Proactive Secret Sharing

Proactive Secret Sharing (PSS) is a cryptographic protocol that enhances standard secret sharing by periodically and automatically refreshing the distributed shares of a secret without altering the underlying secret itself. This process, known as share renewal or refresh, is designed to protect against mobile adversaries who may compromise participants over an extended period. By forcing adversaries to compromise a threshold of participants within a single refresh period, PSS significantly raises the security bar for long-lived secrets, such as private keys in distributed systems.

The core mechanism involves participants in a threshold secret sharing scheme, like Shamir's Secret Sharing, collaboratively generating new random polynomials. These new polynomials are constructed so that they evaluate to zero at the point representing the original secret. Each participant then sends a new share, derived from this polynomial, to every other participant. By summing their old share with the incoming new shares, each participant obtains a fresh share for the same original secret. Critically, the old shares are rendered useless after this process, limiting the window of vulnerability.

PSS is fundamental to securing Distributed Key Generation (DKG) protocols and Threshold Signature Schemes (TSS) used in blockchain networks. For example, a validator set securing a Proof-of-Stake chain using a threshold ECDSA scheme would employ proactive secret sharing to periodically refresh the shares of their collective signing key. This prevents an attacker who slowly corrupts nodes over months from eventually accumulating enough shares to reconstruct the key and forge signatures, a threat model known as a mobile adversary.

The security guarantees of PSS depend on a synchronous refresh period and the assumption that the adversary cannot compromise more than a threshold number of participants between refreshes. If this proactive security assumption holds, the system maintains forward and backward security for the secret. This means compromises from previous periods cannot help recover the secret in future periods, and future compromises cannot retroactively reveal the secret from past periods, confining the damage to a single epoch.

Implementing PSS introduces complexities, including the need for secure communication channels during refresh, robustness against participants who fail or act maliciously during the protocol (requiring verifiable secret sharing), and the operational overhead of coordinating periodic refresh ceremonies. Despite these challenges, it is considered essential for any long-term deployment of threshold cryptography where participant compromise is a realistic threat, making it a cornerstone of modern secure multi-party computation (MPC) and decentralized custody solutions.

Proactive Secret Sharing (PSS) is a cryptographic protocol that periodically and securely refreshes the shares of a distributed secret without ever reconstructing the secret itself. This process, known as share renewal or refresh, is designed to defend against mobile adversaries—attackers who can compromise different participants over time. By refreshing shares, the protocol ensures that an adversary who gains access to some shares during one time period cannot combine them with shares obtained in a future period to reconstruct the secret, thereby establishing a proactive security model.

The core mechanism involves each participant generating new, random shares of the value zero and distributing them to all other participants using a standard secret sharing scheme like Shamir's Secret Sharing. Each participant then adds all the new zero-shares they receive to their existing secret share, creating a refreshed share. Crucially, the sum of all refreshed shares still reconstructs to the original secret, as adding shares of zero does not change the underlying value. This operation is performed over a secure communication channel during a designated refresh period.

A critical component is the proactive resilience threshold. If the underlying secret sharing scheme has a threshold of t-out-of-n, proactive security typically requires that fewer than t participants are compromised during any single refresh period. Adversaries are assumed to be mobile, meaning they can corrupt different sets of participants over time, but the refresh protocol limits the window of vulnerability. This model is far stronger than static secret sharing, which assumes adversaries are fixed after an initial compromise.

Proactive Secret Sharing is foundational for securing long-lived cryptographic keys in distributed systems. Its primary use cases include securing Distributed Key Generation (DKG) protocols, threshold signature schemes (like those used in blockchain validators or multi-party computation wallets), and secure multi-party computation (MPC) sessions that run indefinitely. By routinely refreshing key shares, these systems can remain secure for years, even if some participants are occasionally breached, as old compromised shares become useless after each refresh cycle.

Implementing PSS introduces operational complexities, including the need for synchronized refresh epochs, robust peer-to-peer communication, and verification mechanisms to prevent malicious participants from distributing invalid refresh shares. Protocols often incorporate verifiable secret sharing (VSS) during the refresh phase to ensure consistency. Despite the overhead, the trade-off is essential for applications where the cost of a key compromise is catastrophic, making PSS a cornerstone of modern, intrusion-tolerant distributed cryptography.

Proactive Secret Sharing (PSS) is an advanced cryptographic protocol that periodically refreshes or renews the individual shares of a distributed secret without ever reconstructing or changing the underlying secret itself. This process, known as a share refresh protocol, is designed to defend against mobile adversaries—attackers who can corrupt different participants over time. By frequently updating the shares, any information an adversary may have learned from compromised old shares becomes useless, as the new shares are cryptographically independent of the previous ones. This maintains the security of the secret over an unbounded time horizon.

The core mechanism involves each participant generating new, random shares of the value zero and securely distributing these to other participants. When these zero-shares are added to the existing secret shares, the underlying secret remains unchanged, but the composition of each individual share is completely refreshed. This operation typically requires a communication round where participants broadcast their new zero-shares, often using verifiable secret sharing to ensure correctness and prevent malicious behavior. The protocol ensures that the threshold access structure (e.g., t-out-of-n) is preserved throughout the refresh.

PSS is critical for long-lived cryptographic systems, such as distributed key generation (DKG) for blockchain validators, secure multi-party computation (MPC) wallets, and threshold signature schemes. In these contexts, a private key is the shared secret. Without proactive refresh, an adversary who slowly compromises nodes over months or years could eventually gather enough key shares to reconstruct the secret. PSS renders this share harvesting attack ineffective, providing forward and backward security for the secret across refresh epochs. Its implementation is a cornerstone of robust, enterprise-grade custody solutions and resilient consensus protocols.