# All Questions

61 views

### Coding of unsigned int to prevent guessing next ID

Suppose we are assigning records an unsigned integer ID from a N-bit space (say 32-bit) in a sequential manner. Is there a way we can code this ID before showing it to the public such that someone ...
51 views

### Is it safe to prefix the a key with a known value?

Almost every encryption algorithm is based on a secret key. I wonder when the algorithm is considered safe, does that also imply that I can prefix (or suffix) the key with a known value, as long as ...
35 views

### Creating a new cipher! [closed]

http://flashbackcipher.blogspot.ae/2015/03/introducing-flashback.html Hey guys the above is the blog for a new cipher, if you guys can take a look and suggest problems.. it would be of great help!! ...
48 views

### RC4 , Is it possible to find the key if we know the plaintext and ciphertext?

Is it possible to find the key if we know the plaintext and ciphertext with RC4. How should I write the algorithm?
84 views

### Rijndael S-boxes: Where do the $\mu$ and $\nu$ polynomial ring elements come from?

I've asked some other questions before about Rijndael's S-boxes, and step by step I'm coming to an understanding; but those steps often guide me to new questions. I did some lines of code to ...
50 views

### Information-theoretic bound on leakage by timing measurement

I'm looking for an information-theoretic upper bound on leakage by timing measurement. I'm assuming that an attacker wants to leak out of a black box a secret key of $k$ bits that is secretly ...
58 views

### How can ECDSA signatures be shortened (to be used as a product key)?

So I made my own serial key generation software, using ECDSA, for use in my own applications and it works great so far! To keep the serial key short enough I use a 128 bit EC curve. My final signature ...
58 views

### How dangerous is it to encrypt with AES 256 if the end user knows the unencrypted value?

I am implementing a bit of security in a system that was originally built without encryption on a specific piece of data. The plan was to encrypt this piece of data and include it as part of the ...
131 views

### Why is diffie hellman sha1 used instead of diffie hellman

For SSH why is diffie-hellmangroup1-sha1 used instead of just diffie-hellman? In other words why is the hash function used?
3k views

### Encrypting small values with RSA private key

I'm looking for best practices when it comes to encrypting small (< 128 bytes) amounts of data with the RSA private key. Signing it would make the resulting payload too large.
197 views

### What is the most light-weight symmetric cipher thats still usefull?

I need a very light encryption scheme which costs almost no performance during encryption (while decryption may be slow). I found 5 algorithms: RC4, DES, LED, PRESENT, and Piccolo. But how do I ...
64 views

### MAC in SSH packet encryption, benefits to not including it?

What are the benefits to not including MAC in a SSH packet encryption? I understand what the MAC is there for, but if it was not included would there be an advantage? Is the MAC somewhat redundant ...
16 views

### Behaviour of LDPC code as density of check matrix increases [closed]

My assignment is to implement a Loopy Belief Propagation algorithm for Low-density Parity-check Code. This code uses a parity-check matrix H which is rather sparse (say 750-by-1000 binary matrix with ...
48 views

### Do asymmetric signatures require constant-time verification?

To avoid a timing attack, HMAC signatures are usually compared in constant-time (every byte is compared, and the results aggregated). Is the same necessary for asymmetric signature algorithms such ...
22 views

### Construct rainbow table to cracking DES

I'm facing to a project that is build a rainbow table to cracking DES. I've already collected information about Hellman, DP, rainbow table method. Finally, i choose rainbow table. I've read topic ...
17 views

### Substitution cipher with hexidecimal key and encrypted plaintext? [duplicate]

This is a challenge for a past CTF that I was unable to find a writeup for. The challenge gives two ciphers (http://pastebin.com/raw.php?i=Fq98trFw). The "key" has a length of 32, and the flag has a ...
164 views

### What happens when a RC4 stream gets corrupted?

I want to encrypt a large file using RC4. But what happens if the encrypted file gets corrupted (bytes modified or lost)? Can I still decrypt the rest of the file correctly? If not, what is the best ...
73 views

### Is this DH-based key exchange scheme sound? [closed]

Is the following scheme for generating a shared 512-bit secret sound? Select generator: g = 2 (This is the generator from RFC 3526). Select modulus: p = ...
21 views

### Determining nonce between unsynced systems

It's my first venture into cryptography, and I'm struggling with how to use a nonce in an encryption standard between two digital radios. The two radios support AES encryption, only requiring a ...
60 views

### Deciphering text encrypted with a changing cipher

I know there are many ways to crack basic ciphers were each letter is mapped to some other letter, but what ways are there to decode something that was encrypted using a cipher that changed after ...
30 views

### HMAC tag to verify the key (verification in the reverse direction)

A quick question. Are there any problems with the following HMAC use-case: We have two entities ($A$ and $B$), both of them have a secret ($A$ has $S_0$ and $B$ has $S_1$) $A$ generates a random ...
71 views

### ECDSA for encryption

ECDSA keys are much shorter than RSA keys, but typically only used for signing, not encryption. Is there an accepted way of doing this? (https://bitcointalk.org/index.php?topic=196378.0 ...
13k views

### Can you help me understand what a cryptographic “salt” is?

I'm a beginner to cryptography and looking to understand in very simple terms what a cryptographic "salt" is, when I might need to use it, and why I should or should not use it. Can anyone offer me a ...
101 views

### Deterministically generate a RSA public/private key pair from a passphrase (with sufficient entropy)?

Is it possible to deterministically generate public/private RSA key pairs from passphrases? Would giving the (key generating) algorithm data made from key-stretching the passphrase (instead of a ...
133 views

### 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 ...
44 views

### Improving on a clocked PRNG

I recently developed a PRNG from scratch with a little inspiration from the initialization function used in the HC stream cipher. The state ($S$) of the basic version is an array of 5 bytes and uses ...
301 views

### Secret sharing scheme with possibility to change the secret

Here's a scenario I want to handle with a secret sharing scheme: Alice wants to share the secret $S$ with Bob, Charlie and Dave. Alice generates private shares based on $S$ for Bob, Charlie and ...
129 views

### Can we reverse a hash when we know part of the input?

While I read that hashes are not meant to be reversible, but assuming we have part/half of the message/input of a hash? Is there any method that we can use to recover the remaining message via this ...
937 views

### Why are RSA key sizes almost always a power of two?

I know that other bit sizes are possible, e.g. this HTTPS server seems to have a 9000 bit key https://www.ssllabs.com/ssltest/analyze.html?d=qqq.gg, but it's very rare that one sees a key not of size ...
63 views

### How does SafeNet MobilePASS generate passwords? (TOTP Variant)

This question is similar to How does SafeNet MobilePASS generate passwords? The question and answer there is related to the HMAC Variant of SafeNet Mobilepass. I'm looking for details on the TOTP ...
121 views

### Concatenation of two strong hashes may have striking weakness

For any hash functions $H_0$ and $H_1$, it is easily proved that their concatenation $H_0\|H_1$, defined by $(H_0\|H_1)(X)=H_0(X)\|H_1(X)$, is at least as resistant as the strongest of $H_0$ and $H_1$ ...
32 views

### how to calculate nonlinearity?

I am struggling to understand how to calculate non-linearty of an s-box, could someone show me a detailed example? I found this question which helped a little, but I still didn't get it, if someone ...