GetPublicKey.swift

// if you already have the private key
let publicKey = SecKeyCopyPublicKey(privateKey)

// if you don't have the private key
func getPublicKey() -> SecKey? {
    let query: [String: Any] = [
        kSecClass as String: kSecClassKey,
        kSecAttrApplicationTag as String: "com.cardosoedgar.key",
        kSecReturnRef as String: true]

    var keyRef: CFTypeRef?
    let status = SecItemCopyMatching(query as CFDictionary, &keyRef)
    guard status == errSecSuccess else {
      return nil
    }

    let privateKey = keyRef as! SecKey
    return SecKeyCopyPublicKey(privateKey)
}