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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Avoiding doubled letters in encrypted text

I have an encrypted string based on an "alphabet" of 29 runes and access to successfully decrypted (runes->English) plaintext. Testing for a Vigenere cipher, I tried a Friedman attack (aka Kappa test, ...
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105 views

Does adding more characters to the Enigma rotors improve crypto strength?

This seems like an obvious question but I haven't been able to find it, so here goes: Enigma is based on an alphabet of A-Z (26 characters); obviously this directly affects message content (e.g. ...
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1answer
27 views

Does Kasisky test for Vigenère cyphers assumption always hold?

In the notes from which I'm studying I read about Kasisky test: "Key observation: Two identical segments of $l ≤ L$ plaintext letters, will be encrypted to the same $l ≤ L$ ciphertext letters." ...
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31 views

What is the best approaches to find the type of a Cipher / Encryption? [duplicate]

There are many known and unknown encoding and encryption types / algorithms. When the algorithm is known, then at least it gives some direction or approach on what to do next to obtain the plaintext ...
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2answers
87 views

How to find the keyword of the Playfair text knowing/not knowing the plaintext and having decrypted text?

I know how the Playfair cipher works. I would like to know how can I find the keyword of the play fair cipher in two cases: 1) knowing the plaintext 2) not knowing the plaintext? Some examples: <...
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55 views

Crack multiple One-Time-Pads with same key [duplicate]

I've got 12 pads, all with a length of 50 characters, except the last one. I also know, that the key has got a length of 50 and that the key has been re-used on all of the 12 pads. What do I have to ...
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27 views

Concatenated Key Question - Poly-alphabetic Substitution and Columnar Transposition

I am working on a degree module, and have been tasked with designing an Encryption/Decryption algorithm. I hope to receive some help with a couple of questions, if possible. Apologies if terminology ...
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1answer
39 views

Product of Vigenère ciphers is not a Vigenère cipher with length of LCM

I have always thought that using two Vigenère ciphers leads to another Vigenère cipher with key length of LCM (least common multiple) of the two previous Vigenère ciphers. But I have stumbled upon ...
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2answers
186 views

How secure would hand ciphers be using a Block Cipher Mode?

I've been looking into different ciphers that require very minimal computation power (calculator, deck of cards, index cards and a pencil), i.e. hand ciphers. For the most part all methods typically ...
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41 views

autocorrelation and entropy of historical ciphers

I'm learning cryptography and come across historical ciphers such as vigenere cipher,hill cipher,random substitution and etc.I'm using cryptool for learning purposes and I have a question in mind.I've ...
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1answer
57 views

How would multiple uses of a vigenère cipher be decrypted on paper (classic)?

As far as I can tell (from reading the Wikipedia article about it) decrypting a message encrypted with the vigenère cipher is possible on paper. But what would happen if the message is encrypted ...
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1answer
58 views

VIC cipher author known?

Does anybody know if the design of VIC cipher is attributed to anybody? I've tried Google and found nothing. There are quite a few sites describing the method in detail, a few implementing it ...
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137 views

Best non-digital cipher?

Is there an undisputed cipher that was considered the best before the computer age? (This is not suppose to be a discussion, it's either a yes or no.) Please give a brief description why it is yes ...
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2answers
89 views

Levels of strength: The Caesar Cipher

SITREP: I am deciding what form of "Random" Number generation would fit best for a Caesar Cipher that encrypts each letter with a different integer as a shift. This encryption is being handled via C# ...
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2answers
203 views

How to decode a (Vigenère?) substitution cipher? [closed]

I am trying to decode a message that I suspect has been encrypted with a Vigenère cipher: Utl zluuocp zsc rvzu hbcp ztvibfz vrabzz utl ilry. Utlal flal cb fbaiz, qszu zbsciz. Utvu cllihl vhzb dvil ...
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38 views

Are there any historic examples of breaking the one-time pad because of a careless key re-use? i.e simple attack on a multiple-time pad?

It doesn't have be using a truly random key, (it could be the Enigma) but the attack must be based on XORing two parts of ciphertext.
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1answer
45 views

How would you name and/or describe this algorithm?

First up: this is not a decoding request or anything. This is about terminology. Twice per year, we email another department to ask for a new licence file for their software. They are always slow to ...
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2answers
146 views

Ciphertext with uneven letter distribution

I have a ciphertext encrypted with an unknown method. It is part of a challenge (personal quest), so I can not post it. The letter statistics is the following: ...
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2answers
98 views

Research for video game: Novice cryptography concepts [closed]

I'm making a video game which has some basic cryptography as a main base for puzzle design. I'm hoping players will get great puzzle challenge and learn a few cryptography concepts through playing the ...
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1answer
293 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 ...
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0answers
57 views

Solving ADFGVX decoding with only ciphertext

Hi I'm new to the world of crypto and I'm trying to decode a message. I know it is encrypted using the ADFGVX cipher. I also have a ciphertext, but I have no other information given to me. My question ...
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34 views

How to calculate unicity distance of Fleissner grille?

I need to calculate unicity distance of Fleissners grille of $M \times M$ size. All I know is that plaint text redundancy equals $2$. The alphabet is not specified. Is it possible to calculate its ...
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1answer
314 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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1answer
58 views

What is the viability of a hybrid vigenere cipher?

So a vigenere cipher's main weakness is that you can use frequency analysis on each alternated alphabet, and thus crack the cipher with relative ease... But that implies that you know what the ...
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1answer
113 views

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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2answers
179 views

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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108 views

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: http://thebulletin.org/okinawa-...
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1answer
122 views

Breaking a Modified Caesar Cipher

Does anyone know of an efficient way of breaking a modified Caesar cipher (where the key is a set of numbers (the amount of shift) that is repeated throughout the plaintext, e.g. {1,2,3,4,5}) via ...
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2answers
90 views

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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332 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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1answer
77 views

Help defining a shift cipher mathematically

For an assignment question, I am trying to prove formally that double encryption with two keys by the shift-cipher encryption function results in a shift cipher as well. If the Shift-cipher encryption ...
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2answers
94 views

How strong can multiple encryption be?

Suppose I encrypted a homophonic substitution cipher at the level of syllables, trigrams, digraphs, and individual letter with a Vigenere cipher using the keyword cryptography. I would then encrypt ...
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1answer
102 views

How secure is this modified Bazeries Cylinder?

This device is a modification of the Bazeries cylinder, it has two stacks of 5 rotors which each bear two alphabets plus numbers 0-9, one in correct order and one in random order ( 36 characters in ...
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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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1answer
124 views

Do hand-based hash functions / MACs exist?

Is there research on a hash function or a MAC that can be executed without computers? I know there's a lot of history on hand-based ciphers and the OTP can easily be excecuted by hand. (By using ...
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371 views

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 ...
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1answer
121 views

Which algorithm can be performed by humans?

Suppose I want my grandmother to perform encryption without any help from a computer, and she must encrypt the word 'HELLO' for me. I could ask her to instead of typing the real keys, that she always ...
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2answers
290 views

Is the complexity of Caesar (shift) ciphers “n * n!”?

Can we say that any shift cipher to be decrypted needs an algorithm of complexity “n * n!”? (where n is number of possible ...
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1k 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, <...
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2answers
160 views

What is the keyspace of ROT-13?

Today I was having a discussion with my cryptography lecturer about the keyspace of a ROT-13 cipher. He argues that the keyspace is 0 because it doesn't have a key. Could anyone explain to me why the ...
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1answer
93 views

Is the Vigenère Cryptosystem malleable? [closed]

Is the Vigenère Cryptosystem malleable, and how can we prove that it is (or it is not) malleable?
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1answer
463 views

Monoalphabetic cipher key

I'm new to cryptography and have just completed homework on Monoalphabetic cipher. I managed to decrypt the cipher text successfully but I cannot identify the key from my plaintext to ciphertext ...
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2answers
355 views

Strategy to crack a presumed substitution cipher

The ciphertext given is: ejitp spawa qleji taiul rtwll rflrl laoat wsqqj atgac kthls iraoa twlpl qjatw jufrh lhuts qataq itats aittk stqfj cae I've done ...
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1answer
287 views

For calculating the index of coincidence for each sequence

I was learning about the finding the key length reading the following web site... http://practicalcryptography.com/cryptanalysis/stochastic-searching/cryptanalysis-vigenere-cipher/ and I really don't ...
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1answer
200 views

How hard to break a cipher if it has a different key for each word?

Recently I watched the movie 'The Prestige' again but this time the cipher diary impressed me very well. I like to create my own cipher for my diary because I have some time to kill everyday after ...
2
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2answers
331 views

Perfectly secure shift cipher

Prove that if only one character is encrypted using a shift cipher, then the shift cipher is perfectly secure. I want to show that $P(P=p | C=c)=P(P=p)$. But I don't know how to relate. Can anyone ...
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3answers
564 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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436 views

Decrypting an Affine Cipher with known characters

I am trying to cryptanalyse multiple cipher–texts that I know are encrypted by different Affine ciphers. I have already analysed the frequency that each character occurs, and compared it to a ...
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1answer
152 views

Help deciphering caesar cipher [closed]

I have to to find the shift value and the message for the following caesar cipher. OC_VAWDGPM_VHWTVX_VXJOCVZ_GDX_MWO_VWIZVYGWIZ_NODI_ I have already tried it and with the shift value of 21 i get the ...
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4answers
4k views

How can a Vigenère cipher be broken if the message is short?

I was watching a Stanford lecture on Vigenère cipher and in it the professor said that – to break the cipher – we assume the length of the key is known. We then break the cipher into groups of this ...