5

Classical ciphers operate on letters. If we consider the frequency attack on classical ciphers it considers the frequency of the letters. Modern ciphers, if we consider only block ciphers, operates on blocks - 64, 128, or more bit blocks. Let see how you can perform a frequency attack on modern ciphers to infer some data. Example attack on Databases: Let ...


5

It depends on what you mean by widespread (military) use. The Russian VIC cipher, which used a lagged Fibonacci generator, classified as a "straddling bipartite monoalphabetic substitution superenciphered by modified double transposition," as identified by the FBI in 1953, could have been applied to widespread military intelligence use, especially for ...


4

The cryptosystem enciphers plaintext $m$ as ciphertext $c \gets (m\cdot k) \bmod p$ where $k$ is the secret key, and $p$ is a prime. It is (silently) assumed $0<m<p$ and $k\bmod p\ne 0$; otherwise decryption by $m \gets (c\cdot k^{-1}) \bmod p$ is not possible. Not told, and of paramount importance: is $k$ used just once, or reused? Once: the system ...


4

I suspect it was a semi-deliberate feature. That is, while it probably wasn't a design goal in and of itself, it neatly solved a mechanical issue that would otherwise have required a more complicated and failure-prone solution. What was the issue? Simply, it was making the third wheel only advance one step at a time, rather than 26 steps in a row. That's ...


3

There is a much better explanation available. See Graham Ellsbury's page here for full details, and a summary below: Intuitively, what one does first is to drag the crib (known plaintext) accross the ciphertext and rule out relative positions which map a plaintext letter to itself in the ciphertext (since it is known that the Enigma mapping has no fixed ...


3

It was very likely not put in place deliberately, since it doesn't seems to make sense to have it or not have it in place deliberately. I assume it was just overlooked. Since double-stepping occured only in the middle rotor it just slightly changed the period of the machine. The machine (with 3 rotors) was originally meant to have a period of $26 \times 26 ...


3

As I noted in this earlier answer to a related question, the rotor permutations in your example are written using a shorthand notation that only shows the output alphabet of the permutation, with the implicit assumption that the input alphabet is always ABCDEFGHIJKLMNOPQRSTUVWXYZ. That is to say, your rotor descriptions: EKMFLGDQVZNTOWYHXUSPAIBRCJ | Rotor ...


2

In the VIC cipher, that important five-digit number was created by the user when doing the encryption. It surely had to have been created on the spur of the moment without much equipment, or perhaps without any at all, as you remarked, because these people were spies, and they would not have wanted to get caught with some strange device. The CIA's online ...


2

Yes, Vigenère cipher is vulnerable to frequency analysis. BUT! It requires some pre-processing first. I propose to walk us through a small example of how frequency analysis can help decrypting Vigenère cipher in order to get a better idea of the process. Firstly, it is important to notice that if you were to choose a key length of 1, then Vigenère cipher ...


2

I'd suggest SIGABA. I don't know about the original poster's knowledge, but my knowledge includes the demonstrated fact that one person, working alone, is highly likely to overlook attacks and produce a weak cipher. So if I'm the only one going back in time, I'm going to pick a well-proven system rather than anything I invent myself. If I could bring a ...


2

The concept is called superencipherment or multiple encryption, and works as follows: The original message M is enciphered using the code book, resulting in an enciphered message C1. C1 is then treated as the plaintext, and enciphered using another method, resulting in a new ciphertext C2. C2 is then sent to the receiver The receiver uses the decryption ...


2

The lemma you need for this is that $$((a \bmod m) + b) \bmod m = (a + b) \bmod m \tag{1}$$ for all integers $a$, $b$ and for all positive integers $m$. In other words, this lemma says that, if you're going to add (or subtract) a bunch of numbers together and then reduce the sum modulo $m$, it doesn't matter whether or not you reduce any (or all) of the ...


2

In general, the idea of a Gibbs sampler is to sample from the joint distribution $\Pr(X_1, X_2, \dotsc, X_n)$ on many variables $(X_1, X_2, \dotsc, X_n)$ using samplers for the conditional distributions on each single variable given all the others: $\Pr(X_1 \mid X_2, \dotsc, X_n),$ $\Pr(X_2 \mid X_1, X_3, \dotsc, X_n),$ $\Pr(X_3 \mid X_1, X_2, X_4, \dotsc, ...


2

The Caesar cipher, Vigenere, monoalphabetic substitution, the autokey cipher, columnar transposition, the Playfair cipher, the Rail Fence cipher, disrupted transposition, the ADFGVX cipher, Quagmire III, etc., are all interesting and good to understand, but compared to modern cryptographic systems they are almost always utterly worthless for providing real-...


1

There is also more modern research in order to attack complete ADFGVX ciphertexts (both partly-known-plaintext attacks and ciphertext-only-attacks). See chapter 6 of the following PhD thesis: Lasry, G.: A Methodology for the Cryptanalysis of Classical Ciphers with Search Metaheuristics, Kassel University Press. kassel university press (2018) at https://d-nb....


1

Dooley says that simple substitution or transposition were used by both sides from 1914 to mid-1916--for radio, telegraph, and phone messages. Radio became more important in the summer of 1916, and then cryptanalysis became the name of the game for determining order of battle.[1] Crystal set radio often required messages to be sent repeatedly due to signal ...


1

Anyone could be confused by this, especially in the case of the Zimmermann telegram. Codebooks come in three flavors: one-part, two-part, and hybrid. You described a one-part codebook. These are somewhat vulnerable to guessing because they are alphabetical for both plaintext and ciphertext. For example, using English, if you know that 22667 stands for ...


1

This answer applies to cryptographic algorithms in general, rather than specific cases where the plaintext data must have specific properties. For such a situation, see kelalaka's answer. Frequency analysis is not a new attack, and as such, the encryption functions in use today are designed to resist frequency analysis. Having said this, there are two ...


1

It's actually rather easy to break the Vigenère cipher. The first published method to decrypt Vigenère without having knowledge of the key (breaking the cipher) was done in 1863 and this could be done by hand. Since the keyword is always repeating you would find a lot of repetitions in a ciphertext. For example as described in Wikipedia: If the ...


1

The Playfair cipher would partially avoid this in that it does not encipher two letters that are the same. So the letter pair "LL" would be enciphered as "LX" "L_" (or sometimes just "LX"), producing different ciphertext letters for each pair. It would allow ciphertext letter repeats of the form "AB BC" where "AB" is one pair of letters that are enciphered ...


1

The M-209 cipher machine cannot be expressed as an LFSR because it is not LFSR-based. It is its own construction, using six adjustable key wheels whose initial positions acted as the key for ciphering. Its operation is vaguely similar to a standard substitution cipher, but the substitution positions would change for each letter, making cryptanalysis more ...


1

You have asked about the authentication of a hand-written message, but let's also include an integrity check as a desired service. Let's say that your message was encrypted with a one-time pad. The following methods of early modern cryptography will not do: (1) Russian copulation (which is designed to further conceal names, headers, salutations, etc.)...


1

Transposition and substitution are heavily used in today's block ciphers. S-box means substitution box, and P-boxes transpose bits. When you say transposition cipher you are referring to classical cryptography. The heyday of the transposition cipher was before the end of World War I. The ADFGX and the ADFGVX were fractionated transposition ciphers used by ...


1

Try looking at the Vigenere, or here is a complete list of keyword ciphers: All the ciphers listed here are based on keys, but could be a little difficult to use. Some include making the keyword into a numerical sequence. By numbering each letter according to their place in the alphabet. If a letter appears twice, number them left to right. i.e. CAESAR = ...


1

So, I found a way to do it but it's neither elegant nor a general solution. It relies on the (heavy) assumption that the initial chosen keyword is English and a second assumption that the resulting cipher text is also English. It then becomes a relatively crude bruteforcing exercise: 1) Choose an English word 2) Create a cipher instantiated by that word 3)...


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