Bitcoin/address validation

You are encouraged to solve this task according to the task description, using any language you may know.


Task

Write a program that takes a bitcoin address as argument,
and checks whether or not this address is valid.

A bitcoin address uses a base58 encoding, which uses an alphabet of the characters 0 .. 9, A ..Z, a .. z, but without the four characters:

   0   zero
   O   uppercase oh
   I   uppercase eye
   l   lowercase ell


With this encoding, a bitcoin address encodes 25 bytes:

 the first byte is the version number, which will be zero for this task ;
 the next twenty bytes are a RIPEMD-160 digest, but you don't have to know that for this task: you can consider them a pure arbitrary data ;
 the last four bytes are a checksum check. They are the first four bytes of a double SHA-256 digest of the previous 21 bytes.


To check the bitcoin address, you must read the first twenty-one bytes, compute the checksum, and check that it corresponds to the last four bytes.

The program can either return a boolean value or throw an exception when not valid.

You can use a digest library for SHA-256.



Example of a bitcoin address



1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62i



It doesn't belong to anyone and is part of the test suite of the bitcoin software.

You can change a few characters in this string and check that it'll fail the test.




 
-module( bitcoin_address ).
 
-export( [task/0, validate/1] ).
 
task() ->
	io:fwrite( "Validate ~p~n", ["1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62i"] ),
	io:fwrite( "~p~n", [validate("1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62i")] ),
	io:fwrite( "Validate ~p~n", ["1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW622"] ),
	io:fwrite( "~p~n", [validate("1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW622")] ).
 
validate( String ) ->
	{length_25, <<Address:21/binary, Checksum:4/binary>>} = {length_25, base58:base58_to_binary( String )},
	<<Version:1/binary, _/binary>> = Address,
	{version_0, <<0>>} = {version_0, Version},
	<<Four_bytes:4/binary, _T/binary>> = crypto:hash( sha256, crypto:hash(sha256, Address) ),
	{checksum, Checksum} = {checksum, Four_bytes},
	ok.
 