Go code used to play around FPE / FF1 asking this question https://crypto.stackexchange.com/q/75926/1949

postalcode.go

package main

import (
    "encoding/hex"
    "fmt"
    "strings"
    "github.com/capitalone/fpe/ff1"
    //"alphabets"
)

// I'll be damned: https://stackoverflow.com/a/27516559/591064
func min(a, b int) int {
    if a < b {
        return a
    }
    return b
}

func generateCharactersOfBase(length int) string {
    chars := make([]rune, length)

    i := 0
    soFar := 0
    //Start with digits
    for ; i < min(10, length); i++ {
        chars[i] = rune(int('0') + i)
    }

    soFar = i
    //Then with uppercase letters
    for ; i < min(26+soFar, length); i++ {
        chars[i] = rune(int('a') + i - soFar)
    }

    soFar = i
    //Now with lowercase letters
    for ; i < min(26+soFar, length); i++ {
        chars[i] = rune(int('A') + i - soFar)
    }

    return string(chars)
}

type Alphabet struct {
    characterSet string
    baseXDigits  string
}

func (alphabet *Alphabet) Length() int {
    return len(alphabet.characterSet)
}

func NewAlphabet(characterSet string) *Alphabet {
    a := new(Alphabet)
    a.characterSet = characterSet
    a.baseXDigits = generateCharactersOfBase(len(characterSet))
    return a
}

func (alphabet *Alphabet) Convert(text string) string {
    /* Equivalent Python
    >>> import string
    >>> base36 = list(string.digits+string.ascii_uppercase[:26-10])
    >>> ''.join([base36[string.ascii_uppercase.index(c)] for c in 'ABCHELLO'])
    '01274BBE'
    //*/

    chars := make([]rune, len(text))

    for i := 0; i < len(chars); i++ {
        pos := strings.Index(alphabet.characterSet, string(text[i]))
        chars[i] = rune(alphabet.baseXDigits[pos])
    }

    return string(chars)
}


func (alphabet *Alphabet) Revert(text string) string {
    /* Equivalent Python
    >>> import string
    >>> base36 = list(string.digits+string.ascii_uppercase[:26-10])
    >>> ''.join([base36[string.ascii_uppercase.index(c)] for c in 'ABCHELLO'])
    '01274BBE'
    //*/

    chars := make([]rune, len(text))

    for i := 0; i < len(chars); i++ {
        pos := strings.Index(alphabet.baseXDigits, string(text[i]))
        chars[i] = rune(alphabet.characterSet[pos])
    }

    return string(chars)
}


func encryptFF1(key []byte, tweak []byte, plaintext string, alphabet *Alphabet ) string {
    basedPlaintext := alphabet.Convert(plaintext)

    // Create a new FF1 cipher "object"
    // 10 is the radix/base, and 8 is the tweak length.
    FF1, err := ff1.NewCipher(alphabet.Length(), len(tweak), key, tweak)
    if err != nil {
        panic(err)
    }

    // Call the encryption function on an example SSN
    ciphertext, err := FF1.Encrypt(basedPlaintext)
    if err != nil {
        panic(err)
    }

    //Move the result to the original alphabet
    return alphabet.Revert(ciphertext)
}


func decryptFF1(key []byte, tweak []byte, ciphertext string, alphabet *Alphabet ) string {
    basedCiphertext := alphabet.Convert(ciphertext)

    // Create a new FF1 cipher "object"
    // 10 is the radix/base, and 8 is the tweak length.
    FF1, err := ff1.NewCipher(alphabet.Length(), len(tweak), key, tweak)
    if err != nil {
        panic(err)
    }

    // Call the encryption function on an example SSN
    plaintext, err := FF1.Encrypt(basedCiphertext)
    if err != nil {
        panic(err)
    }

    //Move the result to the original alphabet
    return alphabet.Revert(plaintext)
}


// panic(err) is just used for example purposes.
func main() {
    // Key and tweak should be byte arrays. Put your key and tweak here.
    // To make it easier for demo purposes, decode from a hex string here.
    key, err := hex.DecodeString("EF4359D8D580AA4F7F036D6F04FC6A94")
    if err != nil {
        panic(err)
    }

    tweak, err := hex.DecodeString("D8E7920AFA330A73")
    tweak = nil
    if err != nil {
        panic(err)
    }

    a := NewAlphabet("ABCDEFGHIJKLMNOPQRSTUVWXYZ")

    ciphertext := encryptFF1(key, tweak, "HELLO", a)
    plaintext := decryptFF1(key, tweak, ciphertext, a)

    fmt.Println("Ciphertext:", ciphertext)
    fmt.Println("Plaintext:", plaintext)
}