How does base64 encoding work?

Base64 Fundamentals: From Bytes to Characters

Photo by Alexander Sinn on Unsplash

Have you ever wondered how an image gets embedded directly into a webpage, or how an email can carry a document attachment without breaking?

Chances out, Base64 encoding is playing a crucial role behind the scenes. Often misunderstood as encryption, Base64 is actually a clever way to transform binary data into a text-friendly format.

Let’s dive in and demystify Base64.

1. What Exactly is Base64?

At its core, Base64 is an encoding scheme that translates binary data (which computers understand as sequences of 0s and 1s) into a plain ASCII string. Think of it as a universal translator for data. Its primary purpose is to ensure that data remains uncorrupted when it travels across systems that are primarily designed to handle text, such as:

• Email systems
• Web protocols like HTTP (in URLs, JSON, XML)
• Text-based files and databases

Here are the most common use cases of Base64 in web development:

1. Data URIs for Embedding Assets: This is perhaps the most well-known use. Base64 allows you to embed the actual binary content of small files directly into HTML, CSS, or JavaScript code, rather than linking to separate files.

2. Images:

• HTML: <img src="data:image/png;base64,iVBORw0KGgoAAAANSUh..."/>
• CSS (as background images): background-image: url("data:image/svg+xml;base64,PHN2ZyB2ZXJzaW9uPSIx...")...;
• Fonts: Embedding custom fonts directly into CSS using @font-face.
• src: url('data:font/woff;base64,d09GRgABAAAAAE...'), format('woff');
• SVGs: Often embedded directly without Base64, but can be Base64 encoded if necessary.
• Small Icons/Favicons: Particularly useful for very small, frequently used assets to reduce HTTP requests.

3. API Payloads (JSON/XML): When you need to send or receive binary data (like an image file for upload or a file attachment) via a REST API that primarily deals with text formats like JSON or XML, Base64 is used.

• Sending Data: A client (e.g., a browser uploading a profile picture) will convert the image file to a Base64 string and send it within a JSON object to the server.
• { "username": "user123", "avatar": "data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBD..." }
• Receiving Data: A server might send small binary data (e.g., a dynamically generated QR code) as a Base64 string within a JSON response.

4. Local Storage and Session Storage: The browser’s localStorage and sessionStorage APIs are designed to store string key-value pairs. If you need to store small binary data (e.g., a cached image, small audio clip for offline use), you must convert it to a Base64 string first.

localStorage.setItem('cachedImage', 'data:image/png;base64,iVBORw0KGgoAAAA...');

5. URL Parameters: URLs have limitations on characters that can be used (e.g., spaces, special symbols). While URL encoding handles most, Base64 can be used to encode small pieces of arbitrary binary data (like cryptographic hashes, compressed data, or IDs) so they can be safely passed in a URL query string without corruption.

• https://example.com/item?id=aGVsbG8gd29ybGQ= (where aGVsbG8gd29ybGQ= is "hello world" Base64 encoded)

6. Web Sockets: While WebSockets natively support sending binary frames, sometimes developers might choose to send binary data as Base64 encoded strings within text frames, especially if it simplifies their application logic or integrates better with existing text-processing pipelines.

7. .Client-Side File Handling/Previews: When users select a file using an <input type="file"> element, JavaScript's FileReader API can read the file's content as a Base64 encoded Data URL (reader.readAsDataURL()). This is commonly used to:

Display a preview of an uploaded image before it’s actually sent to the server.

• Perform client-side processing on file content.

8. HTTP Basic Authentication Headers: Although less common for direct developer interaction with Base64, HTTP Basic Authentication sends credentials (username:password) Base64 encoded in the Authorization header.

• Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ== (where QWxhZGRpbjpvcGVuIHNlc2FtZQ== is "Aladdin:open sesame" Base64 encoded). Note: This is encoding, not encryption, and should only be used over HTTPS.

The “64” in Base64 refers to the fact that it uses an alphabet of 64 distinct characters (typically A-Z, a-z, 0–9, +, and /) to represent the encoded binary information.

2. How Does Base64 Work? (A Step-by-Step Example)

The magic of Base64 lies in converting groups of 8-bit bytes into groups of 6-bit units, which then map directly to its 64-character alphabet. Let’s walk through an example using the simple word “Man”:

Original Text: “Man”

Step 1: Convert Each Character to its 8-bit ASCII Binary: Every character in “Man” has a corresponding ASCII (American Standard Code for Information Interchange) decimal value, which we then convert into an 8-bit binary number.

• ‘M’ (ASCII 77) → 01001101
• ‘a’ (ASCII 97) → 01100001
• ’n’ (ASCII 110) → 01101110

Step 2: Concatenate All Binary Strings: Now, we string all these 8-bit binary numbers together to form one continuous stream of bits.

010011010110000101101110 (Total 24 bits)

Step 3: Divide into 6-bit Groups: This is the key step! We divide our long binary string into chunks of 6 bits. Why 6? Because 26=64, which perfectly matches the 64 characters in the Base64 alphabet.

010011 010110 000101 101110

Step 4: Convert Each 6-bit Group to its Decimal Value: Next, we convert each 6-bit binary chunk back into its decimal equivalent.

• 010011 → 19
• 010110 → 22
• 000101 → 5
• 101110 → 46

Step 5: Map Decimal Values to the Base64 Alphabet: Finally, we use the standard Base64 alphabet to find the character that corresponds to each decimal value.

• 19 → ‘T’
• 22 → ‘W’
• 5 → ‘F’
• 46 → ‘u’

The Result: The Base64 encoded string for “Man” is TWFu.

What about Padding? (=) If the original binary data doesn't perfectly fit into groups of 24 bits (which yield 4 Base64 characters), padding characters (=) are added at the end. Each = signifies that 6 bits of the last group were missing/padded with zeros. For example:

• “Ma” (2 bytes) → “TWE=” (one = means 2 bytes short of a full 3-byte block)
• “M” (1 byte) → “TQ==” (two = means 1 byte short of a full 3-byte block)

3. The Advantages of Using Base64

Base64 isn’t just a technical curiosity; it solves real-world problems:

• Data Integrity: It’s the go-to method for ensuring binary data, like images or PDF attachments, doesn’t get corrupted or altered when sent through text-only mediums (like older email servers).
• Text-Friendly: It transforms any type of data into a format compatible with systems designed for plain text, preventing issues with special characters or control codes.
• Embedding Data Directly: Small binary assets, such as icons or small images, can be directly embedded into HTML, CSS, or JavaScript code using Data URIs, reducing the number of HTTP requests.
• URL Safety: A variant exists that replaces the + and / characters (which have special meaning in URLs) with - and _, making Base64 strings safe to use directly in web addresses.

4. The Disadvantages of Base64

While useful, Base64 isn’t without its drawbacks:

• Increased Data Size: The biggest trade-off is the size increase. Base64 encoding makes the original data approximately 33% larger. This means more bandwidth consumption and potentially slower transfers for large files. How? due to addtional padding of 2 bits. we have 8 bit available but we are only using 6 bits.
• Not Encryption: This is a crucial point! Base64 is an encoding, not an encryption. It provides no security or privacy. Anyone can easily decode a Base64 string back to its original form. If you need security, use robust encryption algorithms.
• Reduced Readability: The encoded output is a seemingly random string of characters, making it impossible for humans to read or understand the original content without decoding it.

5. When to Use Base64 (and When to Avoid It)

Knowing when to deploy Base64 effectively is key:

When to Use Base64:

• Email Attachments: This is one of its original and most common uses.
• Data URIs: Embedding small images or fonts directly into web pages (e.g., <img src="data:image/png;base64,...">).
• API Data Transfer: Sending binary data (like user profile pictures) as part of a JSON or XML payload in a web API request.
• Storing Binary in Text Databases: When a database field is strictly text-based, and you need to store small binary blobs.
• URL Parameters: Passing small amounts of binary-like data (e.g., a unique ID or token) safely in a URL query string.

When to Avoid Base64:

• Large Files: For large images, videos, or documents, the 33% size overhead is significant. It’s generally better to serve these files directly or use streaming.
• Security-Critical Data: Never use Base64 as a substitute for encryption. Confidential data should always be encrypted.
• Performance-Critical Applications: If network performance and bandwidth usage are absolutely paramount, and the medium supports raw binary, avoid unnecessary encoding.
• Human-Readable Output: If the output data needs to be easily read and understood by a human without special tools, Base64 is not the right choice.

I hope this taught you to learn something new.

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