instruction stringlengths 15 30k |
|---|
I'm new to Cryptography so please free to edit my question for clarity and add the appropriate tag.
I'm trying to research if the following is possible. Suppose
1. John has a message that he encrypts to share with more than one
recipients.
2. The message is the same, i.e. it's not unique to the recipien... |
One encryption to many unique decryption? |
From this [excellent answer][1] I learned (correct me if I am wrong) that when writing a block cipher with say key size 128 bit, one has to pad the password given (variable size) so that it becomes exactly the 128 bit that is needed. I had thought that one would just add zeros or repeat the password until the length be... |
Which ciphers $c_k$ enable one to define a joining function $f_{c,k}$ such that:
$$
f_{c,k}(c_k(p_1), p_2) = c_k(p_1 + p_2)
$$
That is, the result of joining some known ciphertext $c_k(p_1)$ with a plaintext $p_2$ produces the same ciphertext as the concatenation of the respective plaintexts? Whilst there is s... |
The [original article][1] neglects the cost of DES computations (there are less than $2^{90}$) and everything except memory accesses to its _Table 1_ and _Table 2_. I go one step further: considering that _Table 1_ is initialized only once and then read-only, it could be in ROM, and I neglect all except the accesses to... |
The [original article][1] neglects the cost of DES computations (there are less than $2^{90}$) and everything except memory accesses to its _Table 1_ and _Table 2_. I go one step further: considering that _Table 1_ is initialized only once and then read-only, it could be in ROM, and I neglect all except the accesses to... |
I'm a bit confused, because the title of your question seemed to imply a different scenario than your actual question.
Is your goal, given one message $m$ to produce ciphertexts $c_1, \ldots, c_n$ such that each recipient can decrypt one of the ciphertexts but not check whether the other ciphertexts encrypt the same... |
I'm a bit confused, because the title of your question seemed to imply a different scenario than your actual question.
Is your goal, given one message $m$ to produce ciphertexts $c_1, \ldots, c_n$ such that each recipient can decrypt one of the ciphertexts but not check whether the other ciphertexts encrypt the same... |
The fragment " _what to do about padding the_ ___key___ _?_ " of the question looks scarily like transforming a password into a cryptographic key using some padding mechanism.
Doing this would be a known, serious, often made and often exploited mistake. A standard security assumption for ciphers is that the key is c... |
The fragment " _what to do about padding the_ ___key___ _?_ " of the question looks scarily like transforming a password into a cryptographic key using some padding mechanism.
Doing this would be a known, serious, often made and often exploited mistake. A standard security assumption for ciphers is that the key is c... |
What is the difference in the purpose of DH and RSA? Aren't they both public-key encryption? |
I'm new to Cryptography so please free to edit my question for clarity and add the appropriate tag.
I'm trying to research if the following is possible. Suppose
1. John has a message that he encrypts to share with more than one
recipients.
2. The message is the same, i.e. it's not unique to the recipien... |
I'm new to Cryptography so please free to edit my question for clarity and add the appropriate tag.
I'm trying to research if the following is possible. Suppose
1. John has a message that he encrypts to share with more than one
recipients.
2. The message is the same, i.e. it's not unique to the recipien... |
I'm new to Cryptography so please free to edit my question for clarity and add the appropriate tag.
I'm trying to research if the following is possible. Suppose
1. John has a message that he encrypts to share with more than one
recipients.
2. The message is different, i.e. it's unique to the recipients
... |
Why does the TLS protocol use different symmetric keys for receiving and sending data?
Isn't it enough to have a single key used for both reading and writing? |
There are several systems that sound vaguely similar to your description.
Would either one (or both) of the following systems work for you?
If not, what exactly about systems is unsuitable for your application?
**deniable encryption**
You give a unique key to each of a bunch of people.
You think up a unique ... |
There are several systems that sound vaguely similar to your description.
Would either one (or both) of the following systems work for you?
If not, what exactly about systems is unsuitable for your application?
**deniable encryption**
You give a unique key to each of a bunch of people.
You think up a unique ... |
The fragment " _what to do about padding the_ ___key___ _?_ " of the question looks scarily like transforming a password into a cryptographic key using some padding mechanism.
Doing this would be a known, serious, often made and often exploited mistake. A standard security assumption for ciphers is that the key is c... |
In cryptography, an elliptic curve is a group based on a finite field $GF(p^k)$; this group has $n$ elements on it, and we work on a prime-sized subgroup of size $q$. We denote the value $h = n/q$ as the *cofactor* of the curve.
My question is: why would we ever want to consider using a curve which has a cofactor $... |
Why would anyone use an elliptic curve with a cofactor > 1? |
I do not have any hard data to back this up, but an educated guess is that relaxing the cofactor to be "small" instead of 1 was done to allow Koblitz curves, which in early days looked like an attractive choice for implementation.
[Koblitz curves][1] over binary fields are of the form
$$
y^2 + xy = x^3 + a x^2 ... |
*"Does generating the same ciphertext twice (when given the same plaintext) cause any security flaws?"*
Yes, generating the same ciphertext twice opens up a system to the possibility of [replay attacks][1].
*"Does the same encryption algorithm given the same plaintext always produces the same ciphertext?"*
In ... |
Can anyone provide an extended (and well explained) proof of correctness of the RSA Algorithm?
I can't say that [this](http://en.wikipedia.org/wiki/RSA_%28algorithm%29#Proofs_of_correctness) or [this](http://faculty.cs.tamu.edu/klappi/alg/rsa.pdf) helped me much.
Please consider me as a newbie when answering, if ... |
Can anyone provide an extended (and well explained) proof of correctness of the RSA Algorithm?
And why is it needed?
I can't say that [this](http://en.wikipedia.org/wiki/RSA_%28algorithm%29#Proofs_of_correctness) or [this](http://faculty.cs.tamu.edu/klappi/alg/rsa.pdf) helped me much, I'd like a more detailed and... |
What is the difference between encrypting something and hashing something? in what situations would I want one or the other? |
Where can i find relative software for differential cryptanalysis? |
(Being new to cryptography, I hope the following makes sense).
I will try to follow the Wikipedia article. What I write is pretty much just taken from there. If some of the steps are unclear you might want to look up for example the Chinese Reminder theorem or simply check out a book on basic modular arithmetic.
... |
The [original article][1] rightfully neglects the cost of DES computations (there are less than $2^{90}$) and everything except memory accesses to its _Table 1_ and _Table 2_. I go one step further: considering that _Table 1_ is initialized only once and then read-only, it could be in ROM, and I neglect all except the ... |
Digital certificates are used quite commonly these days, for signing files. They are used by various operating systems to ensure reliability and security. For example, Android requires that each of the apps submitted on the Android Market is signed with a certificate, and any updates are signed with the same certificat... |
How do digital certificates work, and why is it not possible to reverse engineer one from a signed file? |
From this [excellent answer][1] I learned (correct me if I am wrong) that when writing a block cipher with say key size 128 bit, one has to pad the password given (variable size) so that it becomes exactly the 128 bit that is needed. I had thought that one would just add zeros or repeat the password until the length be... |
Such certificates are basically just an implementation of digital signatures. One certificate is used to sign the data using a private key, and a corresponding verification certificate is given to the user along with the signature and the data. The user uses the verification certificate to verify that the file matches ... |
###How does one verify a key revocation?
After revoking a key and sending the revocation to [MIT's keyserver](http://pgp.mit.edu), I noticed that the key is listed as such:
pub 2048R/XXXXXXXX 2011-01-01 *** KEY REVOKED *** [not verified]
Who is responsible for the 'verification of the revocation'? Does ... |
I'm reading a paper about breaking the MD5 hash algorithm using "tunnel attack". The paper is located here: http://cryptography.hyperlink.cz/2006/Tunnels.pdf
But I couldn't understand about the difference between point of verification and point of choice. Aren't they the same? |
Below is the cipher text I am trying to break and as you can see its rather short which is why I am having so much trouble.
WOYFN ZCMSH VUVTG BFUTW ABTZP FHIMF TFOSU UXFQC HKVKG MPUUQ
MHRXI OVBRZ EPJYF KKVJW GEIOV HUKEB JUNSM THIMF TFKUB ULMMQX
What I know about this ciphertext:
- Key is 6 character... |
Suppose I choose $p=7$ and $q=11$.
This gives a public key of:
$p^2q$ = 539
However, decryption occurs using a modulus of $pq=77$
If a person chooses to encrypt 500 using my public key, how do I recover this value if the decryption modulus is so much smaller?
Is this a limitation of the Schmidt-Samoa sc... |
Suppose I choose $p=7$ and $q=11$.
This gives a public key of $p^2·q = 539$.
However, decryption occurs using a modulus of $p·q=77$.
If a person chooses to encrypt $500$ using my public key, how do I recover this value if the decryption modulus is so much smaller?
Is this a limitation of the Schmidt-Samoa ... |
How can the Schmidt–Samoa cryptosystem uniquely decrypt large messages? |
Suppose I choose $p=7$ and $q=11$.
This gives a public key of $p^2·q = 539$.
However, decryption occurs using a modulus of $p·q=77$.
If a person chooses to encrypt $500$ using my public key, how do I recover this value if the decryption modulus is so much smaller?
Is this a limitation of the Schmidt-Samoa ... |
In RSA, the plaintext and ciphertext spaces are the same $\mathbb{Z}_N$ where $N=pq$. This is not, however, true for all cryptosystems. Schmidt-Samoa is one example (Paillier is another). In Schmidt-Samoa $m\in\mathbb{Z}_{pq}$, while $c\in \mathbb{Z}_{N=p^2q}$.
So, if you pick $m=500$ but $500\not\in\mathbb{Z}_{pq}... |
In RSA, the plaintext and ciphertext spaces are the same $\mathbb{Z}_N$ where $N=pq$. This is not, however, true for all cryptosystems. Schmidt-Samoa is one example (Paillier is another). In Schmidt-Samoa $m\in\mathbb{Z}_{pq}$, while $c\in \mathbb{Z}_{N=p^2q}$.
So, if you pick $m=500$ but $500\not\in\mathbb{Z}_{pq}... |
This comes following a discussion with a colleague.
- My plaintext file consists of a about 100,000 lines of "all work and
no play...". It's size is: 2.2mB
- Compressed it is: 5.4kB
- I encrypt the original: `openssl aes-128-cbc -in plain -out
plain.ENC`. `plain.ENC` is marginally bigger than ... |
This comes following a discussion with a colleague.
- My plaintext file `plain` consists of a about 100,000 lines of "all work and
no play...". It's size is: 2.2 MB.
- Compressed it is: 5.4kB
- I encrypt the original:
openssl aes-128-cbc -in plain -out plain.ENC
`plain.ENC` is margin... |
Why is ciphertext from low entropy plaintext not compressible? |
I recently started to follow the cryptography class of Dan Boneh on [coursera.org][1] and the first part is talking about stream ciphers.
In general, we have an encryption which works as follows:
$$c = E(k,m) = m \oplus k$$
and $$D(k,c)=c \oplus k = m \oplus k \oplus k = m \oplus 0 =m $$
Where $k$ is the ke... |
How can I find to strings $m_1$ and $m_2$, knowing that I know $m_1 \oplus m_2$? |
I's going throught the the Paper by Matsui on Linear Cryptanalysis of DES . In that he says
$NS_{5}(16,5)=12$
And then in the next paragraph he says considering the expansion and permutation phases the following equation holds good
$X(15)\bigoplus F(X,K)[7,18,24,29]=k[22]$
Can somebody help me understand th... |
Is it possible to use the RSA algorithm, or a variant, for software licensing? |
I've read definitions of [MAC](http://en.wikipedia.org/wiki/Message_authentication_code) and [HMAC](http://en.wikipedia.org/wiki/HMAC), but can't say I've completely grasped the differences.
- What are principle differences?
- When to use one and when the other?(Typical Use Cases) |
When I was perhaps nine years, I borrowed a book from the library on various maths and CS topics. It outlined various simple ciphers, including one that I used a lot, just for fun. I can't remember the name of the book, or the name of the cipher, so I hope you can help me with the latter one.
It is a monoalphabetic ... |
Toy cipher -- does it have a name? |
Im probably just not reading something again, but:
RFC 4880 says that a OpenPGP Public-Key Encrypted Session Key Packet (tag 1) is made up of
- A one-octet number giving the version number of the packet type.
The currently defined value for packet version is 3.
- An eight-octet number that gives t... |
The combination between addition modulo $2^{32}$ (not modulo $32 = 2^5$) - indicated by $\boxplus$ in the diagram - and XOR (i.e. bitwise addition modulo $2$) - indicated by $\oplus$ - makes the algorithm more non-linear.
Each of them for itself is a linear operation, but over different groups (addition in $GF(2^{32... |
I want to upgrade the security of some existing databases of users' authentication tokens strictly for the purpose of making sure that if the database is stolen, attackers will not be able to guess any but the weakest passwords in a reasonable period of time. I know there's not much I can do for people who choose "pas... |
Is this password migration strategy secure? |
Craig Gentry recently gave the first fully homomorphic cryptosystem. Quite a bit of work has been done since extending his work. It seems, however, that no system is practical for real world use.
- What are the current roadblocks making FHE impractical?
- Which proposed system would you say is currently the most pr... |
What is the most practical fully homomorphic cryptosystem? |
I am trying to make a taxonomy of different purposes of some cryptographic protocols. Generally speaking the purpose of PIR, oblivious transfer and differential privacy it sounds like being invented for the same purpose. "Give me information i want without learning anything about this information". Can we make the afor... |
Are there any differences between PIR, oblivious transfer and differential privacy? |
I'm looking for a scheme where signing may be costly (slow) but batch verification with different signers is the fastest possible.
Modified ECDSA allows batch verification for different signers with a 4X speedup [1].
Is there any signature scheme than can do better?
[1] [Fast Batch Verification of Multiple Sig... |
for example bob send a text to boss1 for symmetric encryption to send to boss2 (only boss1 and boss2 know the key). can bob compare the plain text he sent to boss1 with the encrypted version to guess what is for example AES key? |
does symmetric encryption vulnerable to plain-text-attacks? |
for example bob send a text to boss1 for symmetric encryption to send to boss2 (only boss1 and boss2 know the key). boss1 send encrypted text back to bob to send to boss2. can bob compare the plain text he sent to boss1 with the encrypted version to guess what is for example AES key? |
Imagine that Bob sends a message to Alice for symmetric encryption to send to Charlie. (Only Alice and Charlie know the key.) Alice sends the encrypted message back to Bob to send to Charlie. Can Bob compare the plain text he sent to Alice with the encrypted version to guess, for example, what is Alice's AES key?
|
Is symmetric encryption vulnerable to plain-text-attacks? |
The Goldreich-Goldwasser-Micali construction allows to build a (cryptographically secure) pseudo-random function from of a (cryptographically secure) pseudo-random generator.
More formally, let $G: \{0, 1\}^s \rightarrow \{0, 1\}^{2s}$ be a length-doubling PRNG. Given a seed $s$, $G(s)$ returns a $2s$-bit string $G... |
Are hash functions the mechanism on which Digital Signatures are based, or are they a way to implement a particular scheme of Digital Signature (e.g HMAC provided along with the message)?
In more general terms:
> What is the difference between hashing a document and signing it? |
> Are hash functions the mechanism on which Digital Signatures are
> based, or are they a way to implement a particular scheme of Digital
> Signature (e.g HMAC provided along with the message)?
In more general terms:
> What is the difference between hashing a document and signing it? |
I'm studying Hash functions and Digital Signatures in sequence, and came up with some doubts about their usage.
First of all:
- What is the difference between hashing a document and signing it?
- And why aren't Standard Hash Functions considered fit for message authentication?
More specifically, considerin... |
Identity based encryption schemes [*] seem to have great potential in high-latency Delay-Tolerant and mobile, ad-hoc networks since they apparently seem to avoid the need for key negotiation and exchanges. Used soley for key exchange, they allow any pair of members in the same _group_ (however you choose to define it) ... |
Does Identity-Based Encryption actually solve any problem? |
Suppose that you see all IP communication packets between two computers for a day, as they pass through a connecting router, and this amounts to 10MB. You additionally know that all data transmission follows the standard "RSA-1024 plus AES-128" protocol. To be even more specific, suppose you know only one computer is d... |
I've got a scenario where I need to encrypt many small (16-byte) plaintexts. I want to use AES-128 in ECB mode. Notably, each plaintext is guaranteed to be unique, though each may differ by only a few bits. In this scenario, is ECB secure? |
ECB mode secure if plaintexts guaranteed unique? |
I am trying to get access to my eReader and I managed to get the passwd file.
root:$1$hdhxObPx$TYFuTKsB9GGIgo53rF4bi1:0:0:root:/:/bin/sh
bin:*:1:1:bin:/bin:
daemon:*:2:2:daemon:/sbin:
nobody:*:99:99:Nobody:/:
s3c2440x::507:507:root:/:/bin/sh
I expected to see a standard hash string there o... |
Does anyone know why are there $ signs in my passwd file? |
The [original article][1] rightfully neglects the cost of DES computations (there are less than $2^{90}$) and everything except memory accesses to its _Table 1_ and _Table 2_. I go one step further: considering that _Table 1_ is initialized only once and then read-only, it could be in ROM, and I neglect all except the ... |
Why are there $ signs in my passwd file? |
I've got a scenario where I need to encrypt many small (16-byte) plaintexts. I want to use AES-128 in ECB mode. Notably, each plaintext is guaranteed to be unique, though each may differ by only a few bits.
In this scenario, is ECB secure? |
Is ECB mode secure if plaintexts guaranteed to be unique? |
With [Shamir's secret sharing][1] is it possible to pre-define the points returned by the algorithm?
For simplicity if I have (k, n) where k=2, and n=4, and I have the points X,Y,Z, and Q. Can I create the coefficients so that X,Y,Z, and Q are the points returned from the algorithm.
If yes will it still be secure... |
Well, no, in general, you won't be able to select arbitrary points $X, Y, Z$ and $Q$ that would work in a Shamir secret sharing scheme (unless $k\ge4$).
Here's why; Shamir's scheme assumes that the points are on some polynomial of degree $k-1$ or less; for $k=2$, this means that the points all must be solutions of a... |
Well, no, in general, you won't be able to select arbitrary points $X, Y, Z$ and $Q$ that would work in a Shamir secret sharing scheme (unless $k\ge4$).
Here's why; Shamir's scheme assumes that the points are on some polynomial of degree $k-1$ or less; for $k=2$, this means that the points all must be solutions of a... |
I have an embedded system with AES-128 implemented in hardware. There is very little flash/RAM and the (8-bit) CPU runs relatively slowly. Public key crypto isn't a viable option. This system is connected to the Internet and can have one or more unique AES keys programmed at manufacture.
Are there any standard proto... |
Are there standards for transport layer security using only symmetric keys? |
I know of two standard protocols that support AES encryption without public key cryptography getting involved:
- With TLS, we have [RFC4279][1], and in particular, the ciphersuite TLS_PSK_WITH_AES_128_CBC_SHA. Now, the two sides don't literally have preconfigured AES keys; instead, they have preshared premaster s... |
> **Possible Duplicate:**
> [How does one attack a two-time pad (i.e. one time pad with key reuse)?](http://crypto.stackexchange.com/questions/2249/how-does-one-attack-a-two-time-pad-i-e-one-time-pad-with-key-reuse)
<!-- End of automatically inserted text -->
I recently started to follow the cryptography class of... |
I cite from [Fundamentals of Computer Security](http://www.springer.com/computer/security+and+cryptology/book/978-3-540-43101-5) (Chapter 7 on Digital Signature, Paragraph 7.3 on RSA Signatures, page 289):
> Multiple uses of the RSA Signature scheme tend to weaken it. The way out is to make subsequent signatures dep... |
Let $h$ be a bitstring and let $P(h, n)$ be the n-bit prefix of $h$. A partial collision of length n, for a hash function $H$ is a pair $(x,y)$, such that $P(H(x),n)=P(H(y),n)$.
What is known about this type of collisions? I'm particularly interested in md5. Can rainbow tables be adapted to quickly look for these? W... |
Let $h$ be a bitstring and let $P(h, n)$ be the n-bit prefix of $h$. A partial collision of length n, for a hash function $H$ is a pair $(x,y)$, such that $P(H(x),n)=P(H(y),n)$.
What is known about this type of collisions? I'm particularly interested in md5. Can rainbow tables be adapted to quickly look for these? W... |
It's a statement made by the signer, just like if I say that this message was written on 19 June 2012. Its value is context-dependent. Just like with a pen and paper, you can post-date or pre-date anything.
In general, there's no such thing as verified time. Over the years, there are many people who have tried to cr... |
Im probably just not reading something again, but:
RFC 4880 says that a OpenPGP Public-Key Encrypted Session Key Packet (tag 1) is made up of
- A one-octet number giving the version number of the packet type.
The currently defined value for packet version is 3.
- An eight-octet number that gives t... |
I's going throught the the Paper by Matsui on Linear Cryptanalysis of DES . In that he says
$NS_{5}(16,15)=12$
And then in the next paragraph he says considering the expansion and permutation phases the following equation holds good
$X(15)\bigoplus F(X,K)[7,18,24,29]=k[22]$
Can somebody help me understand thi... |
I'm still a newbie in the field, but trying to learn something about it I ran into these two papers that you surely know, in case not I'll point them out:
- [Blind signatures for untraceable payments, Chaum](http://blog.koehntopp.de/uploads/Chaum.BlindSigForPayment.1982.PDF)
- [Provably secure blind signatures sc... |
I'm still a newbie in the field, but trying to learn something about it I ran into these two papers that you surely know, in case not I'll point them out:
- [Blind signatures for untraceable payments, Chaum](http://blog.koehntopp.de/uploads/Chaum.BlindSigForPayment.1982.PDF)
- [Provably secure blind signatures sc... |
Many sources mention that IVs must not be reused with the same key in CTR mode, for encrypting 2 different pieces of data, because that totally destroys security - but I haven't found an explanation so far as to why this is the case.
The issue is obvious if the attacker can manage to obtain the plain text and its co... |
Why must IV/key-pairs not be reused in CTR mode? |
Ok, here's a hint: suppose you know that $t = s^y$ where you only know the lsbit of $y$.
Consider $t^2 = s^z$ (for some unknown $z$); you can use the oracle to find the lsbit of z. How are $y$ and $z$ related? How can you use your knowledge of the lsbit of $z$ to recover more information about $y$?
And, how can... |
How can I solve the discrete logarithm modulo 2q+1 if I can solve it in the subgroup of order q? |
> **Possible Duplicate:**
> [How does one attack a two-time pad (i.e. one time pad with key reuse)?](http://crypto.stackexchange.com/questions/2249/how-does-one-attack-a-two-time-pad-i-e-one-time-pad-with-key-reuse)
<!-- End of automatically inserted text -->
I recently started to follow the cryptography cla... |
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