The concept of using a smartphone as a wireless hardware security token, akin to a YubiKey, is not new. Yubico themselves provide SDKs and hardware that enable this functionality via NFC or direct connection. However, the security of such a system is fundamentally dependent on the security of the underlying mobile operating system. A standard Android or iOS device, despite its own security measures, presents a larger attack surface than a dedicated hardware token.
This is where an operating system like GrapheneOS becomes a critical component in a feasibility analysis. The inquiry posed by @astrra.space about turning a GrapheneOS phone into a wireless YubiKey is particularly salient because GrapheneOS is engineered with a security model that could, in theory, provide a comparable level of trust.
GrapheneOS's security architecture includes several key features that would be essential for this application:
Secure Element Integration: GrapheneOS leverages the secure element (e.g., the Titan M chip in Google Pixel devices) for its "Weaver" component, which provides hardware-backed throttling of decryption key attempts. This is a crucial feature for protecting stored cryptographic keys from brute-force attacks. Hardware-based Attestation: The operating system supports hardware-based attestation, allowing a remote server to verify the integrity of the device's hardware and software stack. This would be a critical component in any system that uses a GrapheneOS device as a roaming security key. Baseband Isolation: GrapheneOS isolates the cellular baseband, a known source of vulnerabilities, from the main application processor. This reduces the attack surface of the device. Hardened Kernel and Userspace: The OS includes a hardened kernel and userspace, with features like hardened_malloc and a robust sandboxing model, which further mitigate the risk of exploitation.
The primary challenge in implementing such a system would be the development of a secure application that can interface with the secure element to store and manage SSH and PGP keys, and then expose them to a remote client over a secure channel (e.g., Bluetooth LE with end-to-end encryption).
In conclusion, while a standard smartphone is not a direct replacement for a dedicated hardware security token, a device running GrapheneOS, with its robust security model and utilization of hardware-based security features, presents a compelling platform for the development of a secure, wireless alternative to a YubiKey. The feasibility is high, but the implementation would require significant engineering effort to ensure that the application layer is as secure as the underlying operating system.