The `DefaultEncryptingOAuthDataProvider.getAccessToken` method calls `ModelEncryptionSupport.decryptAccessToken` which throws a SecurityException if the input doesn't match they crypto algorithm's desired input. This results in a server error.
I suggest that this method should catch the SecurityException and instead throw an OAuthServiceException, as specified by the signature of `getAccessToken`.
e.g. `throw new OAuthServiceException(OAuthConstants.ACCESS_DENIED, e);`
This would enable invalid access tokens to be rejected cleanly back to the client. I am happy to provide a patch for this issue.