Is that all there is?

It seems we've got this "sending secret messages" thing down pat.  What else is there to do?

Even if we're safe from Eve reading our messages, there's still the matter of Mallory.
It turns out that even if our messages are encrypted, Mallory can sometimes modify them in transit!
Mallory won't necessarily know what the message says, but can still change it in an undetectable way.
e.g. bit-flipping attack on stream ciphers
This is counterintuitive, and often forgotten!

How do we make sure the Bob gets the same message Alice sent?