# All Questions

89 views

### Meet in the middle attack - message and key

I clearly understand the story of Alice and Bob: Alice and Bob are going to use double DES. They know the keys K1 and K2. Alice sends to Bob the encrypted message c= EK1(EK2(m)). Bob decrypts the ...
467 views

### Is the random data at the end of my decrypted message supposed to be there?

This is my PKCS#5 & AES/ECB Python 3.3 code: ...
281 views

### What are some of the major differences between PyNaCl and PyCrypto? [closed]

It seems like one can generate keys and encrypt data with both libraries. Is one better than the other? Do they attempt to solve different problems? Is one more secure than the other?
283 views

### Perfect Secrecy, two Definitions

I'm reading the proof of the implication "Def 2.1 $\Rightarrow$ Def 2.4" in these slides about Adversarial Indistinguishability and Perfectly-Secret Encryption. I have a doubt in the slide 10. Here it ...
415 views

### Why this k parameter is in unary in adversary PPT algorithm? [duplicate]

While reading some text on cryptography, I found that algorithm $A$, that the adversary (called Eve by convention) runs to break the cipher message, needs two parameters, candidate message ($y$) and ...
463 views

### Strong encryption done by hand [duplicate]

One of the distinguishing features of a good encryption algorithm, is that it is easy to encrypt, and hard to crack. Are there any that are easy enough for average folk to remember, and calculate by ...
985 views

### Where to store the private key and the public key in a communication protocol

I want to create a simple secured connection protocol over TCP/IP. I just want to connect from my mobile device to a server securely. Firstly I thought to use an SSL connection, but I changed my ...
208 views

### Because the algorithm is known, it is no longer a trade secret [closed]

In wiki about RC4 cipher they said, Because the algorithm is known, it is no longer a trade secret. Is that mean the cipher text will be more secure if the algorithm unknown to the attacker. ...
198 views

### What is the advantage of an attacker over breaking a 4 digits PIN?

When a hardware system is protected by a 4 digits password, what is the advantage of the attacker into breaking that system? Isn't it $10*10*10*10=10^{4}$? If $\frac{1}{10^{4}}$ is the cost of such ...
52 views

### with RSA encryption, how do I find the md5sum of the private key from the public key?

This is a purely mathematical question on RSA encryption. Having $e$ as the public key exponent and $n$ the modulus of a public key. Can I recover the md5sum of the private key $d$ (inverse of $d$ ...
107 views

### credit card number encryption using aes-ctr mode

I want to encrypt credit card numbers. I want to apply AES-CTR mode. Is it suitable for that? How can I store nonce and counter values for an individual credit card number? How can I send the ...
65 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 ...
50 views

### Reduction of a modular exponentiation's base does not change the result

Can anyone help me to prove this property used in the RSA correctness theorem? $a^b\bmod{n}\equiv (a\bmod{n})^b\bmod{n}$ Specifically, here is what I have done: this is what i mean. i can't ...
65 views

### Complexity of attacks on affine cipher

I am interested in the complexity of attacking affine ciphers under the following two scenarios during a Ciphertext only attack during a Chosen Plaintext attack For an alphabet of size $m$, how ...
39 views

### With wrong IV at receiver side, the CFB in better than OFB?

Assume the receiver have a wrong IV (initialization vector), in the CFB mode only the first block of plaintext is wrong but in OFB mode the second and all blocks will be affected. Is that correct? ...
96 views

### RSA: how does it work and how is it more secure than symmetric systems

Before starting the question, I know that RSA is naturally a key exchange cryptosystem rather than an antilogarithm fully implemented for encrypting and decrypting data since the length of the secret ...
111 views

### Remove padding (CBC) from decrypted text [closed]

I have a bit problem with padding. Everything goes fine when I encrypt the text BUT when I decrypt I see some extra characters. Sometime's one character sometimes more and I dont know how to remove ...
113 views

if a number is said to be a subgroup of a quadratic residue of Z∗p, can I affirm that it is a generator of a cyclic group ? ie, say i am looking for a number which is an element of order q in Z*p, ...
66 views

### Issue about randomness : what if random looks “human” ? [closed]

I read a lot of stuffs about entropy/randomness. But what if a random generator (either digits or strings) once output a "human" patern ? I know it is very unlikely to happen, but if a random code ...
69 views

### Meaning of Multiplicative Group to the power n i.e. Z^n [closed]

What does $Z_q^n$ mean in this notation?      References: on the 2nd paragraph of page: http://en.wikipedia.org/wiki/Learning_with_errors
89 views

### what is SALSA20 Nonce and its requirements [duplicate]

what is the source of nonce in SALSA20 algorithm. basically who provides the value of nonce in SALSA20 algorithm
130 views

### Output of a Feistel network with all “0” round function

What is the output of an r-round Feistel network when the input is $(L_0, R_0)$ and each round function outputs all 0s, regardless of the input?
90 views

### Is One Time Pad something like this? [closed]

Alphabet: [A-Z] = [0-25] Plaintext = HELLO = 7 4 11 11 14 // QUESTION: do this key must be as long as plaintext? KEY: [QWERT] KEY = 16 23 4 17 19 Plaintext + Key = 23 27 15 28 33 Plaintext + Key ...
78 views

### How Could the Music Industry Go About Implementing Crypto? [closed]

So I'm currently doing the Dan Boneh online Crypto course. In the last problem set there was a question on AACS for DVDs and wondered if a similar method could be used for music. Have the music ...
405 views

### Is it possible to find plaintext from ciphertext if (n) and (a) are known for RSA?

I have a couple of questions pertaining to a RSA problem. I need to decipher some ciphertext and find out what the original plaintext was. Modulus: $n = 2537$ and exponent: $a = 11$ Encrypting ...
68 views

### Theory for pre-paid debit card: card-to-card transfer [closed]

In Sweden in the late 90's they deployed a electronic prepaid cash system system, and if I remember correctly you could transfer money between cards without online access. Wikipedia states that the ...
55 views

### The improvement of the private key exponent in the M.Weiner Attack

I am working on attacks on RSA and came across the M. Weiner attack. The limit for $d$ in order for the attack to apply is $d <= (\frac{1}{3})N^{0.25}$. The paper states that Boneh and Durfee ...
65 views

### Error propogation in CBC mode

Going through the wiki for modes of operation I see that the section error propogation says that an error in one block in the ciphertext in CBC mode only impacts two blocks. I do not quite get that. ...
44 views

### How to make sure the pre-agreed information safe for DH-Key Exchange

The definition of DH key exchange was given as the method let two authenticate each other & exchange the crypto key over an insecure channel. DH-key-exchange was innovated to defence ...
71 views

### How do they implement MAC in TLS?

I recently read questions about "MAC then Encrypt/Encrypt then MAC" which made me curious about how they actually use the algorithm. MAC only calculated once either case (according to what I read). ...
290 views

### cryptography and use of python programming language [closed]

why is it recommended to write cryptography programs ,for example crypt-analysis projects with python programming language? what is the advantages of python? why not use c or any other language?
61 views

### What's wrong with this “order-preserving MAC” function?

Please note: this is purely a thought experiment and not intended for any real-world usage! I came up with a simple function $\mathrm{OPF}$ to map the integers $[0,C)$ (where $C$ is the "ChunkSize") ...
37 views

### In Needham Schroeder key distribution what happens if responder's part is not encrypted in step 2 with Kbs but encrypted instead by Kab in step 3

The Needham-Schroeder protocol is: 1 A→S:A,B,Na 2 S→A:{Na,B,Kab,{Kab,A}Kbs}Kas 3 A→B:{Kab,A}Kbs 4 B→A:{Nb}Kab 5 A→B:{Nb−1}Kab Here Na and Nb are nonces. Kab is the key between a and b Kas is ...
200 views

### Hash Based Encryption (fast & simple), how well would this compare to AES? [duplicate]

First of all, I know it's a very bad idea to invent your own encryption algorithm. It's better to use existing known, trusted, extensively tested and studied algorithms with a proven track record. The ...
47 views

### Affine encryption function

Let $f(x) = ax + b$ be an affine encryption function. $a$ and the cardinal of the field of our keys $K$ (in English, 26) must be coprime so that the encryption ...
86 views

### Hypothetical question about AES output

Consider that you stumble upon 5 different AES output strings. One of these strings can be decrypted with a secret phrase, the other four strings are based on the real string but have had random parts ...
88 views

### Assuming that we had a way of finding primes, could RSA be used without computers?

I once asked a question "Would RSA make sense if we used no computers?". The answer was negative, because finding primes that would make secure keys would prove too hard. Assuming that we had found ...
136 views

### How do I cryptanalyze a password field?

In a PostgreSQL database, I can see the following fields in a table (for a specific record): User = administrator Password = "ceMv9Me6go" Now the following are true: The password length is not ...
114 views

### Is there way to combine secrets first and later split them?

Shamir's Secret Sharing allows a secret to be split into multiple shares so that it can be reconstructed later. Is there a way to do the reverse? Can we combine a few secrets first and later split ...
2k views

### Learning to encrypt/encryption algorithms [closed]

I am a beginning programmer interested in coding an application of my own to encrypt sensitive files of mine. For example: lists of passwords, bank accounts, Credit Card numbers etc. I was wondering ...
108 views

### Why is it said that if we know few bits of pseudo random sequence we can predict the whole of it?

I am new to this cryptography and in the source from which i study it is said that if we know the first few bits of a pseudo random sequence we can predict the whole of it.How is it possible? The ...
507 views

### Verify Signature Failed [closed]

I am writing a small module which will verify signed message. Message will be signed using private key in Smart Card. The inputs are: Signed Message Original Message Public Key / Certificate with ...
481 views

### With HMAC, can an attacker recover the key, given many known plaintext/tag pairs?

Given many pairs of $(m, t)$, can the attacker compute the key $k$ satisfying $\text{HMAC}(k,m) = t$? (Assume that $k$ was chosen at random.)
2k views

### How does the rand() function in C work? [closed]

I want to know how rand() works (even when I don't provide any seed how it produces PRNs?) thanks!
215 views

### What are alternatives to the random oracle model for modelling hash functions? [closed]

I was looking for more realistic alternatives to the ROM for describing hash functions in theoretical proofs. I came across the common reference string model (where hash functions can be modeled as ...
166 views

### Cryptographic Primitive Method

Is there any cryptographic primitive bijective (one-to-one and onto) function for creating cryptographic tools like symmetric encryption/decryption, Hash code generator, MAC, HMAC and Random number ...
62 views

### Openssl library problem

I'm using openssl library and I've a problem with RSA_private_encrypt() and RSA_public_decrypt(). I'm using a key size of 256 bit with no padding of the input message. I've noted that when my input ...
54 views

### Explanation of multivariate cryptography [closed]

How do the key generation, encryption, decryption, signing, and verification processes in multivariate cryptography work? How about an example or at least a simplified general one? Also, what is the ...