How to Convert Swift String To C byte array and back to Swift String

Question

I'm trying to take a Swift string as a [UInt8] byte array, and then return the same byte array from C code and convert it back to the original Swift string. I'm trying to preserve unicode (so no lossiness in conversions/manipulations). I'm getting an error "Cannot convert value of type 'String.Encoding' to expected argument type 'UInt' on the conversion line in the decrypt function. Thanks in advance for any help!

    // function to encrypt a string with custom "C" code and encode the result into a hex string
func EncryptString( password: String, stringToEncrypt: String) ->String {

    var hexStr = ""

    // convert the String password into utf8 bytes
    let pw = password.data(using: String.Encoding.utf8, allowLossyConversion:false)
    var passwordBytes : [UInt8] = Array(pw!)

    // convert the string to encrypt into utf8 bytes
    let bytes = stringToEncrypt.data(using: String.Encoding.utf8, allowLossyConversion:false)
    var buf : [UInt8] = Array(bytes!)

    // encrypt the string to encrypt with the Unicode password (both are Unicode in Swift)
    encryptData(&passwordBytes, Int32(password.count), &buf, Int32(stringToEncrypt.count))

    // turn the now encrypted "stringToEncrypt" into two character hex values in a string
    for byte in buf {
        hexStr.append(String(format:"%2X", byte))
    }

    // return the encrypted hex encoded string to the caller...
    return hexStr
}


func DecryptString( password: String, hexStringToDecrypt: String) ->String {

    var decryptedStr = ""

    // convert the String password into utf8 bytes
    let pw = password.data(using: String.Encoding.utf8, allowLossyConversion:false)
    var passwordBytes : [UInt8] = Array(pw!)

    // convert the string to encrypt into utf8 bytes
    let bytes = hexStringToDecrypt.data(using: String.Encoding.utf8, allowLossyConversion:false)
    var buf : [UInt8] = Array(bytes!)

    // encrypt the string to encrypt with the Unicode password (both are Unicode in Swift)

    let bytecount = password.count
    decryptData(&passwordBytes, Int32(password.count), &buf, Int32(hexStringToDecrypt.count))
            // turn the now encrypted "hexStringToDecrypt" into int values here is where I get error: Cannot convert value of type 'String.Encoding' to expected argument type 'UInt'
    var unicode_str = NSString(bytes: buf, length: bytecount, encoding: NSUTF32LittleEndianStringEncoding)

    // return the encrypted hex encoded string to the caller...
    return unicode_str
}

}

Point out the exact line causing the error. You have several lines trying to use utf8. And confirm that you are using Swift 3 or later. — rmaddy
– rmaddy, Commented Dec 13, 2017 at 3:00
Thank you! I am using Swift 4 and the current line of code giving the error is this: var unicode_str = NSString(bytes: buf, length: bytecount, encoding: NSUTF32LittleEndianStringEncoding) — user6096790
– user6096790, Commented Dec 13, 2017 at 3:56
What exactly do you think a "C byte array" is?As far as I can tell a byte does not have a language association, it is simply 8-bits(these days). Nor is Swift "C". — zaph
– zaph, Commented Dec 13, 2017 at 3:56
With Swift 3/4 it is better to use the Data type in place of UInt8. Further NSUTF32LittleEndianStringEncoding is not the correct way to convert data to the String type, you need to specify the encoding type of the bytes and that is utf8 because that is how the data was created. — zaph
– zaph, Commented Dec 13, 2017 at 4:19
Thank you everyone who helped me with this problem! rmaddy's answer (now deleted?!) was actually the most helpful for my problem. I was just needing to cast differently as follows: let unicode_str = NSString(bytes: buf, length: hexStringToDecrypt.count/2, encoding: String.Encoding.utf8.rawValue) Please put back your original post rmaddy! I really appreciate your help! Thanks very much to zaph too! — user6096790
– user6096790, Commented Dec 15, 2017 at 5:34

zaph · Accepted Answer · 2017-12-13 05:08:23Z

3

With Swift 3/4 it is better to use the Data type in place of UInt8.

Convert String to Data:

let dat = string.data(using:.utf8)

Convert Data to String:

let str = String(data:dat, encoding:.utf8)

Here is example encryption using Data in place of UInt8, this is not production code, it is missing error handling at a minimum.

func aesCBCEncrypt(data:Data, keyData:Data, ivData:Data) -> Data {
        let cryptLength = size_t(kCCBlockSizeAES128 + data.count + kCCBlockSizeAES128)
        var cryptData = Data(count:cryptLength)
        var numBytesEncrypted :size_t = 0

        let cryptStatus = cryptData.withUnsafeMutableBytes {cryptBytes in
            data.withUnsafeBytes {dataBytes in
                keyData.withUnsafeBytes {keyBytes in
                    ivData.withUnsafeBytes {ivBytes in
                        CCCrypt(CCOperation(kCCEncrypt),
                                CCAlgorithm(kCCAlgorithmAES),
                                CCOptions(kCCOptionPKCS7Padding),
                                keyBytes, keyData.count,
                                ivBytes,
                                dataBytes, data.count,
                                cryptBytes, cryptLength,
                                &numBytesEncrypted)
                    }}}}

    cryptData.count = (cryptStatus == kCCSuccess) ? numBytesEncrypted : 0

    return cryptData;
}

edited Dec 13, 2017 at 5:08

answered Dec 13, 2017 at 4:30

zaph

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1 Comment

user6096790 Over a year ago

Thanks very much for your help! :-)

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