# All Questions

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### SHA-256 Wrong registers

I have a problem with my sha256 java implementation. The registers (a…h) are correct until j=24; ...
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### Is Dropbox's hashing method cryptographically secure?

Dropbox uses CRC32 and md5 to create a checksum of each 4 mb "block" as part of the their compression/file system. I suspect that Dropbox uses an additional cryptographic hash to check if there's a ...
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### Prove that two MACs with incremendal PRF application are not secure

How do I prove the following MACs are insecure?                                                        1                                                                                   2
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### Diffie-Hellman explicit key confirmation

Suppose I wanted to add explicit key confirmation to Diffie-Hellman key exchange, would the following scheme be secure? Alice selects a random $a$ and sends $g^a \mod p$ to Bob Bob selects a random ...
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### Proof for exponentiation in modular arithemtic

If $e$ is a natural number, then this is true: $$m^e \bmod\ n = (m\bmod\ n)^e\bmod\ n$$ This is often used when encrypting, especially with RSA, since one can avoid directly calculating $m^e$, ...
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### Is knowing the distribution of input domain only way to do cipher text only attacks?

Imagine there is a $n$ bit block cipher. The attacker has harvested some how all possible $2^n$ cipher texts for some given key. Without the knowledge of the distribution of input domain as in ...
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### Z*p Generator (Cyclic Group) in java

I am trying to implement ElGamal in java without using the in-built libraries. The problem occurs when I am trying to find the generators of the number(in this case 11). Everything works perfectly ...
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### Why do we use 1024 / 160 bit primes in DSA?

I am looking at DSA's parameter generation and don't understand why for $p$ a 1024 bit prime is needed if $q$ is chosen as a $160$ bit prime. I thought that the security of DSA relates on the discrete ...
718 views

### Messages exchanged between Bob Alice are encrypted safe?

Can you help me understand the following reasoning? If Alice sends Bob a message and that message is encrypted with two keys simultaneously: a symmetric key (Ks) and Bob's public key. The symmetric ...
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### Is this the mathematical equation for the Vigenere Cipher

Is this the mathematical equation for the Vigenere Cipher or something else and if something else what is it? Where t is the letter of plain text and n is the position of t within the text and c is ...
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### Why does plain RSA not work with big messages ($m>n$)?

From what I can remember, RSA is something like this: Generate 2 distinct prime numbers $p$ and $q$ that have similar bit length. Compute $n=pq$ and $\phi(n)=(p-1)(q-1)$ Compute $e$ such that ...
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### Encoding multiple fields with same AES key and vector?

I'd like some advice on cryptography best practices. I need to store on my web app the users' credentials (login & password) for 3rd-party sites. I'm using AES-256 with random key and ...
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### Applying differential cryptanalysis to ciphers with addition mod $2^{32}$

I am studying differential analysis and have a question. Consider the following simple cipher: $$c_1 = S(m_1 \oplus k_1) \oplus k_2$$ (Plaintext $m_1$ xor with key $K_1$, then result goes into an ...
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### Why routers don't just use Diffie-Hellman protocol?

Routers could just use Diffie-Hellman protocol with any computer who tries to connect to that router. Then it would be impossible to eavesdrop for anyone. Only router and computer would know the ...
152 views

### Public Key encryption /Assymetric encryption

For a start, I 've got a question regarding the Public key encryption. I know the public key encryption overcomes the Key distribution issue with the symmetric encryption and that in the case of ...
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### Testing if a password for an eCryptfs mount is valid [closed]

I'm using "eCryptfs" on several directories on my dedicated Debian server. I mount them manually via mount -t ecryptfs [dir] [mountpoint] But after some weeks ...
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### Is there any restriction in using lossless compression algorithms for huge key sizes?

I'm studying McEliece and Multivariate Public Key cryptographic systems. The main problem here is the huge key size. Is there any restriction in using lossless compression algorithms to fix this ...
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### Definition of cryptographic advantage vs. probability of success

In game-based security definitions, like for example the one defining IND-CPA security, a given cryptosystem is said to be secure if any probabilistic polynomial time adversary has only a negligible ...
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### What does an RSA signature look like?

I am using OpenSSL libs to generate signatures. Internally, I learn that a signature is a hash of the message with some padding added to it. I am trying to understand the structure of a signature. If ...
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### what is an injective trapdoor function?

The answer to this question at Crypto.SE is clear about what a trapdoor function is, but what makes a trapdoor function "injective"?
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### Question about Fermat's little theorem

Why is $g^e \mod p = g^{e \mod (p-1)} \mod p$ if p is prime. I don't get it. It follows from Fermat's little theorem.
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### Is javascript RSA signing safe?

I'm currently working on a secure open source messaging system (https://github.com/DSMNET/DSMNET). Currently I'm using the cryptico.js (https://github.com/wwwtyro/cryptico) library to encrypt all the ...
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### Is it possible to translate a piece of language into your own without knowing the language? [closed]

I understand the the texts within the Voynich Manuscript are of an unknown language, and yet cryptographers are still working on it and still trying to decipher it. Does that mean that it is possible ...
468 views

### RSA smaller number work-out-by-hand not working - I think I made a mistake

I tried out the paper/pencil explanation @ http://sergematovic.tripod.com/rsa1.html, and it seemed to make sense just fine until I came to decryption. Here is what I worked out: Key Creation: Choose ...
385 views

### How terribly flawed is this design for key storage?

Dave here, with a plan for storing secrets. Threat model: Database and keyfile may (will?) be stored on un-trusted servers. Database contains many valuable secrets (passwords), encrypted by key ...
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### Using IV buffer after altered inside a Rijndael CBC Encryption/Decryption process as IV for next message?

When sending a block to be decrypted or encrypted, with RijndaelCBC, we input the data to decrypt/encrypt and an IV for syncing and to prevent identical outputs for identical inputs. This question is ...
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### Lagrange Interpolation for finite field GF(2^8), for Secret Reconstruction

I'm using Lagrange's Interpolation technique to reconstruct the secret from a set of point pairs (x,y). Since I only need the secret, not the whole polynomial, I have simplified the reconstruction ...
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### Quadratic residue problem on composite integers

Its believed that the quadratic residue modulo $n=p·q$ for large primes $p$ and $q$ is intractable, which forms the basis of some cryptosystems. However, it is solvable if the factors of $n$ are ...
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### Three-way key exchange with elliptic curves without pairing

Assume that there are three users, each with their own secret key $d_i$ and the corresponding public key $Q_i = d_i \cdot P$, such that $Q_i$ is a point on an elliptic curve and $P$ is a base point on ...
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### Is the salt value a secret or can it be stored in cleartext just like the IV? [duplicate]

From Wikipedia I read that the IV is used to randomise the mapping of cleartext to cipher text and that the IV is usually stored (in cleartext) together with the encrypted record. ...
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### Brute force attack on RSA - success criteria?

In a hypothetical discussion we ended up with a concrete question I'd like to find the answer to: If you try to brute force an RSA encryption, but the original vale which was encrypted was just ...
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### Why is it a security problem that two RSA moduli are not co-prime?

I found this security problem, for example: Alice has an RSA public key: $e = 37$ and $\phi$ (totient, $N_A = (pa-1)(qa-1)$) is $55$, so the key is $(37, 55)$. Bob has an RSA public key: $e = 37$ ...
526 views

### why public key cryptography (RSA) is use in sites like Google, facebook (what is the need to ecode data in these sites)?

The RSA algorithm is used for encryption (plain text to cipher text) and decryption (cipher text to plain text) of data. What is the need for using public keys in these sites; why is data is encrypted ...
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### Merkle Signature Generation

In the section 2 (page 42) of the book "Post Quantum Cryptography", it says: Then he generates the one-time signature $\sigma_{\text{OTS}}$ of the digest using the $s$-th one-time signature key ...
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### Digital Signature Scheme Count

My question is How I will be able to count digital signature Scheme that can sign many documents with one private key. There are signature scheme as long as exists private keys in the keys spaces? or ...
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### Help with example RSA problem

While learning about RSA, I found this example problem. The answer is supposed to be "a 4-digit number that is a pattern of digits." I have computed it to be 16657 twice. OK, now to see if you ...
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### Amateur question: two-way RSA?

Is this a sensible scheme for 2-way client/server communication? A client connects to a server. The server and client both generate RSA keys, and send over the public ones to each other. If the client ...
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### Acceptable assumptions when proving security

Considering the output of a cryptographic primitive, like an encryption scheme (CBC, ...), a hash function or even the output of any schemes based on number theoretic assumptions, is it reasonable ...
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### Why is the following RSA PRNG cryptographically secure?

One requirement states that the generator has to withstand the next-bit test. Consider the following PRNG, where we calculate next output $x_i$ via the formula $x_i = x_{i-1}^ e\mod n$. I can see ...
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### Indistinguishability attack example

I want solve the next exercise. The author defined the experiment for the cryptosystem $\Pi$, the adversary $A$ and the security parameter $n$ as follows $\mathsf{PRIV_{EAV}}(\Pi,A,n)$ The ...
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### Is it worth applying a MAC on data in a HSM?

I'm building a basic HSM out of an Arduino, and am using the following scheme to store data: Master symmetric key $k_m$ stored in firmware (secure bit set to prevent trivial extraction). Secondary ...
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### Efficient algorithm for remainder calculation over prime field for ECC implementation?

I am working on 224-bit elliptic curve cryptography. In this 224-bit * 224-bit multiplication results 448-bit output. I am reducing 448-bit into prime field range( prime number $2^{224}-2^{96}+1$) ...
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### Is it safe for the ChaCha8 nonce to be deterministic?

ChaCha8 takes a 8 byte nonce (or IV) that should not be the same twice for the same key. Generating this nonce randomly makes me very very nervous for collisions. Is it safe to generate this nonce ...
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### Why are RSA keys encoded with ASN.1 for TLS?

Browser vendors use ASN.1 encoding for RSA certificates in the TLS protocol. RSA public keys are just a number, so why do we need encode them to something else? That increases the risk of security ...