# All Questions

235 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 ...
452 views

### Do companies like CipherCloud really have the option to use homomorphic encryption?

I was reading How is CipherCloud doing homomorphic encryption? and was wondering: Is there a technically feasible way for companies like CipherCloud to use homomorphic encryption (HE) while ...
185 views

### Can an encrypted nonce be used for authentication?

Would a scheme like the following work to allow authentication of a sender: A nonce is appended to data, and the result is encrypted. The encrypted pack is sent to the other party. The sender then ...
178 views

### a possibly stronger type of attack on identity-based encryption

What is known about the security of identity-based encryption schemes against attacks that involve seeing multiple ciphertexts and then receiving the private keys corresponding to some of those ...
864 views

### Digital Signature size requirement

What is an approximate length for a digital signature (SHA) given the following? Using RFID tags, size ranging from 100 to 2000 bytes (8000 bits), our project is setting aside a portion for a digital ...
409 views

### Primality testing (deterministic vs. non-deterministic)

I noticed that non-deterministic primality testing algorithms are more commonly used in practice while there is a deterministic algorithm e.g., AKS which runs in polynomial time? I am right? if so, ...
268 views

### The probability of finding a limited set of collisions for HMACSHA256

I need some input on the use of HMAC-SHA256 in an open source project, any help appreciated. The problem I'm solving is session fixation. We have a username, for that username we'd like to generate a ...
359 views

### plaintext and its encrypted version known [duplicate]

I have a (long) text in two versions, encrypted and plaintext. I think it was encrypted using a substitution cipher method (I'm pretty sure, indeed). I'm not good in this matter, I know little about ...
540 views

### Is OpenPGP vulnerable to the “crypto doom principle”?

From reading RFC 2015 (MIME security with PGP), and from looking at the raw format of (GnuPG) encrypted and digitally signed email messages, it seems that first a signature is calculated from the ...
389 views

### Is this key management scheme secure?

I'm designing a scheme for management of a small number of cryptographic keys (< 100 at any given time) and I had an idea: rather than storing all the keys, could I just store one master key and ...
134 views

### Is Triple-DES a group?

I know for a fact that DES is not a group, but are any of the Triple-Des versions a group? Why, or why not?
328 views

### modular exponentiation as a one-way-hash

As far as I can tell most one-way hashes apply some iterated encryption algorithm to the input data. What would be the issues with a one-way hash based on some fixed large prime p and a generator g ...
136 views

### How to build a secure AE scheme with generic composition?

I am actually looking for using a secure Encrypt-Then-MAC AE scheme , and consider using either an existing "ready-to-use" dedicated AEAD mode (GCM, OCB, CCM, EAX, etc ..) or an alternate composed ...
297 views

### How does Poly1305 in the NaCl library calculate a MAC?

I've been reading about Poly1305 as implemented in the NaCl library, more specifically I've been referring to this Java implementation of NaCl. My understanding of XSalsa20Poly1305 (in this ...
582 views

### length extension attack on an MD5 hash of a text file of about 10K Bytes?

How much computing time on a typical desktop computer would it take to find a new length and new data to extend a text file of about 10K Bytes with a given MD5 hash?
116 views

### Encrypt user email but be able to find user by email

Sorry for my dumb question, but it's better to ask dumb question than to do dumb things silently. I want to encrypt user email in my DB so that if someone stole the DB (and not the key) - he won't be ...
374 views

### Attacking AES/CCM with random IV

A cryptosystem is using AES-128 in CCM mode with random IV. Suppose an attacker capable of: asking the cryptosystem to encrypt a single plaintext as many times as he wants; and asking the ...
207 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 ...
85 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 ...
390 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)) $\oplus$ ...
584 views

### How is ElGamal not secure under chosen ciphertext attack, but semantically secure in some cases?

I know that you can create a ciphertext c' using c and then find the corresponding m' for c' which you can use to find m for c. So, doesn't this mean that it is not semantically secure? But I also ...
303 views

### Is this method for exchanging private key using RSA sound?

Bob know's Alice's public key, and he wants to make sure he's connecting to the one which has that key. Furthermore, Alice wants to verify when she gets a connection from Bob who'll give his public ...
658 views

### What is the usual block cipher mode that goes with CTS

I am wondering: what is the most common, or most used, block cipher mode used that goes with CTS (CypherText Stealing)? Is it ebc or cbc?
192 views

### What information to include when calculating the HMAC of ciphertext

I'm aware that it's crucial to include the IV when calculating the HMAC of a ciphertext (assuming an IV is used). Can anyone explain, in simple terms, why this is? Furthermore, assume that there is ...
269 views

### Is the following key stretching algorithm as memory hard as I think it is?

I'm having some fun designing a key stretching algorithm that can be implemented in pure Python. It's built entirely out of the standard library's hash functions in an attempt to at least wrest some ...
1k views

### Attack XOR encryption of binary data compressed by zlib with known key length (very short key)

I am trying to break a packet format. The packet format simply packs several files into one big file. The file contents are plain. But the index data which contain offsets, file sizes and filenames ...
946 views

### Hashing a password with SHA256 on the client then bcrypt on the server

I'm trying to implement a cryptographically secured storage site (not Mega, or anything similar) and am trying to prevent the user's password from ever touching the server. The password is used to ...
95 views

### Zero-knowledge identification of the majority commitment

Alice is communicating with $n$ people. We do not assume that the $n$ people don't trust each other or trust Alice. Alice doesn't trust them either, and needs to assume that they will not form a ...
636 views

### Decryption a chunk of file with AES

We are having a requirement to store large files (~1GB) encrypted and decrypt them when reading. We are looking to use AES-CTR or AES-GCM mode to encrypt the data. There are scenarios where we want to ...
230 views

### Secure communication between multiple peers on a public channel

I am wondering how can I secure communications between multiple peers over a public channel, like an IRC channel. With two peers it is easy - they exchange their encryption keys and decrypt the ...
572 views

### Why does Fortuna (RNG) require entropy sources to specify accumulator pool?

Re-reading my copy of Cryptography Engineering for the holidays, I was struck by the following question about the Fortuna RNG: Why require an entropy source specify which pool a random event should ...
331 views

### Choosing good parameter for Lenstra's elliptic curve factorization

In Wikipedia, there is an article explaining Lenstra's factorization algorithm. As far as I got it, we choose some $e \in \mathbb{N}$ and a point $P$ on the curve and then calculate $eP$. While ...
96 views

### Individual bits of Textbook RSA

This question relates to the underlying RSA assumption. Forgetting about the fact that Textbook RSA is deterministic, I am curious about the assumed strength of the RSA problem. Does RSA hide all ...
393 views

### Non-Interactive Zero-Knowledge-Proof for discret Logarithm?

In a Non-Interactive $Zero-Knowledge-Proof$, the challenge is chosen by the Prover. I am trying to find a Non-Interactive Zero-Knowledge-Proof based on the following problem: DISCRETE LOGARITHM ...
9k views

### Decrypt digital signature using RSA public key with openssl

I have a digital signature that was created using the following algorithm: a SHA-256 hash of the body of the message is calculated. It is then signed using an RSA private key and the result is ...
2k views

### What is a trapdoor permutation?

Can anyone explain to me what a trapdoor one-way permutation is? Is RSA a trapdoor one-way permutation? Context: I was reading about ring signatures. On page 560, it describes steps to ...
632 views

### Crack SHA1 hash code

Given a set of SHA1 hash code, if we knew a portion of the plaintext for each particular hash code, and also the remaining portion of the plaintext is a constant value for each hash code. Is it ...
1k views

### Optimising Pollard's Rho algorithm for large semi-primes

I have programmed an implementation of Pollard's Rho factoring algorithm using C++ and the GMP library. It is reasonably fast with large numbers, however I haven't implemented any form of cycle ...
457 views

### Is there a method to break an EC curve for all key-pairs (Q,d) such that (Q=d*G) faster than breaking every single key-pair?

Related to this question: Is there any memory trade-off that helps such attack? Obviously if the field size is very small (say 40 bits) it´s possible, but what if the field size is 160 bits long? or ...
384 views

### implications of SSH server key compromission when authenticating users against a public key

Friday we had a disagreement with a colleague of mine about the implications of SSH server key compromission The question was stated as this : What could a hacker do while provided with the SSH ...
1k views

### How can the Diffie-Hellman key exchange be extended to three parties? [duplicate]

Possible Duplicate: Can one generalize the Diffie-Hellman key exchange to three or more parties? How can Alice, Bob, and Charlie share a common secret key using an extended version of the ...
2k views

### Difference in one time key and one time pad and many time key

These terms are confusing me. One time pad is when you use one key for one message. That is what One time key is. Secondly, what is the connection of many time key, can i use one time pad many ...
483 views

### Is there difference between Algebraic Homomorphic Encryption and Fully Homomorphic Encryption Schemes?

Is there difference between Algebraic Homomorphic Encryption and Fully Homomorphic Encryption Schemes?
274 views

### Signing 14 bytes of data

for an embedded device I need to sign a 14-byte string and want to verify that string on the device. Since there is already an AES-Library on the device, I thought about using the following scheme: ...
539 views

### Entropy of system data - use all and hash, or trim least significant bits?

I'm working on a background entropy collector for key generation that monitors hardware and produces an entropy pool. Here's my list of sources: Mouse position Keyboard timings (i.e. time between ...
498 views

### Encryption with private key?

we normally always encrypt by public key and decrypt with private key. If i encrypt with private key, then its still secure as normal PKI ? i mean known-plain-text will not take private key on the ...
323 views

### Vulnerabilities of encrypting data with known regularities

I need to encrypt data that has a pre-defined header and footer structure, and furthermore the data follows pre-defined patterns. The header and footer structure follow a defined structure but cannot ...
2k views

### Deciphering a key from XOR encrypted cypher using boolean logic

Assume there's an unencrypted message A, and an encrypted message B. You know that message B was encrypted using a simple XOR method of A with a private key K, resulting in message B. Thus, B = A ⊕ K ...