# All Questions

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### Serpent cipher : Osvik S-Boxes confusion and test vectors

I'm having hard time with the implementation of the S-Boxes by Osvik found in this paper: Speeding up Serpent. At the end of the paper, all the s-boxes are given and then, I just implement them. ...
266 views

### How difficult is it to brute force d in RSA: d = (1/e) mod φ in a CPT attack?

Given that RSA key generation works by computing: n = pq φ = (p-1)(q-1) d = (1/e) mod φ If I was an attacker who wanted to brute force d, could I brute force d given just the public key, the ...
123 views

### Random session key + predictable IV

I'm using Blowfish in a toy Diffie-Hellman communications scheme. Random session keys are generated for each connection. In this case I can simply feed a null array to the IV right? The same ...
282 views

### Making counter (CTR) mode robust against application state loss

Counter (CTR) mode, which is a block cipher mode of operation, has some desirable qualities (no padding, parallel encryption and decryption), but at the cost of failing badly when non-unique counter ...
131 views

### What does it mean to be simultaneously hardcore?

In this paper the term "simultaneously hardcore" is defined as: "We say that a block of bits of $x$ are simultaneously hard-core for a one-way function $f(x)$, if given $f(x)$ they cannot be ...
165 views

### Salsa20 in Davies-Meyer mode with a fixed key (message): is it one-way?

Say I have a function that calculates a pseudo random permutation and this function is easy to invert. For example $P(i) = AES_k(i)$ where $k$ is a publicly known key. So anyone can compute $P(i)$ ...
243 views

### Proof of elliptic curve difficulty

Are there any proofs that cryptographic functions on an elliptic curve are any more difficult than the analogues over modulo arithmetic? While at present, ECC appears to be more difficult, as it is ...
138 views

### Feedback requested on a method of posting a message without revealing the author

So I was thinking about variations on the Dining Cryptographers problem - In some cases, it's useful to be able to post a message without revealing the source, but with the additional constraint of ...
73 views

### ASN.1 OID of bcrypt

What is the ASN.1 OID associated to bcrypt (the key derivation scheme)? For instance, PBKDF2 has 1.2.840.113549.1.5.12 and it is therefore possible to store a ...
130 views

### salting with password hash to improve security?

Would something like the following improve security (against rainbow attacks, not brute force)? Assume that $P$ is a user-chosen password, and the objective is to obtain a hash $H$ for password ...
135 views

### Finite fields in elliptic curve

I have an elliptic curve defined over finite field where $S_1=aP$ . Is it valid to say that $S_1P$ can also be computed. $P$ is the generator of the group. What my real question is that. Should '$a$' ...
167 views

### How to Compute C^2 in AES MixColumns Matrix?

In mix Columns we have: $$C(x) = \{03\}X^3 + \{01\}X^2 + \{01\}X^1 + \{02\}$$ In Viktor Fischer's Paper on MixColumn and ...
94 views

### Does there exist a two-pass AKE protocol that is secure in eCK model and also has PFS?

As we can know that the best two-pass AKE protocols with DH message can achieve is the weak form of perfect forward security (wPFS) which guarantees security against the passive adversary. But ...
123 views

### Are there any good examples of Contemporary Mechanical Cryptography?

Are there secure mechanical cryptosystems in use today? Not necessarily alphabetic either, ie digital but mechanical?
117 views

### How to compute the attacker's probability?

Given a random bit string $R=r_1r_2\cdots r_n$, let us encrypt each bit $c_i=E(r_i,k_i)$ where $k_i$ is taken from the stream of encryption keys. Suppose that an attacker can guess the correct $r_i$ ...
233 views

### How does partial key escrow work?

It is being speculated that one of the ways that NSA leaker Edward Snowden may have created his "insurance policy"--distributing sensitive documents to various individuals with instructions to ...
170 views

For a stream of packets, where each packet is individually encrypted with a block cipher, it's desirable to have each encrypted packet only valid for that position in the stream. A message number ...
3k views

### OpenSSL AES 256-bit Key Management

I am using C and OpenSSL to encrypt files. After experimenting with the OpenSSL command line utility, it makes you enter a passphrase that can be any length, but uses that to create a 256-bit key. ...
273 views

### Given a private RSA key, how do we get the public key?

Is it possible to pre-choose a private RSA key, then obtain a public key from it?
335 views

### Can you identify the public key used to encrypt something?

If I encrypt a string with a public key, does the encrypted ciphertext reveal the public key I used to encrypt it? Basically, I don't want anyone to know who the ciphertext is addressed to. I'm ...
238 views

### Can i modify data “protected” by a CRC16?

There are 100 bytes with a CRC16. However I only know the first 50. I want to change byte 5 from a known value X to another value Y, and fix up the CRC16 to be valid - without knowing bytes 50-100. ...
259 views

### Difference entity authentication and implicit key authentication

From the Handbook of Applied Cryptography, in discussions of key sharing algorithms, I see definitions: Implicit key authentication is when one party is assured that no other aside from a ...
447 views

### Hill-cipher, disordered alphabet

I am going to apply a simple substitution cipher to my input, then encrypt the result with a Hill cipher. How can this be broken, in a chosen-plaintext threat model? In other words, instead of the ...
778 views

### Winzip AES256 vs PGP

If I use the AES256 option in Winzip to encrypt a file, is it any less safe or less secure than using pgp encryption?
438 views

### 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 ...
257 views

### Counter Mode: static IV but different keys [duplicate]

Given we are using AES counter mode, suppose we randomly generate several keys, all of them are using same IV (say, zeros). Does this lead to any security issue? I know that in CTR mode, same key-iv ...
280 views

### 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 ...
147 views

### GPG and PAR2 error correction data from the plain archive, will it compromise security?

I have the following scenario: Archives compressed with 7z, hundreds of MiB in size GPG to encrypt the archives (binary, without ASCII armor) PAR2 to create error correction data Question 1. ...
517 views

### Why are the Davies-Meyer and Miyaguchi-Preneel constructions secure?

The Davies-Meyer compression function $h(H, m) = E_m(H) \oplus H$ is said to be secure. So too is the Miyaguchi-Preneel compression function $h(H, m) = E_m(H) \oplus m \oplus H$. Why are these ...
258 views

### Can I combine two PRNGs to make use of more seed data?

I have 320 bits of seed data (actually 512 bits of data with 320 bits of entropy, derived from a Diffie-Hellman shared secret and nonces). The PRNG I am using at the moment is the android version of ...
424 views

### Implementing PKCS#7 Padding on a Stream of Unknown Length?

I have a fairly simple Python program using PyCrypto to use AES+CBC to encrypt a stream of input. In order to adhere to the 16-byte input size multiple, I've implemented PKCS#7 by hand. (While I know ...
311 views

### Implementations of Ntru TLS

I posted this in a reply to What is the post-quantum cryptography alternative to Diffie-Hellman? but since it's actually another question, it was deleted. Has anyone come across any implementations ...
463 views

### RSA private key format for Mega

I've been trying to reverse engineer Mega's (mega.co.nz) API calls. And stopped on the step where client needs to decrypt session id with provided RSA key. I can get key data, but I have no idea how ...
438 views

### estimate of time required to crack sha512crypt password with JtR + OpenCL

OK, I have a shadow file with a password that I know, it is 4 letters followed by two numbers. Using John The Ripper with OpenCL support, on a laptop with AMD Radeon Mobility graphics, how long would ...
237 views

### 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?
739 views

### Ideal passphrase length: old diceware method (5 words) vs. your Bitcoin wallet.dat passphrase lenght (8 words) and doubling passwords?

I made a cool 5 word passphrase back then using the old Diceware method and use it as a master password. The question is as computing power increases will we need to add more and more words to our ...
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### Advantage of AES(Rijndael) over Twofish and Serpent

I'm trying to figure out a suitable encryption technique and after reading a bit, I figured the current AES 128-bit encryption is suitable for what I'm trying to do. However, this is more due to the ...
771 views

### Explanation of the Decision Diffie Hellman (DDH) problem.

I'm extremely new to crypto, and very much inexperienced. Lately I've been reading about the Diffie-Hellman key-exchange methods, and specifically about the computational diffie-hellman assumption vs. ...
195 views

### Seed a PRNG with random data and a password

I'd like to combine a random key file with a password to generate a secure seed for a CSPRNG. The key file is assumed to have very high entropy, but the password will be whatever the user provides. ...
680 views

### Figuring out key in hill cipher (chosen-plaintext attack)

I have been wondering what approach to take in order to figure out what key was used to encrypt a message using the hill cipher. I know it is possible to obtain it even if it were just a ...
94 views

### Is the mod_auth_tkt scheme secure?

The third-party Apache plugin mod_auth_tkt uses a tragically-not-HMAC construction: ...
255 views

### Is a continuous stream of encrypted data embedded in garbage more or less secure than only encrypting the data?

Consider a communication channel that needs to be secure (Encryption can not use full "volume" encryption, since future messages are not known). Would it be better to only transmit encrypted ...
278 views

### Is it possible to match encrypted documents using user-defined search terms?

Suppose I am storing a number of encrypted documents in a database. I would like to make it possible to identify the subset of documents whose contents match user-specified search terms without a) ...