How to Generate HMAC in Inbound Policy in Azure APIM

Question

Hi,

Use-Case:

• Need to fetch header parameters from inputs and encode parameter values with backend URI and current timestamp
• generate HMAC hash with secret key and sending hashcode and encoded URI in Query parameters to backend.

find the attached sample gateway script, when I tried in API Connect its working but in APIM when i added in inbound policy, not able to save the code, throwing an error with too may characters

any inputs on how to add HMAC in inbound policy.

114337-hmac-apic.xml

Answer 1

@Vasanth J While not the exact use case, this policy sample covers this. Here are the lines of importance for reference

   @{  
       System.Security.Cryptography.HMACSHA256 hasher = new System.Security.Cryptography.HMACSHA256(<signing-key>);  
       return Convert.ToBase64String(hasher.ComputeHash(System.Text.Encoding.UTF8.GetBytes(<string-to-sign>)));  
   }  

As for the policy code that you've shared, it's in JS whereas APIM uses C# for its policy expressions embedded in XML.