HMACDigest.swift

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

enum HMACAlgorithm {
	case MD5, SHA1, SHA224, SHA256, SHA384, SHA512
	
	func toCCEnum() -> 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)
	}
	
	func digestLength() -> Int {
		var result: CInt = 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 digest(algorithm: HMACAlgorithm, key: String) -> String! {
		let str = self.cStringUsingEncoding(NSUTF8StringEncoding)
		let strLen = UInt(self.lengthOfBytesUsingEncoding(NSUTF8StringEncoding))
		let digestLen = algorithm.digestLength()
		let result = UnsafeMutablePointer<CUnsignedChar>.alloc(digestLen)
		let keyStr = key.cStringUsingEncoding(NSUTF8StringEncoding)
		let keyLen = UInt(key.lengthOfBytesUsingEncoding(NSUTF8StringEncoding))
		
		CCHmac(algorithm.toCCEnum(), keyStr!, keyLen, str!, strLen, result)
		let data = NSData(bytesNoCopy: result, length: digestLen)
		result.destroy()
		
		return data.base64EncodedStringWithOptions(nil)
	}
}

Have you tried your code with http://docs.aws.amazon.com/AWSECommerceService/latest/DG/rest-signature.html? I actually stumbled on this gist searching for a way to do this hash in Swift.