zebra_chain/sapling/
keys.rs

1//! Sapling key types.
2//!
3//! Unused key types are not implemented, see PR #5476.
4//!
5//! "The spend authorizing key ask, proof authorizing key (ak, nsk),
6//! full viewing key (ak, nk, ovk), incoming viewing key ivk, and each
7//! diversified payment address addr_d = (d, pk_d ) are derived from sk,
8//! as described in [Sapling Key Components][ps]." - [§3.1][3.1]
9//!
10//! [ps]: https://zips.z.cash/protocol/protocol.pdf#saplingkeycomponents
11//! [3.1]: https://zips.z.cash/protocol/protocol.pdf#addressesandkeys
12
13use std::{fmt, io};
14
15use rand_core::{CryptoRng, RngCore};
16
17use crate::{
18    error::{AddressError, RandError},
19    primitives::redjubjub::SpendAuth,
20    serialization::{
21        serde_helpers, ReadZcashExt, SerializationError, ZcashDeserialize, ZcashSerialize,
22    },
23};
24
25#[cfg(test)]
26mod test_vectors;
27
28/// The [Randomness Beacon][1] ("URS").
29///
30/// First 64 bytes of the BLAKE2s input during JubJub group hash.  URS
31/// is a 64-byte US-ASCII string, i.e. the first byte is 0x30, not
32/// 0x09.
33///
34/// From [zcash_primitives][0].
35///
36/// [0]: https://docs.rs/zcash_primitives/0.2.0/zcash_primitives/constants/constant.GH_FIRST_BLOCK.html
37/// [1]: https://zips.z.cash/protocol/protocol.pdf#beacon
38pub(super) const RANDOMNESS_BEACON_URS: &[u8; 64] =
39    b"096b36a5804bfacef1691e173c366a47ff5ba84a44f26ddd7e8d9f79d5b42df0";
40
41/// GroupHash into Jubjub, aka _GroupHash_URS_
42///
43/// Produces a random point in the Jubjub curve. The point is
44/// guaranteed to be prime order and not the identity. From
45/// [zcash_primitives][0].
46///
47/// d is an 8-byte domain separator ("personalization"), m is the hash
48/// input.
49///
50/// [0]: https://github.com/zcash/librustzcash/blob/master/zcash_primitives/src/group_hash.rs#L15
51/// <https://zips.z.cash/protocol/protocol.pdf#concretegrouphashjubjub>
52fn jubjub_group_hash(d: [u8; 8], m: &[u8]) -> Option<jubjub::ExtendedPoint> {
53    let hash = blake2s_simd::Params::new()
54        .hash_length(32)
55        .personal(&d)
56        .to_state()
57        .update(RANDOMNESS_BEACON_URS)
58        .update(m)
59        .finalize();
60
61    let ct_option = jubjub::AffinePoint::from_bytes(*hash.as_array());
62
63    if ct_option.is_some().unwrap_u8() == 1 {
64        let extended_point = ct_option.unwrap().mul_by_cofactor();
65
66        if extended_point != jubjub::ExtendedPoint::identity() {
67            Some(extended_point)
68        } else {
69            None
70        }
71    } else {
72        None
73    }
74}
75
76/// FindGroupHash for JubJub, from [zcash_primitives][0]
77///
78/// d is an 8-byte domain separator ("personalization"), m is the hash
79/// input.
80///
81/// [0]: https://github.com/zcash/librustzcash/blob/master/zcash_primitives/src/jubjub/mod.rs#L409
82/// <https://zips.z.cash/protocol/protocol.pdf#concretegrouphashjubjub>
83// TODO: move common functions like these out of the keys module into
84// a more appropriate location
85pub(super) fn find_group_hash(d: [u8; 8], m: &[u8]) -> jubjub::ExtendedPoint {
86    let mut tag = m.to_vec();
87    let i = tag.len();
88    tag.push(0u8);
89
90    loop {
91        let gh = jubjub_group_hash(d, &tag[..]);
92
93        // We don't want to overflow and start reusing generators
94        assert!(tag[i] != u8::MAX);
95        tag[i] += 1;
96
97        if let Some(gh) = gh {
98            break gh;
99        }
100    }
101}
102
103/// Used to derive a diversified base point from a diversifier value.
104///
105/// <https://zips.z.cash/protocol/protocol.pdf#concretediversifyhash>
106fn diversify_hash(d: [u8; 11]) -> Option<jubjub::ExtendedPoint> {
107    jubjub_group_hash(*b"Zcash_gd", &d)
108}
109
110/// A _Diversifier_, as described in [protocol specification §4.2.2][ps].
111///
112/// Combined with an _IncomingViewingKey_, produces a _diversified
113/// payment address_.
114///
115/// [ps]: https://zips.z.cash/protocol/protocol.pdf#saplingkeycomponents
116#[derive(Copy, Clone, Eq, PartialEq)]
117#[cfg_attr(
118    any(test, feature = "proptest-impl"),
119    derive(proptest_derive::Arbitrary)
120)]
121pub struct Diversifier(pub(crate) [u8; 11]);
122
123impl fmt::Debug for Diversifier {
124    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
125        f.debug_tuple("Diversifier")
126            .field(&hex::encode(self.0))
127            .finish()
128    }
129}
130
131impl From<[u8; 11]> for Diversifier {
132    fn from(bytes: [u8; 11]) -> Self {
133        Self(bytes)
134    }
135}
136
137impl From<Diversifier> for [u8; 11] {
138    fn from(d: Diversifier) -> [u8; 11] {
139        d.0
140    }
141}
142
143impl TryFrom<Diversifier> for jubjub::AffinePoint {
144    type Error = &'static str;
145
146    /// Get a diversified base point from a diversifier value in affine
147    /// representation.
148    fn try_from(d: Diversifier) -> Result<Self, Self::Error> {
149        if let Ok(extended_point) = jubjub::ExtendedPoint::try_from(d) {
150            Ok(extended_point.into())
151        } else {
152            Err("Invalid Diversifier -> jubjub::AffinePoint")
153        }
154    }
155}
156
157impl TryFrom<Diversifier> for jubjub::ExtendedPoint {
158    type Error = &'static str;
159
160    fn try_from(d: Diversifier) -> Result<Self, Self::Error> {
161        let possible_point = diversify_hash(d.0);
162
163        if let Some(point) = possible_point {
164            Ok(point)
165        } else {
166            Err("Invalid Diversifier -> jubjub::ExtendedPoint")
167        }
168    }
169}
170
171impl PartialEq<[u8; 11]> for Diversifier {
172    fn eq(&self, other: &[u8; 11]) -> bool {
173        self.0 == *other
174    }
175}
176
177impl Diversifier {
178    /// Generate a new _Diversifier_ that has already been confirmed
179    /// as a preimage to a valid diversified base point when used to
180    /// derive a diversified payment address.
181    ///
182    /// <https://zips.z.cash/protocol/protocol.pdf#saplingkeycomponents>
183    /// <https://zips.z.cash/protocol/protocol.pdf#concretediversifyhash>
184    pub fn new<T>(csprng: &mut T) -> Result<Self, AddressError>
185    where
186        T: RngCore + CryptoRng,
187    {
188        /// Number of times a `diversify_hash` will try to obtain a diversified base point.
189        const DIVERSIFY_HASH_TRIES: u8 = 2;
190
191        for _ in 0..DIVERSIFY_HASH_TRIES {
192            let mut bytes = [0u8; 11];
193            csprng
194                .try_fill_bytes(&mut bytes)
195                .map_err(|_| AddressError::from(RandError::FillBytes))?;
196
197            if diversify_hash(bytes).is_some() {
198                return Ok(Self(bytes));
199            }
200        }
201        Err(AddressError::DiversifierGenerationFailure)
202    }
203}
204
205/// A (diversified) _TransmissionKey_
206///
207/// In Sapling, secrets need to be transmitted to a recipient of funds
208/// in order for them to be later spent. To transmit these secrets
209/// securely to a recipient without requiring an out-of-band
210/// communication channel, the diversified transmission key is used to
211/// encrypt them.
212///
213/// Derived by multiplying a JubJub point [derived][ps] from a
214/// _Diversifier_ by the _IncomingViewingKey_ scalar.
215///
216/// The diversified TransmissionKey is denoted `pk_d` in the specification.
217/// Note that it can be the identity point, since its type is
218/// [`KA^{Sapling}.PublicPrimeSubgroup`][ka] which in turn is [`J^{(r)}`][jubjub].
219///
220/// [ps]: https://zips.z.cash/protocol/protocol.pdf#saplingkeycomponents
221/// [ka]: https://zips.z.cash/protocol/protocol.pdf#concretesaplingkeyagreement
222/// [jubjub]: https://zips.z.cash/protocol/protocol.pdf#jubjub
223#[derive(Copy, Clone, PartialEq)]
224pub struct TransmissionKey(pub(crate) jubjub::AffinePoint);
225
226impl fmt::Debug for TransmissionKey {
227    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
228        f.debug_struct("TransmissionKey")
229            .field("u", &hex::encode(self.0.get_u().to_bytes()))
230            .field("v", &hex::encode(self.0.get_v().to_bytes()))
231            .finish()
232    }
233}
234
235impl Eq for TransmissionKey {}
236
237impl TryFrom<[u8; 32]> for TransmissionKey {
238    type Error = &'static str;
239
240    /// Attempts to interpret a byte representation of an affine Jubjub point, failing if the
241    /// element is not on the curve, non-canonical, or not in the prime-order subgroup.
242    ///
243    /// <https://github.com/zkcrypto/jubjub/blob/master/src/lib.rs#L411>
244    /// <https://zips.z.cash/zip-0216>
245    fn try_from(bytes: [u8; 32]) -> Result<Self, Self::Error> {
246        let affine_point = jubjub::AffinePoint::from_bytes(bytes).unwrap();
247        // Check if it's identity or has prime order (i.e. is in the prime-order subgroup).
248        if affine_point.is_torsion_free().into() {
249            Ok(Self(affine_point))
250        } else {
251            Err("Invalid jubjub::AffinePoint value for Sapling TransmissionKey")
252        }
253    }
254}
255
256impl From<TransmissionKey> for [u8; 32] {
257    fn from(pk_d: TransmissionKey) -> [u8; 32] {
258        pk_d.0.to_bytes()
259    }
260}
261
262impl PartialEq<[u8; 32]> for TransmissionKey {
263    fn eq(&self, other: &[u8; 32]) -> bool {
264        &self.0.to_bytes() == other
265    }
266}
267
268/// An [ephemeral public key][1] for Sapling key agreement.
269///
270/// Public keys containing points of small order are not allowed.
271///
272/// It is denoted by `epk` in the specification. (This type does _not_
273/// represent [KA^{Sapling}.Public][2], which allows any points, including
274/// of small order).
275///
276/// [1]: https://zips.z.cash/protocol/protocol.pdf#outputdesc
277/// [2]: https://zips.z.cash/protocol/protocol.pdf#concretesaplingkeyagreement
278#[derive(Copy, Clone, Deserialize, PartialEq, Serialize)]
279pub struct EphemeralPublicKey(
280    #[serde(with = "serde_helpers::AffinePoint")] pub(crate) jubjub::AffinePoint,
281);
282
283impl fmt::Debug for EphemeralPublicKey {
284    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
285        f.debug_struct("EphemeralPublicKey")
286            .field("u", &hex::encode(self.0.get_u().to_bytes()))
287            .field("v", &hex::encode(self.0.get_v().to_bytes()))
288            .finish()
289    }
290}
291
292impl Eq for EphemeralPublicKey {}
293
294impl From<EphemeralPublicKey> for [u8; 32] {
295    fn from(nk: EphemeralPublicKey) -> [u8; 32] {
296        nk.0.to_bytes()
297    }
298}
299
300impl From<&EphemeralPublicKey> for [u8; 32] {
301    fn from(nk: &EphemeralPublicKey) -> [u8; 32] {
302        nk.0.to_bytes()
303    }
304}
305
306impl PartialEq<[u8; 32]> for EphemeralPublicKey {
307    fn eq(&self, other: &[u8; 32]) -> bool {
308        &self.0.to_bytes() == other
309    }
310}
311
312impl TryFrom<[u8; 32]> for EphemeralPublicKey {
313    type Error = &'static str;
314
315    /// Read an EphemeralPublicKey from a byte array.
316    ///
317    /// Returns an error if the key is non-canonical, or [it is of small order][1].
318    ///
319    /// # Consensus
320    ///
321    /// > Check that a Output description's cv and epk are not of small order,
322    /// > i.e. \[h_J\]cv MUST NOT be 𝒪_J and \[h_J\]epk MUST NOT be 𝒪_J.
323    ///
324    /// [1]: https://zips.z.cash/protocol/protocol.pdf#outputdesc
325    fn try_from(bytes: [u8; 32]) -> Result<Self, Self::Error> {
326        let possible_point = jubjub::AffinePoint::from_bytes(bytes);
327
328        if possible_point.is_none().into() {
329            return Err("Invalid jubjub::AffinePoint value for Sapling EphemeralPublicKey");
330        }
331        if possible_point.unwrap().is_small_order().into() {
332            Err("jubjub::AffinePoint value for Sapling EphemeralPublicKey point is of small order")
333        } else {
334            Ok(Self(possible_point.unwrap()))
335        }
336    }
337}
338
339impl ZcashSerialize for EphemeralPublicKey {
340    fn zcash_serialize<W: io::Write>(&self, mut writer: W) -> Result<(), io::Error> {
341        writer.write_all(&<[u8; 32]>::from(self)[..])?;
342        Ok(())
343    }
344}
345
346impl ZcashDeserialize for EphemeralPublicKey {
347    fn zcash_deserialize<R: io::Read>(mut reader: R) -> Result<Self, SerializationError> {
348        Self::try_from(reader.read_32_bytes()?).map_err(SerializationError::Parse)
349    }
350}
351
352/// A randomized [validating key][1] that should be used to validate `spendAuthSig`.
353///
354/// It is denoted by `rk` in the specification. (This type does _not_
355/// represent [SpendAuthSig^{Sapling}.Public][2], which allows any points, including
356/// of small order).
357///
358/// [1]: https://zips.z.cash/protocol/protocol.pdf#spenddesc
359/// [2]: https://zips.z.cash/protocol/protocol.pdf#concretereddsa
360#[derive(Debug, PartialEq, Serialize, Deserialize, Clone)]
361pub struct ValidatingKey(redjubjub::VerificationKey<SpendAuth>);
362
363impl TryFrom<redjubjub::VerificationKey<SpendAuth>> for ValidatingKey {
364    type Error = &'static str;
365
366    /// Convert an array into a ValidatingKey.
367    ///
368    /// Returns an error if the key is malformed or [is of small order][1].
369    ///
370    /// # Consensus
371    ///
372    /// > Check that a Spend description's cv and rk are not of small order,
373    /// > i.e. \[h_J\]cv MUST NOT be 𝒪_J and \[h_J\]rk MUST NOT be 𝒪_J.
374    ///
375    /// [1]: https://zips.z.cash/protocol/protocol.pdf#spenddesc
376    fn try_from(key: redjubjub::VerificationKey<SpendAuth>) -> Result<Self, Self::Error> {
377        if bool::from(
378            jubjub::AffinePoint::from_bytes(key.into())
379                .unwrap()
380                .is_small_order(),
381        ) {
382            Err("jubjub::AffinePoint value for Sapling ValidatingKey is of small order")
383        } else {
384            Ok(Self(key))
385        }
386    }
387}
388
389impl TryFrom<[u8; 32]> for ValidatingKey {
390    type Error = &'static str;
391
392    fn try_from(value: [u8; 32]) -> Result<Self, Self::Error> {
393        let vk = redjubjub::VerificationKey::<SpendAuth>::try_from(value)
394            .map_err(|_| "Invalid redjubjub::ValidatingKey for Sapling ValidatingKey")?;
395        vk.try_into()
396    }
397}
398
399impl From<ValidatingKey> for [u8; 32] {
400    fn from(key: ValidatingKey) -> Self {
401        key.0.into()
402    }
403}
404
405impl From<ValidatingKey> for redjubjub::VerificationKeyBytes<SpendAuth> {
406    fn from(key: ValidatingKey) -> Self {
407        key.0.into()
408    }
409}
zebra_chain/sapling/keys.rs

zebra_chain/sapling/
keys.rs
