Complex sentence: In modern cryptography, a protocol can only be deemed to be secure if it comes equipped with a "security proof", a mathematical proof that the security of the protocol reduces to that of the security of the underlying primitives.
Split into simpler sentences: In modern cryptography, a protocol can only be deemed to be secure if it comes equipped with a "security proof". This is a mathematical proof that the security of the protocol reduces to that of the security of the underlying primitives.