S-Encrypt(zkey, label, expiration, plaintext):
  PRK_k := HKDF-Extract("gns-xsalsa-ctx-key", zkey)
  PRK_n := HKDF-Extract("gns-xsalsa-ctx-iv", zkey)
  K := HKDF-Expand(PRK_k, label, 256 / 8)
  NONCE := HKDF-Expand(PRK_n, label, 128 / 8)
  IV := NONCE || expiration
  return XSalsa20-Poly1305(K, IV, plaintext)

S-Decrypt(zkey, label, expiration, ciphertext):
  PRK_k := HKDF-Extract("gns-xsalsa-ctx-key", zkey)
  PRK_n := HKDF-Extract("gns-xsalsa-ctx-iv", zkey)
  K := HKDF-Expand(PRK_k, label, 256 / 8)
  NONCE := HKDF-Expand(PRK_n, label, 128 / 8)
  IV := NONCE || expiration
  return XSalsa20-Poly1305(K, IV, ciphertext)