golang mongodb ObjectID base64 encode & decode. https://play.golang.org/p/IACE8_AE-CT

str.go

package main

import (
	"encoding/base64"
	"encoding/hex"
	"fmt"
	"strings"
)

func Hex2bin(s string) []byte {
	ret, err := hex.DecodeString(s)
	if err != nil {
		fmt.Printf("%v", err)
	}
	return ret
}

func Bin2Hex(b []byte) string {
	return hex.EncodeToString(b)
}

func EncodeIdToBase64(input string) string {
	s := base64.StdEncoding.EncodeToString(Hex2bin(input))
	base64Str := strings.Map(func(r rune) rune {
		switch r {
		case '+':
			return '-'
		case '/':
			return '_'
		}

		return r
	}, s)

	base64Str = strings.ReplaceAll(base64Str, "=", "")
	return base64Str
}

func DecodeIdToBase64(input string) string {
	base64Str := strings.Map(func(r rune) rune {
		switch r {
		case '-':
			return '+'
		case '_':
			return '/'
		}

		return r
	}, input)

	if pad := len(base64Str) % 4; pad > 0 {
		base64Str += strings.Repeat("=", 4-pad)
	}

	b, _ := base64.StdEncoding.DecodeString(base64Str)
	return Bin2Hex(b)
}

func main() {
        // here we use mongodb objectID string
	str := "5c9adba7d4579ef73cdc6992"
	
	e := EncodeIdToBase64(str)
	fmt.Printf("encoded = %s \n", e)

	d := DecodeIdToBase64(e)
	fmt.Printf("decoded = %s \n", d)
}