Base64 Decoder

What is Base64 Decoding?

Base64 decoding is the reverse process of Base64 encoding. It converts Base64 encoded ASCII text back into its original binary or text format. This decoder tool helps you quickly decode Base64 strings without any programming knowledge.

Common Uses of Base64 Decoding

• JWT Token Inspection: JSON Web Tokens (JWT) consist of three Base64URL-encoded sections separated by dots: header.payload.signature. Decoding the header reveals the token type and signing algorithm (HS256, RS256, etc.). Decoding the payload exposes claims like user ID (sub), expiration time (exp), issuer (iss), and custom data. This is invaluable for debugging authentication issues, verifying token contents during development, and understanding what information your application is storing in tokens.
• API Response Processing: Many REST APIs return binary data (images, PDFs, encrypted payloads) as Base64 strings within JSON responses. Since JSON is a text format, binary data must be encoded. When consuming these APIs, you need to decode the Base64 string to get the actual file or data. Examples include image generation APIs, document conversion services, and encrypted data transmission endpoints.
• Extracting Data URI Content: Data URIs in HTML/CSS (e.g., data:image/png;base64,iVBORw0KG...) embed files directly in code using Base64. To extract the actual file from a data URI, you decode the Base64 portion. This is useful when scraping web pages, analyzing HTML emails, or converting embedded images back to standalone files for optimization or modification.
• Email Attachment Decoding: MIME-encoded emails use Base64 for attachments. Email clients automatically decode these, but if you're writing email processing scripts, building a custom email client, or analyzing raw email source code (viewing .eml files), you'll need to decode Base64 sections manually to access the actual attachment files.

Security Considerations When Decoding

Base64 decoding is reversible and provides no security. Anyone can decode Base64 strings. Treat decoded content as untrusted unless verified. Learn about encoding security

Base64 Decoding in Programming Languages

Most programming languages provide built-in functions for Base64 decoding. Here are examples with proper error handling:

// Basic decoding
$decoded = base64_decode($encoded);

// Decoding with strict mode (validates input)
$decoded = base64_decode($encoded, true);
if ($decoded === false) {
    // Invalid Base64 string
    throw new Exception("Invalid Base64");
}

// Decoding to UTF-8 string
$text = base64_decode($encoded, true);
if (!mb_check_encoding($text, 'UTF-8')) {
    // Not valid UTF-8
}

// Browser (atob for ASCII only)
try {
    const decoded = atob(encoded);
} catch (e) {
    console.error("Invalid Base64:", e);
}

// Browser with UTF-8 support
function base64Decode(str) {
    try {
        return decodeURIComponent(escape(atob(str)));
    } catch (e) {
        throw new Error("Invalid Base64 or UTF-8");
    }
}

import base64

# Decode to bytes
try:
    decoded_bytes = base64.b64decode(encoded)
    decoded_text = decoded_bytes.decode('utf-8')
except Exception as e:
    print(f"Decoding failed: {e}")

# Validate Base64 before decoding
try:
    decoded = base64.b64decode(encoded, validate=True)
except base64.binascii.Error:
    print("Invalid Base64 string")

import (
    "encoding/base64"
    "fmt"
)

// Standard Base64 decoding
decoded, err := base64.StdEncoding.DecodeString(encoded)
if err != nil {
    fmt.Println("Decoding error:", err)
    return
}

// URL-safe Base64 decoding (for JWT)
decoded, err := base64.URLEncoding.DecodeString(encoded)
if err != nil {
    fmt.Println("Decoding error:", err)
}

import java.util.Base64;
import java.nio.charset.StandardCharsets;

// Basic decoding
try {
    byte[] decodedBytes = Base64.getDecoder().decode(encoded);
    String decoded = new String(decodedBytes, StandardCharsets.UTF_8);
} catch (IllegalArgumentException e) {
    System.err.println("Invalid Base64: " + e.getMessage());
}

// URL-safe decoding
byte[] decoded = Base64.getUrlDecoder().decode(encoded);

require 'base64'

# Basic decoding
begin
  decoded = Base64.decode64(encoded)
rescue ArgumentError => e
  puts "Invalid Base64: #{e.message}"
end

# Strict decoding (validates input)
decoded = Base64.strict_decode64(encoded)

# URL-safe decoding
decoded = Base64.urlsafe_decode64(encoded)

using System;
using System.Text;

// Basic decoding
try
{
    byte[] bytes = Convert.FromBase64String(encoded);
    string decoded = Encoding.UTF8.GetString(bytes);
}
catch (FormatException ex)
{
    Console.WriteLine($"Invalid Base64: {ex.Message}");
}

Related Tools

Need to encode data to Base64? Use our Base64 Encoder to convert text or binary data to Base64 format.

Decoding JWT tokens? Our JWT Decoder automatically decodes all three sections and displays claims in a readable format.

Working with URL-encoded data? Try our URL Decoder to decode percent-encoded strings.