# All Questions

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### TRNG vs PRNG - Entropy?

How much more entropy gives a Hardware Random Number Generator or True Random Number Generator compared to a Pseudo Random Number Generator? I know it depends on which TRNG is used, but I'm thinking ...
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Suppose I have a message $M$ for which I generate an RSA-2048 digital signature as follows: $H = H(M)$, $H(M)$ being the SHA-256 of the message $M$ $S = H^d \bmod N$ Assume $N = pq$ is properly ...
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### Salting in asp.net?

I have spent the past hour reading up on salting and still don't understand how it is achieved. Forgive me if im wrong, but the way I am thinking of salting is, storing an ArrayList of random strings ...
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### What is degree of randomness in individual bits of MD5 hash?

I am researching on Hashing Algorithms and would like to know if we take individual bits of MD5, say we take first 8 bits or any random 8 bits of MD5 then what is the randomness probability of having ...
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### Are there transitive ciphers (either symmetric and asymmetric)

Say Alice has information that's already encrypted with Bob's key. She wants Bob to decrypt it without knowing what he just decrypted. So she encrypts it with her key and sends it to him, he decrypts ...
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### Does AES(x || x) provide secure message authentication when message fits in a single block?

Suppose I have x = <64 bits of data>. I build a 128 bit block $P = x || x$, and transmit a message $M = AES(K, P)$. The receiver has the same $K$. The ...
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### Is there any good reason to chain HMACs in this manner?

I am looking at some decompiled Java code from an Android app. As a security measure, a signature is passed as a parameter to a number of JSON requests. The method that generates the signature is as ...
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### How far is public-key crypto involved in “banking world”? [closed]

I would like to know how public key cryptography is used in the banking world. I am not looking for big details, but for example: Are banking card embedding a private key used for signing ...
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### Encrypt with K1, K2 and decrypt in opposite with K2, K1 [duplicate]

When using symmetric cipher, does the following statement holds: E(x, k) means encrypting x with key k in ECB mode ...
153 views

### One cipher to rule them all?

I learned from Dan Boneh's course that many cryptographic primitives (prngs, stream ciphers, hashes, hmac, key derivation functions) can be built from just one a block cipher or PRF. For example, the ...
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### RSA-PSS salt size in certificate

In RFC 4055, the signature RSAPSS algorithm has the following ASN 1 parameters: ...
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### one complex password as a “seed” for localy generated per-service passwords

I've stumbled upon a project that suggests a way of having to remember one password, from which a “per-service password” could be generated by salting the service name using scrypt. What could be a ...
217 views

### ElGamal against chosen ciphertext attacks

In Hoffstein, Pipher, and Silverman's book An Introduction to Mathematical Cryptography, the authors make the following remark: An attack in which Eve has access to an oracle that decrypts ...
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### Why we need ECDSA when we have ECDH?

ECDSA and ECDH give us the following methods: ...
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### How to crack SHA256 only letters

I recently created a program that hashes a password using SHA256 and potentially using salt. How can I crack that if I know that the password only contains lowercase letters?
132 views

### Are all stream-ciphers IND-CPA?

A stream cipher usually generates a stream of pseudo-random bits which get XORed with the plaintext to form the ciphertext. The stream is generated using a given IV / nonce and a secret key. CTR-mode ...
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### Reversing Rotation + XOR

I have this cypher which is as follows : Take 2 numbers : A=1011 and B=1010 if the ith bit of A is 1 then shift B i times to the left. So in the end you will get ...
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### Diffe-Helman Exchange result is always 1

I watched a video on Khan Academy explaining the Diffe-Hellman exchange. When I try to do an example problem, I get 1 all the time. Does the generator and prime modulus (or base on Wikipedia) have to ...
155 views

### Why have round constants in hashes?

This may seem like basic crypto knowledge, but why do hash functions like SHA-2 and Whirlpool have round constants that are absorbed into their respective states? I can understand that in a cipher ...
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### What is the difference between a PRF and a PRF+?

What is the difference between a PRF and a PRF+? I am understanding them to mean the same thing, but I imagine that is because I don't understand their differences. The IKE RFC defines a PRF as: ...
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### different feistel words in key size

I have confusion on Feistel word in key size. Means, "what is the meaning of key size with different feistel word ?" What does m=2,3,4 represent? Where m represent different size of feistel word or ...