Technical: Using REST web services with 2-legged OAuth on datapower.

2-legged OAuth consists of:
•    The client signs up to the server. The server & client has a shared secret.
•    The client uses these key to gain access to resources on the server

What is REST ?

GET method
The client(secret=password) sends the following request to datapower.
Request: GET http://testname:1010/testname?name=KIM
Header: Authorization: OAuth     oauth_nonce="12345abcde",
                                                  oauth_consumer_key="Kim",
                                                  oauth_signature_method="HMAC-SHA1",
                                                  oauth_timestamp="1319032126",
                                                  oauth_version="1.0",
                                                  oauth_signature="m2A6bZejY7smlH6OcWwaKLo7X4o="

oauth_nonce = A randomly-generated string that is unique to each API call. This, along with the timestamp, is used to prevent replay attacks.
oauth_consumer_key = the client’s key.
oauth_signature_method = the cryptographic method used to sign.
oauth_timestamp = the number of seconds that have elapsed since midnight, January 1, 1970, also known as UNIX time.
oauth_signature = The authorization signature.

On datapower a Multi-protocol gateway needs to be configured allowing requests with an empty message body.

The secret needs to be shared between client and datapower. To make everything easy I stored the shared secret in an xml file in the “local:” file system of datapower.

The next step is verifying the signature. The best way to verify the signature is recreate the signature with the shared secret and then compare the two signatures. Of course to do this, some xslt skills are recommended.
This is the signature base-string that will be signed with the shared secret.

GET&http%3A%2F%2Ftestname%3A1010%2Ftestname&name%3DKIM%26oauth_consumer_key%3DKim

%26oauth_nonce%3D12345abcde%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1319032126%26oauth_version%3D1.0

How do we get to this?
If you look closely you see that there are 3 distinguished parts.

1. GET& 
2. http%3A%2F%2Ftestname%3A1010%2Ftestname& 
3. name%3DKIM%26oauth_consumer_key%3DKim%26oauth_nonce%3D12345abcde%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1319032126%26oauth_version%3D1.0

The first part is fairly straightforward.
    GET = the http method used.

The second part is the URL which is url-encoded.
We’ll need to percent encode all of the parameters when creating our signature. These "reserved" characters must be converted into their hexadecimal values and preceded by a % character. The rest of the characters are "unreserved" and must not be encoded.
The reserved characters are:
!   *   '   (   )   ;   :   @   &   =   +   $   ,   /   ?   %   #   [   ]  white-space
These are the percent-encoded values of the above characters:
%21 %2A %27 %28 %29 %3B %3A %40 %26 %3D %2B %24 %2C %2F %3F %25 %23 %5B %5D %20
Be careful with URL encoding functions built into languages, they may not encode all of the reserved characters, or may encode unreserved characters.
    <xsl:variable
     name="BasicUrl"
     select="dp:encode(string(
     dp:variable('http://testname:1010/testname’)
        ),'url')
    "/>

The third part gets a bit more complicated to do with xslt, because the number of parameters and OAuth header fields can change.
We need to extract the Oauth header from the request.
     <xsl:variable
      name="OAuthParams"
      select="dp:request-header('Authorization')"
     />
The URL parameters are also necessary. The parameters in this example are “name=KIM”. Put them all together and alphabetically organize them. You will get a string like this:
name=KIM&oauth_consumer_key=Kim&oauth_nonce=12345abcde&oauth_signature_method=HMAC-SHA1&oauth_timestamp=1319032126&oauth_version=1.0
And again url-encode this string using the datapower function(dp:encode).If you now concatenate the 3 parts with a “&” you will get the signature base string.

The next thing we need to calculate the signature is the shared secret. OAuth standard uses a concatenation of the shared secret & token(<secret>&<token>). Since we are working with the 2-legged OAuth we don’t need the token and we can leave it empty. So in the example we used the shared secret “password”. Important is that you add the “&” behind the secret: ”password&”. Now we have the necessary information to calculate the signature.
   <xsl:variable name="Calculate_Signature"
    select="dp:hmac('http://www.w3.org/2000/09/xmldsig#hmac-sha1',
    ‘password&’,
   <SignatureBaseString>)"/>

Finally we can compare the signature of the OAuth header with our own calculated signature. Don’t forget to validate the timestamp against a predefined limit e.g. 5 minutes. If all checks out, the client is authorized to use the web service.

link to check OAuth signature

POST method is coming soon…

Author: Kim