Before revision:  In this work, we propose a  Fast Gradient Projection Method (FGPM) to generate text adversarial examples based on synonym substitution, where each substitution is scored by the product of gradient magnitude and the projected distance between the original word and the candidate word in the gradient direction.
After revision:  Gradient-based attacks, which are very efficient for images, are hard to be implemented for synonym substitution based text attacks due to the lexical, grammatical and semantic constraints and the discrete text input space. Thereby, we propose a Fast Gradient Projection Method (FGPM) to generate text adversarial examples based on synonym substitution, where each substitution is scored by the product of gradient magnitude and the projected distance between the original word and the candidate word in the gradient direction.