Can't unwrap Optional.None Error, Yet Not Using an Optional - swift

Question

I am trying to write an HMAC class in Swift by first writing it in Objective C, and then using a Bridging-Header to make that class available to my Swift code. I'm sorry if I'm pasting too much code, but I'd like to learn why this is happening so I can better understand intefacing Swift and Objective C, and how I can resolve the error I'm getting. The "TBGHMAC.calculateWithAlgorithm: forKey: and Data:" function produces a "fatal error: Can't unwrap Optional.None" error.
Starting from the top, here is the snippet in my Swift code that calls the Swift code
let key : String = "" // start with empty string"
let data : String = "" // start with empty string"
let signature : String = TBGHMAC.calculateWithAlgorithm(HMACAlgorithm.SHA256, forKey: key, andData: data)
println(signature)
and here is the TBGHMAC header file
#import <Foundation/Foundation.h>
#import <CommonCrypto/CommonCrypto.h>
#interface TBGHMAC : NSObject
typedef NS_ENUM(NSInteger, HMACAlgorithm)
{
SHA1,
MD5,
SHA256,
SHA384,
SHA512,
SHA224
};
+ (NSString *)calculateWithAlgorithm:(HMACAlgorithm)algorithm forKey:(NSString*)key andData:(NSString *)data;
+ (NSInteger)digestLengthForAlgorithm:(HMACAlgorithm)algorithm;
#end
and finally here is the implementation file
+ (NSString *)calculateWithAlgorithm:(HMACAlgorithm)algorithm forKey:(NSString *)key andData:(NSString *)data
{
NSUInteger keyNumberOfBytes = [key lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
void *keyBuffer = malloc(keyNumberOfBytes);
NSUInteger keyUsedLength = 0;
NSRange keyRange = NSMakeRange(0, [key length]);
BOOL keyResult = [key getBytes:keyBuffer maxLength:keyNumberOfBytes usedLength:&keyUsedLength encoding:NSUTF8StringEncoding options:0 range:keyRange remainingRange:NULL];
NSUInteger dataNumberOfBytes = [data lengthOfBytesUsingEncoding:NSUnicodeStringEncoding];
void *dataBuffer = malloc(dataNumberOfBytes);
NSUInteger dataUsedLength = 0;
NSRange dataRange = NSMakeRange(0, [data length]);
BOOL dataResult = [data getBytes:dataBuffer maxLength:dataNumberOfBytes usedLength:&dataUsedLength encoding:NSUTF8StringEncoding options:0 range:dataRange remainingRange:NULL];
NSInteger digestLength = [self digestLengthForAlgorithm:algorithm];
unsigned char hmac[digestLength];
CCHmac(algorithm, &keyBuffer, strlen(keyBuffer), &dataBuffer, strlen(dataBuffer), &hmac);
NSData *hmacBytes = [NSData dataWithBytes:hmac length:sizeof(hmac)];
NSString* returnStr = [[NSString alloc] initWithData:hmacBytes encoding:NSUTF8StringEncoding];
free(keyBuffer);
free(dataBuffer);
return returnStr;
}
+ (NSInteger)digestLengthForAlgorithm:(HMACAlgorithm)algorithm
{
switch (algorithm)
{
case MD5: return CC_MD5_DIGEST_LENGTH;
case SHA1: return CC_SHA1_DIGEST_LENGTH;
case SHA224: return CC_SHA224_DIGEST_LENGTH;
case SHA256: return CC_SHA256_DIGEST_LENGTH;
case SHA384: return CC_SHA384_DIGEST_LENGTH;
case SHA512: return CC_SHA512_DIGEST_LENGTH;
default: return 0;
}
}
#end
Please point out why this error is coming up, and how I can resolve it. I do not understand how this error is arising, because I'm not using any Optionals, and Xcode isn't trying to correct me into using Optionals.

Answer 1

The Objective-C method
+ (NSString *)calculateWithAlgorithm:(HMACAlgorithm)algorithm forKey:(NSString*)key andData:(NSString *)data;
is exposed to Swift as
class func calculateWithAlgorithm(algorithm: HMACAlgorithm, forKey key: String!, andData data: String!) -> String!
It returns an (implicitly unwrapped) optional string because the NSString returned by the Objective-C method may be nil.
Converting the return value to a String in
let signature : String = BGHMAC.calculateWithAlgorithm(...)
causes a runtime exception if the return value is nil ("no value"). You should therefore test the return value
before using it:
let signature : String! = TBGHMAC.calculateWithAlgorithm(HMACAlgorithm.SHA256, forKey: key, andData: data)
if signature {
println(signature)
} else {
println("failed")
}
But why does calculateWithAlgorithm() return nil? The problem is here:
NSString* returnStr = [[NSString alloc] initWithData:hmacBytes encoding:NSUTF8StringEncoding];
A message digest is a quite arbitrary sequence of bytes, and cannot be interpreted
as an UTF-8 string, therefore returnStr is nil.
To solve your problem, you could for example convert the message digest to
a string using the Base64 encoding:
NSString* returnStr = [hmacBytes base64EncodedStringWithOptions:0];