String Extension to get based 64 signature to sign google api urls in swift 3

HMAC-SHA1-BASE64-SIGNATURE.swift

// String Extension to get based 64 signature to sign google api urls in Swift 3
// FROM https://stackoverflow.com/a/20300625/5965126
extension String {
	
	func signatureBase64(key: String) -> String? {
		if let signature = self.data(using: .utf8) as NSData?, let signingKey = self.decodeURLBase64String(string: key) {
			if let digest = self.hmacSha1(data: signature, key: signingKey) {
				return self.encodeURLBase64Data(data: digest)
			}
		}
		return nil
	}
	
	func decodeURLBase64String(string: String) -> NSData? {
		var replacedString = string.replacingOccurrences(of: "-", with: "+")
		replacedString = string.replacingOccurrences(of: "_", with: "/")

		if let data = Data(base64Encoded: replacedString) as NSData? {
			return data
		}
		return nil
	}
	
	func encodeURLBase64Data(data: NSData) -> String {
		var signatureBase64 = data.base64EncodedString(options: [])
		signatureBase64 = signatureBase64.replacingOccurrences(of: "+", with: "-")
		signatureBase64 = signatureBase64.replacingOccurrences(of: "/", with: "_")
		return signatureBase64
	}
	
	func hmacSha1(data: NSData, key: NSData) -> NSData? {
		if let hmac: NSMutableData = NSMutableData(length: Int(CC_SHA1_DIGEST_LENGTH)) {
			CCHmac(CCHmacAlgorithm(kCCHmacAlgSHA1), key.bytes,  key.length, data.bytes, data.length, hmac.mutableBytes)
			
			return hmac
		}
		return nil
	}
}