如何为Azure Service Bus和Azure IoT Hub生成SharedAccessSignature

很多服务在做验证的时候都会用到SharedAccessSignature，例如Azure Service Bus, Azure IoT Hub等。今天趟了回大坑，这里分享出来，希望对你有所帮助。

SharedAccessSignature的格式如下：

SharedAccessSignature sig={signature-string}&se={expiry}&skn={policyName}&sr={URL-encoded-resourceURI}

因此，凭直觉针对不同的服务只要正确指定policyname,resourceURI 就可以计算出相应的SAS。但实际上，对于不同的服务，他们对如何利用秘钥生成signature会存在细微的差别。如果没有注意到这个细微的差别，会让你抓狂两小时。

下面拿Azure Service Bus和Azure IoT Hub进行举例，如果后续我碰到Azure 其他服务有类似的问题。我会更新此文。

首先是Azure IoT Hub, 针对如何生成signature 官方文档说明如下：

{signature} ：An HMAC-SHA256 signature string of the form: {URL-encoded-resourceURI} + "\n" + expiry. Important: The key is decoded from base64 and used as key to perform the HMAC-SHA256 computation.

再来看Azure Service Bus 的官方文档说明：

The signature for the SAS token is computed using the HMAC-SHA256 hash of a string-to-sign with the PrimaryKey property of an authorization rule.

两者的区别在于是否需要对秘钥进行decode。代码分别如下：

HMACSHA256 hmac = new HMACSHA256(Convert.FromBase64String(key)); // decode the key

HMACSHA256 hmac = new HMACSHA256(Encoding.UTF8.GetBytes(key)); // don't decode the key

在《Azure IoT Hub 入门 - 权限管理》一文中，我跟大家share了生成SAS的source code 和DLL。该DLL 现支持生成Service Bus 和IoT Hub的SAS，两者通过TargetService来区分（源代码在这里）。调用方法如下：

Service Bus:

// servicebus
string targetURL = "sb://<service bus namespace>.servicebus.chinacloudapi.cn";
string keyName = "DefaultFullSharedAccessSignature";
string key = <key>;
double ttlValue = 1;

var sasBuilder = new SharedAccessSignatureBuilder()
{
    TargetService = "servicebus",
    Target = targetURL,
    KeyName = keyName,
    Key = key,
    TimeToLive = TimeSpan.FromDays(ttlValue)
};

string sas = sasBuilder.ToSignature();

IoT Hub: (TargetService是可选项，默认是生成IoT hub的SAS)

//iot hub
string targetURL = "<iot hub name>.azure-devices.cn/devices";
string keyName = <policy name>;
string key = <key>;

var sasBuilder1 = new SharedAccessSignatureBuilder()
{
    Target = targetURL,
    KeyName = keyName,
    Key = key,
    TimeToLive = TimeSpan.FromDays(ttlValue)
};

string sas1 = sasBuilder1.ToSignature();

var sasBuilder2 = new SharedAccessSignatureBuilder()
{
    TargetService="iothub", // optional
    Target = targetURL,
    KeyName = keyName,
    Key = key,
    TimeToLive = TimeSpan.FromDays(ttlValue)
};

string sas2 = sasBuilder1.ToSignature();