Decrypt bearer token in Azure API Management to get acr_values - azure

Question

Is there any way to decrypt a bearer token in an API management policy in order to create a condition it's acr_values, for example a tenant.
Looking at the MS documentation it does not seem possible, I would be looking to achieve something like:
<when condition="#(context.Request.Headers["Authorization"] --DO MAGIC HERE-- .acr_values["tenant"] == "contoso" ">
<set-backend-service base-url="http://contoso.com/api/8.2/" />
</when>
Alternatively something like the example here but for setting the backed service:
http://devjourney.com/blog/2017/03/23/extract-jwt-claims-in-azure-api-management-policy/
Documentation I've read:
https://learn.microsoft.com/en-us/azure/api-management/api-management-transformation-policies#example-4
https://learn.microsoft.com/en-us/azure/api-management/policies/authorize-request-based-on-jwt-claims?toc=api-management/toc.json#policy

Answer 1

Did you try .AsJwt() method (https://learn.microsoft.com/en-us/azure/api-management/api-management-policy-expressions#ContextVariables):
<policies>
<inbound>
<base />
<set-header name="tenant" exists-action="append">
<value>#{
var jwt = context.Request.Headers.GetValueOrDefault("Authorization").AsJwt();
return jwt?.Claims.GetValueOrDefault("tenant") ?? "unknown";
}</value>
</set-header>
<choose>
<when condition="#(context.Request.Headers.GetValueOrDefault("tenant", "unknown") == "some-tenant" )">
<set-backend-service base-url="http://contoso.com/api/8.2/" />
</when>
</choose>
</inbound>
<backend>
<base />
</backend>
<outbound>
<base />
</outbound>
<on-error>
<base />
</on-error>
Also I'm not sure if you need it as a header to backend request, if not consider using set-variable policy.

Answer 2

Ok so I got it working in a very hacky way, you can set vales of the decrypted token in the header and then set conditions on that header.
<policies>
<inbound>
<base />
<set-header name="tenant" exists-action="append">
<value>#{
string tenant = "unknown";
string authHeader = context.Request.Headers.GetValueOrDefault("Authorization", "");
if (authHeader?.Length > 0)
{
string[] authHeaderParts = authHeader.Split(' ');
if (authHeaderParts?.Length == 2 && authHeaderParts[0].Equals("Bearer", StringComparison.InvariantCultureIgnoreCase))
{
Jwt jwt;
if (authHeaderParts[1].TryParseJwt(out jwt))
{
tenant = (jwt.Claims.GetValueOrDefault("tenant", "unknown"));
}
}
}
return tenant;
}</value>
</set-header>
<choose>
<when condition="#(context.Request.Headers.GetValueOrDefault("tenant", "unknown") == "some-tenant" )">
<set-backend-service base-url="http://contoso.com/api/8.2/" />
</when>
</choose>
</inbound>
<backend>
<base />
</backend>
<outbound>
<base />
</outbound>
<on-error>
<base />
</on-error>

Answer 3

A few years have passed since this has been answered, but as I found a less verbose solution, without actually modifying the request headers, i thought it would be nice to share for others:
<set-variable name="tenant" value="#{
var authHeader = context.Request.Headers.GetValueOrDefault("Authorization", "");
return authHeader.AsJwt()?.Claims.GetValueOrDefault("tenant", "");
}" />
...
<choose>
<when condition="#(context.Variables.GetValueOrDefault("tenant", "") == "your-tenant-id")">