Every digital message sent runs the risk of being intercepted, so it is important that messages be encrypted to protect our privacy. The addition of a digital signature can help prove the authenticity of a message or document, but it could nevertheless be intercepted and altered. To prevent this, a message digest is created, forming a kind of fingerprint of the original message. Such fingerprints are mathematical functions that concentrate a message into a fixed-length representation. These are also useful in situations where information is to be kept secret until a later moment, but must be capable of being unmodified and authenticated when it is finally revealed. The aim of this paper is to show how cryptography, digital signatures and fingerprint functions may be linked with a famous curiosity of probability, the birthday paradox.