Decrypting DES with CommonCrypto in Swift 3

Question

I want to decrypt a DES encrypted String with CommonCrypto.

I already imported CommonCrypto with a Bridging Header into my Project.

By try and error I managed to call CCCrypt function and it even returns kCCSuccess.

But after that my result is still empty.

Here is my code:

if let key = "12345678".data(using: .utf8), let data = "inMyOriginalCodeYouWouldSeeADESEncryptedStringHere/ahw==".data(using: .utf8) {
        var numBytesDecrypted: size_t = 0
        var result = Data(capacity: data.count)

        let err = result.withUnsafeMutableBytes {resultBytes in
            data.withUnsafeBytes {dataBytes in
                key.withUnsafeBytes {keyBytes in
                    CCCrypt(CCOperation(kCCDecrypt), CCAlgorithm(kCCAlgorithmDES), CCOptions(kCCOptionPKCS7Padding), keyBytes, kCCKeySizeDES, nil, dataBytes, data.count, resultBytes, data.count, &numBytesDecrypted)
                }
            }
        }

        if err != CCCryptorStatus(kCCSuccess) {
            NSLog("Decryption failed! Error: \(err.description)")
        }

        print(numBytesDecrypted)
        print(result)

        return String(data: result, encoding: .utf8) ?? "???"
    }
    return "???"
}

The output of the two print lines currently is:

56
0 bytes

---

UPDATE:

Corrected code according to accepted answer:

let encrypted = "inMyOriginalCodeYouWouldSeeADESEncryptedStringHere/ahw=="
if let key = "12345678".data(using: .utf8), let data = Data(base64Encoded: encrypted) {
        var numBytesDecrypted: size_t = 0
        var result = Data(count: data.count)

        let err = result.withUnsafeMutableBytes {resultBytes in
            data.withUnsafeBytes {dataBytes in
                key.withUnsafeBytes {keyBytes in
                    CCCrypt(CCOperation(kCCDecrypt), CCAlgorithm(kCCAlgorithmDES), CCOptions(kCCOptionECBMode), keyBytes, kCCKeySizeDES, nil, dataBytes, data.count, resultBytes, data.count, &numBytesDecrypted)
                }
            }
        }

        if err != CCCryptorStatus(kCCSuccess) {
            NSLog("Decryption failed! Error: \(err.description)")
        }

        return String(data: result, encoding: .utf8) ?? "???"
}

This code now returns the correct result. There is only one little problem left: the returned result String looks like the following now: "encryptedString\u{08}\u{08}\u{08}\u{08}\u{08}\u{08}\u{08}\u{08}"

How can I get rid of these last 8 bytes?

I tried to to initialise my result like this:

var result = Data(count: data.count - kCCBlockSizeDES)

but then i get a kCCBufferTooSmall error.

---

UPDATE2:

I now use CCOptions(kCCOptionPKCS7Padding|kCCOptionECBMode) but this only changed the "\u{08}" characters to "\0" characters. So now i simply call .trimmingCharacters(in: CharacterSet.controlCharacters) on my result String before returning it.

Answer 1

When you encrypt or decrypt from String to String, you need 3 steps, as modern encryption algorithms work on only binary data.

Encoding:

[original String]
↓(encode in UTF-8)
[original binary]
↓(encrypt)
[encrypted binary]
↓(encode in base64)
[encrypted String]

(I guess you have a base64 encoded String as your == in the encrypted String is suggesting.)

So, when decoding, you need all these steps in reverse.

Decoding:

[encrypted String]
↓(decode in base64)
[encrypted binary]
↓(decrypt)
[original binary]
↓(decode in UTF-8)
[original String]

You are doing the first step of decoding in a wrong way. (See #1 of the code below.)

---

One more, when you want to receive data into mutable (var) Data, set count (not only capacity) of the Data. (#2 and #3)

---

UPDATED And, as told by zaph, you need to specify IV for CBC mode (default) or use ECB mode (#4).

Your code should be something like this:

if
    let key = "12345678".data(using: .utf8),
    let data = Data(base64Encoded: encrypted, options: .ignoreUnknownCharacters) //<-#1
{
    var numBytesDecrypted: size_t = 0
    var result = Data(count: data.count)    //<-#2
    
    let err = result.withUnsafeMutableBytes {resultBytes in
        data.withUnsafeBytes {dataBytes in
            key.withUnsafeBytes {keyBytes in
                CCCrypt(CCOperation(kCCDecrypt), CCAlgorithm(kCCAlgorithmDES), CCOptions(kCCOptionPKCS7Padding|kCCModeECB), keyBytes, kCCKeySizeDES, nil, dataBytes, data.count, resultBytes, result.count, &numBytesDecrypted) //<-#4
            }
        }
    }
    
    if err != CCCryptorStatus(kCCSuccess) {
        NSLog("Decryption failed! Error: \(err.description)")
    }
    
    print(numBytesDecrypted)
    result.count = numBytesDecrypted //<-#3
    print(result as NSData) //`as NSData` is good for debugging.
    
    return String(data: result, encoding: .utf8) ?? "???"
}
return "???"