Base64 in Go

Go’s encoding/base64 package provides four ready-made encodings and lets you define custom ones.

Strings

package main

import (
	"encoding/base64"
	"fmt"
)

func main() {
	s := base64.StdEncoding.EncodeToString([]byte("Grüße 👋"))
	fmt.Println(s) // R3LDvMOfZSDwn5GL

	b, err := base64.StdEncoding.DecodeString(s)
	if err != nil {
		panic(err) // illegal base64 data at input byte N
	}
	fmt.Println(string(b))
}

Go strings are UTF-8, so no conversion step is needed.

Decode errors

illegal base64 data at input byte 12 gives the exact position of the problem. Typical causes: Base64URL input decoded with StdEncoding, missing padding (use the Raw variant), or a data URI prefix. The decoder ignores \r and \n but nothing else.

Strict mode

enc := base64.StdEncoding.Strict() // rejects non-zero trailing bits

Without it, QQ== and QR== both decode to A. Strict mode matters when Base64 values are compared or used as keys.

Streaming

in, _ := os.Open("big.bin")
out, _ := os.Create("big.b64")
w := base64.NewEncoder(base64.StdEncoding, out)
io.Copy(w, in)
w.Close() // flushes the final partial block and padding

Forgetting Close() silently drops the last 1–2 bytes. Decode streams with base64.NewDecoder(base64.StdEncoding, r).

Appending without allocations (Go 1.22+)

buf = base64.StdEncoding.AppendEncode(buf, data)

Check your output with the Base64 decoder.