It's a secret (3)

(Continuation from Tuesday)
So - cryptography.  Once again two Greek words - cryptos = secret and graphos = writing.  So here there is no attempt to hide the fact that a message is being sent - the idea is that the message ("the plain text") is transformed into some sort of gobbledegook ("the cipher text") which will mean nothing to anybody other than the person the message is intended for.  There are two basic approaches - substitution and transposition. With substitution, each letter, word or phrase of the plain text (depending on which particular "flavour" of substitution you are using) is replaced by something else - something else is substituted for it - hence substitution.  With transposition, each letter (and transposition almost always works at letter level) of the plain text remains the same, but the letters are shuffled around somehow - hence transposition.  We'll see how this works next time.