bcrypt and Laravel: 72 Bytes, Not 72 Characters

I expected bcrypt to silently drop characters past 72. I did not expect it to bake in half an emoji. That's what happens with a specific password combination I tested. The original password still works. But strip the emoji (a password manager, a different keyboard, a Unicode normalizer) and you're locked out. Your Laravel validator passed it as valid the whole time. The 72-Byte Rule bcrypt has a hard input limit of 72 bytes. Not characters - bytes. When you call password_hash($password, PASSWORD_BCRYPT) , PHP silently truncates anything past byte 72. Most developers know this in theory. But for ASCII-only apps, it never bites. 72 ASCII characters is already a very long password, and the silent clip is harmless in practice. The trouble starts with multi-byte scripts. How Many Characters Fit? Character set Bytes per char Effective bcrypt limit ASCII 1 72 chars Cyrillic 2 36 chars CJK (Chinese, Japanese, Korean, common block) 3 24 chars Emoji 4 18 chars Past the byte limit, a longer password adds no security at all. A 200-character Cyrillic password hashes identically to its own first 36 characters. Byte 73 and beyond simply do not exist from bcrypt's point of view. So "longer always produces a stronger bcrypt hash" is not true. A Cyrillic user with a 37-character password gets silently truncated at char 36. The hash is still consistent. The user logs in fine, but any variation past character 36 doesn't matter to bcrypt. Annoying from a security standpoint, but it does not break login. The Split-Byte Trap The 72-byte limit cuts at a byte boundary, not a character boundary. If a multi-byte character falls on that cut, bcrypt bakes in an incomplete UTF-8 sequence. // 35 Cyrillic chars = 70 bytes, emoji = 4 bytes, total = 74 bytes $password = str_repeat ( 'А' , 35 ) . '🔑' ; $hash = password_hash ( $password , PASSWORD_BCRYPT ); password_verify ( $password , $hash ); // ? password_verify ( str_repeat ( 'А' , 35 ), $hash ); // ? password_verify ( str_repeat ( 'А' , 36 ), $h