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transposition.go
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transposition.go
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// transposition.go
// description: Transposition cipher
// details:
// Implementation "Transposition cipher" is a method of encryption by which the positions held by units of plaintext (which are commonly characters or groups of characters) are shifted according to a regular system, so that the ciphertext constitutes a permutation of the plaintext [Transposition cipher](https://en.wikipedia.org/wiki/Transposition_cipher)
// author(s) [red_byte](https://github.com/i-redbyte)
// see transposition_test.go
package transposition
import (
"errors"
"fmt"
"sort"
"strings"
)
var ErrNoTextToEncrypt = errors.New("no text to encrypt")
var ErrKeyMissing = errors.New("missing Key")
const placeholder = ' '
func getKey(keyWord string) []int {
keyWord = strings.ToLower(keyWord)
word := []rune(keyWord)
var sortedWord = make([]rune, len(word))
copy(sortedWord, word)
sort.Slice(sortedWord, func(i, j int) bool { return sortedWord[i] < sortedWord[j] })
usedLettersMap := make(map[rune]int)
wordLength := len(word)
resultKey := make([]int, wordLength)
for i := 0; i < wordLength; i++ {
char := word[i]
numberOfUsage := usedLettersMap[char]
resultKey[i] = getIndex(sortedWord, char) + numberOfUsage + 1 //+1 -so that indexing does not start at 0
numberOfUsage++
usedLettersMap[char] = numberOfUsage
}
return resultKey
}
func getIndex(wordSet []rune, subString rune) int {
n := len(wordSet)
for i := 0; i < n; i++ {
if wordSet[i] == subString {
return i
}
}
return 0
}
func Encrypt(text []rune, keyWord string) ([]rune, error) {
key := getKey(keyWord)
keyLength := len(key)
textLength := len(text)
if keyLength <= 0 {
return nil, ErrKeyMissing
}
if textLength <= 0 {
return nil, ErrNoTextToEncrypt
}
if text[len(text)-1] == placeholder {
return nil, fmt.Errorf("%w: cannot encrypt a text, %q, ending with the placeholder char %q", ErrNoTextToEncrypt, text, placeholder)
}
n := textLength % keyLength
for i := 0; i < keyLength-n; i++ {
text = append(text, placeholder)
}
textLength = len(text)
var result []rune
for i := 0; i < textLength; i += keyLength {
transposition := make([]rune, keyLength)
for j := 0; j < keyLength; j++ {
transposition[key[j]-1] = text[i+j]
}
result = append(result, transposition...)
}
return result, nil
}
func Decrypt(text []rune, keyWord string) ([]rune, error) {
key := getKey(keyWord)
textLength := len(text)
if textLength <= 0 {
return nil, ErrNoTextToEncrypt
}
keyLength := len(key)
if keyLength <= 0 {
return nil, ErrKeyMissing
}
n := textLength % keyLength
for i := 0; i < keyLength-n; i++ {
text = append(text, placeholder)
}
var result []rune
for i := 0; i < textLength; i += keyLength {
transposition := make([]rune, keyLength)
for j := 0; j < keyLength; j++ {
transposition[j] = text[i+key[j]-1]
}
result = append(result, transposition...)
}
result = []rune(strings.TrimRight(string(result), string(placeholder)))
return result, nil
}