Simple random password generator. Bonus: Base62 encoder/decoder

genpswd.rb

#/usr/bin/env ruby

require 'securerandom'

ENTROPY_BITS = 256

# Generate a password with an entropy of #{ENTROPY_BITS} bits by encoding a large number
# from the secure random number generator as Base62. Base64 is avoided because some websites
# reject passwords that contain non-alphanumeric characters. This also makes copy&paste easy
# through automatic word-boundary detection (on doubleclick) in many application.

module Base62
  ALPHABET     = Hash[(0...62).to_a.zip((('0'..'9').to_a + ('a'..'z').to_a + ('A'..'Z').to_a))]
  INV_ALPHABET = ALPHABET.invert

  def self.encode(n)
    to_encode = n.to_i

    raise ::ArgumentError, "Must be a non-negative integer: #{n.inspect}" if to_encode < 0

    result = ""

    begin
      result    << ALPHABET[to_encode % 62]
      to_encode /= 62
    end while to_encode > 0

    return result.reverse!
  end

  def self.decode(str)
    result     = 0
    multiplier = 1

    str.reverse.each_char do |c|
      unless INV_ALPHABET.has_key?(c)
        raise ::ArgumentError, "Invalid character inside Base62 encoded String: #{c.inspect}"
      end

      result     += INV_ALPHABET[c] * multiplier
      multiplier *= 62
    end

    result
  end
end

puts Base62.encode(SecureRandom.random_number(2**ENTROPY_BITS))