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use crate::{ kdf::{extract_and_expand, Kdf as KdfTrait}, kex::{Deserializable, KeyExchange, Serializable, MAX_PUBKEY_SIZE}, util::kem_suite_id, HpkeError, }; use digest::FixedOutput; use generic_array::GenericArray; use rand::{CryptoRng, RngCore}; /// Defines a combination of key exchange mechanism and a KDF, which together form a KEM pub trait Kem: Sized { type Kex: KeyExchange; #[doc(hidden)] type Kdf: KdfTrait; const KEM_ID: u16; /// Deterministically derives a keypair from the given input keying material /// /// Requirements /// ============ /// This keying material SHOULD have as many bits of entropy as the bit length of a secret key, /// i.e., `8* Self::Kex::PrivateKey::size()`. For X25519 and P-256, this is 32 bytes of /// entropy. fn derive_keypair( ikm: &[u8], ) -> ( <Self::Kex as KeyExchange>::PrivateKey, <Self::Kex as KeyExchange>::PublicKey, ) { let suite_id = kem_suite_id::<Self>(); Self::Kex::derive_keypair::<Self::Kdf>(&suite_id, ikm) } /// Generates a random keypair using the given RNG fn gen_keypair<R: CryptoRng + RngCore>( csprng: &mut R, ) -> ( <Self::Kex as KeyExchange>::PrivateKey, <Self::Kex as KeyExchange>::PublicKey, ) { // Make some keying material that's the size of a private key let mut ikm: GenericArray< u8, <<Self::Kex as KeyExchange>::PrivateKey as Serializable>::OutputSize, > = GenericArray::default(); // Fill it with randomness csprng.fill_bytes(&mut ikm); // Run derive_keypair using the KEM's KDF Self::derive_keypair(&ikm) } } // Kem is also used as a type parameter everywhere. To avoid confusion, alias it use Kem as KemTrait; #[cfg(feature = "x25519-dalek")] /// Represents DHKEM(Curve25519, HKDF-SHA256) pub struct X25519HkdfSha256 {} #[cfg(feature = "x25519-dalek")] impl Kem for X25519HkdfSha256 { type Kex = crate::kex::X25519; type Kdf = crate::kdf::HkdfSha256; // §7.1: DHKEM(X25519, HKDF-SHA256) const KEM_ID: u16 = 0x0020; } #[cfg(feature = "p256")] /// Represents DHKEM(P256, HKDF-SHA256) pub struct DhP256HkdfSha256 {} #[cfg(feature = "p256")] impl Kem for DhP256HkdfSha256 { type Kex = crate::kex::DhP256; type Kdf = crate::kdf::HkdfSha256; // §7.1: DHKEM(P-256, HKDF-SHA256) const KEM_ID: u16 = 0x0010; } /// Convenience types representing public/private keys corresponding to a KEM's underlying DH alg type KemPubkey<Kem> = <<Kem as KemTrait>::Kex as KeyExchange>::PublicKey; type KemPrivkey<Kem> = <<Kem as KemTrait>::Kex as KeyExchange>::PrivateKey; /// Holds the content of an encapsulated secret. This is what the receiver uses to derive the /// shared secret. // This just wraps a pubkey, because that's all an encapsulated key is in a DH-KEM pub struct EncappedKey<Kex: KeyExchange>(Kex::PublicKey); // EncappedKeys need to be serializable, since they're gonna be sent over the wire. Underlyingly, // they're just DH pubkeys, so we just serialize them the same way impl<Kex: KeyExchange> Serializable for EncappedKey<Kex> { type OutputSize = <Kex::PublicKey as Serializable>::OutputSize; // Pass to underlying to_bytes() impl fn to_bytes(&self) -> GenericArray<u8, Self::OutputSize> { self.0.to_bytes() } } impl<Kex: KeyExchange> Deserializable for EncappedKey<Kex> { // Pass to underlying from_bytes() impl fn from_bytes(encoded: &[u8]) -> Result<Self, HpkeError> { let pubkey = <Kex::PublicKey as Deserializable>::from_bytes(encoded)?; Ok(EncappedKey(pubkey)) } } /// A convenience type representing the fixed-size byte array of the same length as a serialized /// `KexResult` pub(crate) type SharedSecret<Kem> = GenericArray<u8, <<<Kem as KemTrait>::Kdf as KdfTrait>::HashImpl as FixedOutput>::OutputSize>; // def Encap(pkR): // skE, pkE = GenerateKeyPair() // dh = DH(skE, pkR) // enc = Serialize(pkE) // // pkRm = Serialize(pkR) // kem_context = concat(enc, pkRm) // // def AuthEncap(pkR, skS): // skE, pkE = GenerateKeyPair() // dh = concat(DH(skE, pkR), DH(skS, pkR)) // enc = Serialize(pkE) // // pkRm = Serialize(pkR) // pkSm = Serialize(pk(skS)) // kem_context = concat(enc, pkRm, pkSm) // // shared_secret = ExtractAndExpand(dh, kem_context) // return shared_secret, enc /// Derives a shared secret that the owner of the recipient's pubkey can use to derive the same /// shared secret. If `sk_sender_id` is given, the sender's identity will be tied to the shared /// secret. /// /// Return Value /// ============ /// Returns a shared secret and encapped key on success. If an error happened during key exchange, /// returns `Err(HpkeError::InvalidKeyExchange)`. pub(crate) fn encap_with_eph<Kem: KemTrait>( pk_recip: &KemPubkey<Kem>, sender_id_keypair: Option<&(KemPrivkey<Kem>, KemPubkey<Kem>)>, sk_eph: KemPrivkey<Kem>, ) -> Result<(SharedSecret<Kem>, EncappedKey<Kem::Kex>), HpkeError> { // Put together the binding context used for all KDF operations let suite_id = kem_suite_id::<Kem>(); // Compute the shared secret from the ephemeral inputs let kex_res_eph = Kem::Kex::kex(&sk_eph, pk_recip)?; // The encapped key is the ephemeral pubkey let encapped_key = { let pk_eph = Kem::Kex::sk_to_pk(&sk_eph); EncappedKey(pk_eph) }; // The shared secret is either gonna be kex_res_eph, or that along with another shared secret // that's tied to the sender's identity. let shared_secret = if let Some((sk_sender_id, pk_sender_id)) = sender_id_keypair { // kem_context = encapped_key || pk_recip || pk_sender_id // We concat without allocation by making a buffer of the maximum possible size, then // taking the appropriately sized slice. let (kem_context_buf, kem_context_size) = concat_with_known_maxlen!( MAX_PUBKEY_SIZE, &encapped_key.to_bytes(), &pk_recip.to_bytes(), &pk_sender_id.to_bytes() ); let kem_context = &kem_context_buf[..kem_context_size]; // We want to do an authed encap. Do KEX between the sender identity secret key and the // recipient's pubkey let kex_res_identity = Kem::Kex::kex(sk_sender_id, pk_recip)?; // concatted_secrets = kex_res_eph || kex_res_identity // Same no-alloc concat trick as above let (concatted_secrets_buf, concatted_secret_size) = concat_with_known_maxlen!( MAX_PUBKEY_SIZE, &kex_res_eph.to_bytes(), &kex_res_identity.to_bytes() ); let concatted_secrets = &concatted_secrets_buf[..concatted_secret_size]; // The "authed shared secret" is derived from the KEX of the ephemeral input with the // recipient pubkey, and the KEX of the identity input with the recipient pubkey. The // HKDF-Expand call only errors if the output values are 255x the digest size of the hash // function. Since these values are fixed at compile time, we don't worry about it. let mut buf = <SharedSecret<Kem> as Default>::default(); extract_and_expand::<Kem>(&concatted_secrets, &suite_id, &kem_context, &mut buf) .expect("shared secret is way too big"); buf } else { // kem_context = encapped_key || pk_recip // We concat without allocation by making a buffer of the maximum possible size, then // taking the appropriately sized slice. let (kem_context_buf, kem_context_size) = concat_with_known_maxlen!( MAX_PUBKEY_SIZE, &encapped_key.to_bytes(), &pk_recip.to_bytes() ); let kem_context = &kem_context_buf[..kem_context_size]; // The "unauthed shared secret" is derived from just the KEX of the ephemeral input with // the recipient pubkey. The HKDF-Expand call only errors if the output values are 255x the // digest size of the hash function. Since these values are fixed at compile time, we don't // worry about it. let mut buf = <SharedSecret<Kem> as Default>::default(); extract_and_expand::<Kem>(&kex_res_eph.to_bytes(), &suite_id, &kem_context, &mut buf) .expect("shared secret is way too big"); buf }; Ok((shared_secret, encapped_key)) } /// Derives a shared secret and an ephemeral pubkey that the owner of the reciepint's pubkey can /// use to derive the same shared secret. If `sk_sender_id` is given, the sender's identity will be /// tied to the shared secret. /// All this does is generate an ephemeral keypair and pass to `encap_with_eph`. /// /// Return Value /// ============ /// Returns a shared secret and encapped key on success. If an error happened during key exchange, /// returns `Err(HpkeError::InvalidKeyExchange)`. pub(crate) fn encap<Kem: KemTrait, R>( pk_recip: &KemPubkey<Kem>, sender_id_keypair: Option<&(KemPrivkey<Kem>, KemPubkey<Kem>)>, csprng: &mut R, ) -> Result<(SharedSecret<Kem>, EncappedKey<Kem::Kex>), HpkeError> where Kem: KemTrait, R: CryptoRng + RngCore, { // Generate a new ephemeral keypair let (sk_eph, _) = Kem::gen_keypair(csprng); // Now pass to encap_with_eph encap_with_eph::<Kem>(pk_recip, sender_id_keypair, sk_eph) } // def Decap(enc, skR): // pkE = Deserialize(enc) // dh = DH(skR, pkE) // // pkRm = Serialize(pk(skR)) // kem_context = concat(enc, pkRm) // // shared_secret = ExtractAndExpand(dh, kem_context) // return shared_secret // // def AuthDecap(enc, skR, pkS): // pkE = Deserialize(enc) // dh = concat(DH(skR, pkE), DH(skR, pkS)) // // pkRm = Serialize(pk(skR)) // pkSm = Serialize(pkS) // kem_context = concat(enc, pkRm, pkSm) // // shared_secret = ExtractAndExpand(dh, kem_context) // return shared_secret /// Derives a shared secret given the encapsulated key and the recipients secret key. If /// `pk_sender_id` is given, the sender's identity will be tied to the shared secret. /// /// Return Value /// ============ /// Returns a shared secret on success. If an error happened during key exchange, returns /// `Err(HpkeError::InvalidKeyExchange)`. pub(crate) fn decap<Kem: KemTrait>( sk_recip: &KemPrivkey<Kem>, pk_sender_id: Option<&KemPubkey<Kem>>, encapped_key: &EncappedKey<Kem::Kex>, ) -> Result<SharedSecret<Kem>, HpkeError> { // Put together the binding context used for all KDF operations let suite_id = kem_suite_id::<Kem>(); // Compute the shared secret from the ephemeral inputs let kex_res_eph = Kem::Kex::kex(&sk_recip, &encapped_key.0)?; // Compute the sender's pubkey from their privkey let pk_recip = Kem::Kex::sk_to_pk(sk_recip); // The shared secret is either gonna be kex_res_eph, or that along with another shared secret // that's tied to the sender's identity. if let Some(pk_sender_id) = pk_sender_id { // kem_context = encapped_key || pk_recip || pk_sender_id // We concat without allocation by making a buffer of the maximum possible size, then // taking the appropriately sized slice. let (kem_context_buf, kem_context_size) = concat_with_known_maxlen!( MAX_PUBKEY_SIZE, &encapped_key.to_bytes(), &pk_recip.to_bytes(), &pk_sender_id.to_bytes() ); let kem_context = &kem_context_buf[..kem_context_size]; // We want to do an authed encap. Do KEX between the sender identity secret key and the // recipient's pubkey let kex_res_identity = Kem::Kex::kex(sk_recip, pk_sender_id)?; // concatted_secrets = kex_res_eph || kex_res_identity // Same no-alloc concat trick as above let (concatted_secrets_buf, concatted_secret_size) = concat_with_known_maxlen!( MAX_PUBKEY_SIZE, &kex_res_eph.to_bytes(), &kex_res_identity.to_bytes() ); let concatted_secrets = &concatted_secrets_buf[..concatted_secret_size]; // The "authed shared secret" is derived from the KEX of the ephemeral input with the // recipient pubkey, and the kex of the identity input with the recipient pubkey. The // HKDF-Expand call only errors if the output values are 255x the digest size of the hash // function. Since these values are fixed at compile time, we don't worry about it. let mut shared_secret = <SharedSecret<Kem> as Default>::default(); extract_and_expand::<Kem>( &concatted_secrets, &suite_id, &kem_context, &mut shared_secret, ) .expect("shared secret is way too big"); Ok(shared_secret) } else { // kem_context = encapped_key || pk_recip || pk_sender_id // We concat without allocation by making a buffer of the maximum possible size, then // taking the appropriately sized slice. let (kem_context_buf, kem_context_size) = concat_with_known_maxlen!( MAX_PUBKEY_SIZE, &encapped_key.to_bytes(), &pk_recip.to_bytes() ); let kem_context = &kem_context_buf[..kem_context_size]; // The "unauthed shared secret" is derived from just the KEX of the ephemeral input with the // recipient pubkey. The HKDF-Expand call only errors if the output values are 255x the // digest size of the hash function. Since these values are fixed at compile time, we don't // worry about it. let mut shared_secret = <SharedSecret<Kem> as Default>::default(); extract_and_expand::<Kem>( &kex_res_eph.to_bytes(), &suite_id, &kem_context, &mut shared_secret, ) .expect("shared secret is way too big"); Ok(shared_secret) } } #[cfg(test)] mod tests { use crate::kem::{decap, encap, Deserializable, EncappedKey, Kem as KemTrait, Serializable}; use rand::{rngs::StdRng, SeedableRng}; macro_rules! test_encap_correctness { ($test_name:ident, $kem_ty:ty) => { /// Tests that encap and decap produce the same shared secret when composed #[test] fn $test_name() { type Kem = $kem_ty; let mut csprng = StdRng::from_entropy(); let (sk_recip, pk_recip) = Kem::gen_keypair(&mut csprng); // Encapsulate a random shared secret let (auth_shared_secret, encapped_key) = encap::<Kem, _>(&pk_recip, None, &mut csprng).unwrap(); // Decap it let decapped_auth_shared_secret = decap::<Kem>(&sk_recip, None, &encapped_key).unwrap(); // Ensure that the encapsulated secret is what decap() derives assert_eq!(auth_shared_secret, decapped_auth_shared_secret); // // Now do it with the auth, i.e., using the sender's identity keys // // Make a sender identity keypair let (sk_sender_id, pk_sender_id) = Kem::gen_keypair(&mut csprng); // Encapsulate a random shared secret let (auth_shared_secret, encapped_key) = encap::<Kem, _>( &pk_recip, Some(&(sk_sender_id, pk_sender_id.clone())), &mut csprng, ) .unwrap(); // Decap it let decapped_auth_shared_secret = decap::<Kem>(&sk_recip, Some(&pk_sender_id), &encapped_key).unwrap(); // Ensure that the encapsulated secret is what decap() derives assert_eq!(auth_shared_secret, decapped_auth_shared_secret); } }; } /// Tests that an deserialize-serialize round trip on an encapped key ends up at the same value macro_rules! test_encapped_serialize { ($test_name:ident, $kem_ty:ty) => { #[test] fn $test_name() { type Kem = $kem_ty; // Encapsulate a random shared secret let encapped_key = { let mut csprng = StdRng::from_entropy(); let (_, pk_recip) = Kem::gen_keypair(&mut csprng); encap::<Kem, _>(&pk_recip, None, &mut csprng).unwrap().1 }; // Serialize it let encapped_key_bytes = encapped_key.to_bytes(); // Deserialize it let new_encapped_key = EncappedKey::<<Kem as KemTrait>::Kex>::from_bytes(&encapped_key_bytes).unwrap(); assert!( new_encapped_key.0 == encapped_key.0, "encapped key doesn't serialize correctly" ); } }; } #[cfg(feature = "x25519-dalek")] test_encap_correctness!(test_encap_correctness_x25519, crate::kem::X25519HkdfSha256); #[cfg(feature = "p256")] test_encap_correctness!(test_encap_correctness_p256, crate::kem::DhP256HkdfSha256); #[cfg(feature = "x25519-dalek")] test_encapped_serialize!(test_encapped_serialize_x25519, crate::kem::X25519HkdfSha256); #[cfg(feature = "p256")] test_encapped_serialize!(test_encapped_serialize_p256, crate::kem::DhP256HkdfSha256); }