# All Questions

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### Hash “Preimage by product” resistance

Let H() be a hash function that achieves collision resistance as well as first and second preimage resistance. Let's equip the output set of H of a multiplicative group structure, more precisely a ...
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### What is the period of a cipher?

For the following explanation of RC4: It is a variable key-size stream cipher with byte-oriented operations. The algorithm is based on the use of a random permutation. Analysis shows that the ...
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### 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?
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### What is the polynomial to use in the Massey-Omura cryptosystem?

The Massey-Omura cryptosystem uses "multiplication over the finite field $GF(2^n)$. I'm just starting understand the idea of multiplying polynomials and I've searched for online calculators to use for ...
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### What are the computational benefits of primes close to the power of 2?

Recently I was reading some article about the Bernstein's Curve25519. This is a particular Montgomery curve over $\mathbb{F}_q$ where $q = {2^{255}-19}$. What I missed or was unable to understand is ...
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### How many attempts does it take to crack a 32-bit password hash with this scenario?

How many attempts does it take to crack (match) a 32-bit password hash from a database of 4 million password hashes? Correct me if I'm wrong, but to crack a 32-bit password hash would take roughly ...
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### Crypto hash function vs encryption algorithms

Why is a cryptographic hash function generally faster to execute in software than conventional encryption algorithms such as DES?
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### Can I jettison MAC if I already have SHA1(M)?

I'm currently using SSL with AES-CBC and HMAC for a file transfer containing string M. Now suppose Alice already knows SHA1(M) (and the adversary does not), and she downloads M from Bob using only ...
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### Idea for secure password hash-likes, is this feasible/usable/workable?

(Hand moved from stackoverflow.com,due to suggestion) Since breaking password hashes has become a new passtime for scriptkiddies, I thought of the problem and came up with a novel(?) idea. store ...
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### Good enough deterministic PRNG based on hashes

Lets say I have a seed named iv which has a decent amount of entropy. Are there any glaring issues with the following? First 64 bytes are generated by SHA512(iv) Next 64 bytes are generated by ...
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### Do all stream ciphers (and AES-128-CTR in particular) produce equal output size as the input size?

I'm particulary interested on knowing if the output of the AES-128-CTR ciphertext is always going to have the same size as the input plaintext, or it could be padded somehow. It seems so from my tests ...
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### Encryption - How to find $\sigma$?

Alice communicates with Bill using an encryption system of type Feistel. Specifically, each block has a length of 128 bits, the algorithm has 4 rounds and the common secret key of Alice and Bill is ...
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### Symmetric mutual authentication with client using a derived secret

I'm attempting to find a client/server authentication protocol that allows the client and server to authenticate each other when the client doesn't know the server secret but does have a sensitive key ...
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### Key Length vs KeySpace

I am a newbie here, this is my first post. I have searched the forum but I haven't found a clear answer to this. In general. What is the relationship between key length and the keyspace? (if there ...
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### Is cryptanalysis of CTB-Locker really impossible?

It seems that CTB-Locker make a lot of victims nowadays, and yet, the full encryption scheme of it is now publicly known [1,2]. Would any of you could find a weakness to exploit in this encryption ...
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### IV's for different AES encryption modes

I wanted to know if my understanding is correct! I have been reading up on aes encryption and modes(specifically CTR and CBC) and it took me sometime to understand the concept of IV. If the IV is ...
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### Decoding eAccelerator PHP code? [migrated]

Well I'm unsure if this is the correct site to use but how would I go about decoding/decrypting PHP code which is encoded/encrypted using eAccelerator? ...
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### Why is Poly1305 popular given its 'sudden death' properties?

ECDSA has the undesirable property that if a key pair reuses a nonce in a signing operation, the cryptosystem experiences catastrophic failure in the form of private key leakage. I've heard this ...
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### Decryption of message in IBE without random oracle using bilinear pairig reg

I find the following IBE scheme from the videos posted and i don't understand the decryption algorithm, will any one please elaborate the 6th step scheme S(λ) : (G, GT , g ,P) <-- ...
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### Using hashed password as shared secrets. Good or bad idea?

I built a system that allowed users to generate an unlimited number of email addresses. Instead of storing each one in a db, I created addresses dynamically and made the address an hmac signed ...
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### Anonymous Gravatar Problem

Gravatar is a service that serves avatars for users based on md5(email_address). The hash is embedded on websites, so it's public. One of the problems with this ...
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### What is entropy?

We discuss a lot of topics and use measures of entropy to determine how difficult it is for an attacker to be successful. What does entropy mean in the context of cryptography? How is entropy ...
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### Crack RSA with additional information [duplicate]

Suppose $N=3953$, Alice encryption key is $e_A = 337$ and bobs is $e_B=23$. Eve happens to know the corresponding decryption keys, $d_A=3385$ and $d_B=2663$. Can I use this information to find the ...
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### ECIES with AES-GCM

The Elliptic Curve Integrated Encryption Scheme (ECIES) describes how to derive a "temporal" key, encrypt a message (eg. using AES) and creating a MAC. However, from my understanding using AES in GCM ...
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### What prime lengths are used for RSA?

Let's assume we have an RSA 2048 that was produced by two primes multiplication. What is the digit's difference in those 2 factors? For example one prime is 1024 digits and second one 1024 or, one is ...
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### How to store and use a Cryptographic Seed in a database [closed]

Companies such as Barclays use cryptographic seeds (for their PINsentry system), RSA do the same for RSA SecurID, but how do they safely secure the seeds in the database. With passwords you just hash ...
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### In a group, is it hard to calculate the base $g$ given $g^a$ and $a$?

Discrete logarithm, that is: calculate $a$ given $g$ and $g^a$, is assumed to be a hard problem in some groups. Is it also hard to calculate $g$ given $g^a$ and $a$?
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### 1 Billion Bit Encryption?

So, browsing through YouTube, I stumbled on this video interview of John Draper (Captain Crunch), one of the first "hackers". He talks for about 3 minutes (until 27:48) about his home rolled ...
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### Equivalent key size between HMAC and AES?

I have seem comparison between AES and RSA, e.g., AES 128 bit key is equivalent to RSA modulus 3072 bits. Is there such comparison between AES key size and HMAC?
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### Is it necessary to encrypt nonce in cryptographic communication?

Server and client have a pre-shared key K. Assuming that that pre-shared key is safe, so nobody other than server/client can encrypt/decrypt. Client sends a nonce N to server, server then sends back ...
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### ECC considered secure in OpenSSL?

this is my first question, please bear with me if it comes across silly. If I openssl ecparam -list_curves on my OpenSSL version (1.0.1f), it spits out the ...
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### Pseudorandom number generators based on hard problems in mathematics

I know that Dual_EC_DRBG's security is based on the ellitpic curve discrete logarithm problem, Micali-Schnorr is based on the difficulty of factorization, and MQ_DRBG is based on multivariate ...
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### Questions about updating the secret key of RSA?

I want to update the secret key of RSA. I understand there exists a common modular attack on RSA. Precisely, if adversary knows $(e_1, m^{e_1})$ and $(e_2, m^{e_2})$, where $e_1$ and $e_2$ are the ...