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Calculating Signatures

[Note]Note

The best way to learn how to sign requests is to see the code samples in the Resource Center for examples using Java, C#, PHP, Perl and Ruby.

[Important]Important

Signature version 2 is described below. Previous signature versions were deprecated on May 25, 2011. As of November 25, 2011 you must use signature version 2.

To create the signature

  1. Create the canonicalized query string that you need later in this procedure:

    1. Sort the UTF-8 query string components by parameter name with natural byte ordering.

      The parameters can come from the GET URI or from the POST body (when Content-Type is application/x-www-form-urlencoded).

    2. URL encode the parameter name and values according to the following rules:

      • Do not URL encode any of the unreserved characters that RFC 3986 defines.

        These unreserved characters are A-Z, a-z, 0-9, hyphen ( - ), underscore ( _ ), period ( . ), and tilde ( ~ ).

      • Percent encode all other characters with %XY, where X and Y are hex characters 0-9 and uppercase A-F.

      • Percent encode extended UTF-8 characters in the form %XY%ZA....

      • Percent encode the space character as %20 (and not +, as common encoding schemes do).

      [Note]Note

      Currently all AWS service parameter names use unreserved characters, so you don't need to encode them. However, you might want to include code to handle parameter names that use reserved characters, for possible future use.

    3. Separate the encoded parameter names from their encoded values with the equals sign ( = ) (ASCII character 61), even if the parameter value is empty.

    4. Separate the name-value pairs with an ampersand ( & ) (ASCII code 38).

  2. Create the string to sign according to the following pseudo-grammar (the "\n" represents an ASCII newline).

    StringToSign = HTTPVerb + "\n" +
                   ValueOfHostHeaderInLowercase + "\n" +
                   HTTPRequestURI + "\n" +         
                   CanonicalizedQueryString <from the preceding step>

    The HTTPRequestURI component is the HTTP absolute path component of the URI up to, but not including, the query string. If the HTTPRequestURI is empty, use a forward slash ( / ).

  3. Calculate an RFC 2104-compliant HMAC with the string you just created, your Secret Access Key as the key, and SHA256 or SHA1 as the hash algorithm.

    For more information, go to http://www.ietf.org/rfc/rfc2104.txt.

  4. Convert the resulting value to base64.

  5. Use the resulting value as the value of the Signature request parameter.

[Important]Important

The final signature you send in the request must be URL encoded as specified in RFC 3986 (for more information, go to http://www.ietf.org/rfc/rfc3986.txt. If your toolkit URL encodes your final request, then it handles the required URL encoding of the signature. If your toolkit doesn't URL encode the final request, then make sure to URL encode the signature before you include it in the request. Most importantly, make sure the signature is URL encoded only once. A common mistake is to URL encode it manually during signature formation, and then again when the toolkit URL encodes the entire request.