Fix cleartext leak in signatures of UTF-8 encoded messages

[schbol]

Issue

When signing UTF-8 encoded messages containing multi-byte characters (like German umlauts ö, ü, ä), Ed25519 signatures are incorrectly sized and contain cleartext from the message.

Demonstration:

require 'rbnacl'

key = RbNaCl::SigningKey.generate
message = "Björn Müller from München"

signature = key.sign(message)
puts signature # => "\xE0\x89\xA5\xA1\xA9khߠ\x9E\xF2A\xB3<\xB1\xE3m]\x925\x8ELU\xD0F\xAAǪi\xA6П\xE1ӥ\xD9ֻ\xD0z\xF5\xED䧏\xFC\u0015Cηg%\xAF\xAE\xECg\x80G\xFB\xB5\u0013\xEA\rBjörn Müller from Münche"
puts signature.length # => 90

Security Impact

This is a security issue. The signature leaks portions of the cleartext message:

• Ed25519 signatures should always be exactly 64 bytes of cryptographic data
• With this bug, signatures become 80-100+ bytes and contain readable cleartext
• The cleartext portion starts at byte 64 and continues for the length of the multi-byte character expansion

This violates the fundamental expectation that signatures are opaque cryptographic values and should not leak message content. Also, I did not immediately find any documentation that rbnacl does only accept binary encoded strings. Even if it exists somewhere, not following the documentation should lead to an exception, not leaking parts of the signed message.

Fix

Force messages to binary encoding (ASCII-8BIT) before passing them to libsodium functions:

• SigningKey#sign: Convert message to binary before signing
• VerifyKey#verify: Convert message to binary before concatenation