Optional<String> getHeaderValue(final String key) {
            final String decodedHeader = base64Decode(header);
            return JsonUtil.getValue(decodedHeader, key);
        }
Verify the JSON Web Signature and then extract the user identity from it
stroom-security/stroom-security-common-impl/src/main/java/stroom/security/common/impl/StandardJwtContextFactory.java