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/* This file is part of DarkFi (https://dark.fi)
*
* Copyright (C) 2020-2024 Dyne.org foundation
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License as
* published by the Free Software Foundation, either version 3 of the
* License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*/
use blake2b_simd::{Hash as Blake2bHash, Params as Blake2bParams};
use pasta_curves::group::{GroupEncoding, Wnaf};
use super::{util::fp_mod_fv, PublicKey, SecretKey};
use crate::error::ContractError;
pub const KDF_SAPLING_PERSONALIZATION: &[u8; 16] = b"DarkFiSaplingKDF";
/// Sapling key agreement for note encryption.
/// Implements section 5.4.4.3 of the Zcash Protocol Specification
pub fn sapling_ka_agree(esk: &SecretKey, pk_d: &PublicKey) -> Result<PublicKey, ContractError> {
let esk_s = fp_mod_fv(esk.inner());
// Windowed multiplication is constant time. Hence that is used here vs naive EC mult.
// Decrypting notes is a an amortized operation, so you want successful rare-case note
// decryptions to be indistinguishable from the usual case.
let mut wnaf = Wnaf::new();
PublicKey::try_from(wnaf.scalar(&esk_s).base(pk_d.inner()))
}
/// Sapling KDF for note encryption.
pub fn kdf_sapling(dhsecret: &PublicKey, epk: &PublicKey) -> Blake2bHash {
// The P.to_bytes() for P ∈ ℙₚ function used on affine curves it not perfectly constant time,
// but it's close enough. The function returns 0 when P = ∞ is the identity which is the
// edge case but almost never occurs.
Blake2bParams::new()
.hash_length(32)
.personal(KDF_SAPLING_PERSONALIZATION)
.to_state()
.update(&dhsecret.inner().to_bytes())
.update(&epk.inner().to_bytes())
.finalize()
}