func checkCreds(st *state.State, c params.Creds) (taggedAuthenticator, error) {
entity0, err := st.FindEntity(c.AuthTag)
if err != nil && !errors.IsNotFound(err) {
return nil, err
}
// We return the same error when an entity
// does not exist as for a bad password, so that
// we don't allow unauthenticated users to find information
// about existing entities.
entity, ok := entity0.(taggedAuthenticator)
if !ok {
return nil, common.ErrBadCreds
}
if err != nil || !entity.PasswordValid(c.Password) {
return nil, common.ErrBadCreds
}
// Check if a machine agent is logging in with the right Nonce
if err := checkForValidMachineAgent(entity, c); err != nil {
return nil, err
}
return entity, nil
}