Skip to main content
18 votes
Accepted

Why are the ciphertexts of Ansible Vault's AES256-encrypted files disproportionately composed of '3' and '6'?

Is this a peculiarity of the bytes of the underlying AES ciphertext, or of the way Ansible/Python represents the binary ciphertext as a digit string? It's an artifact of how it represents a binary ...
poncho's user avatar
  • 148k
13 votes

Examples of frauds discovered because someone tried to mimic a random sequence

Following my comment, and even though it's a bit different from what you ask: I really enjoyed the story here where the use of an incorrect pseudorandom number generator led to the arrest of members ...
Geoffroy Couteau's user avatar
12 votes

Examples of frauds discovered because someone tried to mimic a random sequence

Actually, bank and expenses fraud is identified for the very opposite reason. Many of human societal numbers begin with a small digit, with number 1 appearing as the leading significant digit about 30%...
Paul Uszak's user avatar
  • 15.5k
12 votes

Examples of frauds discovered because someone tried to mimic a random sequence

Since you mentioned you're also interested in crypto examples, here's one from probably the most famous cipher machine in history: Enigma. Most of Enigma's key settings were distributed in a keysheet ...
Robert Williams's user avatar
9 votes

Examples of frauds discovered because someone tried to mimic a random sequence

Allan Franklin wrote about this in "The Mendel-Fisher Controversy." As practical tests of results against theoretical distributions became better, Sir R. A. Fisher noted that Mendel's ...
ttw's user avatar
  • 191
7 votes
Accepted

Substitution ciphers amended with cipher block chaining: susceptible to frequency analysis?

CBC in this context means that each letter of the ciphertext is used to help encrypt the next letter. Let's say, making up some numbers, that a certain ciphertext letter is C. The next plaintext ...
Josiah's user avatar
  • 1,042
6 votes
Accepted

How can frequency analysis be applied to modern ciphers?

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 ...
kelalaka's user avatar
  • 49k
6 votes

Developing algorithm for detecting plain text via frequency analysis

My humble contribution to this thread: in order to give some plaintext an "English-score", I found out that the Bhattacharyya coefficient is quite usefull: The Bhattacharyya coefficient is an ...
noamgot's user avatar
  • 297
5 votes
Accepted

How do AES Substitution box's offer any additional security since it's 1 to 1?

Philosophically any invertible cipher (it would be useless if not invertible) is just a one to one map, so no cipher would be secure under your argument. Specifically, the Sbox has been carefully ...
kodlu's user avatar
  • 22.7k
5 votes
Accepted

How to break homophonic substitutions and nomenclators with too many symbols?

Regarding the first part of the question, I will just link to another answer I wrote in the past: Why don't homophones hide multiple-letter patterns? Summary: If you adjust the frequencies so that ...
tylo's user avatar
  • 12.7k
4 votes

Developing algorithm for detecting plain text via frequency analysis

The accepted answer is correct, but I wanted to add an update since the sample set of frequencies given in the accepted answer doesn't always work so well for the Matasano Crypto Challenges. In ...
hft's user avatar
  • 219
4 votes
Accepted

How to break a Quagmire 3 cipher?

All the Quagmire ciphers (see e.g. here for definitions) are combinations of a Vigenère shift cipher and a keyword-based simple substitution cipher, where the substitution cipher is used to scramble ...
Ilmari Karonen's user avatar
4 votes
Accepted

Entropy test vs frequency analysis

I was wondering, would calculating the entropy of the resulting plaintext not serve the same purpose, if not to a better degree seeing as it's not confined to the english language? First note that a ...
SEJPM's user avatar
  • 46.1k
3 votes

Substitution ciphers amended with cipher block chaining: susceptible to frequency analysis?

Encryption process in CBC mode of your amended cipher is performed as \begin{align} C_1 &= Subs(P_1 + IV \bmod 26)\\ C_i &= Subs(P_i + C_{i-1}\bmod 26),\;\; 1 < i \leq nb, \end{align} where ...
kelalaka's user avatar
  • 49k
3 votes

How to do a frequency analysis?

Any course material or book with a title like introduction to cryptography (or similar) will address this topic and provide examples. Alternatives are: Frequency analysis and substitution cipher. The ...
tylo's user avatar
  • 12.7k
3 votes
Accepted

handling junk bytes in chi squared character freq analysis

One approach you might try is applying additive smoothing via pseudocounts. At its most basic, this can be as simple as adding 1 to the number of times each byte (or $n$-tuple, or whatever) occurs in ...
Ilmari Karonen's user avatar
3 votes
Accepted

Cryptoanalysis using Frequency Analysis

This is a monoalphabetic substitution cipher - the same letter in the plaintext will always be represented as a identical letter in the ciphertext (for example the plaintext-letter "E" is ...
AleksanderCH's user avatar
  • 6,462
3 votes

Is a Shift Cipher with Random Insertions Unbreakable?

Frequency analysis would still apply albeit larger quantities of ciphertext might be required. If your system is to introduce a random character with probability $p$ and the plaintext frequency of a ...
Daniel S's user avatar
  • 24.1k
2 votes
Accepted

How to select a good scale for letter frequency in a text?

You can compute a statistical distance measure between the observed letter frequencies in your candidate plaintext and the typical letter frequencies in English text. There are a number of different ...
Ilmari Karonen's user avatar
2 votes
Accepted

Frequency analysis secure language

Substitution cipher (e.g., Caesar) and polyalphabetic substitution cipher (e.g., Vigenere) are deterministic encryption schemes. The same plaintext will always result in the same ciphertext. So, they ...
Shan Chen's user avatar
  • 2,725
2 votes
Accepted

Why I can get a key from frequency in Caesar Cipher?

This will make it more clear: A B C D E F G H I J ... 0 1 2 3 4 5 6 7 8 9 ... The Caesar cipher starts with the letter and goes through the alphabet using the ...
Maarten Bodewes's user avatar
  • 93.2k
2 votes

How can frequency analysis be applied to modern ciphers?

The Caesar cipher, Vigenere, monoalphabetic substitution, the autokey cipher, columnar transposition, the Playfair cipher, the Rail Fence cipher, disrupted transposition, the ADFGVX cipher, Quagmire ...
Patriot's user avatar
  • 3,132
2 votes

How can frequency analysis be applied to modern ciphers?

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 ...
waitaria's user avatar
2 votes

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

The normalizing coefficient makes sure that for a random text the IC will be (close to) 1.0, independent of the size of the alphabet.
Jigg's user avatar
  • 21
2 votes

Examples of frauds discovered because someone tried to mimic a random sequence

Not necessarily discovering fraud but causing it: people who played online casino games in the early days used to predict slot and poker cards by knowing the random number generator logic being used.
edwinc's user avatar
  • 121
1 vote

Frequencies of bigrams in English alphabet(with space)

Does http://storage.googleapis.com/books/ngrams/books/datasetsv2.html help? Not sure how I can get to 30 characters for an acceptable answer length on this one... And hi.
Paul Uszak's user avatar
  • 15.5k
1 vote

Frequency Analysis on DES, AES, 3DES using Cryptool2

My question is - when performing frequency analysis on such ciphers, do I input the hex ciphertext or do I need to transform it into text? How can I comment on that (based on letter frequency or bit ...
Maarten Bodewes's user avatar
  • 93.2k
1 vote

Entropy test vs frequency analysis

I think that letter frequency and entropy can give us some tips if the password source is biased. What advantage can these measures give us if the passwords are chosen with a good percentage of ...
Crypto Learner's user avatar
1 vote

Is there a ready report about character order frequency in most common passwords?

In terms of English language, what you are asking about are digram frequencies. http://www.viviancook.uk/SpellStats/DigFreqs.html
kodlu's user avatar
  • 22.7k
1 vote

Determining susceptibility to frequency analysis

Whatever parameters you pass in will affect the resulting hashes. If you compute only H(Firstname), then two users with the same first name will lead to the same ...
Squeamish Ossifrage's user avatar

Only top scored, non community-wiki answers of a minimum length are eligible