Use this tag for questions about cryptographic algorithms which were invented (and used) before the computer age.

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15
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2answers
2k views

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

What are the main weaknesses, if any, of a Playfair cipher? 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 ...
14
votes
4answers
786 views

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 ...
12
votes
4answers
870 views

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 ...
9
votes
1answer
5k views

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) ...
9
votes
1answer
431 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 ...
7
votes
3answers
2k 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 ...
7
votes
4answers
578 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 ...
7
votes
1answer
528 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 ...
6
votes
2answers
538 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
votes
3answers
437 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 ...
5
votes
1answer
404 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. ...
4
votes
3answers
662 views

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 ...
4
votes
1answer
172 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 ...
4
votes
1answer
504 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
votes
0answers
73 views

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 ...
3
votes
4answers
377 views

Is there a way to generate sound one-time pads out of public information?

Is there a way to 'mangle' a public data-source (for example, the current date in YYYYMMDD or the top New York Times headline) to form a one-time pad that will sufficiently hide the pad's source? ...
3
votes
1answer
130 views

Are there any good examples of Contemporary Mechanical Cryptography?

Are there secure mechanical cryptosystems in use today? Not necessarily alphabetic either, ie digital but mechanical?
2
votes
2answers
1k views

How to get the keyword from a keyword cipher?

I was given a ciphertext and now I am trying to break it via looking for the keyword. This is a keyword cipher. So: PlainEnglish: ABCDEFGHIJKLMNOPQRSTUVWXYZ If ...
2
votes
1answer
95 views

Is it possible to track down copies of WW2-era codebooks?

I've got an academic curiosity regarding some of the old Japanese naval codes used during World War II specifically… but I was just wondering if any of the codebooks from this era had been made ...
1
vote
0answers
57 views

Autocorrelation in Autokey

I know that Vigenère can broken using auto-correlation (as explained at cryptool-online.org). Now my question is: Why does this also work with autokey (tested it using crypttool)? To prove that I ...
0
votes
2answers
601 views

Kryptos : K1. What is the origin of the “palimpsest” keyword?

I'm studying the Kryptos sculpture with its cryptographic puzzles K1 to K4. I understand that the keyword "palimpsest" was reverse-engineered using the tableau (and brute-force computer processing), ...