jali/tri/noise.tri

// ---
// tags: jali, trident
// crystal-type: circuit
// crystal-domain: comp
// ---

//  Noise budget tracking for lattice-based FHE schemes.
//
//  Tracks a log2 upper bound on noise magnitude through homomorphic
//  operations. When the noise budget is exhausted, decryption fails.
//  Conservative (worst-case) bounds -- never underestimates noise.

module jali.noise

/// Log2 upper bound on the noise magnitude.
pub struct NoiseBudget {
    log_bound: U32
}

/// Create a fresh noise budget with an initial bound.
pub fn fresh(initial_bound: U32) -> NoiseBudget {
    NoiseBudget { log_bound: initial_bound }
}

/// Noise budget after homomorphic addition.
/// Addition roughly doubles the noise: max(a, b) + 1.
pub fn after_add(a: NoiseBudget, b: NoiseBudget) -> NoiseBudget {
    let max_log: U32 = if a.log_bound > b.log_bound {
        a.log_bound
    } else {
        b.log_bound
    }
    NoiseBudget { log_bound: max_log + 1 }
}

/// Noise budget after homomorphic multiplication.
/// Multiplication increases noise as: log(a) + log(b) + log2(n).
/// log_n is the log2 of the ring dimension (10 for n=1024).
pub fn after_mul(a: NoiseBudget, b: NoiseBudget, log_n: U32) -> NoiseBudget {
    NoiseBudget { log_bound: a.log_bound + b.log_bound + log_n }
}

/// Noise budget after bootstrapping resets to a fixed noise level.
pub fn after_bootstrap(bootstrap_noise: U32) -> NoiseBudget {
    NoiseBudget { log_bound: bootstrap_noise }
}

/// Check whether bootstrapping is needed.
/// Returns true if current noise meets or exceeds the maximum tolerable budget.
pub fn needs_bootstrap(budget: NoiseBudget, max_budget: U32) -> bool {
    budget.log_bound >= max_budget
}

/// Remaining budget before decryption failure.
/// Returns 0 if already exceeded.
pub fn remaining(budget: NoiseBudget, max_budget: U32) -> U32 {
    if budget.log_bound >= max_budget {
        0
    } else {
        max_budget - budget.log_bound
    }
}