sk_len.rs

use crate::kem::common::kem_type::KemType;

/// A trait to get the length of the secret key
///
/// This is used to determine the length of the secret key
///
/// # Returns
///
/// The length of the secret key in bytes or `None` if the length is variable
pub trait SKLen {
    fn get_sk_len(&self) -> Option<usize>;
}

impl SKLen for KemType {
    /// Get the length of the secret key
    ///
    /// # Returns
    ///
    /// The length of the secret key in bytes or `None` if the length is not fixed
    fn get_sk_len(&self) -> Option<usize> {
        match self {
            // These are Nsk length as per SerializePrivateKey(skX)
            // in RFC 9180
            KemType::P256 => Some(32),
            KemType::P384 => Some(48),
            KemType::X25519 => Some(32),
            KemType::BrainpoolP256r1 => Some(32),
            KemType::BrainpoolP384r1 => Some(48),
            KemType::X448 => Some(56),
            // ML Key secret key sizes
            KemType::MlKem512 => Some(1632),
            KemType::MlKem768 => Some(2400),
            KemType::MlKem1024 => Some(3168),
            // RSA Key secret key sizes: varied
            KemType::RsaOAEP2048 => None,
            KemType::RsaOAEP3072 => None,
            KemType::RsaOAEP4096 => None,

            // In Kem composites, traditional public key is part of the private key
            // pq_sk + trad_sk + pq_overhead + trad_public_key_overhead + trad_overhead + sequence_overhead
            // trad_overhead = 9 + <oid_byte>
            KemType::MlKem768BrainpoolP256r1 => Some(2400 + 32 + 24 + (65 + 6) + 19 + 4),
            KemType::MlKem768X25519 => Some(2400 + 32 + 24 + (32 + 11) + 14 + 4),
            KemType::MlKem1024P384 => Some(3168 + 48 + 24 + (97 + 10) + 16 + 4),
            KemType::MlKem1024BrainpoolP384r1 => Some(3168 + 48 + 24 + (97 + 7) + 19 + 4),
            KemType::MlKem1024X448 => Some(3168 + 56 + 24 + (56 + 12) + 14 + 4),
            KemType::MlKem768Rsa2048 => None,
            KemType::MlKem768Rsa3072 => None,
            KemType::MlKem768Rsa4096 => None,
            KemType::MlKem768P384 => Some(2400 + 48 + 24 + (97 + 10) + 16 + 4),
            KemType::XWing => Some(32),
        }
    }
}