Can't convert NSData to NSString in swift

Question

This is my function. First println print correct hash to console but in the next row program crashes. Can you help me?

func sha256(string: NSString) -> NSString {
    var data : NSData! = string.dataUsingEncoding(NSUTF8StringEncoding)
    var hash = [UInt8](count: Int(CC_SHA256_DIGEST_LENGTH), repeatedValue: 0)
    CC_SHA256(data.bytes, CC_LONG(data.length), &hash)
    let res = NSData(bytes: hash, length: Int(CC_SHA256_DIGEST_LENGTH))
    println(res)
    let resstr = NSString(data: res, encoding: NSUTF8StringEncoding)
    println(resstr)
    return resstr
}

Not all data can be represented by a string. The possibility that a hash (SHA-256) is representable is essentially to zero. This is because the result of SHA-256 (hashs in general) is binary data. Even in the unlikely event that it it could it would not be "reasonable" text. — zaph, Aug 21 '14 at 11:59

Martin R · Accepted Answer · 2014-08-21T13:20:06.437

41

let resstr = NSString(data: res, encoding: NSUTF8StringEncoding)

returns nil if the data does not represent a valid UTF-8 sequence (which is very likely). Then the following println() crashes.

A possible string representation for arbitrary binary data would be a hex string or a Base-64 encoded string.

A Base-64 encoded string can simply be obtained with

let resstr = res.base64EncodedStringWithOptions(nil)

There is (as far as I know) no built-in method to convert binary data to a hex string. A possible implementation in Swift (inspired by the lots of available Objective-C solutions) is

extension NSData {
    func hexString() -> NSString {
        var str = NSMutableString()
        let bytes = UnsafeBufferPointer<UInt8>(start: UnsafePointer(self.bytes), count:self.length)
        for byte in bytes {
            str.appendFormat("%02hhx", byte)
        }
        return str
    }
}

But you could integrate that into your hash method directly, without using an intermediate NSData object:

func sha256(string: NSString) -> NSString {
    let data = string.dataUsingEncoding(NSUTF8StringEncoding)!
    var hash = [UInt8](count: Int(CC_SHA256_DIGEST_LENGTH), repeatedValue: 0)
    CC_SHA256(data.bytes, CC_LONG(data.length), &hash)
    let resstr = NSMutableString()
    for byte in hash {
        resstr.appendFormat("%02hhx", byte)
    }
    return resstr
}

edited Aug 21 '14 at 13:20

answered Aug 21 '14 at 11:39

Martin R

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how can i convert data to UTF-8? console shows res as "74696c22 2d3233cf c5dc040e 70ec9f5b dbcea3cf 53946368 65bb4382 64c782e5", which is exactly what i need, but your method gives "dGlsIi0yM8/F3AQOcOyfW9vOo89TlGNoZbtDgmTHguU=", which is wrong – Yury Alexandrov Aug 21 '14 at 11:55
@ЮрикАлександров: `<74696c22 2d3233cf …>` is the *description* of the NSData object. There is no built-in method to convert NSData to a hex string, but you will find lot's of solutions if you search for "NSData to hex string", for example http://stackoverflow.com/questions/1305225/best-way-to-serialize-a-nsdata-into-an-hexadeximal-string or http://stackoverflow.com/questions/7520615/how-to-convert-an-nsdata-into-an-nsstring-hex-string. – Martin R Aug 21 '14 at 12:10
1

@ЮрикАлександров: I have added some Swift code for your convenience :) – Martin R Aug 21 '14 at 12:29
"Then the following println() crashes." `println()` does not crash. `println()` can print optionals with value `nil`. The crash happens on the previous line, because the compiler infers `resstr` to have type `NSString`, which unwraps the optional. If he had explicitly declared `resstr` to have type `NSString!` or `NSString?`, it would not crash. – newacct Aug 21 '14 at 18:47
@newacct: Well, I have single-stepped it, and it *definitely* crashes at `println(resstr)`. The reason is probably that the initializer `init(data: NSData, encoding: UInt)` does *not* return an optional, see "Swift does not support object initializers that fail by returning null" in the beta 6 release notes. - If you change the assignment to `let resstr : NSString? = NSString(data: res, encoding: NSUTF8StringEncoding)` then `println(resstr)` prints "nil". – Martin R Aug 21 '14 at 19:04
@MartinR: Hmm, you are right. That is very interesting. It looks like `foo` really has the value `nil` (it seems that they don't check when assigning the result of an initializer), but it is of a type that doesn't allow `nil`, causing undefined behavior. In my opinion this is a hole in the type system that shouldn't be there. – newacct Aug 21 '14 at 20:35
@MartinR: I had originally thought that initializers return optional but the compiler infers non-optional when you assign it to a variable unless you explicitly specify optional, thus causing an unwrapping error. But now I finally realize that the compiler thinks that initializers return non-optional, and if it returns `nil`, it's actually an invalid value of that non-optional type. The reason the "workaround" of assigning to optional works is because assignment from non-optional object pointer to optional object pointer is a simple assignment between object pointers at runtime. – newacct Aug 21 '14 at 22:28

Can't convert NSData to NSString in swift

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