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Questions tagged [cryptography]

Questions on the mathematics behind cryptography, cryptanalysis, encryption and decryption, and the making and breaking of codes and ciphers.

Please only post questions about the mathematics of cryptography here.

  • Coding and implementation specific questions should go to Stackoverflow with encryption or cryptography tags.
  • You may also consider asking at Cryptography Stack Exchange which is for asking questions about the mathematics and properties of cryptographic systems, their analysis ("cryptanalysis") and subsidiary topics that generally make up cryptology.
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What does it mean "polynomial related"?

Taken from Cambridge University Press 0521830842 - Foundations of Cryptography: Basic Applications, Volume 2 CHAPTER FIVE - Encryption Schemes which can be downloaded here Notation. In the rest of this text, we write $E_e (\alpha)$ instead of…
user6830
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Encrypting with Vigenere procedure without a key

I'm learning about Vigenere cipher,and I came across an exercise asking to encrypt a plain text without a key. How would that work exactly?
DS_UNI
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common encryption exponent attack - Match Encryption and public key sets.

I have five encryption mods ($n_1$ through $n_5$) with the same encryption exponent ($e$) and the same message encrypted using the five public keys giving $E_1$ through $E_5$ that were not given in a particular order. I am trying to use the Chinese…
MathIsHard
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Solve x*(x+B) = y mod n for x

I am in cryptography class, working on homework that is due tomorrow, and I came across the following problem: Modified Rabin Cryptosystem Consider modification to Rabin cryptosystem in which ek(x) = x*(x+B) mod n, where B (in integers modulo n) is…
Mike Warren
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Diffie Hellman Problem

I know that in Diffie Hellman, the final key (from Bob's point of view the final key is calculated as follows) KB = (gx mod n)y mod n, where x represents Alice's private no. y represents Bob's private no. g and n the two public nos. which can be…
Zaid Khan
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RSA: Calculate $p$, having $n$, $e$ and half $q$

I need to calculate the $d$ private key in RSA. The data I know is $n$, $e$ and part of $q$. For calculating that d, I need to calculate $\phi = (p-1)(q-1)$, but, before I can calculate $\phi$ I need to know $q$ and $p$. Knowing that $n = p\ \cdot…
DrChacal
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Monoalphabetic Cipher

I am not sure how to get the key for the following Monoalphabetic Cipher question. This is a textbook question and I know the answer, but I juts dont know how they got the key. Question: Decrypt the following Cipher texts, and give the key. (Note…
Tom
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Factoring of composite numbers of two primes

Let n=pq, with primes $p=x^a +1$ and $q=x^b+1$, for $x$, $a$, $b$ integers with $a$ not equal to $b$. Is $n$ hard to factor? If not what would be an algorithm and its complexity?
Crypto-enthusiast
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Involutory key for the substitution cipher

I need to find all the involutory keys for the substitution cipher over $\mathbb{Z}_7$. I wasn't sure what can be the key for the substitution cipher. For example, for the affine cipher, $e(x) = kx + a$, $(k,a)$ is the key. However for the…
BBbbBB
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Fermat's theorem as primality tester when powers are too large

As part of cryptography, if I wish to test whether a given number is probably prime I use the formula: $$ a^{p-1} \equiv 1 \bmod p $$ where $p$ is (potentially) a prime number. However, when it comes to testing a number such as 2341, i.e. $$2^{2341}…
user3168961
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Fermat's Theorem as a primality tester doesn't work for all primes?

I'm studying cryptography. According to Fermat's theorem... $$a^{p-1} \pmod p = 1$$ .. when $p$ is a prime number. The above should prove whether a number is prime or not yet it doesn't work for simple primes like $7$... Having ran through a…
user3168961
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Why is this prime a bad choice for the ElGamal cryptosystem?

Using the ElGamal cryptosystem in $\mathbb{Z}_{p}^{\times}$, the proposed prime is $p = 2^{1947}\cdot 5 + 1$. The exercise asks me to show why this is a poor choice, and I can't quite do it. In my textbook, the keyspace is $K = \{(p, \alpha, a,…
Auclair
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Application of fixed point theory in cryptography

Does fixed point theory has an application in Cryptography? I really don't know about Cryptography. If you mention any books on this matter it really helpful
Snehasish Bose
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RSA public encryption: Finding p and q given $\phi(pq)$

I have a quick question: My book asks me to show that if someone were to find that value of $\phi(pq)$ then they would be able to find out p and q. Is this possible? I've seen many examples of finding p and q given $\phi(pq)$ and the value of pq.…
The Physics Student
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Suggestion for a book on cryptography

Can someone suggest me a good book on cryptography with a lot of solved examples.
david watson
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