Program obfuscation, which aims to obscure the inner workings of a computer program while maintaining its functionality, is a central goal in cryptography and software protection. An emerging line of research explores the possibility of applying obfuscation to quantum programs. So far, however, researchers have only achieved obfuscation in specific quantum circuits — falling short of supporting obfuscation of general quantum input-output functionalities.
Enter Allen School Ph.D. student Er-Cheng Tang and collaborator Mi-Ying (Miryam) Huang, a Ph.D. student at the University of Southern California. The duo recently developed the first quantum state obfuscation scheme for unitary quantum programs, which are the backbone of quantum computing, in the classical oracle model.
“We achieve program obfuscation in the fully quantum setting for the first time, enabling software that runs on quantum data to be provably protected,” said Tang, who is advised by Allen School professors Andrea Coladangelo and Huijia (Rachel) Lin.
Tang and Huang presented their research at the 66th IEEE Symposium on Foundations of Computer Science (FOCS 2025) last December in Sydney, Australia, where they received the Machtey Award for Best Student Paper.
“Obfuscation of quantum programs secure against quantum adversaries is a significantly more powerful extension of obfuscation in the classical world. It’s a wonderful feat achieved by two Ph.D. students,” said Lin.
Building off of previous frameworks for quantum obfuscation, Tang and Huang offer several improvements in their scheme to extend obfuscation to quantum programs with quantum inputs and outputs. They start by building a strengthened cryptographic tool, called a functional quantum authentication scheme, for protecting quantum programs while enabling their execution. To support quantum programs with quantum inputs and outputs, the researchers also integrate quantum teleportation into the framework, allowing the transitions of input and output quantum states between protected and unprotected forms.
At the core of their obfuscation scheme is a novel compiler which serves two major purposes. First, it represents an arbitrary quantum circuit as a projective linear-plus-measurement quantum program, as the functional quantum authentication scheme natively works under that format. The attained projective property provides a basis for analyzing the program’s execution trace. The researchers then prove that their obfuscated unitary quantum program can only be used to compute the implemented unitary transformation and its inverse; nothing else can be derived from the obfuscated program.
“The significance of the result is that it relates obfuscation of general quantum programs — that can take as input quantum states and output quantum states — to obfuscation of classical programs. At a high level, it says that there is a generic way to bootstrap the latter to obtain the former,” said Coladangelo.
Read the full paper on obfuscation of unitary quantum programs here.