(Swift 3,4,5...) RSA Encrypt and Decrypt Strings

Swift 3,4,5... sample code to RSA public-key encrypt and decrypt strings using public and private keys.

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func chilkatTest() {
    // This example assumes the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    let rsa = CkoRsa()!

    // This example also generates the public and private
    // keys to be used in the RSA encryption.
    // Normally, you would generate a key pair once,
    // and distribute the public key to your partner.
    // Anything encrypted with the public key can be
    // decrypted with the private key.  The reverse is 
    // also true: anything encrypted using the private
    // key can be decrypted using the public key.

    // Generate a 1024-bit key.  Chilkat RSA supports
    // key sizes ranging from 512 bits to 4096 bits.
    var success: Bool = rsa.generateKey(1024)
    if success != true {
        print("\(rsa.lastErrorText!)")
        return
    }

    // Keys are exported in XML format:
    var publicKey: String? = rsa.exportPublicKey()
    var privateKey: String? = rsa.exportPrivateKey()

    var plainText: String? = "Encrypting and decrypting should be easy!"

    // Start with a new RSA object to demonstrate that all we
    // need are the keys previously exported:
    let rsaEncryptor = CkoRsa()!

    // Encrypted output is always binary.  In this case, we want
    // to encode the encrypted bytes in a printable string.
    // Our choices are "hex", "base64", "url", "quoted-printable".
    rsaEncryptor.encodingMode = "hex"

    // We'll encrypt with the public key and decrypt with the private
    // key.  It's also possible to do the reverse.
    success = rsaEncryptor.importPublicKey(publicKey)

    var usePrivateKey: Bool = false
    var encryptedStr: String? = rsaEncryptor.encryptStringENC(plainText, bUsePrivateKey: usePrivateKey)
    print("\(encryptedStr!)")

    // Now decrypt:
    let rsaDecryptor = CkoRsa()!

    rsaDecryptor.encodingMode = "hex"
    success = rsaDecryptor.importPrivateKey(privateKey)

    usePrivateKey = true
    var decryptedStr: String? = rsaDecryptor.decryptStringENC(encryptedStr, bUsePrivateKey: usePrivateKey)

    print("\(decryptedStr!)")

}