far the simplest to implement and therefore to understand and audit. It is reasonably fast if based on AES.
**GCM** seems quite popular, but I personally see a number of issues with it:
* it is very difficult to implement in software (which is not surprising, since it was develope... |
SSL 3.0 and TLS 1.0 used an insecure scheme to generate implicit IVs when encrypting records in CBC mode: they used the last part of the previous record, a value that can be predicted by the attacker. This attack was demonstrated in the [BEAST attack][1].
To avoid these issues, TLS 1.1 and later use explicit IVs, wh... |
let's suppose we have a asymmetric cryptosystem H which is homomorphic with respect to some function F.
- Alice encrypts a message m with her private key e in the cryptosystem H and obtains the ciphertext C = H(M)
- Alice sends C to Bob
- Bob computes F over C and obtains C' = F(C)
- Bob sends C' to Alice
-... |
Homomorphic Encryption: how does the equality test on ciphertexts work? |
I would pick **EAX** as it is by far the simplest to implement and therefore to understand and audit. It is reasonably fast if based on AES.
**GCM** seems quite popular, but I personally see a number of issues with it:
* it is very difficult to implement in software (which is not surprising, since it was develope... |
How do we verify practically the strength of a block ciphers ? Most of the security analysis of block ciphers is usually around theoretical analysis and proofs in some random oracle model etc.
For example, for our recently designed [FNR encryption][1] scheme, we have only given results based on previously done theo... |
How do we verify practically the strength of a block ciphers? Most of the security analysis of block ciphers is usually around theoretical analysis and proofs in some random oracle model etc.
For example, for our recently designed [FNR encryption][1] scheme, we have only given results based on previously done theor... |
Plenty of ciphers come out of the USA from government research or selection competitions. AES and DES are examples.
Are there any public ciphers produced by other states, China or Iran for example? Do you really think they trust AES?
**EDIT: List of ciphers from different nations:**
- USA: [Skipjack][1]
- J... |
Assume:
* We have a centralized data store that can store whatever we need
* Every user is online
* Users can securely communicate
* Function $f : File \to Key$ gets access to the store, and is the same for all users.
* $g : Key \to File$ should retrieve the file so that $g \circ f = id$.
* Eve can't break mod... |
Is there a asymmetric algorithm especially suited for a use case where we only use the private key once, to sign a small cleartext (1024 bytes in my case) and we only use the public key to verify said signature?
|
I want to generate a key pair such that the private key can be used once to sign a small message (1024 bytes) at some indeterminate point in the future and the public key can be used to verify that signature, what can I do to get better security than a "regular" asymmetric algorithm (say, RSA)?
The use case is allow... |
I want to generate a key pair such that the private key can be used once to sign a small message (1024 bytes) at some indeterminate point in the future and the public key can be used to verify that signature, what can I do to get better security than a regular asymmetric algorithm (e.g. RSA)?
The use case is allowin... |
> Plenty of ciphers come out of the USA from government research or selection competitions. AES and DES are examples.
Indeed, the US is known from some crypto-related competitions that were/are ope to anyone and they surely will do ample of government research related to cyptology, but you need to be sure that you d... |
I want to generate a key pair such that the private key can be used once to sign a small message (1024 bytes) at some indeterminate point in the future and the public key can be used to verify that signature, what can I do to get better security than a regular asymmetric algorithm (e.g. RSA)?
The use case is allowin... |
What do you mean by better security? RSA was proved to be as difficult as prime factorisation and is thus one of the best understood public key cryptosystems. Also, since its widely used their implementations have withstood years of attacks. Since you are going for a one-time usage of the keys, speed is not of importan... |
I want to generate a key pair such that the private key can be used once to sign a small message (1024 bytes) at some indeterminate point in the future and the public key can be used to verify that signature, what can I do to get better security than a regular asymmetric algorithm (e.g. RSA)?
The use case is allowin... |
What do you mean by better security? RSA was proved to be as difficult as prime factorisation and is thus one of the best understood public key cryptosystems. Also, since its widely used their implementations have withstood years of attacks. Since you are going for a one-time usage of the keys, speed is not of importan... |
What do you mean by better security? RSA was proved to be as difficult as prime factorisation and is thus one of the best understood public key cryptosystems. Also, since its widely used their implementations have withstood years of attacks. Since you are going for a one-time usage of the keys, speed is not of importan... |
What do you mean by better security? <strike>RSA was proved to be as difficult as prime factorisation and is thus one of the best understood public key cryptosystems.</strike> Also, since its widely used their implementations have withstood years of attacks. Since you are going for a one-time usage of the keys, speed i... |
I want to manage authorization, as 3rd-party permissions, through asymetric cryptography.
I'm concerned about how is it possible to share access of encrypted data with N entities, and be able to revoke any of them at anytime, and this securely.
I've thought about a cryptography function that I'm asking here some ... |
I want to manage authorization, as 3rd-party permissions, through asymmetric cryptography.
I'm concerned about how is it possible to share access of encrypted data with N entities, and be able to revoke any of them at anytime, and this securely.
I've thought about a cryptography function that I'm asking here some... |
I want to generate a key pair such that the private key can be used once to sign a small message (1024 bytes) at some indeterminate point in the future and the public key can be used to verify that signature, what can I do to get better security than a regular asymmetric algorithm (e.g. RSA)?
The use case is allowin... |
One algorithm that is especially suited to one-use key pars is [lamport signatures](http://en.wikipedia.org/wiki/Lamport_signature). Like many (all?) other signature functions, lamport signatures first hash the message to get it down to a size that is more reasonable to sign.
For this use case, if you are willing to... |
I want to manage authorization, as 3rd-party permissions, through asymmetric cryptography.
I'm concerned about how is it possible to share access of encrypted data with N entities, and be able to revoke any of them at anytime, and this securely.
I've thought about a cryptography function that I'm asking here some... |
One algorithm that is especially suited to one-use key pars is [lamport signatures](http://en.wikipedia.org/wiki/Lamport_signature). Like many (all?) other signature functions, lamport signatures first hash the message to get it down to a size that is more reasonable to sign.
For this use case, if you are willing to... |
> Plenty of ciphers come out of the USA from government research or selection competitions. AES and DES are examples.
Indeed, the US is known from some crypto-related competitions that were/are ope to anyone and they surely will do ample of government research related to cyptology, but you need to be sure that you d... |
Let's suppose we have a asymmetric crypto-system $H$ which is homomorphic with respect to some function $F$.
- Alice encrypts a message $m$ with her private key $e$ in the crypto-system $H$ and obtains the ciphertext $C = H(M)$
- Alice sends $C$ to Bob
- Bob computes $F$ over $C$ and obtains $C' = F(C)$
- Bob... |
Assume:
* We have a centralized data store that can store whatever we need
* Every user is online
* Function $f : File \to Key$ gets access to the store, and is the same for all users.
* $g : Key \to File$ should retrieve the file so that $g \circ f = id$.
* Eve can't break modern algorithms.
Then:
3. Eve... |
In my question (and answer) about a [partial hash code used within a signature application](http://crypto.stackexchange.com/questions/18828/partial-hash-code-protocol-for-security-tokens-providing-signatures) I found out that partial hash codes are sometimes used within protocols with a relatively restricted security t... |
Does creating a partial hash value make sense? |
In my question (and answer) about a [partial hash code used within a signature application](http://crypto.stackexchange.com/questions/18828/partial-hash-code-protocol-for-security-tokens-providing-signatures) I found out that partial hash codes are sometimes used within protocols with a relatively restricted security t... |
Are partial hashes used in cryptographic protocols? |
Yes, standard GC are not re-usable, thus by means of GC you may outsource the computation of a single function on a single input (i.e. you delegate a function described by a Boolean circuit and later you may ask the evaluation of the function on a single input not fixed in advance).
Indeed this is the approach to Veri... |
Yes, standard GC are not re-usable, thus by means of GC you may outsource the computation of a single function on a single input (i.e. you delegate a function described by a Boolean circuit and later you may ask the evaluation of the function on a single input not fixed in advance).
Indeed this is the approach to Ve... |
If Alice, Bob et al. can share a secret, for example, using some sort of key exchange protocol, it is possible to use convergent encryption in a way that fulfills the requirements. Otherwise it probably isn't.
---
First, assume the users know a secret key $s$. They use [convergent encryption][1] with the key der... |
Why is the block cipher in the CBC-MAC taken to be a pseudo random permutation (PRP) and not a pseudo random function (PRF)? We don't need the bijective property of the PRP. So why go with it? |
# Desirable Properties
For simplicity, I’m skipping some of the details here… but the main criteria of a good s-box are:
- It should have balanced component functions,
- The non-linearity of its component functions should be high,
- The non-zero linear combinations of its component functions should be balanced ... |
Some cryptographers feel that the ultimate goal for an encryption scheme is [semantic security][1] or, even better, [perfect security][2].
An encryption scheme that supports de-duplication also allows the backup server -- and the attacker, who we assume steals every backup tape that the server sysadmins send to the ... |
I am using ElGamal ECC to encrypt my plain text data, I want to ensure that my data must be safe from Man In Middle attack,
what methodology I can adopt to achieve this goal...?
Thanks in advance |
I am using ElGamal ECC to encrypt my plain text data. I want to ensure that my data is safe from a Man-In-The-Middle attack. What methodology I can adopt to achieve this goal?
How can we prevent a Man-In-The-Middle attack while encrypting data by using ElGamal ECC?
|
Prevent MITM attack while encrypting data by using ElGamal ECC? |
in general ,the idea of multiple encryption sounds interesting - for creating stronger cyphers(especially against implementation bugs), one could always get the configuration details wrong and mess it up.
So i wonder : if one start with nacl , which is supposed to have a default config that's very hard to mess up ... |
Is multiple encryption using tls + nacl better? |
If the goal is to demonstrate that you are who you say you are, I think the following will work: Bob wants to know that you are Amy. Bob encrypts a number using Amy's private key and then asks her what that number is. If Bob gets back the number, "Amy" at least had Amy's private key.
From what I have read, this is n... |
How is the following different than RSA Signing of a document? |
In general, the idea of multiple encryption sounds interesting - for creating stronger cyphers (especially against implementation bugs), one could always get the configuration details wrong and mess it up.
So I wonder: if one starts with NaCL, which is supposed to have a default configuration that's very hard to me... |
Is multiple encryption using NaCL and TLS better? |
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