3
votes
1answer
117 views

Can substrings of a long string be efficiently authenticated?

Suppose one has a (strong) shared secret key and wishes to authenticate a very long string in a way that will allow the verifier to verify small substrings without reading much of the stored data and ...
3
votes
2answers
281 views

differential and linear cryptanalysis

I have been reading about differential and linear cryptanalysis. They were mainly introduced by Adi Shamir and Biham to show weakness of DES. However, many of articles state that they have been ...
3
votes
1answer
200 views

When truncating an AES MAC value by “w” , how do I justify that “w” is still negligible?

I'm taking an online class on cryptography at corsera.org / Stanford, and the professor is explaining that it's OK to truncate an AES MAC to $w$ bits as long as $1/2^w$ is still negligible (say $w ...
3
votes
1answer
794 views

Adding and multiplication in jacobian coordinates

How can I derive formulas for adding 2 points and multiplication by a scalar in Jacobian coordinates $(x,y) = (\frac{X}{Z^2},\frac{Y}{Z^3})$ over an elliptic curve?
3
votes
1answer
1k views

Why are the Davies-Meyer and Miyaguchi-Preneel constructions secure?

The Davies-Meyer compression function $h(H, m) = E_m(H) \oplus H$ is said to be secure. So too is the Miyaguchi-Preneel compression function $h(H, m) = E_m(H) \oplus m \oplus H$. Why are these ...
3
votes
1answer
257 views

How to use salt if I am sending hashed password?

If have application that is sending username and hashed password to server for authentication, how would I use salt for this scenario ?
3
votes
2answers
2k views

Known-plaintext attack on Blowfish in ECB mode

The protection scheme I faced recently seems so weak nowadays that a simple exhaustive key search would be enough to recover the user key in an acceptable amount of time (it's OK, since almost no ...
3
votes
1answer
460 views

What does the expression $1^n$ mean as a function argument?

In a paper about predicate encryption or attribute based encryption, the setup function is mentioned with the $setup(1^n)$ or $setup(1^l)$. I want to know what is meant here. Is it multiples of ones ...
3
votes
3answers
185 views

How can two UProve token holders prove to a 3rd party that they aren't the same user?

Suppose I have two users who are issued two different UProve IDs. The Issuer has guaranteed that one UProve token bearer will never have more than one UProve token ID. How can I use UProve to ...
3
votes
1answer
552 views

How to compute the dot product on encrypted values?

Is there a practical homomorphic encryption scheme that can give reasonable execution time results in computing a dot product: $$a_1*b_1 + a_2*b_2 +a_3*b_3 +\ldots+ a_n*b_n$$ I imagine the scheme will ...
3
votes
1answer
157 views

How to obtain a one-value share in Shamir's secret sharing

This is a trivial question, but I had to ask: since each generated share in a Shamir's secret sharing scheme initially consists of a pair of values (representing the coordinates of a point on the ...
3
votes
1answer
405 views

How are Elliptic Curve Cryptography and Pairing Based Cryptography related?

I have been doing a project that uses the PBC library developed by Ben Lynn. But I am still not clear on how PBC is related to ECC. I know that this is a site for complex crypto QA, but I did not know ...
3
votes
1answer
703 views

Symmetric encryption mode where ciphertext size is plain text size

I've had many questions on Stackoverflow on how to minimize the output of a cipher - during encryption of course - to the same size as the input. Obviously this is possible for a single block of ...
3
votes
1answer
1k views

Extract private RSA key from USB cryptographic token using Bardou et al. attack (variant of “million message attack”)

There is a side channel attack on tamper-resistant USB cryptographic tokens using padding-oracle, described by Bardou, Focardi, Kawamoto, Simionato, Steel and Tsay, titled "Efficient padding oracle ...
3
votes
1answer
309 views

How to find an element of high-order in an RSA group?

Is this even possible? The RSA group is not cyclic, so usually you wouldn't find a generator for accessing all group elements. What happens if you use the RSA group in a scenario where you want that ...
3
votes
2answers
665 views

RSA private key format for Mega

I've been trying to reverse engineer Mega's (mega.co.nz) API calls. And stopped on the step where client needs to decrypt session id with provided RSA key. I can get key data, but I have no idea how ...
3
votes
1answer
937 views

How can I calculate the time complexity of modular arithmetic?

I'm doing modular arithmetic in a Java program, and I want to calculate the time complexity of the individual operations. $$ c= a · b \bmod n $$ $$ m = a^{-1} · b \bmod n $$ How do I get an ...
3
votes
1answer
343 views

Mental Poker with untrusted server (i.e. no peer to peer)

The question of implementing peer to peer mental poker has already been asked on stackoverflow. And there appears to be an implementation called LibTMCG. Also on this site a question was asked about ...
3
votes
1answer
1k views

Ideal passphrase length: old diceware method (5 words) vs. your Bitcoin wallet.dat passphrase lenght (8 words) and doubling passwords?

I made a cool 5 word passphrase back then using the old Diceware method and use it as a master password. The question is as computing power increases will we need to add more and more words to our ...
3
votes
2answers
188 views

Security model for privacy-preserving aggregation scheme.

Suppose that $S=(E,D)$ is an additively homomorphic encryption scheme. Now I want to design a protocol $P$ such that given inputs $x_1,x_2,..,x_n$, the adversary $A$ (who can decrypt) can only learn ...
3
votes
2answers
259 views

Probability that an attacker wins the discrete logarithm game when exponents are drawn from a subset

Suppose $g$ is a generator of an order $p$ cyclic group in which discrete logarithm is hard and $p$ is a prime (i.e., given $g^x$ for a random $x \in \{0,1,\ldots, p-1\}$, it is hard to recover $x$ ...
3
votes
2answers
859 views

How much extra information is in an RSA public key?

I'm trying to calculate the size of an RSA public key in Ruby. I've retrieved the key in PEM format, and once I've decoded the base64 part from the PEM format, I get the size in bytes. What I find is ...
3
votes
1answer
189 views

SSL Key Exchange

I read briefly over the TSL/SSL key exchange protocol. I have to wonder about the whole pre-master-key procedure. Wouldn't it be sufficient if the client or server encrypted a generated session key ...
3
votes
1answer
2k views

How to prove that the concatenation of two secure PRG is secure?

Given $G:\{0,1\}^s \rightarrow \{0, 1\}^n$ a secure PRG, how can one prove that $G'(k_1, k_2) = G(k_1) \cdot G(k_2)$ is secure ($\cdot$ means concatenation)? In other words, I'd like to show that if ...
3
votes
1answer
511 views

Efficient setup for a Montgomery multiplication

Montgomery described an efficient method to compute a modular multiplication. This works by using a special constant $R$ and assumes the inputs $a$ and $b$ have been made into a special representation ...
3
votes
2answers
957 views

Key derivation from a random seed

The main problem is to use a block cipher to generate a random key. I would like to generate 256-bits key which can be as random as possible. I generate it in the following way: Pick a plaintext ...
3
votes
2answers
490 views

Can I save space for short messages by using encryption with private key instead of a signature?

Let's say I have a message, a Private Key and a Public Key. Normally if you want to see if the message is unaltered and is from the sender you would have the message part + signature part, which you ...
3
votes
3answers
300 views

Existing works on pre-computing ElGamal ephermal keys

I was playing around with a problem in e-voting schemes that use additive homomorphic encryption to tally votes, namely that at the end of the day somebody (or somebodies, if the secret material has ...
3
votes
2answers
534 views

Is it possible to match encrypted documents using user-defined search terms?

Suppose I am storing a number of encrypted documents in a database. I would like to make it possible to identify the subset of documents whose contents match user-specified search terms without a) ...
3
votes
1answer
451 views

Is truncating a hashed private key with SHA-1 safe to use as the symmetric key for AES for data at rest?

I realize this is mixing the purposes between asymmetric and symmetric crypto, but I was wondering if it is safe to use a hashed, truncated private key (asymmetric) as the symmetric key for encrypting ...
3
votes
1answer
266 views

Stream ciphers based on discrete logs

Blum Blum Shub is a stream cipher that is provably reducible to the difficulty of factoring integers. I'm wondering whether there is a similar construction for discrete logs? For example, I could ...
3
votes
1answer
358 views

Where can i find relative software for differential cryptanalysis?

Is there any publicly known software for differential cryptanalysis over some cryptographic building blocks as blocks ciphers or cryptographic hash functions
3
votes
1answer
1k views

Okay to use OpenSSL to encrypt then sign a message?

I am planning on encrypting a message then signing the message so the recipient can authenticate the source. Reading this article has me concerned that I'm doing something wrong. My current plan is ...
3
votes
1answer
353 views

How to properly add ECDSA private keys?

I'm currently working on an application that requires me to add two ECDSA private keys in order to make a new private key. The result has to have the property, that its corresponding public key is the ...
3
votes
2answers
142 views

When to prefer exchanging cryptographic certificates over exchanging only public keys?

Let's think of the following case: A group of peers want to exchange messages with each other. They use public-key cryptography to sign and encrypt messages. Anyone with any name can join the ...
3
votes
1answer
845 views

How do I generate a session key using the Diffie Hellman algorithm?

How to generate a session key between two nodes in two different subnets when the nodes don't know each other directly, using diffie hellman algorithm?
3
votes
2answers
279 views

Algorithm Design for only Mutual Information Sharing

Bob and Alice each have a bit string they want to keep private. They each want to know what the bitwise AND of their two strings would be without telling the other or anyone else listening to their ...
3
votes
1answer
193 views

DIfferent inner and outer hash functions for NIST Recommended HMAC?

The NIST recommended HMAC uses $$\operatorname{HMAC}_k(text) = H_\mathrm{out}( (k \oplus \mathrm{opad}) \operatorname\| H_\mathrm{in}((k \oplus \mathrm{ipad}) \operatorname\| text) )$$ Is it ...
3
votes
1answer
259 views

Is there a known vulnerability when using identical key and salt with PBKDF2?

For optimum security, a random salt should be chosen for PBKDF2. I came across a scenario where I produce a hash of a random file that serves as the "password". Due to the nature of files, this has ...
3
votes
1answer
395 views

What type of hash functions provides non-malleability of hash digests?

I want to use a hash function for commitments. I don't want an attacker to construct a commitment related to a previously published (but still unopened) commitment. A simple deterministic commitment ...
3
votes
1answer
323 views

OCB - brute force against unknown IV

I read that brute force attacks against a plaintext encrypted in OCB with unkown key and IV has approximately the same complexity as an attack where only the key is unknown. Why is that ? Is there a ...
3
votes
2answers
807 views

Inverses in Truncated Polynomial Rings

I've been trying a long time to understand a thing which is obviously extremely simple, but I just can't get it. Read this, please: The NTRUEncrypt PKCS uses the ring of truncated polynomials $R$ ...
3
votes
2answers
2k views

Proof of security for RSA signatures

I am new to here, I was having some problems with a problem I am working on: "For each of the following variants of the definition of security for signatures, state whether textbook RSA is secure ...
3
votes
1answer
2k views

Demonstrating the insecurity of an RSA signature encoding scheme

I'm working on problem 12.4 from Katz-Lindell. The problem is as follows: Given a public encoding function $\newcommand{\enc}{\operatorname{enc}}\enc$ and a textbook RSA signature scheme where ...
3
votes
1answer
736 views

KeeLoq showing that decryption is indeed the inverse of encryption

In some text I am reading, there is an exercise asking to show that KeeLoq decryption function is the inverse of the encryption function. Details about KeeLoq are given in the Wikipedia article. As I ...
3
votes
1answer
71 views

What is the private key in RSA?

I'm new to cryptography and I have a doubt: I read some pages a bit different definition for the RSA private key: In 1 - (n, d) In 2 - ...
3
votes
1answer
55 views

Do you need a HMAC if decryption attempts are severely limited?

I am considering an application where data bandwidth is extremely limited, and considering removing message authentication codes from encrypted transmissions. The encryption method is currently ...
3
votes
1answer
76 views

Is there any function that does not suffers birthday problem?

I am eager to know that if there is any function that does not suffer birthday problem and how to prove it formally that the function is not suffering the birthday problem.
3
votes
1answer
40 views

About ORAM initalization and running out of space

I have two questions about ORAM Since at the beginning,the server allocates unused storage for ORAM cient,there are no user's data in the storage.if now client access the storage the server will ...
3
votes
1answer
44 views

what is the difference between proofs and arguments of knowledge?

What is the difference between proofs and arguments of knowledge in the context of zero-knowledge? I have read this sentence in this ePrint: It is useful to distinguish between zero-knowledge ...

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