HMAC digest in Swift

// Make sure you add #import <CommonCrypto/CommonCrypto.h> to the Xcode bridging header!

enum CryptoAlgorithm { case MD5, SHA1, SHA224, SHA256, SHA384, SHA512

var HMACAlgorithm: CCHmacAlgorithm {
    var result: Int = 0
    switch self {
    case .MD5:      result = kCCHmacAlgMD5
    case .SHA1:     result = kCCHmacAlgSHA1
    case .SHA224:   result = kCCHmacAlgSHA224
    case .SHA256:   result = kCCHmacAlgSHA256
    case .SHA384:   result = kCCHmacAlgSHA384
    case .SHA512:   result = kCCHmacAlgSHA512
    }
    return CCHmacAlgorithm(result)
}

var digestLength: Int {
    var result: Int32 = 0
    switch self {
    case .MD5:      result = CC_MD5_DIGEST_LENGTH
    case .SHA1:     result = CC_SHA1_DIGEST_LENGTH
    case .SHA224:   result = CC_SHA224_DIGEST_LENGTH
    case .SHA256:   result = CC_SHA256_DIGEST_LENGTH
    case .SHA384:   result = CC_SHA384_DIGEST_LENGTH
    case .SHA512:   result = CC_SHA512_DIGEST_LENGTH
    }
    return Int(result)
}

}

extension String {

func hmac(algorithm: CryptoAlgorithm, key: String) -> String {
    let str = self.cStringUsingEncoding(NSUTF8StringEncoding)
    let strLen = Int(self.lengthOfBytesUsingEncoding(NSUTF8StringEncoding))
    let digestLen = algorithm.digestLength
    let result = UnsafeMutablePointer<CUnsignedChar>.alloc(digestLen)
    let keyStr = key.cStringUsingEncoding(NSUTF8StringEncoding)
    let keyLen = Int(key.lengthOfBytesUsingEncoding(NSUTF8StringEncoding))

    CCHmac(algorithm.HMACAlgorithm, keyStr!, keyLen, str!, strLen, result)

    let digest = stringFromResult(result, length: digestLen)

    result.dealloc(digestLen)

    return digest
}

private func stringFromResult(result: UnsafeMutablePointer<CUnsignedChar>, length: Int) -> String {
    var hash = NSMutableString()
    for i in 0..<length {
        hash.appendFormat("%02x", result[i])
    }
    return String(hash)
}

}