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Is it possible to construct an (asymmetric) encryption scheme from a signature scheme?
If the signature scheme is deterministic and allows existential forgery (e.g. RSA), then the answer is yes (create a forgery (m, sig); encrypt plaintexts using AES with sig as the key; send m in clear to the signer, who can also c... |
Reduction from signatures to encryption? |
Is it possible to construct an (asymmetric) encryption scheme from a signature scheme?
If the signature scheme is deterministic and allows existential forgery (e.g. RSA), then the answer is yes (create a forgery (m, sig); encrypt plaintexts using AES with sig as the key; send m in clear to the signer along with ciph... |
Is it possible to construct an (asymmetric) encryption scheme from a signature scheme?
If the signature scheme is deterministic and allows [existential forgery][1] (e.g. [RSA][2]), then the answer is yes (create a forgery (m, sig); encrypt plaintexts using [AES][3] with sig as the key; send m in clear to the signer ... |
SAT solvers are very important on [algebraic attack][1].
There are some SAT solvers such as [walksat][2] and [minisat][3].
These tow sat solvers solve benchmark problems available [here][4] in enormous different duration? Walksat is very faster than minisat in satisfiable problems.
Why did not exist [t... |
Why is there a enormous difference between SAT solvers? |
SAT solvers are very important in [algebraic attacks][1], for example [walksat][2] and [minisat][3].
However, when solving the benchmark problems available [here][4] there is an enormous performance difference between the two - Walksat is much faster than minisat for these problems. Why is this?
[This][5] imple... |
Let's first define a few things. Precise definitions are needed because your question is a bit ill-defined, and it seems that you are somewhat cheating.
**Some definitions**
Traditionally, we define a signature system as the combination of three algorithms:
- _G_: key generation; given a "security parameter" ... |
Why is there an enormous difference between SAT solvers? |
Plaintext that consists of an RSA key is easily recognisable as such, because it satisfies certain mathematical properties, in paticular (See [answer][1] / [question][2])
d = e^-1 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 was successful... |
Is it possible to create an encrypted key whose plaintext is not easily verifiable? |
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 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 was successfu... |
Is it possible to create a cryptosystem where the private keys are not easily verifiable as such? |
I have the need for a hexadecimal token that is smaller than the normal length of the hexadecimal representation of a sha-256 hash.
Should I take the first bits or the last bits? Which of them contain the most entropy? |
Should I use the first or last bits from a sha-256 hash? |
bcrypt uses [Blowfish][1], which is a block cipher (albeit with a much enlarged key schedule). As such, Blowfish implements a permutation of the space of 64-bit blocks; and there should be no way to distinguish Blowfish (using a random key) from a permutation extracted at random, with uniform probability, from the set ... |
This answer is not entirely on topic but not off topic either.
I read the question as, is it possible to design an asymmetric cryptosystem in which the private key is non-verifiable. I believe such a system is possible but it requires changes to the typical asymmetric encryption assumptions.
Trivial example: Pub... |
This answer is not entirely on topic but not off topic either.
I read the question as, is it possible to design an asymmetric cryptosystem in which the private key is non-verifiable. I believe such a system is possible but it requires changes to the typical asymmetric encryption assumptions.
Trivial example: Pub... |
This answer is not entirely on topic but not off topic either.
I read the question as, is it possible to design an asymmetric cryptosystem in which the private key is non-verifiable. I believe such a system is possible but it requires changes to the typical asymmetric encryption assumptions. *Edit*: My third exampl... |
Before it was *the* standard, the NSA proposed some changes to the S-boxes and didn't explain them. The explanation (which turned out to be correct when differential cryptanalysis was "rediscovered" by the non-spy community) was that if you changed a single bit of the input, every bit of output should have a 50% chance... |
I believe that the accepted tactic to "prove" a system as secure is to allow the crypto-community to review it and if no vulnerabilities are found over a long period of time (5 or 6 years), then a new system can be accepted as secure until proven otherwise. Please correct me if I'm wrong.
That aside, are there any s... |
Tactics available to help prove security of a new system? |
**Prove resistance to differential cryptanalysis**. For example this presentation [Provable security against Impossible Differential Cryptanalysis][1] or this paper Provable [Security Against a Differential Attack (1995)][2]
**Prove resistance to linear cryptanalysis:**
**Run a bunch of statical tests against the... |
In the computer security class (in which cryptography is a big chapter) that I took, I remembered the professor said about current asymmetric cryptography algorithms are based on integer factorization (i.e. prime numbers) and discrete logarithm. So my question is, are there any asymmetric cryptographic algorithms that ... |
In the computer security class (in which cryptography is a big chapter) that I took, I remembered the professor said about current asymmetric cryptography algorithms are based on integer factorization (i.e. prime numbers) and discrete logarithm.
So my question is, are there any asymmetric cryptographic algorithms th... |
"Discrete logarithm" is a wide class. Originally, this means that we work in a finite field (e.g. integers modulo a big prime) and, given _g_, _p_ and _g<sup>x</sup>_ mod _p_, it is computationally difficult to recover _x_ (it becomes impossible with today's technology once _p_ is big enough).
At some point, someone... |
Most of the time, when some data must be encrypted, it must also be protected with a [MAC][1], because encryption protects only against passive attackers. There are some nifty encryption modes which include a MAC ([EAX][2], [GCM][3]...) but let's assume that we are doing old-style crypto, so we have a standalone encryp... |
Should we MAC-then-encrypt or encrypt-then-MAC ? |
Yes, it is possible under certain circumstances to determine the encryption method used purely from ciphertexts. The first question that a cryptanalysis faced with such a problem must answer what general type the cipher is. Does the cipher appear to be a classical cipher, asymmetric cipher, or symmetric cipher?
-... |
Yes, it is possible under certain circumstances to determine the encryption method used purely from ciphertexts. The first question that a cryptanalysis faced with such a problem must answer what general type the cipher is. Does the cipher appear to be a classical cipher, asymmetric cipher, or symmetric cipher?
-... |
Related to [this question][1].
Given ciphertexts generated by any of the major asymmetric ciphers ([RSA][2], [ElGamal][3], [ECC][4], etc..) can these ciphertexts be distinguished from random noise? Justify why, why not?
That is do the major asymmetric ciphers have any statical bias?
If you knew which asymmet... |
Are any of the major asymmetric ciphers distinguishable (EG, RSA, ECC) ? |
Prime theory is of great interest to me! It is currently used in many cryptosystems to protect data (in making public keys, for example). There are always a few obscure researchers studying how to make prime factorization easier (or stronger I suppose).
There are so many branches of math that we use in cryptography ... |
It may be useful to define what is meant by noise. Noise is data that is selected randomly from some distribution. @Thomas deals with comparing to random bitstrings (which each bit is equally likely a zero or a one) and notes that because not all bitstrings of an appropriate length are possible values in most asymmetri... |
It may be useful to define what is meant by noise. Noise is data that is selected randomly from some distribution. @Thomas deals with comparing to random bitstrings (where each bit is equally likely 0 or 1) and notes that because not all bitstrings of an appropriate length are possible values in most asymmetric encrypt... |
Yes. DES is no longer considered an approved algorithm by NIST, and breaking a 56-bit key can be done quickly - in 1998 the [EFF DES Cracker](http://en.wikipedia.org/wiki/EFF_DES_cracker) managed to break a DES key in 56 hours. The average time it required to break any DES key was 4½ days. You could use triple DES inst... |
Can some one tell me which Mode out of `ECB`, `CBC` is better and how to decide what mode to use? Is there any other Modes that are better? |
"Normal" discrete logarithm based cryptosystems (DSA, Diffie-Hellman, ElGamal) work in the finite field of integers modulo a big prime _p_. However, there exist other finite fields out there, in particular binary fields _GF(2<sup>n</sup>)_. There is a specific attack described by Coppersmith for discrete logarithm in a... |
How robust is discrete logarithm in GF(2^n) ? |
I am aware that MD5 has a known collision vulnerability and should not be relied upon when uniqueness is required, but in the environment I am working on I only have access to MD5 hash function. Background detail below, but in general **how much and how bad is the change in the probability of hash collision if I remove... |
I've read a lot of summaries of block ciphers particularly with regards to the NIST competitions stating that reduced-round block ciphers are, for example, vulnerable to differential cryptanalysis.
I have a general idea that the application of differential cryptanalysis is to look at the difference between inputs; [... |
What is a white-box implementation?
Does White-box implementation have specific properties?
|
Generally, algorithms based on discrete logarithm specify that private keys are chosen as scalars between 1 and the order of the group (denoted q here). For instance IEEE P1363 and FIPS 186-3 both specify this range for DL private keys. Is it a problem if the key is not uniformly chosen from this range?
One possibil... |
Are there security issues with discrete logarithm keys not being uniformly distributed? |
So according to [wikipedia][1] unbalanced feistel ciphers provide greater provable security. Specifically, they state:
> The Thorp shuffle is an extreme case of an unbalanced Feistel cipher in which one side is a single bit. This has better provable security than a balanced Feistel cipher but requires more rounds.
... |
Does unbalancing a feistel cipher always improve security? Does it improve security at all? |
Inspired slightly by the Encrypt-then-MAC question. The most obvious message authentication code is probably [HMAC][1] or [RFC 2104][2] which is basically a hash of the input, an xor with a key... you get the idea.
However, I've also discovered [UMAC][3] which sounds very much like a hash function/lookup table to se... |
UMAC: to what extent is it in use today? |
Actually the article you link to does not says that a balanced Feistel cipher is less secure than an unbalanced one; it says that the security of an unbalanced Feistel cipher is more easily proven, _given enough rounds_.
Luby and Rackoff have [shown in 1988][1] that a balanced Feistel scheme with only 4 rounds is "p... |
I understand my group theory (allegedly), so I can make partial sense of [The Hidden Subgroup problem][1]:
> Given a group $G$, a subgroup $H \leq G$, and a set $X$, we say a function $f : G \Rightarrow X$ separates cosets of $H$ if for all $g_1, g_2 \in G$, $f(g_1) = f(g_2)$ if and only if $g_1H = g_2H$.
> Hidd... |
What exactly is the impact of the hidden subgroup problem on cryptography? |
So we've already had a question on [replacing the Rijndael S-Box][1]. My question is - can we use a different finite field other than the one given by $x^8 + x^4 + x^3 + x + 1$ in $GF(2^8)$. In other words, would any irreducible polynomial over this field do the trick or are the special considerations and properties of... |
The first step In [Algebraic-attack][1] is expressing the cipher operations as a system of equations. Is there an automated tool to do this?
[1]: http://en.citizendium.org/wiki/Algebraic_attack |
Are there any tools for expressing the cipher operations as a system of equations? |
So I've often looked at serpent and thought it was a very strong contender in AES. Not so long ago I was looking for evidence as to why it didn't beat rijndael. So far, the closest I've got answering that is this:
> The 32 rounds means that Serpent has a higher security margin than Rijndael; however, Rijndael with 1... |
XSL on serpent and rijndael - which is most affected? |
For security of ECC, the choice of the irreducible polynomial is unimportant, because all finite fields with the same cardinal are isomorphic to each other (and the isomorphisms are easy to compute); this is why we can say "_the_ finite field GF(2<sup>163</sup>)" even though there are many irreducible polynomials of de... |
So we've already had a question on [replacing the Rijndael S-Box][1]. My question is - can we use a different finite field other than the one given by $x^8 + x^4 + x^3 + x + 1$ in $GF(2^8)$. In other words, would any irreducible polynomial over this field do the trick or are the special considerations and properties of... |
Is there a point where increasing the complexity of an encryption algorithm, will make it easier to break using a timing attack? Or is there no connection here at all? |
About X.509 certificates serial numbers the [RFC 5280][1] says:
> The serial number MUST be a positive integer assigned by the CA to each certificate. It MUST be unique for each certificate issued by a given CA (i.e., the issuer name and serial number identify a unique certificate). CAs MUST force the serialNumber... |
This only occurred to me this morning. Suppose I had a message in Chinese (or some other non-phonetic language) and that I wanted to encipher it. Some of the simplest encryptions in English are simple substitution ciphers, but these don't seem to be a viable option for such a language (it's more inherently a code...). ... |
For designing a cipher, one first has to decide about the alphabet.
This is a bit problematic for a language like Chinese, since it is not really clear how many (and which) characters should be used. The number of signs known by people differs greatly. You don't want that your encrypted message is un-decryptable jus... |
I understand my group theory (allegedly), so I can make partial sense of [The Hidden Subgroup problem][1]:
> Given a group $G$, a subgroup $H \leq G$, and a set $X$, we say a function $f : G \Rightarrow X$ separates cosets of $H$ if for all $g_1, g_2 \in G$, $f(g_1) = f(g_2)$ if and only if $g_1H = g_2H$.
> Hidd... |
I understand my group theory (allegedly), so I can make partial sense of [The Hidden Subgroup problem][1]:
> Given a group $G$, a subgroup $H \leq G$, and a set $X$, we say a function $f : G \Rightarrow X$ separates cosets of $H$ if for all $g_1, g_2 \in G$, $f(g_1) = f(g_2)$ if and only if $g_1H = g_2H$.
> Hidd... |
Lets say that I am using 128 bit HMAC. How many operations are needed to find "non secure" message. Is birthday attack possible? |
**The short answer is:** 2<sup>128</sup> operations, no known birthday-like attack.
**The long answer:** when HMAC was [first published][1], it came with a security proof, tailored for iterated constructions like [Merkle-Damgård][2]. In a MD hash function (MD4, MD5 and the whole SHA family are MD hash functions), th... |
Assume I want to design a protocol (or data format or similar) including some cryptographic hash, and want it to be as future-proof as possible, i.e. I want to avoid that breakthroughs in cryptography make my old data insecure.
If I use only one hash algorithm, I can have bad luck to pick just the one which will be ... |
Combining is what [SSL/TLS][1] does with MD5 and SHA-1, in its definition of its internal "PRF" (which is actually a [Key Derivation Function][2]). For a given hash function, TLS defines a KDF which relies on HMAC which relies on the hash function. Then the KDF is invoked twice, once with MD5 and once with SHA-1, and t... |
Assume I want to design a protocol (or data format or similar) including some cryptographic hash, and want it to be as future-proof as possible, i.e. I want to avoid that breakthroughs in cryptography make my old data insecure.
If I use only one hash algorithm, I can have bad luck to pick just the one which will be ... |
Combining is what [SSL/TLS][1] does with MD5 and SHA-1, in its definition of its internal "PRF" (which is actually a [Key Derivation Function][2]). For a given hash function, TLS defines a KDF which relies on HMAC which relies on the hash function. Then the KDF is invoked twice, once with MD5 and once with SHA-1, and t... |
Regarding the German Enigma machines, if I recall correctly, the reason they were defeated was because the Allies were able to generate a massive database of possible rotor settings, and because the day key was encoded twice in the beginning of each message.
Given that computers of today can simulate an Enigma-ish m... |
Can an Enigma-style cipher of sufficient complexity be considered secure in today's world? |
If I'm hashing alphanumeric strings (chars in the set '0'-'9', 'a'-'z', and 'A'-'Z') with a CRC-64 hashing function, how long can my strings be while guaranteeing no hash collisions? Stated differently: if I have a set of all CRC-64 checksums computed from alphanumeric strings 0 to N bytes, how large can N be before ... |
What alphanumeric string length can be used to guarantee no hash collisions from CRC-64? |
Yes, if you are willing to throw enough resources at it. Only the most fatally flawed schemes cannot be rescued (in practical terms) given enough additional computation. Since you are even willing to enhance the rotor complexity, you could actually use it to implement a modern algorithm exactly. The ability for a rotor... |
Walksat is an incomplete solver. This means that it tries to find a solution for a number of iterations. If it does find a solution it answers with the solution, otherwise it answers "don't know". Walksat uses a form of random walk to search for solutions with heuristics to guide its step.
Minisat on the other hand,... |
I have an ID-Secret scheme and I'd like to hear if there are any vulnerabilities present.
Party 1 and Party 2 hold some credentials, an ID and a Secret.
Party 1 Creates this message:
HMAC(ID, SECRET)
IV + ENCRYPT(MESSAGE, SECRET, IV)
Using: SHA-256 and AES-CBC-256
Party 2 can then verify the use... |
I've always been interested in encryption but I have never found a good explanation (beginners explanation) of how **Encryption** with public key and **Decryption** with private key works.
How does it encrypt something with one key and decipher it with another key?.
Thanks in advance. |
How does SSL encryption work? |
The message format includes a datetime field in the clear. Is it okay to also use this field (or some hash thereof) as the initialization vector? |
Is it okay to use a hash of a timestamp as the IV for AES? |
Well, I see two clean ways of having practical resistance to these vulnerabilities.
If you want to use two hash functions, make sure you feed back the original data in a HMAC to the second function:
hash = algo1(data)
hash = hmac(algo2, data, hash)
The benefit here is that any collisions for algo1 wil... |
I'm not sure which of the inputs in your HMAC is your key and which one the message. The secret should be in the key position here.
Also, you might want to include the message (either plaintext or ciphertext) itself in the MAC - otherwise you are vulnerable to an attacker which can change the data (even if he can't ... |
I understand this may not be the best place to ask a question like this, but I believe that this community may be the best/only place I can ask such a question.
I have inputs and outputs from an in-house hash developed years ago and the developer is no longer working with us any more. I need to determine the proces... |
Reverse engineering a hash? |
It's been said that CRC-64 is bijective for a 64-bit block. It this true for SHA-2? |
Is SHA-512 bijective when hashing a single 512-bit block? |
Since the input is base-10 and the output is base-16, the output actually looks bigger than the input.
Do you have access to a binary code implementation? Disassembling or debugging that would be easiest.
How many hashed pair samples do you have access to? Can you generate more with chosen inputs?
What was the... |
Let's say I'm on an open wireless network that's being actively sniffed and I connect to an HTTPS site. Even though my subsequent traffic is encrypted, couldn't the sniffer use the data from the initial (non-secure) handshake to decrypt my subsequent data? |
Is HTTPS secure if someone snoops the initial handshake? |
Wait, is the goal for Bob to encrypt messages to Alice (who holds the secret signing key) or for Alice to encrypt messages to Bob (who knows Alice's public key)? If it is the former, then I can't think of any way around Thomas Pornin's excellent argument above. If it is the latter, and if we grant my assumption that no... |
My primary question is:
- Is there an easy way to create a *bijective* mapping from points on an elliptic curve E (over a finite field) to the integers (desirably to $\mathbb{Z}_q$ where $q$ is the order of E)?
To phrase it a second way, given a point on the curve that is chosen with uniform randomness, can you t... |
My primary question is:
- Is there an easy way to create a *bijective* mapping from points on an elliptic curve E (over a finite field) to the integers (desirably to $\mathbb{Z}^*_q$ where $q$ is the order of E)?
To phrase it a second way, given a point on the curve that is chosen with uniform randomness, can you... |
I've always been interested in encryption but I have never found a good explanation (beginners explanation) of how **encryption** with public key and **decryption** with private key works.
How does it encrypt something with one key and decipher it with another key? |
How does asymmetric encryption work? |
I know that there is a [formula][1] to calculate the result of any input but is the logic of how the machine actually worked documented?
I have seen schematics for the circuitry and [even a how to make your own at home][2] site. But has the logic been broken down? Is there source for a virtua Enigma Machine that w... |
Is the logic for how the enigma machine worked documented somewhere? |
It's been said that [CRC-64 is bijective for a 64-bit block][1].
It the corresponding statement true for SHA-2?
[1]: http://crypto.stackexchange.com/questions/282/what-alphanumeric-string-length-can-be-used-to-guarantee-no-hash-collisions-from/286#286 |
Are there any evidence (other than not being cracked so far) that the [Bitcoin][1] protocol is secure? "How secure" is it?
(I realize that this might not qualify as a meaningful question - feel free to edit if you can improve it. Also, if you liked this question, you might like the [Bitcoin Proposal][2])
[1]:... |
How secure is the Bitcoin protocol? |
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