Bitcoin/address validation: Difference between revisions

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 Write a program that takes a [[wp:bitcoin|bitcoin address]] as argument, and checks whether or not this address is valid.  The program can either return a boolean value or throw an exception when not 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, O, I and l.
+With this encoded, a bitcoin address encodes a string 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 bytes, compute the checksum, and check that it corresponds to the last four bytes.
 You can use a digest library for [[SHA-256]].