# All Questions

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I'm working on implementing a cryptographic system and I'm trying to understand the Zero Knowledge Password Proof concept. So here's some background: To generate a secret key I am: Doing an ECDH ...
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### Brute forcing CRC-32

I'm working on a cryptosystem which uses IDEA. The designer made the mistake of including a CRC-32B hash of the password unencrypted in the header, so that the system can quickly reject bad ...
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### How can two different passphrases unlock the same content?

I have heard that in enterprises it is common that the IT admin has a master passphrase that can unlock any content that an employee encrypts. It would be temping to think, that the IT admin's ...
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### How to calculate inverse affine transform in AES from forward affine transform?

In AES the forward affine tranformation matrix is defined as:  \begin{bmatrix} 1& 0& 0& 0& 1& 1& 1& 1 \\ 1& 1& 0& 0& 0& 1& 1& 1 \\ 1& ...
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### Should HMAC or NMAC or plain Keccak be used for a secure MAC?

After some digging around in the HMAC spec I found this (paraphrased): Step 1: If the length of Key equals the block size of the hash function (512 bits/64 Bytes for SHA-256), set the key equal ...
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### Difference between lightweight, online and low memory Authenticated Encryption schemes

What is the basic difference between the following types of Authenticated Encryption (AE) schemes? Lightweight Authenticated encryption Online Authenticated Encryption Authenticated Encryption for ...
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### How can k3d3/ed25519-java's performance be improved?

I tried asking this on stackoverflow because I thought it might be a simple coding question and didn't want to clutter up this amazing stack, but I received no responses. CodesInChaos showed me that ...
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### In RSA, why is it important choosing e so that it is coprime to φ(n)?

When choosing the public exponent e, it is stressed that $e$ must be coprime to $\phi(n)$, i.e. $\gcd(\phi(n), e) = 1$. I know that a common choice is to have $e = 3$ (which requires a good padding ...
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### Elliptic Curve Encryption Ciphertext Size

I'd like to know how much bigger is the ciphertext when encrypting a message using ECC encrytpion? ECIES (or ElGamal)
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### Is there a cryptographic hash function that can be performed with pencil and paper?

Imagine I'm signing up for the 99th new web site this month. I somehow take my secret key (which I have written down on a card in my wallet) and the domain name of the site and feed them both into ...
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### Why is the discrete log problem easy when the exponent comes from a binomial distribution?

I read in http://epubs.surrey.ac.uk/7219/2/esorics06.pdf that in exponential El Gamal the discrete log problem for recovering $m$ from $g^m$ can be made tractable when $m$ is drawn from a binomial ...
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### Recommended way of adding a pepper/secret key to password before hashing?

There have been several questions regarding password hashing here and on Security.SE. A "pepper" is sometimes mentioned – an application-specific secret key. The canonical answer on password hashing ...
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### 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 ...
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### Are hash trees an alternative, quantum-resistant signature scheme which can replace RSA?

Can hash trees provide quantum resistant signatures to replace RSA for signing securely? What is the key size and how many times can we use same key?
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### iterated discrete log problem

Consider the following problem: given $g_1 \ldots g_i,h_1 \ldots h_i \in G$, $\forall i$ find $x_i$ such that $g_i^{x_i}=h_i$ For $i=1$ this is the discrete log problem and is assumed to to have ...
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### Cryptanalysis of Linear Feedback Shift Registers

It is well known that simple m-sequence linear feedback shift registers have a linear algebraic structure and therefore the generator seed can easily be deduced using the Berlekamp-Massey algorithm. ...
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### FIPS 140-2 Compliant Algorithms

Is there any reference to check the list of encryption & signing algorithms which are compliant to FIPS 140-2. After an exhaustive search I could find only "AES". Any suggestions would be much ...
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### Is it possible to pick your Ed25519 public key?

Is it possible to generate an Ed25519 keypair that has a very similar public key as another keypair (fooling a casual visual comparison) or is this as hard as solving one of SHA-512 or the discrete ...
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### What is the name of this kind of protocol

There is a communication protocol that I believe creates the equivalent of a one time pad, with the downside that the secret message must be transferred multiple times. The protocol is so simple that ...
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### Difficulty of breaking RSA for a given key size

Is it true that breaking a 1024-bit RSA key is as difficult as breaking a 128 bit symmetric key (e.g. AES)? I know that breaking a RSA key is equivalent to factoring the modulus $N$. To factor it, you ...
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### How to perform file encryption using 128-Bit AES?

I am confused, how can I encrypt a file using 128 Bit Advanced Encryption Standard? Do I need only to encrypt the file name and it's content or is there something that I need to do to encrypt it? Is ...
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### are CFB and OFB really meant for streaming?

CFB, OFB and other modes are meant for streaming and don't require padding. Are there still limitations such as the text needs to be greater than key length?
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### How can I break REDSHIRT / REDSHIRT2 encryption?

Recently, a user on Gaming.SE asked a question about whether the user password in the video game Uplink could be modified after being initially set. The game does not contain an option to change the ...
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### How is de-synchronisation of HOTP solved?

From RFC 4226 I understand how HOTP generates one-time passwords by incrementing a counter and uses the 'look-ahead' window to try to resynchronise (from this counter), if the user tries a few wrong ...
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### Is there an oblivious decryption scheme?

Alice has $K$; Bob has $E(K, m)$; Is there such a scheme that enables Alice decrypts $E(K, m)$ without knowing $m$, and Bob gets $m$ ?
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### Message lengths with AES CTR mode?

Im working on my own AES implementation as a personal educational exercise (not for production use), and a book im using as a resource states: In the case of CTR, we are merely XORing the output ...
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### SSL-like protocol with public-key hard-coded in the client

Imagine a SSL-like protocol that instead of using certificates signed by a trusted CA, has the server's public key hard-coded in the client. My question is: what happens if the server's private key is ...
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### HMAC-SHA1 vs HMAC-SHA256

I have three questions: Would you use HMAC-SHA1 or HMAC-SHA256 for message authentication? How much HMAC-SHA256 is slower than HMAC-SHA1? Are the security improvements of SHA256 (over SHA1) enough ...
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### Non-standard signature security definition conforming ed25519 malleability

According to the paper “High-speed high-security signatures”… Malleability. We also see no relevance of "malleability" to the standard definition of signature security. Aside from the example, ...
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### How do I test my encryption? (absolute amateur)

I am a hobby programmer with a background in biology and have developed an encryption program based on DNA. I tried to make it hard to crack, but it's essentially a substitution cipher and uses the ...
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### secret sharing of product of 2 numbers

Does evaluating the products of shares $a_jb_j$ give a secret sharing of $ab$? I think it's a no, but I am not sure on how to prove it.
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### Is OTP useful in modern electronic communication?

I have seen from time to time questions about various OTP schemes and "OTP" (i.e. stream cipher) schemes. The most common OTP troubles appear to be: How to extend size of previously generated key or ...
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### 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 ...
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### Break double encryption

Let $E_k$ : {0,1}$^l$ be a block cipher encryption function with block-size $l$ and key-length $n$. In class, we saw that a double encryption with two independent keys $E{}'_{k_1k_2}(x)$ = ...
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### How can I use eulers totient and the chinese remainder theorem for modular exponentiation?

I'm trying to implement modular exponentiation in Java using Lagrange and the Chinese remainder theorem. The example we've been given is: Let $N = 55 = 5 · 11$ and suppose we want to compute ...
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### One-time pad and zero key

I'm doing some exercises before my exam, and I am stuck with task number 4 in this file: http://www.cs.umd.edu/~jkatz/crypto/f10/hw1.pdf Could you help me with this task? When using the one-time ...
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### Malleability attacks against encryption without authentication

Suppose there is a message that is encrypted with AES-128-CBC. The message is as follows, new lines are used to delimit the 16 byte boundary for each block: ...
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### 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 ...
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### Is the Blum Blum Shub PRNG suitable to create initialization vectors?

Is it a good idea to use a Blum Blum Shub Generator to create my initialization Vector (IV) in AES-OFB (Output Feedback Mode)?
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Sorry if this is a noob questions, but I finally figured out how to implement the Tiny Encryption Algorithm in C++. My question is what to do about padding the key and the plaintext? I know that ...
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### DGK Cryptosystem Encryption Speedup

Following @poncho's nice clarification of the RSA speedup here, let's see if I'm able to do the same in the case of the DGK cryptosystem: We have pk = (n, g, h, u), sk = (p, q, $v_p$, $v_q$) which ...
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### Cipher Feedback Mode

I can't understand what CFB really is. It said in Wikipedia that CFB is same as CBC, but I find that CFB is more difficult than CBC. Can someone explain to me how CFB works. Such as how ...
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### RSA finding the inverse of the public exponent

I have a very basic doubt in RSA key generation and its usage. In RSA key generation you choose two large prime numbers of a very large order. Then you multiply them.(eq $p \cdot q = N$) Now, ...
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My question is related to PBKDF2. I am unsure how to validate user entered password with already existing hashes and salts. I have heard of the so called "length-constant" comparison which basically ...
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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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### One-Time Pad Distribution of Keys Between Two Parties

Consider the One-Time Pad (OTP). Suppose two parties, A and B, generate a completely random secret key in a real-life meeting, and they keep this secret. Now A wants to send some message to B using ...
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### Derive a key from public data using a secret master key

Users in my system will be given a device that contains a 128-bit AES user key. The user key will be derived from 64 bits of public data related to the user, which is unique but very predictable (i.e. ...
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### definition and meaning of semantic security

I'm taking coursera cryptography course. The definition of semantic security is hard to understand. I tried to slightly restate it (the word "efficient" was in the original definition). Do I get it ...
### What does $(\mathbb{Z}_n^*)^2$ mean?
In a paper they write once, $(\mathbb{Z}_n^*)^2$. Is this the group of quadratic residues or is it something else? Here the theorem: Under the strong RSA assumption, given a modulus $n$, along with ...