Comment on “an Efficient Identity-Based Signature Scheme with Provable Security”

In this comment paper, we reveal a security issue in an efficient identity-based signature (IBS) scheme that has been proven secure in the standard model. More specifically, we perform a key-only attack on the IBS scheme to break its universal unforgeability. Since the security notion of universal unforgeability under key-only attack (uuf-koa) is a weaker notion than the existential unforgeability under chosen message attack (euf-cma), this invalidates the euf-cma security claimed by the IBS scheme. Subsequently, we propose a fix to achieve a stronger seuf-cma security without a random oracle.