title: Why is senders private key used in encryption?
body: <p>I'm looking through libsodium-examples of public-key-cryptography and it seems the senders private key is used in addition to the receivers public key when encrypting the plaintext.</p>
  
  <p>Extract from the relevant <a href="https://download.libsodium.org/libsodium/content/public-key_cryptography/authenticated_encryption.html" rel="nofollow">example</a>:</p>
  
  <blockquote>
    <p>The crypto_box_easy() function encrypts a message m whose length is
    mlen bytes, with a recipient's public key pk, a sender's secret key sk
    and a nonce n.</p>
  </blockquote>
  
  <p>What is the point of this? My understanding was that the senders private key only was used when signing a message? </p>