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Request signatures

Last Updated: Oct 08, 2021

You must sign all API requests to ensure security. Alibaba Cloud uses the request signature to verify the identity of the API caller. Therefore, each API request must contain signature information, regardless of whether it is sent by using HTTP or HTTPS.

Note: Alibaba Cloud provides Alibaba Cloud service SDKs and third-party SDKs in multiple languages. This helps you easily sign API requests. Click hereto view more information about Alibaba Cloud service SDKs.


To sign a RESTful API request, you must add the Authorization parameter to the request header in the following format:

  • acs: the abbreviation for Alibaba Cloud Service. This is a fixed field and cannot be modified.

  • AccessKeyId: the AccessKey ID that is used to call the API.

  • Signature: the signature generated after the request is symmetrically encrypted by using the AccessKey secret.


The signature encryption algorithm complies with RFC 2104 HMAC-SHA1 specifications. The AccessKey secret is used to calculate the hash-based message authentication code (HMAC) value of the encoded and sorted query string, and the HMAC value is used as the signature string. Request signatures include operation-specific parameters. Therefore, the signature of a request varies depending on the request parameters.

Signature = Base64( HMAC-SHA1( AccessSecret, UTF-8-Encoding-Of(StringToSign)) )
To calculate a signature, perform the following steps:
  1. Compose and encode a string-to-sign.

    The string-to-sign is a string assembled by using related elements in an API request. It is used to calculate the signature and contains the following elements:
    • HTTP header

    • Alibaba Cloud protocol headers (CanonicalizedHeaders)

    • Canonicalized resource (CanonicalizedResource)

    • Body

    The string-to-sign must be created in the following format:

    StringToSign =       //HTTP header        HTTP-Verb + "\n" +        Accept + "\n" +        Content-MD5 + "\n" +//Specify the request body that is encrypted by using the MD5 algorithm.        Content-Type + "\n" +        Date + "\n" +       //Alibaba Cloud protocol headers (CanonicalizedHeaders)        CanonicalizedHeaders +       //Canonicalized resource (CanonicalizedResource)        CanonicalizedResource

    Sample original request:

    POST /stacks? name=test_alert&status=COMPLETE HTTP/1.1Host: *.aliyuncs.comAccept: application/jsonContent-MD5: ChDfdfwC+Tn874znq7Dw7Q==Content-Type: application/x-www-form-urlencoded;charset=utf-8Date: Thu, 22 Feb 2018 07:46:12 GMTx-acs-signature-nonce: 550e8400-e29b-41d4-a716-446655440000x-acs-signature-method: HMAC-SHA1x-acs-signature-version: 1.0x-acs-version: 2016-01-02

    Sample canonicalized request:

    POSTapplication/jsonChDfdfwC+Tn874znq7Dw7Q==application/x-www-form-urlencoded;charset=utf-8Thu, 22 Feb 2018 07:46:12 GMTx-acs-signature-nonce: 550e8400-e29b-41d4-a716-446655440000x-acs-signature-method:HMAC-SHA1x-acs-signature-version:1.0x-acs-version:2016-01-02/stacks? name=test_alert&status=COMPLETE
  2. Add the signature string.

    Add the calculated signature string to the request header in the following format:

    Authorization: acs AccessKeyId:Signature

HTTP header

The HTTP header in the string-to-sign must contain the values of the following parameters. The parameters must be sorted in alphabetical order. If a parameter has no value, set it to \n.

  • Accept: the response type required by the client. Valid values: application/json and application/xml.

  • Content-MD5: the Base64-encoded 128-bit MD5 hash value of the request body.

  • Content-Type: the type of the HTTP request body. The type is defined in RFC 2616.

  • Date: the GMT time specified in HTTP 1.1. Example: Wed, 05 Sep. 2012 23:00:00 GMT.

    Note: You do not need to specify your AccessKey pair in the HTTP header.
Sample original header:
Accept: application/jsonContent-MD5: ChDfdfwC+Tn874znq7Dw7Q==Content-Type: application/x-www-form-urlencoded;charset=utf-8Date: Thu, 22 Feb 2018 07:46:12 GMT
Sample canonicalized header:
application/jsonChDfdfwC+Tn874znq7Dw7Q==application/x-www-form-urlencoded;charset=utf-8Thu, 22 Feb 2018 07:46:12 GMT

Alibaba Cloud protocol headers (CanonicalizedHeaders)

CanonicalizedHeaders are non-standard HTTP headers of Alibaba Cloud. They are parameters prefixed with x-acs- in a request. A request must contain the following parameters:
  • x-acs-signature-nonce: a unique, random number used to prevent replay attacks. You must use different numbers for different requests.

  • x-acs-signature-version: the version of the signature encryption algorithm. Set the value to 1.0.

  • x-acs-version: the version number of the API. For more information, see the API documentation of each service.

To construct Alibaba Cloud canonicalized headers, perform the following steps:

  1. Convert the names of all HTTP request headers prefixed with x-acs- to lowercase letters. For example, convert X-acs-OSS-Meta-Name: TaoBao into x-acs-oss-meta-name: TaoBao.

  2. Arrange all HTTP request headers that are obtained from the preceding step in alphabetical order.

  3. Delete spaces from each side of a delimiter between the request header and its content. For example, convert x-acs-oss-meta-name: TaoBao,Alipay into x-acs-oss-meta-name:TaoBao,Alipay.

  4. Separate all headers and content with delimiters (\n) to form the final CanonicalizedHeaders.

Sample original header:
x-acs-signature-nonce: 550e8400-e29b-41d4-a716-446655440000x-acs-signature-method: HMAC-SHA1x-acs-signature-version: 1.0x-acs-version: 2016-01-02GMT
Sample canonicalized header:

Canonicalized resource (CanonicalizedResource)

CanonicalizedResource is the canonical description of the resource that you want to access. Sort sub-resources along with the query parameters in ascending lexicographic order and separate them with ampersands (&) to generate a sub-resource string. The sub-resource string consists of all parameters that follow the question mark (?).

Sample original request:
Sample canonicalized request:


Encrypt the request body by using the MD5 algorithm, Base64 encode the encrypted request body, and add the Base64-encoded string to the value of the Content-MD5 parameter.