Pre-signature

Pre-signatures let you precompute the expensive multi-round MPC computation (between mobile and first server) before the transaction is ready. When the user submits a transaction, preSignatureFinal returns the signature instantly — no MPC round-trip at that point.

For the SDK contract see Pre-signature (Kotlin) and Pre-signature (Swift).

How the example does it

Step 1: Generate the pre-signature (expensive, do this in advance)

Android

vault/.../session/VaultSessionManager.kt

suspend fun preSignature(keyId: String): ByteArray {
    val dao = readDao(keyId)
    val keyshare = dao.currentKeyshare
        ?: throw Exception("No active keyshare found for keyId: $keyId")
    return sessionFor(dao.keyType).preSignature(keyshare).getOrThrow()
}

iOS / macOS

Vault/Session/VaultSessionManager.swift

func preSignature(keyId: String) async throws -> Data {
    let record = try loadRecord(keyId: keyId)
    let keyshare = try requireActiveKeyshare(record)
    return try await sessionForKeyType(record.keyType)
        .preSignature(keyshare: keyshare).get()
}

Step 2: Finalize when the transaction is ready (instant)

Android

vault/.../session/VaultSessionManager.kt

suspend fun preSignatureFinal(
    keyId: String,
    preSignature: ByteArray,
    message: ByteArray,
    derivationPath: String,
): ByteArray {
    val keyType = readDao(keyId).keyType
    return sessionFor(keyType)
        .preSignatureFinal(
            preSignature = preSignature,
            message = message.toHex(),
            derivationPath = derivationPath,
        ).getOrThrow()
}

iOS / macOS

Vault/Session/VaultSessionManager.swift

func preSignatureFinal(preSignature: Data, keyId: String,
                       message: Data, derivationPath: String) async throws -> Data {
    let keyType = try loadRecord(keyId: keyId).keyType
    let messageHex = message.map { String(format: "%02x", $0) }.joined()
    return try await sessionForKeyType(keyType).preSignatureFinal(
        preSignature: preSignature, message: messageHex, chainPath: derivationPath
    ).get()
}

Single-use

Each pre-signature can only be finalized once. Consuming it via preSignatureFinal invalidates it — the example resets the UI state so the user generates a fresh one for the next transaction. The pre-signature is held in memory (not persisted to disk), so it's also lost if the app is killed before finalization.

The example wires this up under the Pre-Signature screen, available from the dashboard for ECDSA (Ethereum) tokens only.