DecodeJWS policy

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What

Decodes the JWS header without verifying the signature on the JWS, and writes each header to a flow variable. This policy is most useful when used in concert with the VerifyJWS policy, when the value of a header from within the JWS must be known before verifying the signature of the JWS.

A JWS can have an attached payload, as in the form:

header.payload.signature

Or, the JWS can omit the payload, called a detached payload, and be in the form:

header..signature

The DecodeJWS policy works with both forms because it only decodes the header portion of the JWS. The DecodeJWS policy also works regardless of the algorithm that was used to sign the JWS.

See JWS and JWT policies overview for a detailed introduction and an overview of the format of a JWS.

Video

Watch a short video to learn how to decode a JWT. While this video is specific to a JWT, many of the concepts are the same for JWS.

Sample: Decode a JWS

The policy shown below decodes a JWS found in the flow variable var.JWS. This variable must be present and contain a viable (decodable) JWS. The policy can obtain the JWS from any flow variable.

<DecodeJWS name="JWS-Decode-HS256">
    <DisplayName>JWS Verify HS256</DisplayName>
    <Source>var.JWS</Source>
</DecodeJWS>

For each header in the header portion of the JWS, the policy sets a flow variable named:

jws.policy-name.header.header-name

If the JWS has an attached payload, then it sets the jws.policy-name.header.payload flow variable to the payload. For a detached payload, payload is empty. See Flow variables for a complete list of the variables set by this policy.

Element reference for Decode JWS

The policy reference describes the elements and attributes of the Decode JWS policy.

Attributes that apply to the top-level element

<DecodeJWS name="JWS" continueOnError="false" enabled="true" async="false">

The following attributes are common to all policy parent elements.

<DisplayName>

<DisplayName>Policy Display Name</DisplayName>

Use in addition to the name attribute to label the policy in the management UI proxy editor with a different, natural-language name.

<Source>

<Source>JWS-variable</Source>

If present, specifies the flow variable in which the policy expects to find the JWS to decode.

Flow variables

Upon success, the Verify JWS and Decode JWS policies set context variables according to this pattern:

jws.{policy_name}.{variable_name}

For example, if the policy name is verify-jws, then the policy will store the algorithm specified in the JWS to this context variable: jws.verify-jws.header.algorithm

Error reference

This section describes the fault codes and error messages that are returned and fault variables that are set by Edge when this policy triggers an error. This information is important to know if you are developing fault rules to handle faults. To learn more, see What you need to know about policy errors and Handling faults.

Runtime errors

These errors can occur when the policy executes.

Deployment errors

These errors can occur when you deploy a proxy containing this policy.

Fault variables

These variables are set when a runtime error occurs. For more information, see What you need to know about policy errors.

Example error response

For error handling, the best practice is to trap the errorcode part of the error response. Do not rely on the text in the faultstring, because it could change.

Example fault rule

<FaultRules>
    <FaultRule name="JWS Policy Errors">
        <Step>
            <Name>JavaScript-1</Name>
            <Condition>(fault.name Matches "TokenExpired")</Condition>
        </Step>
        <Condition>JWS.failed=true</Condition>
    </FaultRule>
</FaultRules>