Github user alopresto commented on the issue:

    https://github.com/apache/nifi/pull/2047
  
    I continued reviewing with a custom IdP from Auth0. This provider can 
integrate with other providers, including Google, Facebook, etc. and also have 
its own custom users stored locally. @mcgilman and I were able to receive a 
success response from Auth0 for a custom user, but the token signature could 
not be validated. We traced this to determine that on startup, when the 
`TokenValidator` is initialized, if no 
`nifi.security.user.oidc.preferred.jwsalgorithm` is present in 
`nifi.properties`, the first value of `id_token_signing_alg_values_supported` 
in the response from the OpenID Connect Discovery Document URI is used. In 
Auth0's case, this was `HS256` as opposed to `RS256`, but the tokens returned 
were signed with `RS256`, resulting in NiFi returning an error "Unable to 
exchange authorization for ID token: Unable to parse the response from the 
Token request: Signed JWT rejected: Another algorithm expected, or no matching 
key(s) found". 
    
    The Auth0 OIDC Discovery Document response:
    
    ```
    
{"issuer":"https://apachenifi.auth0.com/","authorization_endpoint":"https://apachenifi.auth0.com/authorize","token_endpoint":"https://apachenifi.auth0.com/oauth/token","userinfo_endpoint":"https://apachenifi.auth0.com/userinfo","mfa_challenge_endpoint":"https://apachenifi.auth0.com/mfa/challenge","jwks_uri":"https://apachenifi.auth0.com/.well-known/jwks.json","registration_endpoint":"https://apachenifi.auth0.com/oidc/register","scopes_supported":["openid","profile","offline_access","name","given_name","family_name","nickname","email","email_verified","picture","created_at","identities","phone","address"],"response_types_supported":["code","token","id_token","code
 token","code id_token","token id_token","code token 
id_token"],"response_modes_supported":["query","fragment","form_post"],"subject_types_supported":["public"],"id_token_signing_alg_values_supported":["HS256","RS256"],"token_endpoint_auth_methods_supported":["client_secret_basic","client_secret_post"],"claims_supported":
 
["aud","auth_time","created_at","email","email_verified","exp","family_name","given_name","iat","identities","iss","name","nickname","phone_number","picture","sub"]}
    ```
    
    We decided to change the initialization logic to default to `RS256` as it 
is required by the [OpenID Connect 
Specification](https://openid.net/specs/openid-connect-discovery-1_0.html#ProviderMetadata)
 to always be present in this response. 
    
    > id_token_signing_alg_values_supported
    > REQUIRED. JSON array containing a list of the JWS signing algorithms (alg 
values) supported by the OP for the ID Token to encode the Claims in a JWT 
[JWT]. **The algorithm RS256 MUST be included**. The value none MAY be 
supported, but MUST NOT be used unless the Response Type used returns no ID 
Token from the Authorization Endpoint (such as when using the Authorization 
Code Flow).
    
    I manually provided `RS256` as a configuration value for 
`nifi.security.user.oidc.preferred.jwsalgorithm` in `nifi.properties` and was 
able to successfully sign in as the user. 


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