All Questions
8
votes
2answers
550 views
Are any of the major asymmetric ciphers distinguishable (EG, RSA, ECC) ?
Related to this question.
Given ciphertexts generated by any of the major asymmetric ciphers (RSA, ElGamal, ECC, etc..) can these ciphertexts be distinguished from random noise? Justify why, why ...
8
votes
1answer
314 views
Is H(k||length||x) a secure MAC construction?
If $H$ is a typical secure hash function, then $(k,x) \mapsto H(k \mid\mid x)$ is not a secure MAC construction, because given a known plaintext $x_1$ and its MAC $m_1$, an attacker can extend $k ...
8
votes
1answer
278 views
Can one efficiently iterate valid bcrypt hash output values?
bcrypt is an intentionally slow hash algorithm. In my last protocol idea, I wanted to use it to expand a password and then only transfer the bcrypt-hashed password.
An efficient attack on this would ...
8
votes
1answer
237 views
Making ECDSA public keys one bit shorter
In compressed form, ECDSA public keys are one bit larger than the curve size. There are some situations where this extra bit is extremely inelegant. So I had the following idea as a way to remove it.
...
8
votes
1answer
224 views
How did the Koblitz/Menezes papers affect the cryptography community?
Two highly-critical papers by Koblitz and Menezes (two well-regarded mathematicians who've contributed to the crypto community) were published years ago: Another Look at “Provable Security” in 2004, ...
8
votes
1answer
402 views
How can I make my cipher show the avalanche effect?
I am a beginner in cryptography. I designed an password based encryption-decryption algorithm, which uses a random salt and a password to encrypt a message. I'm using SHA-512 for hashing, matrix ...
8
votes
1answer
556 views
Duration for attacking Two-Key Triple-DES Encryption using all RAM ever built?
I am considering attacks on Two-Key Triple-DES Encryption assuming $2^{32}$ known plaintext/ciphertext pairs (that's a mere 32 Giga Bytes of ciphertext) by the method devised by Paul C. van Oorschot ...
8
votes
0answers
133 views
What changed in PKCS#1 v2.2, and why?
PKCS#1 is one of the most used (de-facto) standard for real-world use of RSA.
That's for good reasons: PKCS#1 is well thought, versatile, understandable, has been relatively stable for over two ...
7
votes
7answers
3k views
Is Diffie-Hellman mathematically the same as RSA?
Is the Diffie-Hellman key exchange the same as RSA?
Diffie Hellman allows key exchange on a observed wire – but so can RSA.
Alice and Bob want to exchange a key – Big brother is watching everything.
...
7
votes
7answers
662 views
How can I improve a password generation scheme based on a shared secret and URL?
I currently use the following method to generate a different password on every website I have to login:
password = SHA1 ( mainPassword . domainName . number )
...
7
votes
1answer
314 views
Why do all SSH-RSA Keys begin with “AAAAB3NzaC1yc”?
My Friend and I have been generating a few ssh2-rsa keys and noticed that all the public keys began with "AAAAB3NzaC1yc". The similarity extended to "AAAAB3NzaC1yc2EAAAABIwAAAQEA" between two keys I ...
7
votes
3answers
306 views
Is it safe to use file's hash as IV?
I'm encrypting some files using AES in CBC mode.
I'm also using file's digest (SHA-1) to check that data is decrypted correctly (so I need to store it with file).
Is it safe to use this digest as ...
7
votes
3answers
2k views
Can one generalize the Diffie-Hellman key exchange to three or more parties?
Does anyone know how to do a Diffie-Hellman or ECDH key exchange with more than two parties?
I know how to do a key exchange between 2 parties, but I need to be able to have a key agreement between 3 ...
7
votes
4answers
292 views
Can I determine if a user has the wrong symmetric encryption key?
We're using the Objectivity/DB object database with a custom encryption plugin that encrypts serialized objects on disk. Encryption uses AES with a shared secret key held by all users. I would like to ...
7
votes
2answers
895 views
Unpredictability of X.509 serial numbers
About X.509 certificates serial numbers the RFC 5280 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 ...
7
votes
4answers
1k views
What is the effect of the different AES key lengths?
How does a changing key length affects the ciphertext, not only in case of AES, but in general? I know that the key spaces become much larger and the number of rounds in case of AES changes, but is ...
7
votes
3answers
508 views
Using a Non-Random IV with modes other than CBC
The weakness CWE-329 is an interesting problem with CBC mode. However, does this same weakness affect the other modes of operation that rely upon an IV such as: PCBC, CFB and OFB? My gut feeling is, ...
7
votes
2answers
416 views
Does AES have any fixed-points?
Is there any pair of 128-bit strings M and K such that AES$_K$(M) = M?
If yes, how do I go about (efficiently) finding such a pair?
7
votes
3answers
372 views
Asymmetric cryptographic algorithms that are not based on integer factorization and discrete logarithm
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 ...
7
votes
3answers
504 views
Modern integer factorization software
What are the modern software packages that can be used to factoring large numbers into primes. By modern I mean developed and made public within the last 5 years. I'm interested in things that are ...
7
votes
2answers
403 views
How does the MOV attack work?
What exactly is the MOV attack, how does it actually work, and what is it used for?
It's explained briefly here and I'd like to know what it is more / what is it fully used for.
7
votes
2answers
592 views
Are there practical upper limits of RSA key lengths?
Suppose one wanted to use RSA encryption for the sole purpose of sending key bits for use in symmetric crypto systems, a dedicated key exchange system so to speak.
And say you didn't think that the ...
7
votes
1answer
360 views
Should I use the first or last bits from a sha-256 hash?
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 ...
7
votes
3answers
293 views
Is key size the only barrier to the adoption of the McEliece cryptosystem, or is it considered broken/potentially vulnerable?
A recent paper showed that the McEliece cryptosystem is not, unlike RSA and other cryptosystems, weakened as drastically by quantum computing because strong Fourier sampling cannot solve the hidden ...
7
votes
3answers
1k views
What is pre-image resistance, and how can the lack thereof be exploited?
What is preimage resistance, and how can the lack thereof be exploited?
How is this different from collision resistance?
Are there any known preimage attacks that would be considered feasible?
7
votes
1answer
1k views
Can CBC ciphertext be decrypted if the key is known, but the IV not?
Let's say that there is a binary file encrypted with AES in CBC mode (i.e. using a key and initialization vector). If key is known, but IV is not, is it easy to fully decrypt the file?
How hard is ...
7
votes
2answers
481 views
Selecting a large random prime
Say I want a random 1024-bit prime $p$. The obviously-correct way to do this is select a random 1024-bit number and test its primality with the usual well-known tests.
But suppose instead that I do ...
7
votes
1answer
279 views
Does the elliptic curve (EC) cryptosystem outperform RSA and DL cryptosystems?
Throughout the literature, it is stated that EC cryptosystems outperform RSA and Discrete logarithm cryptosystems, but I cannot understand how ECC would be more efficient than RSA and DL in terms of ...
7
votes
8answers
364 views
Two mutually untrusted parties want to exchange data: how to ensure each one gets the data it needs?
I am trying to come up with what could maybe be a novel algorithm for an application I am writing. Client A has a file fA. Client B has file fB. Each party is untrustworthy and will try to rip off the ...
7
votes
2answers
312 views
Using same keypair for Diffie-Hellman and signing
Are there any security risks using a single key-pair for both key-exchange and signing?
I'm mainly interested in using Curve25519 for key-exchange and Ed25519 for signing. But similar combinations, ...
7
votes
3answers
757 views
Is it okay to use a hash of a timestamp as the IV for AES?
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?
In this case, CBC is the mode being used.
7
votes
2answers
242 views
What is the purpose of using different hash functions for the leaves and internals of a hash tree?
I just learned that the THEX hash tree specification which is widely used in P2P requires that two different hash functions be used: one for the leaf nodes (hashes of input data) and one for the ...
7
votes
2answers
480 views
Desirable S-box properties
What desirable properties should an S-box have?
My current standard selection process is to just pick them at random and verify that they fit the following criteria:
The probability that any random ...
7
votes
2answers
308 views
What is a hard-core predicate?
I read this article on Wikipedia: Hard-core predicate.
Still I don't understand what exactly is a hard-core predicate. Is it possible to put this in simple English terminology, and perhaps with a ...
7
votes
2answers
2k views
AES in ECB mode weakness
In a project that I'm currently working on, we are encrypting some data using AES with ECB mode in a database. Each piece of data being encrypted is very small, no more than 10 characters long.
Very ...
7
votes
3answers
3k views
How does one calculate a primitive root for Diffie-Hellman?
In the Diffie-Hellman key exchange, one of the steps involves calculating a primitive root of a prime number $p$. How would one go about doing so, considering that $p$ could be very large?
Is there ...
7
votes
4answers
418 views
How do you find a cheater in Shamir Secret Sharing?
If there are 4 people involved, and every two of them should be able to know the secret (the polynomial is just a line) and you are given f(x) and x for each of those people, and you know one of them ...
7
votes
2answers
177 views
Streaming API to authenticated encryption
In regards to NaCl, I asked DJB he had any intent to add a streaming API to an authenticated cipher. His response was obvious in retrospect, that one should never release a decrypted plaintext before ...
7
votes
1answer
1k views
Why has the RSA factoring challenge been withdrawn?
Wikipedia states that RSA challenge has been withdrawn.
Does it mean that an efficient factoring algorithm is "just around the corner"?
or are there some other reasons?
If the challenge was still ...
7
votes
2answers
546 views
How should I store passwords that need to be available in plain text?
Suppose I need to store login information for a third-party website for a few users, how would I go about doing it?
Since I am logging into a third party website, I need the password in plain-text, ...
7
votes
1answer
3k views
How does the index of coincidence work in the Kasiki test?
I'm starting to learn about cryptanalysis and I am having a bit of difficulty understanding the Kasiski test's index of coincidence. I have a book (Cryptography Theory And Practice by Douglas Stinson) ...
7
votes
1answer
366 views
Change in probability of collision when removing digits from MD5 hexadecimal hash values
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. ...
7
votes
2answers
546 views
Measuring entropy for a ciphertext only attack
When bruteforcing a password (e.g. the common attacks on DES), where you have ciphertext only, you need a way to assess whether a decrypted plaintext is the right one. I believe the EFF DES machine ...
7
votes
4answers
483 views
Cracking WWII-era codes - code found on a pigeon's leg in Surrey
A recent BBC article entitled WWII code 'may never be cracked' posted a code:
AOAKN HVPKD FNFJW YIDDC
RQXSR DJHFP GOVFN MIAPX
PABUZ WYYNP CMPNW HJRZH
NLXKG MEMKK ONOIB AKEEQ
WAOTA RBQRH DJOFM TPZEH
...
7
votes
1answer
5k views
What is safer: ZipCrypto or AES-256?
Like in title: which one of these encryption methods (ZipCrypto, AES-256) is more secure and why? I am asking about it because I'd like to know which should be preferred when compressing files with ...
7
votes
1answer
1k views
RIPEMD versus SHA-x, what are the main pros and cons?
RIPEMD is a family of cryptographic hash functions, meaning it competes for roughly the same uses as MD5, SHA-1 & SHA-256 do. The Wikipedia page for RIPEMD seems to have some nice things to say ...
7
votes
2answers
1k views
Differences Between White-Box Cryptography and Code Obfuscation
I have read this question regarding white-box Cryptography, and it led to this short article / Q&A which states in question 2:
Q2: What is the difference with code obfuscation?
Related ...
7
votes
1answer
449 views
Replacing the Rijndael S-Box?
The Rijndael S-Box design generates a permutation cycle of type $2+27+59+81+87$. What effect would replacing that permutation with a cycle of type $256$ have on the security of AES?
7
votes
1answer
190 views
Alice trusts Bob only when Bob trusts Alice
some story first: Alice and Bob both have public/private key pairs. Now Bob wants Alice to sign his public key id. Alice agrees but only when Bob signs the public key id of her.
Is this something ...
7
votes
1answer
253 views
Is there any known malleability of scrypt outputs?
The scenario is single-use passphrase-based non-interactive message authentication.
The obvious try to do this is to chose a random salt of the appropriate length and send:
...
