Base64 Decode Guide

What is Base64 Decoding?

Base64 decoding is the reverse process of Base64 encoding. It converts Base64-encoded ASCII text back to its original binary or text format. This is essential for reading data that was encoded for transmission or storage.

How Base64 Decoding Works

The decoding process reverses the encoding steps:

1. Takes 4 Base64 characters
2. Converts each character to its 6-bit value
3. Combines 4 × 6 bits = 24 bits (3 bytes)
4. Removes padding (=) and returns original data

Examples

Basic Text Decoding

Email Address

Authentication Credentials

Common Use Cases

1. Reading Email Attachments

Decode Base64-encoded attachments from email messages:

// Email MIME part
Content-Transfer-Encoding: base64

SGVsbG8gV29ybGQ=

// Decode to get original file

2. Processing Data URIs

Extract image or file data from data URIs:

// Extract Base64 from data URI
const dataURI = "data:image/png;base64,iVBORw0KGgo...";
const base64 = dataURI.split(',')[1];
const decoded = atob(base64); // Binary image data

3. API Response Processing

Decode Base64 data received from APIs:

{
  "filename": "report.pdf",
  "content": "JVBERi0xLjQKJeLjz9MK..."
}

// Decode content to get actual file

4. Reading JWT Tokens

Decode JWT (JSON Web Token) payloads:

// JWT format: header.payload.signature
const jwt = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIn0...";
const parts = jwt.split('.');
const payload = JSON.parse(atob(parts[1]));
console.log(payload); // {"sub":"1234567890"}

Programming Examples

JavaScript

// Browser (built-in)
const decoded = atob('SGVsbG8gV29ybGQ=');
console.log(decoded); // Hello World

// Node.js
const decoded = Buffer.from('SGVsbG8gV29ybGQ=', 'base64').toString('utf-8');
console.log(decoded); // Hello World

Python

import base64

encoded = "SGVsbG8gV29ybGQ="
decoded = base64.b64decode(encoded)
print(decoded.decode('utf-8'))  # Hello World

PHP

$encoded = "SGVsbG8gV29ybGQ=";
$decoded = base64_decode($encoded);
echo $decoded;  // Hello World

Java

import java.util.Base64;

String encoded = "SGVsbG8gV29ybGQ=";
byte[] decodedBytes = Base64.getDecoder().decode(encoded);
String decoded = new String(decodedBytes);
System.out.println(decoded);  // Hello World

Error Handling

Invalid Base64 Strings

Not all strings are valid Base64. Watch for these issues:

• Invalid characters: Only A-Z, a-z, 0-9, +, /, = are allowed
• Wrong length: Must be a multiple of 4 (with padding)
• Incorrect padding: = padding must be at the end

// JavaScript error handling
try {
  const decoded = atob('Invalid@Base64!');
} catch (error) {
  console.error('Invalid Base64 string:', error.message);
}

Security Considerations

Best Practices

1. Validate input: Check if string is valid Base64 before decoding
2. Handle errors: Use try-catch blocks for decoding operations
3. Character encoding: Specify UTF-8 when converting to strings
4. Memory limits: Be cautious with large Base64 strings
5. URL-safe variant: Use Base64url decoder for URL-encoded data

Common Errors and Solutions

"Invalid character" Error

Solution: Remove whitespace and newlines from Base64 string:

const base64 = input.replace(/\s/g, '');
const decoded = atob(base64);

"Invalid length" Error

Solution: Ensure proper padding:

// Add padding if missing
while (base64.length % 4 !== 0) {
  base64 += '=';
}

Try It Yourself

Use our free Base64 decoder to convert encoded strings instantly:

Related Encoding Tools

• Base64 Encode - Encode text to Base64
• URL Decode - Decode URL-encoded text
• HTML Entity Decode - Decode HTML entities