LP-141: Threshold VM

Abstract

The Threshold VM runs the T-chain, a dedicated chain for **executing

threshold cryptographic protocols** — signing sessions, threshold FHE

operations, session management, quotas. It is the hot path. The K-chain

(Key VM, LP-140) is the cold path that owns keys; the T-chain borrows them

to sign.

Precompile address nibble: C=7 (see LP-129 Registry).

Scope

T-chain supports these threshold signing protocols:

T-chain also hosts threshold FHE: threshold decryption of FHE ciphertexts

that were encrypted under a T-chain-held FHE key. Used by Z-chain for

encrypted rollup state disclosure.

Session Lifecycle

type SignSession struct {
    ID              [32]byte
    KeyRecordID     [32]byte    // Reference to K-chain KeyRecord
    Protocol        Protocol     // CGGMP21 | FROST | BLS | ...
    Message         []byte
    Committee       []ids.NodeID
    Status          Status       // Pending | InProgress | Complete | Failed | Expired
    StartedAt       uint64       // Block timestamp
    ExpiresAt       uint64       // Block timestamp + SignTimeout
    Signatures      map[ids.NodeID][]byte  // Partial sigs
    Result          []byte       // Combined signature (when Complete)
}

Sessions run for at most SignTimeout (default 5 min). Expired sessions

release their reserved committee seats and are marked Failed.

Committee Selection

For each session, T-chain samples a committee from the validators that hold

a share of the referenced key (as recorded by K-chain).

Committee size rules:

• Small threshold (T ≤ 6): full group signs — use Threshold ML-DSA, FROST
• Medium (T in [7, 32]): deterministic sampling with VRF-based unbiasable

selection

• Large (T > 32): SHOULD NOT use a single session. Use hierarchical

quorum certs (LP-142).

See LP-142 for why thousand-validator monolithic sessions are the wrong shape.

FHE Subsystem

T-chain hosts:

• Threshold FHE decryption (parties jointly decrypt without revealing plaintext

to any single party)

• FHE parameter registry (network-wide CRS)
• FHE operation quotas (bounded compute per caller)

Threshold FHE integrates with Z-chain (ZK rollups) for **private disclosure

of encrypted state**: Z-chain commits to a ciphertext on-chain, authorized

parties request threshold decryption via T-chain, result is revealed only

once quorum is reached.

Boundary with K-chain

Reference