BCrypt uses first 72 bytes instead of 72 chars.

irb log.txt

irb(main):011:0> pass1 = "あ" * 50
=> "ああああああああああああああああああああああああああああああああああああああああああああああああああ"

irb(main):012:0> pass1.size
=> 50

irb(main):013:0> pass1.bytes.size
=> 150

irb(main):014:0> secret1 = BCrypt::Password.create pass1
=> "$2a$10$HvRPl.mj8fdRidaEtj9qK.8ecHygryxsKEkhBUNbo.GEzHA0UdoYC"

irb(main):015:0> pass2 = "あ" * 51
=> "あああああああああああああああああああああああああああああああああああああああああああああああああああ"

irb(main):017:0> pass2.bytes.size
=> 153

irb(main):018:0> secret1 == pass2
=> true