S-Encrypt(zkey, label, expiration, plaintext):
  PRK_k := HKDF-Extract("gns-aes-ctx-key", zkey)
  PRK_n := HKDF-Extract("gns-aes-ctx-iv", zkey)
  K := HKDF-Expand(PRK_k, label, 256 / 8)
  NONCE := HKDF-Expand(PRK_n, label, 32 / 8)
  BLOCK_COUNTER := 0x0000000000000001
  IV := NONCE || expiration || BLOCK_COUNTER
  return CTR-AES256(K, IV, plaintext)

S-Decrypt(zkey, label, expiration, ciphertext):
  PRK_k := HKDF-Extract("gns-aes-ctx-key", zkey)
  PRK_n := HKDF-Extract("gns-aes-ctx-iv", zkey)
  K := HKDF-Expand(PRK_k, label, 256 / 8)
  NONCE := HKDF-Expand(PRK_n, label, 32 / 8)
  BLOCK_COUNTER := 0x0000000000000001
  IV := NONCE || expiration || BLOCK_COUNTER
  return CTR-AES256(K, IV, ciphertext)