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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What are the main weaknesses of a Playfair cipher, if any?

What are the main weaknesses of a Playfair cipher, if any? I know that they depend on none of the letters missing, but that is an easy fix if a letter gets dropped. Besides that, are there any other ...
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Can an Enigma-style cipher of sufficient complexity be considered secure in today's world?

Regarding the German Enigma machines, if I recall correctly, the reason they were defeated was because the Allies were able to generate a massive database of possible rotor settings, and because the ...
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About Cryptography in a Character Language

Suppose I had a message in Chinese (or another non-phonetic language) and I wanted to encipher it. Some of the simplest encryptions in English are substitution ciphers, but such ciphers don't seem ...
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1answer
696 views

How to build an electro-mechanical public key cipher machine?

It is generally assumed that asymmetric encryption schemes were invented in 1973 at GCHQ in Britain and, independently, in 1976 at the MIT. Imagine, if the abstract idea of having a public key and a ...
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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
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How does the index of coincidence work in the Kasiki test?

I'm starting to learn about cryptanalysis and I am having a bit of difficulty understanding the Kasiski test's index of coincidence. I have a book (Cryptography Theory And Practice by Douglas Stinson) ...
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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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Possible ways to crack simple substitution ciphers

We had a quiz in class today where we had to break the ciphertext with the key given, but not the algorithm. Suffice to say that I wasn't able to decrypt it within the allotted time of 12 mins and ...
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609 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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670 views

Cracking WWII-era codes - code found on a pigeon's leg in Surrey

A recent BBC article entitled WWII code 'may never be cracked' posted a code: AOAKN HVPKD FNFJW YIDDC RQXSR DJHFP GOVFN MIAPX PABUZ WYYNP CMPNW HJRZH NLXKG MEMKK ONOIB AKEEQ WAOTA RBQRH DJOFM TPZEH ...
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Historic Authentication Schemes Before Computers

The Bulletin of the Atomic Scientists recently ran an article about an alleged near miss with rockets in 1962, which raises some interesting cryptographic questions: ...
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1answer
835 views

How to deduce enigma settings given a partial plaintext?

Assuming some large block of text is encrypted with an enigma machine and I only know a small subset of letters before and after encryption, how do I go about figuring out the enigma settings from ...
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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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2answers
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Finding a keylength in a repeating key XOR cipher

In an old cryptography FAQ, I found the following step described for determining a the length of the key a cipher was repeatedly XORed against: Discover the length of the key by counting ...
6
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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, ...
6
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2answers
652 views

Using a mono-alphabetic substitution cipher with a different language per word

How much harder is it to determine the secret key for a mono-alphabetic substitution cipher, if each word is translated into a different language before the cipher is applied? If somehow computers ...
6
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3answers
474 views

Good challenges for a crypto competition for teenagers

I'm holding a cryptography workshop for teenagers (around 16 years old) at our university. As part of the workshop, I'm planning to run a crypto competition with prizes: there will be different tasks, ...
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825 views

Is the logic for how the enigma machine worked documented somewhere?

I know that there is a formula to calculate the result of any input but is the logic of how the machine actually worked documented? I have seen schematics for the circuitry and even a how to make ...
6
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520 views

How does cryptanalysis of the Playfair cipher work?

I have a set of Playfair-enciphered data that I'm trying to crack without the key. I know I need to analyse bigrams; I've currently worked out what decrypts to th, ...
6
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1answer
642 views

What is the best method to determine the language used in a monoalphabetic substitution cipher?

Working on a cipher (which I assume to be a mono-alphabetic substitution cipher due to the letter frequency) I struggle with the fact that I don't know which language the plain text is written in. ...
6
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1answer
151 views

Why was the Navajo code not broken by the Japanese in WWII?

In reading about this topic recently, to my understanding, the encryption schemes used on top of the Navajo language were very simple and definitely could have been broken (my research shows they ...
5
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1answer
229 views

Affine Cipher over an Affine cipher

I would like to know your view about the title, encrypting a plain text with an affine cipher then encrypting that ciphertext once more using the same cipher, but of course different keys. Would it be ...
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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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1answer
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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 ...
5
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1answer
163 views

How would you encrypt-then-MAC when using pen-and-paper and a Caesar cipher?

I'll probably get shot for asking this, but I've got some kids (aged 8-10) in my neighbourhood that I've been showing/teaching the simple pen-and-paper Caesar cipher and they're successfully playing ...
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Comparison Vigènere vs. Monoalphabetic cipher

I have a question for a class of secure communication, but I have no idea about how to proceed in order to answer it. I'll be very grateful to whom replies to me. Assuming that brute force attack ...
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135 views

How to break a columnar transposition cipher?

I'm currently studying for a cryptography exam. I've been given ciphertext that has been encrypted by a columnar transposition cipher. I've been given no shift key length or key word, the only thing I ...
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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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Alphabetic Substitution with Symbols

I was reading on a site about the Zodiac Killer and how he used a basic substitution cipher, but instead of substituting english letters and characters he substituted symbols. I was wondering, if you ...
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1answer
238 views

Solving Vignere Encryption

I'm currently trying to crack a cypher that I believe is Vignere Encrypted and I'm currently stuck. I calculated the key length by finding repeated sequences in the cypher and calculating the the ...
4
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2answers
479 views

Is Chaocipher a secure cipher under ciphertext-only attack?

Chaocipher was invented by John F. Byrne in 1919. The algorithm was recently revealed – see Moshe Rubin's Chaocipher Revealed, the Algorithm (PDF). While a known plaintext attack successfully finds ...
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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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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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Best modern cipher for morse code?

I am looking for a modern cipher to encode morse code, ie. input is [A-Z 0-9] (base36 as I learned in the comments below) and output is the same, one character at a ...
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1answer
118 views

Question about the definition of a perfect cipher

I need to prove that the following encryption scheme is a perfect cipher: Let $p$ be a prime. The secret key is a pair $(a,b)$ sampled uniformly at random from $\mathbb{Z}_p^* \times \mathbb{Z}_p$. ...
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1answer
578 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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2answers
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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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551 views

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 ...
4
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2answers
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How were semagrams encrypted in the pre-digital era?

Historically messages in languages that use alphabets have been encrypted manually according to some kind of algorithm (e.g. mono- and poly-alphabetic ciphers). But how wew messages encrypted in a ...
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1answer
715 views

Can two cipher letters per plaintext letter easily defeat character frequency analysis?

For a class 5 years ago I wrote a paper about "defeating character frequency analysis by using two cipher letters per plaintext letter" ...
4
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1answer
651 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 ...
4
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Would the encryption of bigrams and trigrams weaken the code?

Supposing a rotor machine of the 1940s or 50s, with 36 characters instead of 26. Would the encryption of the ten additional characters as the most frequent bigrams or trigrams weaken the code? What ...
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Looking for C++/Python Open Source code library for cryptanalysis of classical ciphers [closed]

I've done a significant amount of coding over the years working on classical ciphers (e.g., Chaocipher, D'Agapeyeff). My main programming languages today are C++ and Python, although there was a time ...
4
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How has the “power” of cryptography evolved from its beginnings till today? [closed]

I'm looking for a historical overview for the change in cryptographic power, which I hereby define as How far state-of-the-art cryptography is ahead of state-of-the-art cryptanalysis. In other ...
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Using one-way hash functions as the encryption method

Suppose two parties want to communicate securely with each other (Bob and Alice) using a simple messaging system in English. There are approximately 180,000 currently used words in the English ...
3
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2answers
646 views

security of Felix cipher

I just find this pencil and paper cipher 'Felix' and I want to know how secure it is? EDIT From http://web.archive.org/web/20110825142054/http://topcat.hypermart.net/papers/felix.txt Felix a ...
3
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2answers
640 views

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 very insecure very slow. This is an attempt to make a relatively secure, keyable, and ...
3
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Is there a name for this 'enhanced' caeser shift and if so, is it trivial to break?

This cipher shifts the letter that it will shift to, after each shift... I used to play with this when I was a kid. I was thinking about it recently and realized that it wasn't as simple as I ...
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Cracking the Beaufort cipher

Is there any easy way to crack a Beaufort cipher? We have a Vigenère table, and are trying to guess the keyword. Any easier way?
3
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1answer
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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 ...