EXERCISE 3 — Match the primitive to the job
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(a) Encrypting a 4 GB video stream efficiently.
    PRIMITIVE: Symmetric encryption (AES), ideally an AEAD mode (AES-GCM).
    GUARANTEE: Confidentiality.
    Why: only symmetric crypto is fast enough for bulk data; asymmetric
    would be hopelessly slow at that volume.

(b) Letting strangers send you a secret with nothing pre-shared.
    PRIMITIVE: Asymmetric / public-key encryption (RSA, or ECDH key
    exchange).
    GUARANTEE: enables a shared secret -> ultimately Confidentiality.
    Why: you publish a public key anyone can use; only your private key
    can unlock what they send. This is the only way to bootstrap a secret
    between parties who share nothing in advance.

(c) Proving a downloaded file wasn't corrupted, given a TRUSTED digest.
    PRIMITIVE: Cryptographic hash (SHA-256).
    GUARANTEE: Integrity (keyless).
    Why: re-hash the file and compare to the published digest. Works here
    BECAUSE the digest is stated as trusted/untampered — no key needed for
    integrity alone when the reference value is trustworthy.

(d) Proving a message both is intact AND came from someone sharing your key.
    PRIMITIVE: MAC / HMAC (hash + secret key).
    GUARANTEE: Integrity + sender authenticity.
    Why: only a key-holder can produce a valid tag, so a match proves the
    message wasn't altered and was produced by someone with the shared
    key — integrity bound to a sender, which a bare hash can't do.

BIG PICTURE:
  No single primitive covers everything. TLS combines them:
    - asymmetric (b) to establish a shared key + authenticate the server,
    - then symmetric/AEAD (a) for fast confidential data,
    - with MAC/AEAD integrity (d) on every record,
    - and hashing (c) used throughout (in signatures, key derivation, etc.)