  We show in details the four quantum key distribution protocols which
initiated the important field of quantum cryptography, using an accessible
language for undergraduate students. We begin presenting the BB84 protocol,
which uses polarization states of photons in order to transmit cryptographic
keys. Thereupon we show the E91 protocol, whose security is based on the use of
singlet states to generate a random sequence of bits. We end the paper with the
BBM92 and the B92 protocol. These last two protocols can be seen as simplified
versions of the first two.
