Documented in Volume 1 of the UNIX Programmers Manual.
% crypt < infile > outfile
will first prompt you for an encryption key (with no echo, similar
to passwd). Then the contents of infile are encrypted using the key,
and the results are stored into outfile. Take care to specify
redirection of the output via '>' or '|' otherwise you may scramble
your terminal definition.
% crypt < outfile > clearfile
will effectively reverse the above, storing clear text into clearfile.
WARNING: crypt is not secure from a determined attack, but using compact(1)
on a file before using crypt is rumored to make it a little more secure.