The secret note
You want to pass a secret note to a friend, but a nosy kid sits between you and handles every note on the way. That gives you two problems.
AES-GCM fixes both at once. It has two parts.
Part 1 · Nobody can read it
AES is a very strong way to scramble a message. You and your friend share one secret key, like a password only the two of you know.
With the key, your friend unscrambles the note. Without it, the note is random junk: x7#Qp!9zL…. Guessing every possible key would take all the computers on Earth longer than the universe has existed.
Part 2 · Nobody can change it
Scrambling alone has a weakness. The nosy kid cannot read the junk, but they could flip a few characters. Your friend would unscramble a slightly wrong message and never notice.
So GCM adds a tag: a wax seal only you and your friend know how to make, worked out from the key and every letter of the scrambled note. Change even one letter and the seal no longer matches. Your friend sees that and throws the note away.
One extra rule
Every note also gets a fresh random ticket number. It stops two notes that say the same thing from scrambling into the same junk, so nobody can spot patterns like “every Friday’s note looks identical, so it must say pizza day.”
The golden rule: never use the same nonce twice with the same key. Break it and GCM breaks badly. An attacker can start working out messages and even forge fake seals.
Putting it together
The shared secret password
Only you and your friend have it.
The scrambler
Nobody else can read the note.
The wax seal
Nobody can change the note without getting caught.
A fresh ticket number on each note
Two identical notes never look identical.
Where you already meet it
AES-GCM is popular because it is fast (most computer chips have hardware built for it) and it does both jobs in one step. In Verndr, your files are encrypted on your device with AES-GCM before anything is sent. If a file were tampered with, it would not open.
