ถอดรหัส Base64 ใน Go — คู่มือ

package main

import (
	"encoding/base64"
	"fmt"
)

func main() {
	// Raw bytes → Base64 encoded → decoded back to raw bytes
	original := []byte("service_token:xK9mP2qR")
	// Encoded: "c2VydmljZV90b2tlbjp4SzltUDJxUg=="

	encoded := base64.StdEncoding.EncodeToString(original)
	decoded, _ := base64.StdEncoding.DecodeString(encoded)
	fmt.Println(string(decoded) == string(original)) // true
}

package main

import (
	"encoding/base64"
	"fmt"
	"log"
)

func main() {
	// Standard Base64 — the alphabet uses + and / with = padding
	encoded := "eyJob3N0IjoiZGItcHJvZC51cy1lYXN0LTEiLCJwb3J0Ijo1NDMyfQ=="
	decoded, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		log.Fatalf("decode error: %v", err)
	}
	fmt.Println(string(decoded))
	// {"host":"db-prod.us-east-1","port":5432}
}

package main

import (
	"encoding/base64"
	"fmt"
	"log"
)

func main() {
	src := []byte("eyJob3N0IjoiZGItcHJvZCIsInBvcnQiOjU0MzJ9")
	dst := make([]byte, base64.RawStdEncoding.DecodedLen(len(src)))

	n, err := base64.RawStdEncoding.Decode(dst, src)
	if err != nil {
		log.Fatalf("decode: %v", err)
	}
	fmt.Println(string(dst[:n]))
	// {"host":"db-prod","port":5432}
}

package main

import (
	"encoding/base64"
	"fmt"
)

func main() {
	// JWT header payload — URL-safe, no padding
	jwtHeader := "eyJhbGciOiJSUzI1NiIsImtpZCI6IjIwMjMtMDkifQ"

	// Wrong: StdEncoding fails on URL-safe input without padding
	_, err1 := base64.StdEncoding.DecodeString(jwtHeader)
	fmt.Println("StdEncoding error:", err1)
	// StdEncoding error: illegal base64 data at input byte 43

	// Correct: RawURLEncoding — no padding, URL-safe alphabet
	decoded, err2 := base64.RawURLEncoding.DecodeString(jwtHeader)
	fmt.Println("RawURLEncoding ok:", err2, "→", string(decoded))
	// RawURLEncoding ok: <nil> → {"alg":"RS256","kid":"2023-09"}
}

package main

import (
	"encoding/base64"
	"fmt"
	"log"
	"os"
	"strings"
)

func main() {
	raw, err := os.ReadFile("certificate.pem.b64")
	if err != nil {
		log.Fatalf("read file: %v", err)
	}

	// Strip newlines — Base64 files often have line breaks every 76 chars
	cleaned := strings.ReplaceAll(strings.TrimSpace(string(raw)), "\n", "")

	decoded, err := base64.StdEncoding.DecodeString(cleaned)
	if err != nil {
		log.Fatalf("decode: %v", err)
	}

	if err := os.WriteFile("certificate.pem", decoded, 0600); err != nil {
		log.Fatalf("write: %v", err)
	}
	fmt.Printf("decoded %d bytes → certificate.pem\n", len(decoded))
}

package main

import (
	"encoding/base64"
	"encoding/json"
	"fmt"
	"log"
	"net/http"
)

type SecretResponse struct {
	Name    string `json:"name"`
	Payload string `json:"payload"` // Base64-encoded secret value
	Version int    `json:"version"`
}

func fetchAndDecodeSecret(secretURL string) ([]byte, error) {
	resp, err := http.Get(secretURL)
	if err != nil {
		return nil, fmt.Errorf("http get: %w", err)
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("unexpected status: %d", resp.StatusCode)
	}

	var secret SecretResponse
	if err := json.NewDecoder(resp.Body).Decode(&secret); err != nil {
		return nil, fmt.Errorf("decode json: %w", err)
	}

	value, err := base64.StdEncoding.DecodeString(secret.Payload)
	if err != nil {
		return nil, fmt.Errorf("decode base64: %w", err)
	}
	return value, nil
}

func main() {
	value, err := fetchAndDecodeSecret("https://api.example.com/secrets/db-password")
	if err != nil {
		log.Fatalf("fetch secret: %v", err)
	}
	fmt.Printf("secret value: %s\n", value)
}

package main

import (
	"encoding/base64"
	"fmt"
	"io"
	"log"
	"os"
)

func streamDecodeFile(srcPath, dstPath string) error {
	src, err := os.Open(srcPath)
	if err != nil {
		return fmt.Errorf("open source: %w", err)
	}
	defer src.Close()

	dst, err := os.Create(dstPath)
	if err != nil {
		return fmt.Errorf("create dest: %w", err)
	}
	defer dst.Close()

	decoder := base64.NewDecoder(base64.StdEncoding, src)
	written, err := io.Copy(dst, decoder)
	if err != nil {
		return fmt.Errorf("stream decode: %w", err)
	}
	fmt.Printf("written %d bytes to %s\n", written, dstPath)
	return nil
}

func main() {
	if err := streamDecodeFile("backup.tar.b64", "backup.tar"); err != nil {
		log.Fatal(err)
	}
}

# Decode a Base64 string (Linux / macOS)
echo "eyJob3N0IjoiZGItcHJvZCIsInBvcnQiOjU0MzJ9" | base64 --decode
# {"host":"db-prod","port":5432}

# Decode and pretty-print with jq (pipe the JSON output)
echo "eyJob3N0IjoiZGItcHJvZCIsInBvcnQiOjU0MzJ9" | base64 --decode | jq .
# {
#   "host": "db-prod",
#   "port": 5432
# }

# Decode a Base64-encoded file to binary
base64 --decode < encrypted_payload.b64 > encrypted_payload.bin

# macOS uses -D flag instead of --decode
echo "c2VjcmV0LXRva2Vu" | base64 -D

go get filippo.io/base64

package main

import (
	"fmt"
	"log"

	"filippo.io/base64"
)

func main() {
	// Drop-in replacement — same API as encoding/base64
	encoded := "eyJob3N0IjoiY2FjaGUtcHJvZCIsInBvcnQiOjYzNzl9"
	decoded, err := base64.StdEncoding.DecodeString(encoded)
	if err != nil {
		log.Fatalf("decode: %v", err)
	}
	fmt.Println(string(decoded))
	// {"host":"cache-prod","port":6379}
}

package main

import (
	"encoding/base64"
	"encoding/json"
	"fmt"
	"log"
	"strings"
)

type JWTPayload struct {
	Subject  string   `json:"sub"`
	Issuer   string   `json:"iss"`
	Expiry   int64    `json:"exp"`
	Roles    []string `json:"roles"`
}

func decodeJWTPayload(token string) (*JWTPayload, error) {
	parts := strings.Split(token, ".")
	if len(parts) != 3 {
		return nil, fmt.Errorf("invalid JWT: expected 3 segments, got %d", len(parts))
	}

	// JWT uses RawURLEncoding — URL-safe alphabet, no = padding
	raw, err := base64.RawURLEncoding.DecodeString(parts[1])
	if err != nil {
		return nil, fmt.Errorf("decode payload: %w", err)
	}

	var payload JWTPayload
	if err := json.Unmarshal(raw, &payload); err != nil {
		return nil, fmt.Errorf("unmarshal payload: %w", err)
	}
	return &payload, nil
}

func main() {
	token := "eyJhbGciOiJSUzI1NiJ9.eyJzdWIiOiJ1c3ItNDQyIiwiaXNzIjoiYXV0aC5leGFtcGxlLmNvbSIsImV4cCI6MTc0MTk1NjgwMCwicm9sZXMiOlsiYWRtaW4iLCJhdWRpdG9yIl19.SIGNATURE"

	payload, err := decodeJWTPayload(token)
	if err != nil {
		log.Fatalf("jwt: %v", err)
	}
	fmt.Printf("Subject: %s\n", payload.Subject)
	fmt.Printf("Issuer:  %s\n", payload.Issuer)
	fmt.Printf("Roles:   %v\n", payload.Roles)
	// Subject: usr-442
	// Issuer:  auth.example.com
	// Roles:   [admin auditor]
}