Glossary | Little-Endian
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Little Endian

"Little-endian" refers to the order in which bytes of information can be processed by a computer.

For example, here's a hexadecimal number:

12345678

And here's that same number in little-endian:

78563412

A computer processor will read this number in the same way that we read it as 12345678… it's just two different ways of reading the same thing.

Bitcoin likes using little-endian format for some data, so when working with code, you often have to get things in to little-endian format for them to work.

How does it work?

Let's look at that first number again:

12345678

When you read it from left to right, you start with the most-significant value first (i.e starting with 10000000 and ending with 8). So as you're processing this number in your head, you could say you're starting with the big-end first.

But if you think about it, it could make just as much sense to start with the little-end first.

87654321

In this format, you're working your way up from the least-significant value and finishing with the most-significant value.

Sure, it's a complete shift in the way you're used to reading numbers, but if you're a computer, this is arguably a much more logical way to do it. And that's why many computer processors read data the "little-endian" way.

But wait, this still isn't the same format as the actual little-endian number from the start…

Bytes.

Computers read through data in chunks. Or to be more precise, they process data in "bytes".

Now, 1 byte is just some space in a computer's memory, and holds 2 hexadecimal characters. So as you're reversing the data in to being little-end first, you actually do it 1 byte at a time:

Big Endian:
             +------+------+------+------+
Byte Number: |  0   |  1   |  2   |  3   |
             +------+------+------+------+
Data:        |  12  |  34  |  56  |  78  |
             +------+------+------+------+

Little-Endian:
             +------+------+------+------+
Byte Number: |  0   |  1   |  2   |  3   |
             +------+------+------+------+
Data:        |  78  |  56  |  34  |  12  |
             +------+------+------+------+

And so by reversing 2 characters (1 byte) at a time, we get 12345678 in little-endian:

78563412

Ta da.

So the little end is still first, but you're taking 1 byte of that little-end at a time (and not simply 1 character at a time).

Why is little-endian used in bitcoin?

Because that's the way bitcoin was designed.

It may not be the most user-friendly (or popular1) choice, but modern computers almost always use the little-endian format internally, so this decision is a way of improving speed.2

Example of little-endian in bitcoin.

The majority of the fields in transaction data are in little-endian format.

Remember, when converting from little-endian to big-endian, swap each pair of characters first, then reverse the string.


Code

Bash

Here's how you can swap endianness on the command line (using grep to match every 2 characters, tac to reverse the order, then tr to remove the line breaks to give you a string):

echo 12345678 | grep -o .. | tac | echo $(tr -d '\n')

PHP

Here's a similar method in PHP (switching every 2 characters again):3

// converts a string to little-endian
$string = 12345678;
$little-endian = implode('', array_reverse(str_split($string, 2)));
echo $little-endian;

Gif

Resources

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