4
votes
1answer
162 views

Would a symmetric cipher with a keylength a big as the data length be information theoretically secure?

One-Time-Pad is information theoretically secure as long as the random number stream is evenly long or longer than the data stream it encrypts, for a "decyphered" message could have been any message ...
4
votes
1answer
316 views

How can k3d3/ed25519-java's performance be improved?

I tried asking this on stackoverflow because I thought it might be a simple coding question and didn't want to clutter up this amazing stack, but I received no responses. CodesInChaos showed me that ...
4
votes
1answer
142 views

Private Messaging

Suppose Alice and Bob wants to send messages through Sue without revealing to Eve that the exchange took place. Slightly more formally, I don't want Eve to have any advantage telling if Alice is ...
4
votes
1answer
170 views

How HOTP values are validated according to RFC 4226

In Section 7.2 "Validation of HOTP Values" of the HOTP spec (RFC 4226) it says, in part, The HOTP client (hardware or software token) increments its counter and then calculates the next HOTP ...
4
votes
1answer
110 views

Hardness of CDH in different groups

What is the difference of the CDH problem in different groups? In particular, given a group $\mathbb{G}_1$ of order $q$ that is a subgroup of $\mathbb{Z}_q^*$, $q$ prime, and another group ...
4
votes
1answer
219 views

How can I tell which curve a given ECDSA implementation uses? (P-521 or something else)

I'd like to test and see if certain software uses P-521 ECC curves, or if it uses another variant. Without having access to the sourcecode, or the specification, is there any way for me to test which ...
4
votes
1answer
197 views

Do Gap-CDH groups exist?

A Gap-CDH group is such that, given group elements $g, a = g^x, b = g^y$, it is hard to compute $g^{xy}$, but, given a group element $c$, easy to verify if $c = g^{xy}$. While such groups have been ...
4
votes
1answer
214 views

How to design a practical and secure MAC scheme?

I am sorry, but I need to introduce some concepts which are not directly related to cryptography to make myself clear, I hope I won't stun you with this ... (I'd rather explain it here than redirect ...
4
votes
2answers
468 views

How to calculate y value from ((y*y) mod prime) efficiently

i am working ECC-224 bit. can any one tell me, how to calculate y value from ((y*y) mod prime) efficiently for large bit numbers.
4
votes
2answers
809 views

Security of Deterministic Encryption Scheme

A deterministic encryption scheme is a cryptosystem which always produces the same ciphertext for a given plaintext and key, even over separate executions of the encryption algorithm. Although we ...
4
votes
1answer
185 views

Changing SRP-6a message order

In SRP-6a, the public key $B$ of the server is normally sent after receiving the public key of the client $A$. Is it okay to send $B$ and $s$ after the client sends its username $I$, but before the ...
4
votes
1answer
324 views

Why does the SRP-6 calculation of B include a multiplier k = 3?

In SRP-6 $B$ is calculated as $B=kv+ g^b, k=3$. What is the purpose of $k$, and why was it fixed as $3$? (In SRP-6a, this value $3$ is replaced by $k = H(N,g)$, but this question is about SRP-6.)
4
votes
1answer
259 views

Does Grover's algorithm effect block size or only key size?

We know that Grover's algorithm can speed up cracking symmetric keys. Basically the key space is halved. This means we have to use at least a 256 bit key (to get 128 bit security). I heard somewhere ...
4
votes
4answers
512 views

Efficient Symmetric Mutual Entity Authentication Protocol

I am looking for an Authentication protocol for access control. Let's say both the client and server would share a symmetric secret (256 bit key) and the client wants to access the server. Access ...
4
votes
3answers
368 views

Can I use PGP to sign a message without providing cryptographic non-repudation?

The difference between a digital signature and a MAC is non-repudiation. A message with a digital signature proves that only the sender could have signed the message, whereas a message with a MAC ...
4
votes
1answer
435 views

Can ElGamal encryption and ElGamal signatures be used together sharing the same key-pairs?

I'm working on a encryption system where each party can store exactly a single ElGamal private key in a device. This is a hardware limit. The system must be expanded to support signatures and ...
4
votes
2answers
228 views

Detect tampering of data

I have the following problem: There are $n$ users and each users fills in three data points per day, this are three integers between say $-10$ and $10$. I have access to the database and hence I am ...
4
votes
1answer
369 views

Memory-hard operations in work-factor hash functions

I'm playing around with work-factor hash functions, and I'm looking for a memory-hard operation to make it resistant to GPU / parallel hardware attacks. I considered a very large (i.e. 64K) s-box that ...
4
votes
2answers
469 views

Can I use guids / uuids as counters for key derivation?

I have a 512 bit master key, and 128 bit unique identifiers. I'm going to derive 256 bit keys to for use with deterministic and block ciphers. For reference, The KDF I'm going to use is Hash( C | ...
4
votes
1answer
286 views

How to construct a zero-knowledge proof of a number of the form $n=p^a q^b$

Let $n = p^a$$q^b$ where p and q are distinct primes and a and b are positive integers. How to construct a zero knowledge proof that n is of such form? This is actually a homework problem with a ...
4
votes
1answer
365 views

How can one share information using the 'host-proof' paradigm?

I am attempting to make a web-based secure password management and sharing utility, both as an academic exercise and to fully understand and feel safe about using it. I really like the idea of a ...
4
votes
1answer
286 views

Undecrypt using the OpenSSL EVP API?

I'm writing a client-server encryption scheme for homework, and I've stumbled upon what might be a fatal blow to my current implementation. When using AES-256-CBC on the two sides, it's important ...
4
votes
1answer
291 views

Best choice of finite field for AES on a 4-bit microcontroller?

As the finite field of $GF(2^8)$ are isomorphic to $GF((2^4)^2)$, $GF((2^2)^4)$ and $GF(((2^2)^2)^2)$, which of the fields is best suited and most efficient for 4-bit MCU and why? Would it be ...
4
votes
1answer
108 views

Is there any serious discussion about using blinding intermediaries in digital currency scenarios?

A digital currency system like Lucre (OpenTransaction) creates a coin by a mint blind signing the output of a hash function, which the payer then unblinds and pairs with the input to the has function. ...
4
votes
1answer
307 views

How is the MJH double-block-length hash function constructed?

I'm looking for information on the MJH double-block-length hash function, but the best free source I could find is the diagram on page 18 of Efficient hashing using the AES Instruction Set (submitted ...
4
votes
1answer
84 views

64bit nonce in AES-CTR, why not 128?

I'm just a bit confused about the nonce/IV in AES-CTR ... it seems the suggested way to go is have a 64bit random nonce, and use the last 64bits as the counter, "starting at 0". I'm just wondering ...
4
votes
1answer
127 views

Quantum computing vs AES s-box equation

Do quantum computers have any affect on the ability to solve the non-linear AES s-box algebraic expression, or does solving it still fall under search algorithms (Grover's)? If quantum computer do ...
4
votes
1answer
141 views

Parallelization of CFB mode encryption and decryption

While reading Evaluation of Some Blockcipher Modes of Operation by Rogaway (someone just linked it somewhere), I noticed two claims about CFB that I'm not sure I understand: Regarding whether CFB ...
4
votes
1answer
73 views

Parallel authentication of encrypted data. What AE type to choose?

I've got a system with a slow data transfer channel. Transferring a message through several channels does increase the transfer speed significatnly. AES-CBC+HMAC is good except that verifying the MAC ...
4
votes
1answer
69 views

On Proving That a Primitive Does Not Exist

In the paper of Hsiao and Reyzin, Section $1.4$: Note that to show that no general reduction from $P$ to $Q$ exists requires proving that $Q$ does not exist Since the statement is about trying ...
4
votes
1answer
88 views

Implementing AES MixColumns with fewest XOR gates

I'm playing with Contiki (O.S. for constrained devices such as sensors and micro-controllers) and particularly with the AES encryption in software (the one available in Contiki source code). I'm ...
4
votes
1answer
113 views

EC: Why does $h>200$ need to hold?

From BSI-TR-03111 (PDF), on page 15: The class number of the principal order belonging to the endomorphism ring of E SHOULD be at least 200. This value commonly is referred to as $h$ in that ...
4
votes
1answer
66 views

possible MITM attack against servers that used to support weak DH?

from reading the TLS 1.1 RFC, it looks like it would be possible to break a previously recorded TLS <1.2, 512-bit DHE ServerKeyExchange and then send it (unmodified, with the original, valid ...
4
votes
1answer
90 views

Hard-core predicates: should the adversary be given $1^n$?

In most (all?) classical sources such as the book of Goldreich (2001), hard-core predicated are defined thus: A polynomial-time computable predicate $b : \{0,1\}^* \to \{0,1\}$ is a hard-core of a ...
4
votes
2answers
82 views

Discrete logarithm over prime modulo: small input, large exponent, larger prime

I am considering using modular exponentiation as a one-way hash function. More specifically, here is the scenario. 1) Input ($m$): the input messages are small (16-bit) 2) Exponent ($e$): the ...
4
votes
1answer
130 views

Prime factorization

What is the largest integer that can be factored by modern algorithm like Msieve and GGNFS in a time less than 5 hours with normal computers? For example, can an integer like ...
4
votes
1answer
239 views

AES as Integer Program

I read that AES can be broken by solving a Linear Integer Program (IP). From a given encrypted text, how can one construct this IP? How big is the system?
4
votes
1answer
217 views

Factoring two RSA modulus with known $|p_1 - p_2| < \ell $

I have two 1024bit RSA modulus $N_1 = p_1q_1$ and $N_2 = p_2q_2$ such that $0 <|p_1 - p_2| < \ell$, and $\ell$ is at most 64bit integer. Can I factorise $N_1$ and $N_2$? What's the answer when ...
4
votes
2answers
269 views

How does cryptanalysis of the Playfair cipher work?

I have a set of Playfair-enciphered data that I'm trying to crack without the key. I know I need to analyse bigrams; I've currently worked out what decrypts to th, ...
4
votes
1answer
200 views

What cryptography is suitable to motivate High School students to learn math?

A High School math teacher in the Bronx asked me if I knew of any way to use cryptography in a way that motivates students (15 to 18 years old) to learn math. I do know that many of them want to ...
4
votes
2answers
169 views

Power analysis and exponentiation by squaring

I am trying to understand how one can retrieve the secret exponent via a simple power analysis. Lets suppose that the exponentiation by squaring algorithm is implemented in its simplest form : ...
4
votes
1answer
120 views

How did the power measurements translate to AES Key?

A side channel attack is described here. I did not quite get , how the power measurements are used to get the decryption key , can any body explain it?
4
votes
1answer
274 views

reducing false positives in searchable encryption

Practical techniques for searchable encryption proposed by Song et al. [1] have a false positive rate of $l/2^m$ where $l$ is left half of encrypted bits and $m$ is right half and $l+m=n$ where $n$ is ...
4
votes
1answer
99 views

OCB and GCM security

Is OCB as secure as GCM or CCM ? Since OCB design is quite different from GCM and CCM, I was wondering if the security properties of these latters are satisfied by OCB, as well. Thank you.
4
votes
1answer
138 views

Public key in fully homomorphic encryption over the integers

I'm reading “Fully Homomorphic Encryption over the Integers” by van Dijk et al. I wonder why $x_0$, which is a component of the public key, should be an odd number?
4
votes
1answer
172 views

Key Derivation from random salted seed, HMAC and HMAC based KDF

I would like to generate a key which would be good enough to be used for encryption with AES in a mobile environment. My idea is to use as a seed ''random attributes'' from a mobile device. Is hashing ...
4
votes
2answers
186 views

Allowing multiple password reset tokens, and a question on random number predictability

I have a few requirements for a password reset system, along with some questions at the end as to its security. Requirements The aim is to provide a password reset mechanism satisfying the following ...
4
votes
1answer
133 views

Deterministic Rand function for Winternitz One Time Signatures

Suppose you are implementing a Generalized Merkle Signature Scheme, using the Winternitz One-Time Signature Scheme for the node signatures. Furthermore, suppose the implementation is to be stateless ...
4
votes
1answer
112 views

Length-preserving all-or-nothing transform

Is there any known way to construct a length-preserving all-or-nothing transform? In other words, a secure all-or-nothing transform where the length of the output is the same as the length of the ...
4
votes
1answer
198 views

Why does a perfect secrecy can be achieved when decryption correctness is not totally required?

By Shanon theorem, a perfect secrecy encryption scheme must use a key space of equal size as the message space. But when the correctness requirement is weakened such that $Pr[Dec_k(Enc_k(m))=m]=1/2$ ...

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