Questions tagged [classical-cipher]

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.

Filter by
Sorted by
Tagged with
0 votes
0 answers
14 views

Implement cypher algorithm from observation [closed]

I have an interesting challenge that I think I kind of know how it works, but I am not quite sure how to implement it and decrypt strings automatically. It seems that, it starts by: 40 and then, adds +...
  • 101
2 votes
1 answer
257 views

Keyless cipher proof

I answered a question on StackOverflow yesterday that was asking why the following keyless encoding/decoding scheme for 16-bit numbers worked: Encoding Note that the "shift" operation is ...
  • 123
1 vote
0 answers
28 views

Is there any known/standard encoding or cipher method that uses a 64-character alphabet from byte 30 (ASCII `0`) to byte 6F (ASCII `o`)?

Is there any known/standard encoding or cipher method that uses a 64-character alphabet from byte 30 (ASCII 0) to byte 6F (ASCII ...
  • 111
2 votes
1 answer
69 views

How do different types of ciphers relate to each other?

I understand that there are symmetric and asymmetric (public-key) cyphers. The first have the same key used for encryption and decryption, while the second use a public key for encryption and a ...
  • 121
2 votes
1 answer
229 views

Mathematical definition of scytale

Most cryptographers know the scytale. It is that cipher where you roll a leather strip around a rod and then write text on it. For encryption you roll it off the stick and for decryption you roll it ...
  • 1,692
2 votes
1 answer
111 views

Shift cypher, perfectly secure?

I know that if only one character is encrypted using a shift cipher, then the shift cipher is perfectly secure. But what if the space of keys is greater than the space of messages? Would it still be ...
  • 23
1 vote
1 answer
119 views

Analysis of the Vigenere cipher

Im just starting out learning some cryptoanalysis techniques. I came across an idea which analyzes the vigenere cipher. Essentially the video explains that there is a standard english probability ...
2 votes
0 answers
54 views

Using ML to detect what classical cipher the ciphertext is encrypted with

I was considering creating an ML project where it is fed some ciphertext by any classical cipher and would return possible ciphers that encrypted the text. I would have to create a sizeable dataset ...
  • 21
0 votes
0 answers
34 views

Given a key, ciphertext and alphabet, what ciphers should we be looking at?

I have the following information: ...
  • 117
4 votes
2 answers
123 views

Does machine cryptography only consist of rotor machines?

For the chronological categorisation of cryptography, I have proceeded as follows: Cryptography by hand (e.g. Caesar, Vigenére, etc., till around 1900/1920) Cryptography using machines (e.g. Enigma, ...
  • 1,692
0 votes
0 answers
51 views

How to encrypt affine cipher in block?

Assuming the alphabet of capital letters, if an affine linear cipher $v\to Av+b \bmod 26$ with block length $3$ maps plaintext ENCRYPTAGAIN to ciphertext ...
0 votes
1 answer
107 views

How are the cipher, the key and the initial message (that is not encrypted) are releted?

Suppose that $m$ is a message that someone player $i$ wants to send to a network of other players $j\neq -i$. The player to prevent his message from cheating by others uses an encyrpstion scheme. Say $...
  • 131
1 vote
1 answer
151 views

What are the equations (Gen, Enc, Dec) for the vigenere cipher?

Long story short, trying to understand how I would write the vigenere cipher mathematically when using Gen, Enc, Dec and I can't figure it out. This is what I've come up with so far. $$\mathrm{Gen}: k ...
  • 11
1 vote
0 answers
68 views

Vigenère cipher with switching keys

I am looking for possible ways of attacking a modified Vigenère cipher. Let's say we have two keys e.g. 'stackoverflow' and 'Vigenère'. The V cipher starts with one of those keys but switches as ...
11 votes
3 answers
4k views

Common Classical Ciphers

When studying cryptography, the first thing every student learns is some historical ciphers. There are way too many of those ciphers to name them all. So my question is: What are the most important ...
  • 1,692
6 votes
4 answers
2k views

Make a Strong, Easy-to-Remember Password Using Classical Cryptography?

Passwords can be tough to remember. For example: H7535637353959595*9608J614625C1313^398583I0397897j^ So Bob wants to make and use a good password for GPG that he ...
  • 3,077
0 votes
1 answer
286 views

What is a ciphertext only attack and how OTP is unbreakable using this attack

I was wondering that what actually is cipherText-only attack and how the One Time Pad Encryption cant be broken by this attack?
1 vote
1 answer
256 views

Key space of a substitution cipher

I was going through a cryptography course, and I found a question there that: What is the size of the key space of the substitution cipher with 26 letters? Its answer was 26! I am not sure what this ...
2 votes
0 answers
59 views

Need some help with decryption code for monoalphabetic substitution

I am working on decrypting ciphertext in monoalphabetic substitution algorithm. I tried to decrypt using frequency histogram of English alphabets individually. The percentage of these frequencies can ...
1 vote
2 answers
107 views

Can all affine cyphers be expressed with this formula

There are $26!$ permutations of the English alphabet. In Modern Cryptography textbook, William Easttom states that the basic formula for any affine cipher is $ax + b \equiv \pmod{26}$. I'm wondering ...
  • 113
1 vote
0 answers
104 views

How can I determine the number of times I have to reapply the Scytale until the original plaintext comes back?

Given "①②③④⑤⑥⑦⑧⑨⑩⑪⑫" as plain text and a 4 rows Scytale inscription with 3 as the length of the longest row, the original plaintext is back after 5 runs: ①④⑦⑩②⑤⑧⑪③⑥⑨⑫ ①⑩⑧⑥④②⑪⑨⑦⑤③⑫ ①⑥⑪⑤⑩④⑨③⑧②⑦...
4 votes
1 answer
131 views

Does a one-time pad of digits 0-9 have any weaknesses?

I'm working on one of those unsolved puzzles, and one theory I have is that one or more sections of it are using digits of pi, e, or some other irrational number as a one-time pad. (The obvious ...
  • 141
1 vote
2 answers
136 views

Does anyone have any hard ciphers to be cracked for fun?

The title says it all. Do you have any ciphers or encoded messages that you could post as a brainteaser? Thanks!
3 votes
2 answers
726 views

How to determine which type of transposition cipher I have?

I have some ciphertext that I have determined using frequency analysis to be most likely encrypted using a transposition cipher, as the letter frequencies are similar to plain English language text. I ...
1 vote
1 answer
57 views

Can I use a Friedman test when decrypting a Playfair cipher?

I’d like to decrypt a Playfair ciphertext; however, I need to find the key word first. Is it possible to use the Friedman test here to find the length of the key word? My cipher has no J’s in it would ...
0 votes
1 answer
93 views

Tranposition Ciphers

I am studying classical ciphers and have the following doubts: Is the row and columnar transposition cipher the same? It is similar since the process of encryption and decryption is the same for ...
's user avatar
1 vote
1 answer
203 views

How to go about decrypting a cipher with known input and output? [closed]

I have a known input and a known output but how would I go about working out what the cipher was used. Below are a few examples of input and output. On the last one I noticed a repeating pattern. ...
  • 117
1 vote
0 answers
129 views

How can I break a Vigenère cipher with partial plain key? [closed]

I've got the cypher text: cöimamiçknsyznhaczstş and a partial plain key: ?ğre??i I've used "?" instead of unknown ...
1 vote
0 answers
58 views

What does the normalizing coefficient do in the Index of Coincidence?

In school we only learned the formula without the normalizing coefficient: But Wikipedia has one that says: I am using IC in a cryptography class to find the key length and am checking various key ...
0 votes
1 answer
71 views

Index of Coincidence Averages List

I am looking for a comprehensive list of the IOC values of any or all classical ciphers. I can't find a list, only pieces. Each cipher has an average IOC value, and I was wondering if there was a ...
  • 220
3 votes
1 answer
112 views

LAT of an SBox, values are even

This is probably a silly question, but I could not find any reference. For the DDT of an SBox, it is easy to see why all the values are even. Is there any related result for LAT (i.e., all its values ...
  • 593
16 votes
7 answers
4k views

Which is the simplest cryptographic algorithm which is close to commercial-level security?

An algorithm is simpler than another if It is easier to explain to a noob with basic high-school math knowledge. It is easier to securely self-implement by an intermediate programmer (Yes I know one ...
  • 271
2 votes
1 answer
110 views

Is there a name for strong human-computable encryption schemes? Are there definitions for "human-computable"?

Background: There is some interest in encryption schemes meant for humans to perform without computational aid. (Relevant questions here, here, and here.) People often cite the VIC pen-and-paper ...
  • 135
1 vote
1 answer
123 views

Can someone help me understand this?

In the book Serious Cryptography It says for a cipher's permutation to be secure "Different keys should result in different permutations. Otherwise it becomes easier to decrypt without the key: ...
1 vote
1 answer
110 views

How to decrypt a short ciphertext that is said to be immune to attacks?

I need to decrypt a ciphertext which is short, and I have the first word of the plaintext. I thought of doing a probable-word attack, but I already know the position of the known word. In addition, ...
  • 13
0 votes
0 answers
69 views

Packing technique for large data storage

Packing multiple plaintext messages into a single ciphertext block is known to increase the effeciency of homomorphic encryption. Can someone let me know if this technique can support large data ? For ...
  • 61
1 vote
2 answers
1k views

How to decipher an ADFGVX cipher?

I got introduced to the ADFGVX cipher, and it seems very tedious to decipher such ciphertexts. If I do not have access to the key square and the keyword, and I am just given the ciphertext and a ...
2 votes
1 answer
107 views

Why was it easier to decode messages that you had several copies of?

During WW1 the German ambassador in Washington, von Bernstorff, blamed their numerous transmissions of the same messages as the cause for the successful decoding done by the British. I don't see the ...
2 votes
2 answers
580 views

How were codes in WW1 reciphered (to enhance security levels) without them turning meaningless?

In WW1, the Germans made their communications secret by encoding their messages. This meant that the message's sender would have a codebook with all possible words and phrases he might use listed in ...
0 votes
0 answers
36 views

Hill cipher. Any way around the not invertible matrices [duplicate]

Question: The plaintext JULY is encrypted using Hill Cipher with m = 2, to give the ciphertext DELW, then determine the encryption and decryption matrix My approach: We will form a 2x2 matrix of ...
0 votes
2 answers
286 views

Prove monoalphabetic cipher is a valid symmetric cipher

I should prove mathematically that: $D_k(E_k(p))=p$ I know $E_k(p)=c=(p+k)\bmod26$ and $D_k(c)=p=(c-k)\bmod26$ Substituting the two formulas to the initial assertion I have $((p+k)\bmod26-k)\...
  • 51
1 vote
0 answers
411 views

How to find/determine the Polybius Square Key and Additive Key, if only the Cipher Text is known, in Nihilist Cipher?

I was solving a Crypto Challenge in a CTF, and I got a Nihilist Cipher Encoded String. Luckily the additive key was given as a hint, and for the polybius Square key, I kinda bruteforced or I just got ...
1 vote
1 answer
106 views

How to make this cipher strong?

Suppose I have an arbitrary 256 bit number $m$ another secret number $k$ of the same bit length, and then I multiply them both modulo a 256 bit prime number $p$ to get $c$ as follows: $$ c = (m\cdot k)...
1 vote
0 answers
30 views

Classic cipher method [closed]

I am struggling with these two cipher texts, I cannot identify which king of cipher was used. First: otnpeetrnneepeufofutisrtrllohcdmetoeheeatyngonrmrm And second: ...
2 votes
1 answer
86 views

Running Key Gibbs Sampling

I am trying to implement the algorithm described in this paper. It uses blocked Gibbs sampling to perform a ciphertext-only attack on text encrypted with a running key cipher. However, I am confused ...
  • 21
0 votes
0 answers
454 views

Is it possible to crack the trifid cipher without the keyword?

I was learning about the classical ciphers and was looking up the methods to break them without the keywords and when I searched up "cryptanalysis of the trifid cipher" I couldn't find anything. I ...
  • 1
1 vote
2 answers
174 views

Concatenating "words," is it possible to create a key that is indistinguishable from one made in a truly random manner?

By concatenating "words" and adding them Modulo 26, can a key be created that is indistinguishable from one generated in a truly random manner? Let's say the key is more than about ten ...
  • 3,077
6 votes
3 answers
2k views

How can frequency analysis be applied to modern ciphers?

I am building a computer program that deciphers Caesar, Vigenere and monoalphabetic substitution ciphers. All of those are susceptible to frequency analysis. However, it does not seem to be real-world ...
  • 165
1 vote
0 answers
64 views

What is the strength of this machine? [closed]

This machine has the stator, with one half of the alphabet, and three rotors with replications of the other half in different order and repeated twice. There can have two rotors in use or the full ...
  • 220
0 votes
2 answers
96 views

Is there a way to easily encode a message without a computer on either end?

I enjoy making notes and passing them to my friends, and I also like making them encrypted for no reason. My friends enjoy it too, but it's a bit annoying to use the Caesar cipher for everything ...

1
2 3 4 5