tex: update encryption defs

tex/text.tex

@@ -72,12 +72,14 @@ the author usually means the Password Compromise Monitoring Tool program.
 Encryption is the process of transforming certain data, called a
 \emph{message}, using, as Aumasson writes in Serious Cryptography, ``an
 algorithm called a \emph{cipher} and a secret value called the
-key''~\cite{seriouscryptography}.
+key''~\cite{seriouscryptography}. Its purpose is to protect the said message so
+that only its intended recipients that know/hold the key are able to
+\emph{decipher} and read it.
 
 \n{3}{Symmetric encryption}
 
-Symmetric encryption is simply when the \emph{key} used during both
-encryption and decryption is shared.
+Symmetric encryption is simply when the \emph{key} used is to facilitate both
+encryption and decryption operations.
 
 \n{3}{Asymmetric encryption}