TL;DR

Always explicitly set encoding for Base64 decoded input as it defaults to ASCII.

Base64.decode64(input).force_encoding('utf-8')

Base64 encoding problem

Base64 comes handy when we need to represent binary data in ASCII. For example when transferring a file or storing encryption key.

The other use case when Base64 is useful is when we can handle only ASCII characters but user may provide non-ascii strings. In such case we encode string on one end transfer encoded input and decode on the other end. You may however be surprised that this may not work straight in Ruby.

encoded = Base64.encode64('Zażółć gęślą jaźń.')
=> "WmHFvMOzxYLEhyBnxJnFm2zEhSBqYcW6xYQu\n"
Base64.decode64(encoded)
=> "Za\xC5\xBC\xC3\xB3\xC5\x82\xC4\x87 g\xC4\x99\xC5\x9Bl\xC4\x85 ja\xC5\xBA\xC5\x84."

Decoding what was previously encoded with standard Base64 class doesn’t return the same value which was encoded. It is because Base64 can’t determine input’s encoding and decodes in 8-bit ASCII:

Base64.decode64(encoded).encoding
=> #<Encoding:ASCII-8BIT>

It is very difficult task to guess input’s encoding but surprisingly decode64 doesn’t accept any parameter allowing to set the desired encoding and just defaults to ASCII.

To make above example work string encoding needs to be forced manually:

Base64.decode64(encoded).force_encoding('utf-8')
=> "Zażółć gęślą jaźń."