[Keystone] How to retrieve secret key for a given access key id

Hi All, I have a scenario where I need to retrieve the secret key of a given access key id (of EC2 credentials) from Keystone. I know that this can be done by sending a GET request to the following URL: v3/users/{user_id}/credentials/OS-EC2/access_key_id I don't have the user_id required for the above request, but I have the admin username and admin password, that was used to create the EC2 credentials. Is there a way to get the user_id, and then get the secret key from Keystone? Thanks, Pritha