The rsa_verify code was vulnerable to a BB'06 attack, allowing to forge
signatures for arbitrary messages if and only if the public key exponent is
3. Since the updates key is hardcoded to 65537, there is no risk for
youtube-dl, but I don't want vulnerable code in the wild.
The new function adopts a way safer approach of encoding-and-comparing to
replace the dangerous parsing code.
Filippo Valsorda