2
votes
1answer
143 views

Can you help me understand PFS and wPFS?

Every time I encounter the concepts of PFS (perfect forward secrecy) and wPFS (weak perfect forward secrecy), I feel uncertain about them. My understanding is that: PFS ensures that, if the ...
2
votes
1answer
521 views

What key exchange do OpenSSL and CryptoAPI prefer by default?

Here is what I know. When I connect to a brand-new Linux Web server with Chrome and click the "lock" icon, the key exchange algorithm is described as "ECDHE_RSA". (Yes I know what that is.) When I ...
2
votes
1answer
404 views

Hide a weakness in ECC by choosing the prime or one of the curve coefficients

Suppose you are given a value $c$. Can you find a prime $p$ and an integer $b$ such that the elliptic curve $$E: y^2 \equiv x^3 -3x + b \pmod p$$ is cryptographically weak? You need to choose ...
9
votes
4answers
970 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 ...
4
votes
1answer
157 views

Can a PRNG with a fixed amount of entropy always be detected?

Given a fixed seed (i.e. no ongoing source on entropy), is there (practically or theoretically) any PRNG algorithm that can generate an infinite sequence of random numbers such that an observer cannot ...
1
vote
1answer
159 views

Is the “secure-as-worst-case” version of NTRU patented?

Does the improved version of NTRU — which is described in "Making NTRU as Secure as Worst-Case Problems over Ideal Lattices" by Stehlé and Steinfeld — fall under the same patents as the original?
3
votes
4answers
308 views

Is it possible to anonymize web traffic so that the IP Address cannot be determined while still being able to determine distinct IPs?

I'd like to find a way to anonymize IP addresses in web logs to ensure user privacy given the following requirements: If given an IP address, there is no way to look up which requests came from the ...
1
vote
1answer
179 views

General-purpose non-interactive multi-party computation protocol

Most of general purpose multi-party computation protocols are highly interactive. Especially the ones with Shamir's secret scheme, as there is a degree reduction step in multiplication that needs the ...
1
vote
1answer
335 views

Why are the first few bytes of a gpg encryption always the same?

If I run the command 'gpg -a -e < input.txt > output.txt' with completely different inputs (e.g. from /dev/urandom), the first 19 characters are always the same. What is this header? Is it ...
1
vote
1answer
104 views

Question about Fermat's little theorem

Why is $g^e \mod p = g^{e \mod (p-1)} \mod p$ if p is prime. I don't get it. It follows from Fermat's little theorem.
2
votes
2answers
162 views

Security of S/MIME in case of CA compromise

Suppose Alice and Bob are exchanging messages using S/MIME, protected by certificates that have been issued by either the same CA or by two mutually independent CAs. There exists an adversary Mallory ...
1
vote
1answer
155 views

Security of (cryptographic) padding compared to other methods for blockcompletion for block ciphers

As said in the heading, I want to know how the security of different padding methods, e.g. ANSI X.923, ISO 10126 and PKCS7, is compared to other methods to reach the needed block size, like ciphertext ...
12
votes
1answer
399 views

Security of pairing-based cryptography over binary fields regarding new attacks

In the last week, the discrete logarithm problem was broken for the binary fields $\mathbb{F}_{2^{(14 \times 127)}}$ and $\mathbb{F}_{2^{(27 \times 73)}}$. Pairing-based cryptography using binary ...
4
votes
1answer
210 views

OMAC/CMAC constant for different block sizes

OMAC/CMAC only specifies constants for 64-bit (0x1B) and 128-bit (0x87) block size. I would like to know how to get constants ...
3
votes
1answer
72 views

Should I use distinct MACs for each user, or should I use a *direction* flag when communicating?

Alice and Bob want to communicate using a stream cipher. At the beggining, they create a session key $K_s$ and exchange it via some secure channel (using some asymmetric algorithm). After every ...
15
votes
1answer
884 views

What NIST protocol was allegedly backdoored by NSA in 2006?

From a recent NY Times article: Cryptographers have long suspected that the agency planted vulnerabilities in a standard adopted in 2006 by the National Institute of Standards and Technology ...
4
votes
2answers
178 views

Can S/MIME be still considered secure?

Previoulsy I had asked this question at http://stackoverflow.com/questions/18235983/can-s-mime-be-still-considered-secure but I feel this forum is topic-wise the right place. Recently there has been ...
0
votes
1answer
117 views

Distributed Asymmetric Key Decryption with threshold

Threshold decryption of public key encryption schemes allows the decryption key to be distributed among trustees. Then, to decrypt the cipher text it requires at least threshold t trustees to run the ...
17
votes
3answers
1k views

Is using slow password hashing on the client side easier attackable than on the server side?

As we know, one should use a slow password hashing algorithm instead of a fast one for storing passwords, to hinder brute force attacks when the database is compromised. The problem with this is that ...
-1
votes
2answers
178 views

What are the weaknesses of this message exchange protocol?

Here is a protocol for a key exchange: Bob encrypts a message $m$ with his key $K_B$: $m_1 = E_{K_B}(m)$. Bob send the result $m_1$ to you. Alice encrypts my encrypted above message with her own ...
3
votes
1answer
748 views

RSA key pair generation using PRNG with same seed

I see a lot of Q/A where persons are trying to generate a specific key pair using static data such as a password. Now say we use a known PRNG (dangerous assumption), seeded with a the data as a static ...
4
votes
2answers
296 views

Proof of elliptic curve difficulty

Are there any proofs that cryptographic functions on an elliptic curve are any more difficult than the analogues over modulo arithmetic? While at present, ECC appears to be more difficult, as it is ...
6
votes
2answers
416 views

A lower bound on the insecurity of CipherCloud?

CipherCloud claims to support , among other things, searchable encryption. A bunch of speculation seems to suggest they did this via some breathtakingly incompetent means( unfortunately such ...
1
vote
2answers
2k views

Is javascript RSA signing safe?

I'm currently working on a secure open source messaging system (https://github.com/DSMNET/DSMNET). Currently I'm using the cryptico.js (https://github.com/wwwtyro/cryptico) library to encrypt all the ...
8
votes
5answers
483 views

RSA leak bits to factor N

Suppose you randomly generate large primes p and q as in RSA, and then tell me N=pq but not p or q. Then, you would like to actually let me factor N, except you should tell me as few bits of ...
29
votes
5answers
4k views

How can we reason about the cryptographic capabilities of code-breaking agencies like the NSA or GCHQ?

I have read in Applied Cryptography that the NSA is the largest hardware buyer and the largest mathematician employer in the world. How can we reason about the symmetric ciphers cryptanalysis ...
2
votes
1answer
139 views

Can stream ciphers (usually) be “run backwards”?

Is it possible to reconstruct the previous output bits of a stream cipher, e.g. RC4, when only the current state is known, or is that computationally hard, or even impossible (due to ambiguous ...
-1
votes
1answer
259 views

What are some of the major differences between PyNaCl and PyCrypto? [closed]

It seems like one can generate keys and encrypt data with both libraries. Is one better than the other? Do they attempt to solve different problems? Is one more secure than the other?
2
votes
2answers
548 views

Is there an encryption/decryption algorithm that can give two different outputs?

The recent revelations that airport staff can detain travellers in an interrogation room and force them to give over passwords to encrypted-HDD laptops under threat of jail time has made me think ...
3
votes
1answer
179 views

Can GCM be used in incremental mode?

I want to sync blobs over the network and the most frequent change will be appending to blob. To minimize data amount sent I would prefer to encrypt appended data, send it over the network, append to ...
3
votes
1answer
553 views

What is an active S-Box?

I don't understand what an active s-box is (in reference of design criteria 8 of the DES). Because according to DES, all 8 s-boxes are used every round. But this is not what active means. So my ...
5
votes
1answer
573 views

Will rehashing an SHA256 hash continually, eventually produce every possible value?

So let's say you had infinite time and energy. You have a hashed string of some sort. Because you have infinite time and energy, you can produce a collision(or the original value) easily enough. But, ...
1
vote
2answers
136 views

One-way function and $EXP$

All examples of one-way functions I have see till now are closely related to the assumption that $NP\neq P $ (or even weaker ones, such as $UP\neq P$), but why not considering the theorem $P\neq EXP$? ...
0
votes
2answers
901 views

RSA PCKS1 v2.1 RSAES-OAEP algorithm

Using RSA public key (n,e) the encryption of a message is c= (m^e) mod n and the corresponding message is decrypted as ...
4
votes
5answers
4k views

Are there hash algorithms with variable length output?

I understand that for example MD5 produces a 128 bit hash value from a given text of variable size. My question is if there is a hash-like algorithm that will produce a hash value where one can ...
1
vote
1answer
246 views

Encryption schemes and pseudorandom permutations

Can an encryption scheme (deterministic or not) be viewed as a pseudorandom permutation taking as input a message and returning a value of the same bit-encoding length ? Thank you.
1
vote
1answer
185 views

Key exchange and man in the middle issue. Secure solution

I am trying to design MiTM protected key exchange algorithm. For instance I am using RSA asymmetric keys. Alice and Bob respectively have each their own pair of keys. And they should be able to ...
2
votes
2answers
248 views

PBKDF2 uses HMAC-SHA1 to generate keys, but what is the key for the HMAC?

I was reading up on PBKDF2 and noted the following: DK = PBKDF2(PRF, Password, Salt, c, dkLen) PRF is a keyed HMAC, but I can't seem to find out what they use ...
2
votes
5answers
1k views

Using one-way hash functions as the encryption method

Suppose two parties want to communicate securely with each other (Bob and Alice) using a simple messaging system in English. There are approximately 180,000 currently used words in the English ...
2
votes
1answer
734 views

Self-expiring symmetric keys, or: cryptography in absence of secure deletion

I can encrypt some data D using a random symmetric key K, obtaining a ciphertext C, and then encrypt K with my public key Pub and obtain H. So far so good: I can only decrypt C if I have H and my ...
6
votes
3answers
387 views

Why the symmetric key layer in PGP?

I've been familiarizing myself with the basics of PGP. If I understand correctly, PGP symmetrically encrypts the data of interest using a random single-use key, then encrypts the encrypted data and ...
8
votes
1answer
161 views

Any problems with this secure time synchronization scheme?

I have a time authority and I want to securely set a client's time to this authority's time/date within a precision of $\delta$ seconds. The authority's public key is known to the client. This was my ...
6
votes
2answers
1k views

What is the progress on the MIT LCS35 Time Capsule Crypto-Puzzle?

Ron Rivest posed a puzzle in 1999. MIT LCS35 Time Capsule Crypto-Puzzle. The problem is to compute $2^{2^t} \pmod n$ for specified values of $t$ and $n$. Here $n$ is the product of two large ...
1
vote
2answers
361 views

How terribly flawed is this design for key storage?

Dave here, with a plan for storing secrets. Threat model: Database and keyfile may (will?) be stored on un-trusted servers. Database contains many valuable secrets (passwords), encrypted by key ...
3
votes
2answers
144 views

Feedback requested on a method of posting a message without revealing the author

So I was thinking about variations on the Dining Cryptographers problem - In some cases, it's useful to be able to post a message without revealing the source, but with the additional constraint of ...
1
vote
2answers
135 views

Is it possible to translate a piece of language into your own without knowing the language? [closed]

I understand the the texts within the Voynich Manuscript are of an unknown language, and yet cryptographers are still working on it and still trying to decipher it. Does that mean that it is possible ...
8
votes
3answers
348 views

AES CTR with similar IVs and same key

Lets say there is a piece of software that uses AES CTR to encrypt different messages using the same key but with slightly different IVs So for example, a 16 byte IV, the 2nd 8 bytes are always the ...
3
votes
4answers
2k views

Initialization vector in symmetric-key encryption

Can we use symmetric-key algorithms without an initialization vector? I am making an app where both the sender and receiver share a key and there is no way to create an initialization vector for each ...
3
votes
1answer
86 views

ASN.1 OID of bcrypt

What is the ASN.1 OID associated to bcrypt (the key derivation scheme)? For instance, PBKDF2 has 1.2.840.113549.1.5.12 and it is therefore possible to store a ...
2
votes
1answer
95 views

Where can I double check my elliptic curve results?

I am trying to do some elliptic curve calculations by hand, just to refresh myself on how the system works. I calculated some points and did some operations by hand. I am trying to double check my ...

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