2
votes
2answers
136 views

How to compare two datasets „anonymously”?

Ok, I hope this question makes some sense because I am not so sure how to word it any differently… Imagine the following situation: There are 10 defined colors (blue, orange, yellow etc.) There are ...
-1
votes
3answers
203 views

Padding of Original Message in MD5

what should I do if my original message is greater than 64 bits? what is the process of padding in MD5? As described in Internet Security: Cryptographic Principles, Algorithms and Protocols, page ...
1
vote
1answer
181 views

chaining rsa with ecies

In an answer to a previous question it was suggested that one way to protect your asymmetrically encrypted AES-256 keys, from say a solution to prime factorization, would be to chain asymmetric ...
5
votes
1answer
271 views

Proof that padded RSA is CPA-secure

I'm referring to page 383 of J. Katz and Y. Lindell's Introduction to Modern Cryptography. The book presents a padded RSA: ${\bf Key Generation:}$ same as Textbook RSA (given security parameter ...
1
vote
1answer
74 views

Adversary Two Stages

I am read this paragraph and I have a doubt. "An adversary to PKC $\Pi$ is given by two probabilistic polynomial time algorithms, $A = (A1; A2)$. In the first stage, the "find" stage, the ...
3
votes
1answer
255 views

Why does key generation take an input $1^k$, and how do I represent it in practice?

In my lecture, the lecturer said: Let $K$ be the key generation algorithm. Given a security parameter represented in unary, $1^k$, $K(1^k)$ will output a keypair $(pk; sk)$, known as the public ...
0
votes
1answer
83 views

Sematically Secure McEliece

I am read the Lemma 2 (pp13) in the paper "Kazukuni Kobara and Hideki Imai: Semantically Secure McEliece Public-Key Cryptosystems –Conversions for McEliece PKC– (PKC 2001)". Related to the question ...
4
votes
2answers
331 views

toy hash algorithm

I'm looking for a toy hash function, where the idea is to have high school students break (i.e. find a collision) a hash function by hand, in order to teach them how one way functions and hashing ...
1
vote
2answers
809 views

Custom crypto library in C

I will be making (in the near future) an app that will contain some pretty heavy crypto. As it is possible that my app will be ported to different devices (initially it will be iOs only, but Mac, ...
2
votes
1answer
559 views

Secure order preserving hash function

Is there a construction of an order preserving hash function that keeps the preimage property of a crypto hash function? By order preserving hash function (OPHF) i mean for $x<y$ then $OPHF(x) < ...
6
votes
2answers
1k views

What is the difference between Scrypt and PBKDF2?

After reading these two resources I am wondering am I getting all the differences between Scrypt and PBKDF2. As far as I understood, the similarity is: both are password-based key derivation ...
0
votes
1answer
121 views

Certificate == signed public key

For primes p and q used to create a keypair, I understand that the following operation is used to create a signature: $M^d (\bmod\ N)$ where d and N have their usual meanings and M is the message. ...
2
votes
2answers
502 views

risk of attacker decrypting RSA ciphertext without public or private key

As I describe in my previous question I am trying to decide if it's worth it for me to use the Offline Private Key Protocol in creating some long term private archives, instead of just going with a ...
3
votes
1answer
268 views

Counter Mode: static IV but different keys [duplicate]

Given we are using AES counter mode, suppose we randomly generate several keys, all of them are using same IV (say, zeros). Does this lead to any security issue? I know that in CTR mode, same key-iv ...
1
vote
2answers
156 views

standard way to authenticate parties with public keys without certificates

Is there some standard protocol for mutual authentication of two parties when there is possibility to use public key cryptography? The problem is that two parties can use public key cryptography but ...
2
votes
1answer
306 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?
2
votes
0answers
101 views

Question about proof of knowledge defintion?

I am just reading the "soundness"-definition for proofs of knowledge by Bellare / Goldreich. A proof of knowledge is a proof between a prover $P$ and a verifier $V$. $P$ convinces $V$ to know a secret ...
0
votes
1answer
157 views

RSA - Ecrypting the same data with the same public key = same ciphertext?

If an adversary knows my public key and guesses what was my plaintext, can he test for it somehow? The most obvious way is encrypting the guessed plaintext with my public key and the same parameters ...
3
votes
1answer
790 views

Secure encrypt-then-sign with RSA

I understand that when you want to encrypt and sign data with RSA the generally recommended approach is sign-then-encrypt. However, I have encrypted data that I need to sign, to prove the author of ...
1
vote
0answers
105 views

Is OAEP reversible?

Given nothing more than some integer $m =$ OAEP($M$), is it possible to recover the original plaintext $M$? In other words, without being given the hash functions or the random string used for ...
3
votes
1answer
139 views

implementing long term archive encryption

Let's say I want to create private archives for the long term (e.g. more than 30 ). The archives' sizes could be anywhere from 1 GB to 30 GB. As far as I understand I could go down two ways: The ...
2
votes
1answer
158 views

How can a key pair be derived from an arbitrary hash?

If I correctly understand the concept of a "brain wallet" in BitCoin, you start with a passphrase, generate the hash of the passphrase, then somehow derive a public / private key from that to use as ...
1
vote
0answers
88 views

Algorithm digipass-like

I'm pretty sure there are famous algorithms for my problem, but I don't have enough knowledge in this field, so I need your lights. Here si the problem : Alice sends key X to Bob. Bob has an ...
5
votes
2answers
1k views

AES plaintext is smaller than 128 bits - how to expand?

We are currently developing a little AES implementation in a crypto-course at university. As far as I know, AES uses a 128 bit block length, so all data that will be encrypted gets split up into ...
1
vote
1answer
175 views

Password verification

I'm looking for feedback on whether or not this is the proper way to approach password verification without transmitting the actual password. Are there any problems with this scheme. We are working ...
9
votes
1answer
2k views

repeating-key xor and hamming distance

I read that to break repeating-key xor you can do the following: try a keysize $n$ and compute the hamming distance between the first $n$ bits of the encrypted string and the bits $n+1$ to $2n$ of the ...
0
votes
1answer
299 views

Is this scheme a provably fair random number generation?

I have thought up a method for generating random numbers between a client and a server which I hope is fair: The client and server decide on a range in advance, $0$ trough $n-1$. The server ...
1
vote
1answer
71 views

What does “securely realize” mean?

I was wondering what "securely realizes" means. I see this in some cryptographic papers but I don't know what it means for a protocol to "securely realize" a function $F$. Is it just a fancy way of ...
4
votes
1answer
178 views

Combining AEAD with RSA

'Hybrid' encryption, where we combine symmetric encryption with public-key cryptography, is pretty 'tried and tested'. To summarise, we generate a symmetric key and encrypt it using RSA. We would ...
1
vote
1answer
507 views

Is it possible to determine a PGP public key from encrypted data?

Is it possible to find out which public key was used to generate the message? Scenario is like this : I got data which is encrypted with a public key. Now a "bad guy" hast the encrypted data, and ...
3
votes
4answers
450 views

Tamper-proofing log files

Problem Overview I want to securely store log files so the contents are secret, and they can't be modified without detection. The files will be encrypted using authenticated encryption (AES in GCM ...
2
votes
1answer
94 views

What happen if a asymmetric crypto-system deals with only one key

Assume there is a crypto algorithm that deals with matrices to encrypt and decrypt. Regardless of the specification of such algorithm, what if the algorithm assumes that two parties can securely agree ...
5
votes
2answers
382 views

Are cryptographic hash functions perfect hash functions?

For a cryptographic hash function and input values of shorter length than the hash function output, it's pretty obvious that there should be as few collisions as possible. But are there guaranteed to ...
3
votes
1answer
299 views

Attacking historical ciphers methodology

It's more a theoretical question of how would you approach it. All you know about the ciphertext it's was generated with a historical cipher. The ciphertext appears to be random, BUT it's divided into ...
1
vote
4answers
294 views

Using the output of a stream cipher, how to guarantee the integrity of 4 bytes of data?

I am designing a simple and secure stream communication protocol. My idea was to build each message sent to the wire as: (message size || clear text || UHASH(message size || clear text)) $\oplus$ ...
2
votes
1answer
1k views

SHA256 HMAC brute force with chosen plaintext attacks

This is a follow up to Is It Possible To Reconstruct a Cryptographic Hash's Key I am using a SHA-256 HMAC function on a single-word input: sha256hmac(privatekey,word) = output. The private key length ...
6
votes
1answer
826 views

Quality of randomness on a Linux system with haveged

Has anyone checked if using haveged, a Linux daemon which uses the HAVEGE algorithm, changes the non-deterministic properties of the random data from ...
6
votes
2answers
2k views

Using the same secret key for encryption and authentication in a Encrypt-then-MAC scheme

Is it a weakness to use a single shared secret for protecting messages using a Encrypt-then-MAC scheme? Assuming a system is using AES-256-CBC and a SHA1-HMAC and the same secret key for both ...
3
votes
3answers
271 views

Is it possible to work out the hash algorithm from a list of known message-hash pairs?

For example, in my situation I know hash(20) = 486e9638177faf1f34e49910491b77af. I also know the hashes for all values from 0 to 20. Is it possible to work out the ...
4
votes
0answers
198 views

LT codes with Homomorphic hashing

I have been working on a project implementing LT codes with Homomorphic hashing (inspired from http://blog.notdot.net/2012/08/Damn-Cool-Algorithms-Homomorphic-Hashing and ...
4
votes
3answers
373 views

The utility of elliptic curve cryptography

Suppose that the only public key cryptography schemes that we knew were Diffie Hellman, RSA and ElGamal. How much would this set civilization back? Are there important applications of elliptic curve ...
1
vote
1answer
119 views

Connections between Instance Hiding and Fully Homomorphic Encryption

Another approach taken by researchers for carrying out computations over encrypted data is Instance Hiding. In brief, If a user wants to outsource the computation of a function for a particular input ...
2
votes
3answers
244 views

Requiring a “supervisor” key pair and a “user” key pair to decrypt multiple-recipient messages

I've been toying with some encryption scenarios recently. One of the hard ones I came across is a multi-party system. So we have Bob -- The person who sends the message (and knows it's recipients) ...
6
votes
1answer
346 views

Prove that textbook RSA is susceptible to a chosen ciphertext attack

Given a ciphertext $y$, describe how to choose a ciphertext $\hat{y} \neq y$, such that knowledge of the plaintext $\hat{x}=d_K(\hat{y})$ allows $x=d_k(y)$ to be computed. So I use the fact that the ...
3
votes
1answer
194 views

Addition with Shamir secret sharing

When performing Shamir secret sharing I'm trying to find $z_i$, such that $z = x + y$. Where $n = 6$ and $t = 3$. I believe this would be the correct solution (correct me if I'm wrong): Each party ...
3
votes
1answer
739 views

Triple-DES Brute Force

Shannon's paper states: A system is pure if all keys are equally likely and if for any three transformations, $T_i$, $T_j$, $T_k$ in the set, the product $T_iT_j^{-1}T_k$ is also a ...
6
votes
2answers
371 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 ...
2
votes
2answers
247 views

Chain several RSA encryptions without increasing the message size

I would like to be able to encrypt the output of RSA with RSA again without having the output grow in size over time. In other words, I have some data $D_0$ which I want to encrypt with RSA: $D_1 = ...
17
votes
1answer
895 views

Should I trust CipherCloud? [closed]

Should I trust CipherCloud's system for "homomorphic encryption" of data in the cloud? Has the security of their system been subject to peer review or other cryptanalysis? Is there any known analysis ...
5
votes
2answers
787 views

Is encrypting a single 128 bit block with AES ECB “safe”

I want to encrypt a small piece of data that is less that 16 bytes in size (think SSN), and I'll be using a 256bit encryption key. The typical suggestion is to never use ECB, but if there is just a ...

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