mfa.go

package common

import (
	"bytes"
	"crypto/hmac"
	"crypto/sha1" //nolint:gosec // G501: Blocklisted import crypto/md5: weak cryptographic primitive
	"encoding/base32"
	"encoding/binary"
	"fmt"
	"strconv"
	"strings"
	"time"
)

// Append extra 0s if the length of otp is less than 6
// If otp is "1234", it will return it as "001234".
func prefix0(otp string) string {
	if len(otp) == 6 {
		return otp
	}
	for i := 6 - len(otp); i > 0; i-- {
		otp = "0" + otp
	}
	return otp
}

func getHOTPToken(secret string, interval int64) (string, error) {
	// Converts secret to base32 Encoding. Base32 encoding desires a 32-character
	// subset of the twenty-six letters A–Z and ten digits 0–9
	key, err := base32.StdEncoding.DecodeString(strings.ToUpper(secret))
	if err != nil {
		return "", err
	}
	bs := make([]byte, 8)
	binary.BigEndian.PutUint64(bs, uint64(interval))

	// Signing the value using HMAC-SHA1 Algorithm
	hash := hmac.New(sha1.New, key)
	hash.Write(bs)
	h := hash.Sum(nil)

	// We're going to use a subset of the generated hash.
	// Using the last nibble (half-byte) to choose the index to start from.
	// This number is always appropriate as it's maximum decimal 15, the hash will
	// have the maximum index 19 (20 bytes of SHA1) and we need 4 bytes.
	o := h[19] & 15

	var header uint32
	// Get 32 bit chunk from hash starting at the o
	r := bytes.NewReader(h[o : o+4])
	err = binary.Read(r, binary.BigEndian, &header)
	if err != nil {
		return "", err
	}
	// Ignore most significant bits as per RFC 4226.
	// Takes division from one million to generate a remainder less than < 7 digits
	h12 := (int(header) & 0x7fffffff) % 1000000

	// Converts number as a string
	otp := strconv.Itoa(h12)

	return prefix0(otp), nil
}

func getTOTPToken(secret string) (string, error) {
	// The TOTP token is just a HOTP token seeded with every 30 seconds.
	interval := time.Now().Unix() / 30
	return getHOTPToken(secret, interval)
}

// MfaValidation validates TOTP mfa with given secret and token.
func MfaValidation(secret string, token string) error {
	curToken, err := getTOTPToken(secret)
	if err != nil {
		return err
	}
	if curToken != token {
		return fmt.Errorf("token is invalid or expired")
	}
	return nil
}