Raw Input

Cryptography is the art, or science, of secret writing.
It has been widely used as a way to provide private
conversions very early in human history. In the context
of modern computing, it is associated with private
electronic communications. It may be used to protect
files, e-mail, and various types of network
connections.

Desired Output

is or of
a
in In
of it is
It be to
e
connections

Script and Comments

Script₁

[ 1] s/[^A-Za-z]*\([A-Za-z][A-Za-z]*\)[^A-Za-z]*/\n\1/g
[ 2] /\n/!d
[ 3] s/[^\n]/#&/g
[ 4] :loop
[ 5] s/\(\n[^\n]*\)#/\1/g
[ 6] /\n[^#]/!b loop
[ 7] s/\n#[^\n]*//g
[ 8] s/^.//
[ 9] s/\n/ /g

Comments

Here a word is defined as '[A-Za-z]+'.
Step [1] will insert a newline character at the beginning of every word. Non-word part of the line will also be removed.
Lines that do not have any word will be discarded by Step [2].
Step [3] will insert a '#' before every character of each word of the line.
Step [4] thru [6] constitute a loop. Each iteration of the loop will delete a '#' from every word. The loop will repeat until some word(s) of the line do not have any '#'.
The words do not have any '#' are the shortest ones. Step [7] is used to delete non-shortest words.
Step [8] is used to remove the newline character at the beginning of the line.
We use Step [9] to change every newline character to a space.