Ciphers invented before the era of computers and modern cryptographic theory. Classical ciphers typically operate on letters instead of bits and are usually designed to be implemented by hand or using simple mechanical devices.
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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 ...
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95 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 ...
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Is this hand cipher any more secure than the Vigenère cipher?
I know that inventing one's one crypto always sucks, but the problem is that hand ciphers are usually 1. very insecure 2. very slow. This is an attempt to make a relatively secure, keyable, and ...
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What are some approaches to attack an triple key polyalphabetic cipher when the IV key and PT key are known?
The cipher is an triple key polyalphabetic cipher also known as an Quagmire IV by the ACA. An example would be here: Quagmire IV example and info (pardon the ads)
I posses the IV key and the PT key. ...
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2answers
276 views
Benefit of combining classical substitution ciphers with modern cryptography
I know that a simple monoalphabetic substitution cipher is considered extremely weak, on account of linguistic frequency-analysis attacks. However, assume the following:
cleartext is encrypted ...
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security of Felix cipher
I just find this pencil and paper cipher 'Felix' on this site
and I want to know how secure it is?
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142 views
Can a shift cipher attain perfect secrecy?
On a practice question for my intro cryptography exam, it asks the following:
Assuming that keys are chosen with equal likelihood, the shift cipher provides:
A) computational security
...
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2answers
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How can I break a Vigenère cipher with partial plain text?
I have an exercise to do regarding Vigenère encryption.
I've got the cipher text: WNZTNVIEEGTJYKRRWYUELWNZTNV
and a partial plain text: ...
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437 views
Cracking the beaufort cipher
Is there any easy way to crack a beaufort cipher?
We have a vigenere table, and are trying to guess the keyword.
Any easier way?
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How weak/strong is this hand cipher? (updated) [closed]
In simple steps here the system:
first write text.
hand cipher
then divide into groups of three letters:
han dci phe r
then choose a key consisting of three letters, for example ...
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2answers
565 views
Assistance Cracking Classical Cipher
Below is the cipher text I am trying to break and as you can see its rather short which is why I am having so much trouble.
...
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1answer
203 views
Is frequency analysis a useful tool against encryption by multiplication?
If I transform natural plaintext by:
making each letter two decimal digits, considering the whole as a decimal number;
multiplying by the key (some integer constant), giving the ciphertext;
would ...
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771 views
How many keys does the Playfair Cipher have?
I was just studying the Playfair cipher and from what I've understood, it is just a slightly better version of a Caesar cipher, in that it isn't actually mono-alphabetic but rather the 'digrams' are ...
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6answers
514 views
Why does ROT13 provide no cryptographic security?
I can understand that ROT13 is not secure for obvious reasons, but I'm looking for the theoretical answer. Wikipedia says "The algorithm provides no cryptographic security.." What does it mean to ...
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1answer
318 views
Cryptanalysing Affine cipher
I am trying to cryptanalyse a cipher–text encrypted by Affine cipher.
The encryption formula is: $c = f(x) = (ax+b)\bmod m$, where $a$ and $b$ are unknown constants; $x$ is a plain-text symbol, and ...
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2answers
263 views
Toy cipher — does it have a name?
When I was perhaps nine years, I borrowed a book from the library on various maths and CS topics. It outlined various simple ciphers, including one that I used a lot, just for fun. I can't remember ...
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How secure is the Vigenère cipher in file encryption if you encrypt the password first?
I created a basic file encryption method, that seems pretty secure to me (based on an answer below, it seems to be an implementation of the Vigenère cipher) , but one of my colleagues claims that it ...
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Has there been any cryptanalysis of RC4-52?
Several websites ( such as Is there a secure cryptosystem that can be performed mentally? ) briefly mention RC4-52 as a modification of standard RC4.
RC4-52 has only with 52 instead of 256 elements ...
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202 views
Brute forcing Cardan grille
Having a "rotating" square Cardan Grill with sides long n cells, how could i determinate the cost of a brute-force attack?
How many configurations should be considered to perform an exhaustive ...
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2answers
578 views
How to attack a general polyalphabetic cipher?
I am able to decrypt vigenere cipher text using the index of coincidence and chi test.
However out of interested how do you go about attacking ciphertext that was encrypted using a mix alphabet ...
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3answers
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Encryption/ciphers/codes in Chinese
I am quite curious as to how you can perform simple encryption for the Chinese language.
Saw a similar question related to encryption/Chinese here: About cryptography in a character language, ...
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2answers
803 views
What is the most secure hand cipher?
By "hand cipher", I mean a symmetric cipher for which encryption and decryption can can both be performed with a pencil on graph paper, consuming about 10-20 seconds per character by a proficient ...
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1answer
523 views
Obtaining the key length from the ciphertext of an auto-encipher
I'm revising for a cryptography exam at the moment and I'm having some problems with a question.
The question looks for the key length of a cipher given that the word "earthquake" appears in the ...
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3answers
524 views
Is a book cipher provably secure?
I've seen ciphers (usually in spy drama shows) that involve taking a book and writing down an index to individual characters. Essentially it's a keyed substitution cipher, where the key is the name ...
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1answer
548 views
Example of CHI Square test on Caesar Cipher?
I am trying to get my head round the chi square test, when used with the Caesar cipher.
I started off using this formula,
$$ X = \sum_{i = 1}^k \frac{f_i · f'_i}{n · n'} $$
Where ...
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2answers
857 views
How to attack a classical cipher using known partial plaintext?
I have a ciphertext generated by a classical cipher. I do not know what was cipher used to generate it. I do however have the beginning of the plaintext.
What are the cryptanalysis approaches for ...
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How can one break a monoalphbetic substitution chipher at pseudorandom text?
Does anybody know how to break monoalphbetic substitution cipher, if it is applied to some pseudorandom text (for example to some surrogate key filed in a database)?
Let us assume that we have only ...
