Function add_3_bitstrings

ciphercore-base/src/mpc/mpc_conversion.rs:381–410

Adds 3 bit strings contained in nodes a, b and c of a graph g. This function calls 2 graphs: - adder_g that is the full adder graph for 2 bit strings, - shift_g that performs the left shift of a bit string by 1 position.

(
    g: Graph,
    adder_g: Graph,
    shift_g: Graph,
    a: Node,
    b: Node,
    c: Node,
    prf_for_mul_keys: Node,
)

/// - adder_g that is the full adder graph for 2 bit strings,
/// - shift_g that performs the left shift of a bit string by 1 position.
fn add_3_bitstrings(
    g: Graph,
    adder_g: Graph,
    shift_g: Graph,
    a: Node,
    b: Node,
    c: Node,
    prf_for_mul_keys: Node,
) -> Result<Node> {
    // Reduce addition of three values to two values via the full adder circuit.
    // Given 3 bitstrings a, b and c, Add(a,b,c) = Add(carry << 1, xor_123),
    // where carry = a AND b XOR c AND (a XOR b), xor_123 = a XOR b XOR c
    // and Add is the binary adder.
    // a XOR b
    let xor_12 = add_mpc(a.clone(), b.clone())?;
    // a AND b, don't reshare the result
    let and_12 = multiply_mpc(a, b, prf_for_mul_keys.clone(), false)?;
    // (a XOR b) AND c, don't reshare the result
    let xor_12_and_3 = multiply_mpc(xor_12.clone(), c.clone(), prf_for_mul_keys.clone(), false)?;
    // a XOR b XOR c
    let xor_123 = add_mpc(xor_12, c)?;
    // (a AND b) XOR (a XOR b) AND c and reshare the result
    let carry = add_mpc(and_12, xor_12_and_3)?;

    // carry << 1
    let shifted_carry = reshare(&g.call(shift_g, vec![carry])?, &prf_for_mul_keys)?;

    // xor_123 + (carry << 1) = Add(a,b,c)
    g.call(adder_g, vec![prf_for_mul_keys, xor_123, shifted_carry])
}

#[cfg(test)]
mod tests {