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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Is it possible to perform a rudimentary asymmetric key encryption algorithm by hand?

I have at least three, distinct motivations for having a simple way to use asymmetric key encryption by hand or without modern computers. First, I was recently demonstrating to a 12-year-old how ...
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0answers
74 views

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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497 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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5answers
2k 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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3answers
2k 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 ...
2
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1answer
180 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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1answer
98 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 ...
4
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2answers
268 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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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 ...
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0answers
169 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
46 views

Affine cipher: calculate the key from a known plaintext/ciphertext pair

Suppose I have a ciphertext that I know is encrypted using an affine cipher in $\mathbb{Z}_{26}$. The plaintext begins with es and the corresponding ciphertext is ...
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1answer
93 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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1answer
157 views

Calculation of time needed to crack DES with my CPU

I have a laptop that has the CPU and the RAMs shown below in the images. My question is that, using this CPU and the RAMs, how much time would it take for me to crack an old school DES algorithm ...
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2answers
129 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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2answers
116 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
80 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
167 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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3answers
4k 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
197 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
224 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
143 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 ...
5
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1answer
156 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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3answers
372 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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1answer
244 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
118 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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0answers
374 views

Hill cipher key space

Key space is the set of all possible keys that can be used to generate a key. We using the number of valid key to describe it. I've given a hill cipher of block size $k$ over alphabet of size $p$, ...
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1answer
112 views

Number of different substitution alphabet ciphers possible with given conditions?

If you consider a 26 letter alphabet, and a cipher where 24 of the letters are sent to themselves, and only 2 letters switch, how many different substitution alphabet ciphers are there, and what ...
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2answers
110 views

Correct way to read a given permutation cipher?

In some literature I am reading – where they are explaining the permutation cipher – they have two examples, which encrypt a small block of text. But they seem to contradict each other: First ...
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2answers
2k views

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?
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2answers
270 views

What is the limit of plaintext required to break the Vigenère encryption?

A theoretical question about the Vigenère cipher: Without any knowledge about the key (not even it's length) can we tell how much known or chosen plaintext is needed for the adversary to completely ...
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4answers
1k 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. ...
3
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1answer
1k 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 ...
2
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1answer
199 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 ...
2
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1answer
159 views

Security of permutation cipher

I would like to understand how secure the permutation cipher is. I would specifically like to understand the following concrete setup: If the alphabet is $L = \{0,1,\dots , 255\}$ and $\sigma_1, ...
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1answer
203 views

How to solve custom made ciphers? [closed]

How would you go about solving basic custom made ciphers - e.g. one generated from this website http://www.wikihow.com/Write-a-Custom-Cipher-Carefully? This is solving it without knowing the method ...
2
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1answer
1k views

Hill cipher cryptanalysis - known plaintext known key size

Hello I want to know how to go about this problem I know the plaintext "abcdef" and the ciphertext. The key size is 2. I really can't figure out how to find the key for decrypting and encrypting.
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2answers
1k 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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4answers
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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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2answers
461 views

What are requirements for this grille?

I'm trying to program a Cardan grille, but I can't find requirements for grille. In my case, grille is square matrix A with $N$ x $N$ size, and elements are $0$ or $1$ (where $1$ means that it is ...
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1answer
339 views

What is total key space in transposition algorithms

How we can measure key space in transposition algorithms? Should we specify the method, like rail fence?
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151 views

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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3answers
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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 ...
3
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1answer
146 views

Is there a general method to crack this type of fractionating cipher?

I've seen a few puzzles based on a type of cipher in which letters in the plaintext are substituted with groups of characters in the ciphertext. The ciphertext only uses a handful of unique ...
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2answers
161 views

Can anyone tell me the name of this cipher please?

Can anyone tell me the name of this cipher please? I know it's a simple substitution cipher, I just don't know the name of it. Cipher Key: help Cipher alphabet: ...
3
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2answers
560 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 ...
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1answer
2k views

Find the key to a Vigenère cipher, given known ciphertext and plaintext

I have the ciphertext and the plaintext. Is it possible to know the key? If not, what do I need to find it?
3
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1answer
226 views

Any historical accounts of cryptanalysis of Jefferson's wheel cipher?

David Kahn in his book "The Codebreakers" wrote about Jefferson's wheel cipher, saying that To this day the Navy uses it… (the book was first published in 1967) Are there any historical accounts ...
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2answers
579 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 ...
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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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Feasibility of using a base 26 LFSR for cryptography by hand

I have been playing with base 26 LFSRs (i.e. using the alphabet) and noticed that the XOR operation for base 26 is just the tabula recta and so can be done very quickly. This made me wonder whether a ...