Base64 length calculation

base64formulapaddingstring

After reading the base64 wiki …

I'm trying to figure out how's the formula working :

Given a string with length of n , the base64 length will be enter image description here

Which is : 4*Math.Ceiling(((double)s.Length/3)))

I already know that base64 length must be %4==0 to allow the decoder know what was the original text length.

The max number of padding for a sequence can be = or ==.

wiki :The number of output bytes per input byte is approximately 4 / 3 (33%
overhead)

Question:

How does the information above settle with the output length enter image description here ?

Best Answer

Each character is used to represent 6 bits (log2(64) = 6).

Therefore 4 chars are used to represent 4 * 6 = 24 bits = 3 bytes.

So you need 4*(n/3) chars to represent n bytes, and this needs to be rounded up to a multiple of 4.

The number of unused padding chars resulting from the rounding up to a multiple of 4 will obviously be 0, 1, 2 or 3.

Related Solutions

Javascript – How to you encode a string to Base64 in JavaScript

You can use btoa() and atob() to convert to and from base64 encoding.

There appears to be some confusion in the comments regarding what these functions accept/return, so…

btoa() accepts a “string” where each character represents an 8-bit byte – if you pass a string containing characters that can’t be represented in 8 bits, it will probably break. This isn’t a problem if you’re actually treating the string as a byte array, but if you’re trying to do something else then you’ll have to encode it first.
atob() returns a “string” where each character represents an 8-bit byte – that is, its value will be between 0 and 0xff. This does not mean it’s ASCII – presumably if you’re using this function at all, you expect to be working with binary data and not text.

See also:

How do I load binary image data using Javascript and XMLHttpRequest?

Most comments here are outdated. You can probably use both btoa() and atob(), unless you support really outdated browsers.

Check here:

Java – Decode Base64 data in Java

As of Java 8, there is an officially supported API for Base64 encoding and decoding. In time this will probably become the default choice.

The API includes the class java.util.Base64 and its nested classes. It supports three different flavors: basic, URL safe, and MIME.

Sample code using the "basic" encoding:

import java.util.Base64;

byte[] bytes = "Hello, World!".getBytes("UTF-8");
String encoded = Base64.getEncoder().encodeToString(bytes);
byte[] decoded = Base64.getDecoder().decode(encoded);

The documentation for java.util.Base64 includes several more methods for configuring encoders and decoders, and for using different classes as inputs and outputs (byte arrays, strings, ByteBuffers, java.io streams).

Related Topic