Poisoning-Based Backdoors Are Not Cryptographic

Abstract

Defending against a backdoor requires recovering the verifier that decides whether an input is a trigger, since blocking individual triggers fails against an attacker who can generate new ones. If a verifier could be installed with the asymmetry of cryptographic authentication, where only the attacker can produce triggers and the defender cannot reconstruct the verifier, defense that learns the verifier from observations would be impossible in principle. We show that no such asymmetry can arise when the verifier enters training only through the labels of poisoned data, which covers the dirty-label backdoors that dominate the literature on language models. A training procedure that installs a verifier from poisoned data is itself a learner for that verifier. Hence any verifier that is hard to learn cannot be installed through poisoning, and any verifier that poisoning can install is reconstructible by a white-box defender holding as many labeled trigger observations as the attacker used. Hiding a secret key in the input or the weights does not change this. Experiments on instruction-tuned large language models confirm the dichotomy, with no configuration yielding a backdoor that is both effective for the attacker and irrecoverable for the defender.