Intermediate
Cryptography
Symmetric encryption
Encryption where the same key both locks and unlocks the data.
In simple terms
One shared secret protects the message in both directions. AES is the standard algorithm; GCM is the standard mode.
Analogy
A padlock you can snap shut yourself: whoever has the matching key opens it — so the hard part is handing over that key safely.
Going deeper
AES processes 128-bit blocks with 128/192/256-bit keys; in GCM mode it doubles as a GHASH-based authenticator. Reusing a nonce with the same key can leak the XOR of plaintexts and even expose the authentication key — libraries enforce nonce discipline for you.
Common mistake
Choosing the mode matters more than choosing the cipher: ECB leaks patterns even though AES itself is unbroken.