email encryption

Email, one of the most pervasive and enduring technologies of computer
networking, was invented in about a dozen places by dozens of people in the
1960s. It's hard to lay out a clear history of the technology because it's just
so obvious—pretty much as soon as more than one person could use a computer,
there was some kind of mail facility. These ranged from mainframe-centric
systems where all of the users of a single computer could write messages to each
other, to PC-centric systems where workstations would mount a network share to
store and retrieve messages. Pretty much any scheme you can come up with for
moving messages was probably in use somewhere from roughly the 1960s to the
1990s, by which time the ARPANET-derived family of email implementations had
taken hold.

This form of email has a clearer heritage, to Ray Tomlinson, who came up with
the core idea that addresses could identify both a user and a host, and that
some kind of open protocol could be used to send messages to another host when
necessary. Over time, and with many revisions, Tomlinson's design became SMTP
and was joined by protocols like IMAP that built out the form of email we use
today. This is a form of email that is in some ways decentralized (any user is
free to choose a host) and in other ways centralized (each host assumed to be
continuously online to store-and-forward messages for its users). Tomlinson's
design was flexible enough that we have not had to totally get rid of it, but
enough has changed about the modern Internet that we have had to take a new
approach.

Email has many vexing limitations, artifacts of its age. For example, email
handling should not be assumed to be "8-bit clean"—email protocols were
originally defined over 7-bit ASCII and ran on many machines that used the
eighth bit as a checksum. These machines were prone to changing the last bit of
each byte, or otherwise mishandling email with 8-bit content. That wasn't a
problem when text was completely limited to that…