instruction stringlengths 15 30k |
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Is there an authenticated encryption scheme which also provides non-repudiation? |
If the private exponent $d$ has very low Hamming weight, it will be easy to break the scheme.
Suppose that $d$ has weight $w$, and is $m$ bits wide. @Antimony already explained how to break it in $m \choose w$ time. I'll show how to break it even faster, using a birthday-style technique. My algorithm takes about ... |
Let $E$ be a family of Pseudo-Random Permutations of $b$ bits, with $E_K$ indistinguishable from random permutation with effort less than $O(2^b)$, easily computable as well as the inverse permutation, e.g. as instantiated by AES-192 for $b=128$. Let $E_A$ and $E_B$ be two public members of this family.
For an $E_K$... |
There are no non-repudiation services without using of digital signature.
> Or maybe I just need to encrypt (using a plain randomized mode,
> provides confidentiality only) then sign the ciphertext+associated
> data (which provides integrity, authentication and non-repudiation)?
Yes, it will be enough. Or you c... |
Let $E$ be a family of Pseudo-Random Permutations of $b$ bits, with $E_K$ indistinguishable from random permutation with effort less than $O(2^b)$, easily computable as well as the inverse permutation, e.g. as instantiated by AES-192 for $b=128$. Let $E_A$ and $E_B$ be two public members of this family.
For an $E_K$... |
>Or maybe I just need to encrypt (using a plain randomized mode, provides confidentiality only) then sign the ciphertext+associated data (which provides integrity, authentication and non-repudiation)?
That is not enough. You need to encrypt (confidentiality only) then sign
<br>
a commitment to the cipher key $\:$ ... |
What if i use CBC mode using this approach
- Use a fixed IV(like a block of 0's)
- Before encrypting, generate a random block, and prepend it to the plain messagem
- After decrypting , ignore first block
Is it exactly the same in terms of security as using a random IV?
Would it be different if instead of us... |
This is a follow-up question to [Relative merits of AES ECB and CBC modes for securing data at rest.][1]
I need to store encrypted Personal Account Numbers (PANs) in a database. The only encryption option I have available is CFB mode with a fixed (0x00) IV. I can choose my cipher, and have chosen AES-256.
Encrypt... |
Will varying plaintext compensate for a fixed initialisation vector? |
Is there a formal security proof in the shape of reduction that states that if an attacker manages to break the collision resistance property of a cryptographic hash function (a random oracle) he will break the pre-image attack as well? To rephrase it: is first pre-image resistance as hard as collision resistance s.t i... |
Does a break in a collision resistance property of a hash function by definition implies an attack at the first pre-image attack? |
> Use the master key in ECB mode. I have heard/read that this is cryptographically weak (Why?).
>
> The reason I include ECB mode here is that it could allow me to save half the storage space. Since the messages are only one block long, I suspect this may affect the 'traditional' arguments against ECB.
Because any... |
The scenario in Alice/Bob/Cindy terms:
Alice approaches Bob, who does not know Alice, and asks for some secret information. Bob doesn't trust Alice, but Alice says she knows Cindy. To prove that, Bob calls Cindy, who does know Bob, and asks for half of an asymmetric key pair. Bob then challenges Alice to encrypt a "... |
Is this a valid real-time authentication scheme? |
The scenario in Alice/Bob/Cindy terms:
Alice approaches someone she doesn't know, but thinks is Bob, and asks for some secret information. Bob doesn't know and doesn't trust Alice, but Alice says she knows Cindy.
To prove that, Bob calls Cindy, who does know Bob, and asks for half of an asymmetric key pair. Bob ... |
In S/MIME, are the same certificates always used to sign and encrypt messages? |
CFB with a fixed IV? Yikes! That is completely insecure: it is even worse than ECB mode, and that's saying something.
It sounds like the baseline is to use CFB mode with a fixed all-zeros IV, to encrypt a 16-bit (one-block) message. In this case, the encryption of the PAN $P$ will be
$$C = E_K(0) \oplus P.$$
... |
CFB with a fixed IV? Yikes! That is completely insecure: it is even worse than ECB mode, and that's saying something.
It sounds like the baseline is to use CFB mode with a fixed all-zeros IV, to encrypt a 16-bit (one-block) message. In this case, the encryption of the PAN $P$ will be
$$C = E_K(0) \oplus P.$$
... |
CFB with a fixed IV? Yikes! That is completely insecure: for the first 16 bytes of plaintext, it is even worse than ECB mode, and that's saying something. Please go enlighten whoever thought it was a good idea to expose this as the only mode of encipherment available (or even one among multiple options).
Let me e... |
CFB with a fixed IV? Yikes! That is completely insecure: for the first 16 bytes of plaintext, it is even worse than ECB mode, and that's saying something. Please go enlighten whoever thought it was a good idea to expose this as the only mode of encipherment available (or even one among multiple options).
Let me e... |
Is there anyway to detect that a cipher is encrypted with ROT13 Or Base64 without decrypting it using the algorithm and check that the result means (works)?
I mean does an algorithm exists that test the cipher and tell YES it is ROT13/Base64 (or NO it isn't)?
Thanks
|
This is a follow-up question to [Relative merits of AES ECB and CBC modes for securing data at rest.][1]
I need to store encrypted Personal Account Numbers (PANs) in a database. The only encryption option I have available is CFB mode with a fixed (0x00) IV. I can choose my cipher, and have chosen AES-256.
Encrypt... |
To get non-repudiation, you must have a "proof" which can be used _against the signer himself_. Symmetric cryptography, by itself, will not help you there, because a cryptographic proof can only come from some "secret" which is under exclusive control of the signer, and the symmetric model assumes that there is a _shar... |
When having a company computer that is full disk encrypted, it is common that the boss have a master password / backdoor, so the data always can be decrypted.
I have heard some call this for "scrow" or something similar.
Does anyone know the correct name?
|
What can a master password also be called? |
Also, where do I store my salt (can I just store it at the beginning of the encrypted file)?
|
Can I use my random IV (for AES) as a salt for PBKDF2? |
I _suppose_ that what you are trying to do is password-based encryption of some data; you use PBKDF2 to derive the password into an encryption key, and then use the key with AES to encrypt the data. The AES encryption needs an _IV_, and the PBKDF2 function needs a _salt_. Both IV and salt should be generated anew for e... |
Consider the following problem: given $g_1 \ldots g_i,h_1 \ldots h_i \in G$, $\forall i$ find $x_i$ such that $g_i^{x_i}=h_i$
For $i=1$ this is the discrete log problem and is assumed to to have some level of security based on the order of $G$ for certain groups. Let's call this security level $\lambda$.
Wh... |
With a standard authenticated encryption scheme (or MAC), Alice and Bob share a symmetric key. When Alice sends something to Bob, Bob can check that it is authentic. At that point, Bob can deduce that the message must have been sent by someone who knows the key, i.e., it must have been sent by Alice or Bob.
How ca... |
Is there an authenticated encryption scheme where the recipient can attribute the message to a single sender? |
CFB with a fixed IV? Yikes! That is completely insecure: for the first 16 bytes of plaintext, it is even worse than ECB mode, and that's saying something. Please go enlighten whoever thought it was a good idea to expose this as the only mode of encipherment available (or even one among multiple options).
Let me e... |
Here's a different attack, one that runs in $O(2^{b/2})$ time. I'll also present a theoretical framework for how to think about the security of these sort of schemes. The bottom line is that it looks like no scheme of this form can be secure; I'll try to make more precise what I mean by that, below.
----
We can... |
I need to know if it is secure to store the hash of a symmetric cipher key along with data encrypted with that key. Are there any circumstances or algorithms in which this combination could lead to potential weaknesses - or is it always a bad idea? Assume the cipher is AES-128 or -192 and the hash is SHA-256, and also ... |
Is storing the hash of a key together with cipher-text encrypted with that key secure? |
I need to know if it is secure to store the hash of a symmetric cipher key along with data encrypted with that key. Are there any circumstances or algorithms in which this combination could lead to potential weaknesses - or is it always a bad idea? Assume the cipher is AES-128 or -192 and the hash is SHA-256, and also ... |
Is it secure to store the hash of a symmetric cipher key along with data encrypted with that key? Are there any circumstances or algorithms in which this combination could lead to potential weaknesses -- or is it always a bad idea? Assume the cipher is AES-128 or -192 and the hash is SHA-256, and also assume that the k... |
Is storing the hash of a key together with ciphertext encrypted with that key secure? |
We want to explicitly construct a _good_ (as tentatively defined below) Pseudo-Random Function $F$ with $b$-bit input and output, from _one_ Pseudo-Random Permutation $E$ of $b$-bit, as instantiated in practice by TDEA for $b=64$ or AES-256 for $b=128$, and a fixed random secret key.
I tentatively define a _good_ PR... |
How to construct a good PRF from a block cipher? |
We want to explicitly construct a _good_ (as tentatively defined below) Pseudo-Random Function $F$ with $b$-bit input and output, from _one_ Pseudo-Random Permutation $E$ of $b$-bit, as instantiated in practice by TDEA for $b=64$ or AES-256 for $b=128$, and a fixed random secret key.
I tentatively define a _good_ PR... |
First off, if you're doing William's p+1 test, then also doing Pollard's p-1 is redundant, since the p+1 test covers both cases, right?
Second, why is the recurrence $V_{n+1} = aV_n - V_{n-1}$ used? Using $V_{n+1} = V_n + bV_{n-1}$ instead would give you a provable $\frac{1}{2}$ chance of getting a quadratic nonresi... |
On page 7 of http://csrc.nist.gov/groups/ST/hash/sha-3/Round1/Feb2009/documents/Nandi.pdf, I see that, at least at this point, NIST planned on offering SHA-3 having eTCR security (defined on page 3). $\:$ However, I never saw any subsequent references to the eTCR property for the SHA-3 candidates.
Does SHA-3 have an... |
Does Keccak have an eTCR mode? |
What you're looking for is often called a construction of a PRF with "beyond the birthday bound security," and you can probably find some constructions by searching on variants of that term.
For concreteness, this [paper by Iwata][1] (alternate [link][2]) almost gives a solution to your problem: The only deficiencie... |
We want to explicitly construct a _good_ (as tentatively defined below) Pseudo-Random Function $F$ with $b$-bit input and output, from (preferably just) _one_ Pseudo-Random Permutation $E$ of $b$-bit, as instantiated in practice by TDEA for $b=64$ or AES-256 for $b=128$, and _one_ fixed random secret key.
I tentativ... |
I'm crossposting a [StackOverflow.com question][1] there since it belongs here as well:
> I'm wondering if it would make sense to record a whole HTTPS session,
> publish its encryption keys and present it to third parties as a proof
> that this particular data was sent by a given server identifying
> itself with ... |
Python, mechanize & HTTPS: socket dump as a proof of data authenticity? |
I'm crossposting a [StackOverflow.com question][1] there since it belongs here as well:
> I'm wondering if it would make sense to record a whole HTTPS session,
> publish its encryption keys and present it to third parties as a proof
> that this particular data was sent by a given server identifying
> itself with ... |
HTTPS: socket dump as a proof of data authenticity? |
Is there any easy way to crack a beaufort cipher?
We have a vigenere table, and are trying to guess the keyword.
Any easier way?
|
We often want to send messages that are both (a) encrypted, so passive attackers can't discover the plaintext of the message, and (b) signed with a private-key digital signature, so active attackers can't trick Alice into thinking that some message came from Bob, when really the message is some (accidental or malicious... |
Should we sign-then-encrypt, or encrypt-then-sign? |
In Public key infra structure, The MD5 of the data is encrypted with private key of sender and this encypted md5 along with the data is again encryted using AES or 3DES algo. here key used to encrypt with AES is encrypted with private key of sender. Is this correct?
> DATA// text which should be send through PKI has... |
yes (guessing ur doing the cypher challenge...?)
[This link][1] has a really good decoding tool (saves you time), then trial and error keywords. also [this tool;][2] can be used to find the length of the keyword; paste the texts you're decoding then the number of the column(s) with the most x's is the length of the ... |
I think this paper may help:
M. Bellare, T. Krovetz and P. Rogaway (1998),
["Luby-Rackoff backwards: Increasing security by making block ciphers non-invertible"](http://www.cs.ucdavis.edu/~rogaway/papers/p2f-abstract.html), _Advances in Cryptology - EUROCRYPT '98_, Lecture Notes in Computer Science, Vol. 1403.
>... |
I think this paper may help:
M. Bellare, T. Krovetz and P. Rogaway (1998),
["Luby-Rackoff backwards: Increasing security by making block ciphers non-invertible"](http://www.cs.ucdavis.edu/~rogaway/papers/p2f-abstract.html), _Advances in Cryptology - EUROCRYPT '98_, Lecture Notes in Computer Science, Vol. 1403.
>... |
A recent BBC article entitled [WWII code 'may never be cracked'][1] posted a code:
<pre>
AOAKN HVPKD FNFJW YIDDC
RQXSR DJHFP GOVFN MIAPX
PABUZ WYYNP CMPNW HJRZH
NLXKG MEMKK ONOIB AKEEQ
WAOTA RBQRH DJOFM TPZEH
LKXGH RGGHT JRZCQ FNKTQ
KLDTS FQIRW AOAKN 27 1525/6</pre>
According to the article the... |
I'm wondering if it would make sense to record a whole HTTPS session,
publish its encryption keys and present it to third parties as a proof
that this particular data was sent by a given server identifying
itself with some signed certificate. Could such proof be forged in any
way? Should this proof be convincing t... |
Does a trace of SSL packets provide a proof of data authenticity? |
To complete what @CodesInChaos explains:
- If the server has a RSA key in a certificate which is suitable for encryption, then _anybody_ can forge a completely fake conversation without the server being involved at all. In the [SSL/TLS][1] protocol, when using a "RSA" cipher suite, the _client_ generates the random... |
A recent BBC article entitled [WWII code 'may never be cracked'][1] posted a code:
<pre>
AOAKN HVPKD FNFJW YIDDC
RQXSR DJHFP GOVFN MIAPX
PABUZ WYYNP CMPNW HJRZH
NLXKG MEMKK ONOIB AKEEQ
WAOTA RBQRH DJOFM TPZEH
LKXGH RGGHT JRZCQ FNKTQ
KLDTS FQIRW AOAKN 27 1525/6</pre>
According to the article the code was fo... |
I'm wondering if it would make sense to record a whole HTTPS session,
publish its encryption keys and present it to third parties as a proof
that this particular data was sent by a given server identifying
itself with some signed certificate. Could such proof be forged in any
way? Should this proof be convincing t... |
When you try to reduce the security of one primitive (H') to the security of another primitive (H), you assume that there exists an adversary that could break H'. Then you show that existence of said adversary implies an adversary against H by describing an algorithm (the reduction) that uses the adversary to break H.
... |
I have a FPGA and am curious what cryptographic applications I can use with it. I'm just a hobbyist / security researcher and not a professional cryptographer.
Since I'm creating my own curriculum, what cryptographic goals should I have with regards to the FPGA device? (note I'm not looking for FPGA programming ad... |
I have a question about the way to store a key, that was used for encryption, so that the application can check if the user put in the right key for decryption. If I make a mistake, please advise me and I will try to avoid such a mistake in the future.
Normally the key, that was used for encryption, is stored hashed o... |
Lets take your idea to the extreme to see its weakness. For simplicity I'll scale things down. Let's assume an 8 bit key chosen randomly. Call this key $k$. If I break into your database and get $d=H(k,s)$ along with $s$ (where $H$ is a hash function and $s$ is the salt), it would take on average $2^{8-1}=2^7=128$ comp... |
Give a 256 bit key space and 128 message space would AES block cipher as the encryption scheme be CPA secure? |
Is the AES encryption scheme CPA secure? |
* In a brute force attack on DES, it seems to me that *one* plaintext-ciphertext pair suffices to launch the attack. (I.e. suppose we are given a message $x$ and a ciphertext $c$ such that $DES_k ( x ) = c$, where the $56$-bit key $k$ is unknown. Then going through all $2^{56}$ keys will eventually give the result.) I ... |
How would you go about generating a list of distinct random strings, where all strings consist of alphanumeric digits and are of a fixed length? |
I'm looking for a solution to use in a context where I need to be able to generate new symmetric key pairs quickly (using a widely recognized algorithm, and EC-DSA is not applicable). It sounds like DSA would be the adequate solution.
According to the documentation, just generating DSA parameters once allows then to... |
I'm designing a messaging system where the sender *A* sends a message *m* with a signature *s* to *n* Receivers.
A Receiver *Ri* should then be able to prove to a Verifier *V* that he is one of the receivers of the message without disclosing *m* and the identity of the other receivers. He will however disclose *A* ... |
How to prove membership of a list without disclosing the list of members? |
How to prove membership of a list without disclosing the list members? |
RFC 4880 may be full of information, but it can be incredibly vague at times, so im looking for someone who actually knows the answer to this.
Given this public key:
-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: BCPG v1.39
mQGiBE5B0h8RBAD533Z5bK1IpBx02QyQL0QoJE4uFRIMGDiwXuwmZzVl+R7Vlurd
G... |
On page 7 of [NIST's views on SHA-3's security requirements and Evaluation of attacks](http://csrc.nist.gov/groups/ST/hash/sha-3/Round1/Feb2009/documents/Nandi.pdf), I see that, at least at this point, NIST planned on offering SHA-3 having eTCR security (defined on page 3):
> **eTCR**: Find $M$ and then for a random... |
So I actually asked the theoretical version of this question a while [ago][1]: what happens if you choose multiple keys from the same group?
The answer, as best as I determined, is its still secure. First, this practice is used both in the Internet Key Agreement Protocole (IKE) in IPSEC, and for SSH.
Second, th... |
I'm looking for a solution to use in a context where I need to be able to generate new asymmetric key pairs quickly (using a widely recognized algorithm, and EC-DSA is not applicable). It sounds like DSA would be the adequate solution.
According to the documentation, just generating DSA parameters once allows then t... |
[Shamir's original paper][1] describing a threshold secret sharing scheme states:
> To make this claim more precise, we use modular arithmetic instead of
> real arithmetic. The set of integers modulo a prime number $p$ forms a
> field in which interpolation is possible. Given an integer valued
> data $D$, we pick... |
[Shamir's original paper][1] describing a threshold secret sharing scheme states:
> To make this claim more precise, we use modular arithmetic instead of
> real arithmetic. The set of integers modulo a prime number $p$ forms a
> field in which interpolation is possible. Given an integer valued
> data $D$, we pick... |
[Shamir's original paper][1] describing a threshold secret sharing scheme states:
> To make this claim more precise, we use modular arithmetic instead of
> real arithmetic. The set of integers modulo a prime number $p$ forms a
> field in which interpolation is possible. Given an integer valued
> data $D$, we pick... |
One of the necessary components to the Schnorr Digital Signature scheme is a pair of prime numbers p and q such that q divides p-1. However, there is never a modular inverse taken of q so why is there an extra constraint on q? Couldn't q be a number that divides p-1 but not prime? |
Why does Schnorr's Digital Signature scheme necessitate two prime numbers? |
My question relates to the original KP-ABE paper:
http://research.microsoft.com/en-us/um/people/vipul/abe.pdf
I'm having trouble understanding the proof (page 10-13) that the scheme is secure in the Selective Set Model. In particular, when the simulator simulates Phase 1 and constructs the key for $T$ such that ... |
I'm designing a messaging system where the sender *A* sends a message *m* with a signature *s* to *n* Receivers.
A Receiver *Ri* should then be able to prove to a Verifier *V* that he is one of the receivers of the message without disclosing *m* and the identity of the other receivers. He will however disclose *A* ... |
Instead of storing a value based on the key, look at the problem you're trying to solve. You're not actually interested in the value of the key, you just need to ensure decryption works. If you feel you must test the user's input immediately, might I suggest instead storing a value that was encrypted with the key, an... |
My question relates to the original KP-ABE paper:
http://research.microsoft.com/en-us/um/people/vipul/abe.pdf
I'm having trouble understanding the proof (pages 10–13) that the scheme is secure in the Selective Set Model. In particular, when the simulator simulates Phase 1 and constructs the key for $T$ such that... |
I have a question about the way to store a key or password that was used for encryption, so that the application can check if the user put in the right key for decryption. If I make a mistake, please advise me and I will try to avoid such a mistake in the future.
Normally, the key that was used for encryption is sto... |
Is there an advantage to storing keys split between several hashes? |
FIPS 186-3 specifies a method to generate DSA parameters. Is there anything similar (official standard or widely-accepted recommendation) that shows how to generate the primes for multi-prime RSA? |
Is there any standards of multi-prime RSA key generation? |
Instead of storing a value based on the key, look at the problem you're trying to solve. You're not actually interested in the value of the key, you just need to ensure decryption works. If you feel you must test the user's input immediately, might I suggest instead storing a value that was encrypted with the key, an... |
I'm building a small VoIP client which transmits encrypted audio data via UDP, and some packet loss is expected, so each packet will need to be decrypted separately even if others in the stream don't arrive. I also don't enforce strong ordering of data, since re-ordering is done only if a packet is significantly out of... |
What cipher mode is suitable for independantly decryptable short messages? |
What about using CTR in that case ? As long as you know the index of the incoming packet you can decrypt it without needing any of the previous or subsequent packets.
Do you need integrity as well ? |
I was reading the paper $[1]$ and came across the scheme that I show below. While I understand the scheme well, I don't understand why they prepend a 0 to the block containing $r$ and a 1 to all other blocks. What is achieved by this? They never explain why they do it. Here is the scheme as presented in the paper:
h... |
Ok hypothetical scenario. Lets say I want to generate a lot of key material quickly for a one-time pad. This key material will be divided up and used for many different messages in the future.
**1)** I select 50-100+ songs out of a custom playlist of all my music. The music has been converted to basic Waveform Audio... |
Does a playlist of songs or movies mixed together contain enough random enough for key material? |
As part of a puzzle I was given an RSA 256-bit public key and an encrypted message.
The key itself is very weak, having exponent e = 65537 and modulus N = 00:c0:fb:55:b3:ed:f5:19:bf:8d:3a:2a:60:e8:bc:6e:1c:94:f0:5c:17:19:f8:38:ff:45:0b:01:0f:47:96:27:fb
I can determine p and q by using an arbitrary precision libr... |
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