What is Base64?

Base64 is an encoding scheme that converts binary data into a text-only format using 64 safe ASCII characters (A-Z, a-z, 0-9, +, /, =). It's useful for transmitting binary data through text-based systems like email, JSON, or HTTP headers.

Think of it as a translation layer:

NOT encryption: It's encoding, not security. Anyone can decode it.
Text-safe: Safe to include in JSON, XML, or HTTP headers
Reversible: Can be decoded back to original binary data
Standard: Widely supported across all programming languages

⚠️ Important: Base64 is NOT encryption. Don't use it to hide sensitive data. Use it only for data format compatibility.

How Base64 Works

Base64 works by converting groups of three bytes (24 bits) into four characters (6 bits each).

Simple Example:

Original text: "Hello"
Binary: 01001000 01100101 01101100 01101100 01101111

Grouped into 6-bit chunks:
010010 000110 010101 101100 011011 000110 1111 (padded to 1111 00 = 110000)

Base64 values: 18 6 21 44 27 6 48
Base64 chars:  S G V s b G w=

Result: "SGVsbG8="
             ^^-- padding

Base64 Character Set

The 64 characters used:

0-25: A-Z (uppercase)

26-51: a-z (lowercase)

52-61: 0-9

62-63: + and / (or - and _ in URL-safe variant)

Padding

If the input length isn't divisible by 3, padding with = characters ensures the output length is always divisible by 4.

Real-World Use Cases

1. Email Attachments

Email systems work with text only. Binary files are Base64-encoded before sending in emails, then decoded when received.

2. API File Uploads

When uploading files via JSON APIs, files are often Base64-encoded as string values:

{
  "filename": "profile.jpg",
  "data": "iVBORw0KGgoAAAANSUhEUgAAAAUA..." // Base64 encoded image
}

3. HTTP Headers

Basic authentication encodes credentials in Base64:

Authorization: Basic dXNlcm5hbWU6cGFzc3dvcmQ=
// Decodes to: username:password

4. Data URLs (Embedded in HTML)

Embed small images directly in HTML without separate requests:

<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUg..." />

5. Certificates & Keys

SSL certificates, SSH keys, and cryptographic material are often stored in Base64 format (PEM files).

6. Database Blobs

Storing binary data in text databases (XML, JSON) often requires Base64 encoding.

How to Encode to Base64

JavaScript

// String to Base64
const str = 'Hello, World!';
const encoded = btoa(str);
console.log(encoded); // "SGVsbG8sIFdvcmxkIQ=="

// File to Base64 (using FileReader)
const file = document.getElementById('input').files[0];
const reader = new FileReader();

reader.onload = (e) => {
  const base64 = e.target.result.split(',')[1];
  console.log(base64); // Base64-encoded file
};
reader.readAsDataURL(file);

Python

import base64

# String to Base64
text = "Hello, World!"
encoded = base64.b64encode(text.encode()).decode()
print(encoded)  # "SGVsbG8sIFdvcmxkIQ=="

# File to Base64
with open('image.jpg', 'rb') as f:
    file_content = f.read()
    encoded = base64.b64encode(file_content).decode()
    print(encoded)

Go

package main

import (
    "encoding/base64"
    "fmt"
)

func main() {
    text := "Hello, World!"
    encoded := base64.StdEncoding.EncodeToString([]byte(text))
    fmt.Println(encoded) // "SGVsbG8sIFdvcmxkIQ=="
}

How to Decode from Base64

JavaScript

// Base64 to String
const encoded = 'SGVsbG8sIFdvcmxkIQ==';
const decoded = atob(encoded);
console.log(decoded); // "Hello, World!"

// Base64 to Bytes
const bytes = new Uint8Array(encoded.split('').map(c => c.charCodeAt(0)));
console.log(bytes);

Python

import base64

# Base64 to String
encoded = 'SGVsbG8sIFdvcmxkIQ=='
decoded = base64.b64decode(encoded).decode()
print(decoded)  # "Hello, World!"

# Base64 to File
with open('image.jpg', 'wb') as f:
    f.write(base64.b64decode(encoded_data))

Go

package main

import (
    "encoding/base64"
    "fmt"
)

func main() {
    encoded := "SGVsbG8sIFdvcmxkIQ=="
    decoded, err := base64.StdEncoding.DecodeString(encoded)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(decoded)) // "Hello, World!"
}

File Uploads with Base64

Browser Upload Example

Convert file to Base64 and send via API:

const input = document.getElementById('fileInput');
input.addEventListener('change', async (e) => {
  const file = e.target.files[0];
  const reader = new FileReader();
  
  reader.onload = async (event) => {
    const base64 = event.target.result.split(',')[1];
    
    // Send to server
    const response = await fetch('/api/upload', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({
        filename: file.name,
        content_type: file.type,
        data: base64
      })
    });
    
    console.log(await response.json());
  };
  
  reader.readAsDataURL(file);
});

Server-Side Processing (Python)

import base64
from flask import Flask, request

app = Flask(__name__)

@app.route('/api/upload', methods=['POST'])
def upload():
    data = request.json
    
    # Decode Base64
    file_data = base64.b64decode(data['data'])
    
    # Save file
    with open(f"uploads/{data['filename']}", 'wb') as f:
        f.write(file_data)
    
    return {'success': True, 'message': 'File uploaded'}

Common Mistakes to Avoid

❌ Using Base64 for Security

Base64 is NOT encryption. It's easily decoded by anyone. Never Base64-encode passwords or sensitive data thinking it's secure.

❌ Large File Uploads

Base64 increases file size by ~33%. For large files, use multipart/form-data instead:

// ❌ Bad: Base64 encoding large files
const base64Data = "iVBORw0KGgoAAAANSUhEU..."; // 33% larger

// ✅ Good: Use FormData for files
const formData = new FormData();
formData.append('file', file);
fetch('/upload', { method: 'POST', body: formData });

❌ Forgetting Padding

Ensure proper padding with = characters. Most libraries handle this automatically, but custom implementations might miss it.

❌ Wrong Base64 Variant

Know when to use standard Base64 (+, /) vs URL-safe Base64 (-, _). Use URL-safe for URLs and file names.

❌ Not Handling Errors

Always validate and handle decoding errors:

try {
  const decoded = atob(userInput);
} catch (e) {
  console.error('Invalid Base64', e);
  // Handle error properly
}

Quick Reference

Format: Uses 64 safe ASCII characters
Size: ~33% larger than original binary
Speed: Fast to encode/decode
Security: NOT secure (it's just encoding)
Reversible: Can be perfectly decoded
Standard: Supported everywhere

Base64 Encoding Guide: When & How to Use It

What is Base64?

How Base64 Works

Base64 Character Set

Padding

Real-World Use Cases

1. Email Attachments

2. API File Uploads

3. HTTP Headers

4. Data URLs (Embedded in HTML)

5. Certificates & Keys

6. Database Blobs

How to Encode to Base64

JavaScript

Python

Go

How to Decode from Base64

JavaScript

Python

Go

File Uploads with Base64

Browser Upload Example

Server-Side Processing (Python)

Common Mistakes to Avoid

❌ Using Base64 for Security

❌ Large File Uploads

❌ Forgetting Padding

❌ Wrong Base64 Variant

❌ Not Handling Errors

Quick Reference

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