SHRINCS: 324-byte stateful post-quantum signatures with static backups

The revised SHRINCS scheme introduces a more user-friendly approach to managing cryptographic keys and signing devices, primarily by simplifying the process of device initialization and seed management. In the traditional setup, each new device required a backup of a fresh secret seed, complicating the user experience, especially in scenarios involving lost or additional devices. The modified scheme, however, requires only a single static master seed to be backed up once. This master seed coupled with a public per-device identifier (id) enhances both security and convenience.

In practice, when a user initializes a new signing device under the revised scheme, they no longer need to generate and secure a new secret seed for each device. Instead, they generate a unique id for each device, which is publicly storable and does not compromise the security of the signature scheme. This identifier acts somewhat like a descriptor, which can be safely stored in more accessible locations, potentially even on the blockchain, although this might not be an optimal use of blockspace. The device uses this id to compute the SHRINCS seed through a pseudorandom function (PRF), using the master seed as the key. This process ensures that the master seed remains protected, as it alone cannot derive the necessary SHRINCS seed without the corresponding id.

The core algorithms of the modified SHRINCS scheme include MasterSeedGen, which generates the master seed; KeyGen, which produces the device-specific id, the public key, and sets the initial state; and Sign, which performs the actual signing operation. If the current device id matches the state's id, the signing proceeds normally. If not, it indicates that the device state might have been lost, triggering a fallback signing mechanism. The flexibility of where MasterSeedGen can be executed—such as through non-electronic means like dice rolls—further adds to the robustness of the scheme.

This redesigned approach not only simplifies the backup and recovery processes but also maintains rigorous security standards crucial for effective cryptographic operations. The collaboration and discussions with various contributors in the field underscore the scheme’s innovative adaptation to practical needs while adhering to stringent security requirements.