tychon / node-vigenere

Small library to crypt, decrypt and crack something in the vignere-code.

This module should be a little helper if you found an exercise sheet instructing the reader to solve a short paragraph of vigenere encoded text.

It provides encryption and decryption of texts with normal vigenere and the autokey cipher. You may also use the brute force search if you know the key size. After doing that you can sort the results by analysing the relative letter frequency.

Concept

The strings are converted to number arrays before calculation. So the offsets can be applied more efficiently.

Furthermore belongs to every text an alphabet that defines the positions of the characters and the overlappingIndex that indicates where the end of the alphabet is glued to the beginning. It

Supported Functions

Encryption and Decryption

Texts can be encrypted and decrypted in the normal mode with the repeating key or in the autokey mode with the plaintext appended to the initial key.

Brute Force

You may try a brute force attack on a ciphertext if you know or guessed the key length. The result can be filtered by keywords.

Lexicographic Analysis

The combinations resulted from the brute force attack can be analysed for the frequency of letters and compared to a given table of relative frequencies. For this method you have to try every combination so the complexity is (key-length ^ alphabet-length) .

An other way is to split the text up into blocks with the same length as the key: For example TRMQICEQEMVCT with the given key length 4 is splitted up into

TRMQ
ICEQ
EMVC
T

Then the lexicographic analysis can be executed over the characters in one column because each column was encrypted with the same key. This method results in a smaller correctness because the actual analysed text is shorter, but it has only the complexity of (key-length * alphabet-length) .

Reference

Search for exports.(functionname) in the code to find detailed documentation.

Constants:

• ALPHABET
• TOP_10_OF_MOST_COMMON_ENGLISH_WORDS
• TOP_10_OF_MOST_COMMON_GERMAN_WORDS
• RELATIVE_FREQUENCIES_OF_LETTERS_IN_THE_ENGLISH_LANGUAGE
• RELATIVE_FREQUENCIES_OF_LETTERS_IN_THE_GERMAN_LANGUAGE

Functions:

• toNumbers(alphabet, text)
• toString(alphabet, numbers)
• encryptText(plaintext, key, alphabet, autokey)
• encrypt(text, key, overlappingIndex, autokey)
• decryptText(ciphertext, key, alphabet, autokey)
• decrypt(text, key, overlappingIndex, autokey)
• bruteForce(text, keyLength, alphabet, autokey, keywords)
• analyseBruteForceAttackResult(combinations, frequenciesTable)
• ciphertextAnalysis(ciphertext, keyLength, alphabet, frequencyTable)
• letterFrequencies(text, frequencyTable)
• sortByLetterFrequencies(combinations, ascending)
• printCombinationsWithFrequencyTable(combinations)