Exercise 1: Which Key Does Which Operation — Possible Solution
====================================================================

ENCRYPT  -> uses the RECIPIENT's PUBLIC key.
  Anyone can encrypt a message TO a given recipient, because the
  recipient's public key is, by definition, public.

DECRYPT  -> uses the RECIPIENT's PRIVATE key.
  Only the recipient can reverse the encryption and read the message,
  because only they hold the matching private key.

SIGN     -> uses the SIGNER's PRIVATE key.
  Only the signer can produce a valid signature, because producing one
  requires their private key, which only they hold.

VERIFY   -> uses the SIGNER's PUBLIC key.
  Anyone can check whether a signature is valid, because the signer's
  public key is, by definition, public.

Summary table:

  Operation | Whose key?  | Public or private? | Who can do it?
  ----------|-------------|---------------------|------------------
  Encrypt   | Recipient's | Public               | Anyone
  Decrypt   | Recipient's | Private              | Only the recipient
  Sign      | Signer's    | Private              | Only the signer
  Verify    | Signer's    | Public               | Anyone

WHY THIS WORKS AS AN ANSWER
------------------------------
This reuses the chapter's own comparison table directly. The pattern
worth internalizing is that "public key" operations (encrypt, verify)
are always the ones anyone can perform, while "private key" operations
(decrypt, sign) are always restricted to the key's one legitimate
owner -- and critically, encryption and signing use the PRIVATE key at
opposite ends of the process (private key decrypts vs. private key
signs), which is exactly the "opposite direction" relationship the
chapter's intro paragraph describes.