Symmetric- and asymmetric-key encryption algorithms.
Encryption algorithms transform plaintext in some way that
is dependent on a key or key pair, producing ciphertext.
Symmetric algorithms
Encryption can easily be reversed, if (and, hopefully, only if)
one knows the same key.
In other words, sender and receiver share the same key.
The symmetric encryption modules here all support the interface described in PEP
272, "API for Block Encryption Algorithms".
If you don't know which algorithm to choose, use AES because it's
standard and has undergone a fair bit of examination.
Asymmetric algorithms
For asymmetric algorithms, the key to be used for decryption is totally
different and cannot be derived in a feasible way from the key used
for encryption. Put differently, sender and receiver each own one half
of a key pair. The encryption key is often called public whereas
the decryption key is called private.