sapling-crypto 0.5.0

Cryptographic library for Zcash Sapling
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165

//! Helper functions defined in the Zcash Protocol Specification.

use blake2s_simd::Params as Blake2sParams;
use group::{cofactor::CofactorGroup, ff::PrimeField, Curve, GroupEncoding, WnafBase, WnafScalar};

use super::{
    constants::{
        CRH_IVK_PERSONALIZATION, KEY_DIVERSIFICATION_PERSONALIZATION,
        NOTE_COMMITMENT_RANDOMNESS_GENERATOR, NULLIFIER_POSITION_GENERATOR, PRF_NF_PERSONALIZATION,
    },
    group_hash::group_hash,
    pedersen_hash::{pedersen_hash, Personalization},
};

const PREPARED_WINDOW_SIZE: usize = 4;
pub(crate) type PreparedBase = WnafBase<jubjub::ExtendedPoint, PREPARED_WINDOW_SIZE>;
pub(crate) type PreparedBaseSubgroup = WnafBase<jubjub::SubgroupPoint, PREPARED_WINDOW_SIZE>;
pub(crate) type PreparedScalar = WnafScalar<jubjub::Scalar, PREPARED_WINDOW_SIZE>;

/// $CRH^\mathsf{ivk}(ak, nk)$
///
/// Defined in [Zcash Protocol Spec § 5.4.1.5: CRH^ivk Hash Function][concretecrhivk].
///
/// [concretecrhivk]: https://zips.z.cash/protocol/protocol.pdf#concretecrhivk
pub(crate) fn crh_ivk(ak: [u8; 32], nk: [u8; 32]) -> jubjub::Scalar {
    let mut h: [u8; 32] = Blake2sParams::new()
        .hash_length(32)
        .personal(CRH_IVK_PERSONALIZATION)
        .to_state()
        .update(&ak)
        .update(&nk)
        .finalize()
        .as_bytes()
        .try_into()
        .expect("output length is correct");

    // Drop the most significant five bits, so it can be interpreted as a scalar.
    h[31] &= 0b0000_0111;

    jubjub::Fr::from_repr(h).unwrap()
}

/// Defined in [Zcash Protocol Spec § 5.4.1.6: DiversifyHash^Sapling and DiversifyHash^Orchard Hash Functions][concretediversifyhash].
///
/// [concretediversifyhash]: https://zips.z.cash/protocol/protocol.pdf#concretediversifyhash
pub(crate) fn diversify_hash(d: &[u8; 11]) -> Option<jubjub::SubgroupPoint> {
    group_hash(d, KEY_DIVERSIFICATION_PERSONALIZATION)
}

/// $MixingPedersenHash$.
///
/// Defined in [Zcash Protocol Spec § 5.4.1.8: Mixing Pedersen Hash Function][concretemixinghash].
///
/// [concretemixinghash]: https://zips.z.cash/protocol/protocol.pdf#concretemixinghash
pub(crate) fn mixing_pedersen_hash(
    cm: jubjub::SubgroupPoint,
    position: u64,
) -> jubjub::SubgroupPoint {
    cm + (NULLIFIER_POSITION_GENERATOR * jubjub::Fr::from(position))
}

/// $PRF^\mathsf{nfSapling}_{nk}(\rho)$
///
/// Defined in [Zcash Protocol Spec § 5.4.2: Pseudo Random Functions][concreteprfs].
///
/// [concreteprfs]: https://zips.z.cash/protocol/protocol.pdf#concreteprfs
pub(crate) fn prf_nf(nk: &jubjub::SubgroupPoint, rho: &jubjub::SubgroupPoint) -> [u8; 32] {
    Blake2sParams::new()
        .hash_length(32)
        .personal(PRF_NF_PERSONALIZATION)
        .to_state()
        .update(&nk.to_bytes())
        .update(&rho.to_bytes())
        .finalize()
        .as_bytes()
        .try_into()
        .expect("output length is correct")
}

/// Defined in [Zcash Protocol Spec § 5.4.5.3: Sapling Key Agreement][concretesaplingkeyagreement].
///
/// [concretesaplingkeyagreement]: https://zips.z.cash/protocol/protocol.pdf#concretesaplingkeyagreement
pub(crate) fn ka_sapling_derive_public(
    sk: &jubjub::Scalar,
    b: &jubjub::ExtendedPoint,
) -> jubjub::ExtendedPoint {
    ka_sapling_derive_public_prepared(&PreparedScalar::new(sk), &PreparedBase::new(*b))
}

/// Defined in [Zcash Protocol Spec § 5.4.5.3: Sapling Key Agreement][concretesaplingkeyagreement].
///
/// [concretesaplingkeyagreement]: https://zips.z.cash/protocol/protocol.pdf#concretesaplingkeyagreement
pub(crate) fn ka_sapling_derive_public_prepared(
    sk: &PreparedScalar,
    b: &PreparedBase,
) -> jubjub::ExtendedPoint {
    // [sk] b
    b * sk
}

/// This is defined implicitly by [Zcash Protocol Spec § 4.2.2: Sapling Key Components][saplingkeycomponents]
/// which uses $KA^\mathsf{Sapling}.\mathsf{DerivePublic}$ to produce a diversified
/// transmission key with type $KA^\mathsf{Sapling}.\mathsf{PublicPrimeSubgroup}$.
///
/// [saplingkeycomponents]: https://zips.z.cash/protocol/protocol.pdf#saplingkeycomponents
pub(crate) fn ka_sapling_derive_public_subgroup_prepared(
    sk: &PreparedScalar,
    b: &PreparedBaseSubgroup,
) -> jubjub::SubgroupPoint {
    // [sk] b
    b * sk
}

/// Defined in [Zcash Protocol Spec § 5.4.5.3: Sapling Key Agreement][concretesaplingkeyagreement].
///
/// [concretesaplingkeyagreement]: https://zips.z.cash/protocol/protocol.pdf#concretesaplingkeyagreement
pub(crate) fn ka_sapling_agree(
    sk: &jubjub::Scalar,
    b: &jubjub::ExtendedPoint,
) -> jubjub::SubgroupPoint {
    ka_sapling_agree_prepared(&PreparedScalar::new(sk), &PreparedBase::new(*b))
}

/// Defined in [Zcash Protocol Spec § 5.4.5.3: Sapling Key Agreement][concretesaplingkeyagreement].
///
/// [concretesaplingkeyagreement]: https://zips.z.cash/protocol/protocol.pdf#concretesaplingkeyagreement
pub(crate) fn ka_sapling_agree_prepared(
    sk: &PreparedScalar,
    b: &PreparedBase,
) -> jubjub::SubgroupPoint {
    // [8 sk] b
    // <ExtendedPoint as CofactorGroup>::clear_cofactor is implemented using
    // ExtendedPoint::mul_by_cofactor in the jubjub crate.

    (b * sk).clear_cofactor()
}

/// $WindowedPedersenCommit_r(s)$
///
/// Defined in [Zcash Protocol Spec § 5.4.8.2: Windowed Pedersen commitments][concretewindowedcommit].
///
/// [concretewindowedcommit]: https://zips.z.cash/protocol/protocol.pdf#concretewindowedcommit
pub(crate) fn windowed_pedersen_commit<I>(
    personalization: Personalization,
    s: I,
    r: jubjub::Scalar,
) -> jubjub::SubgroupPoint
where
    I: IntoIterator<Item = bool>,
{
    pedersen_hash(personalization, s) + (NOTE_COMMITMENT_RANDOMNESS_GENERATOR * r)
}

/// Coordinate extractor for Jubjub.
///
/// Defined in [Zcash Protocol Spec § 5.4.9.4: Coordinate Extractor for Jubjub][concreteextractorjubjub].
///
/// [concreteextractorjubjub]: https://zips.z.cash/protocol/protocol.pdf#concreteextractorjubjub
pub(crate) fn extract_p(point: &jubjub::SubgroupPoint) -> bls12_381::Scalar {
    // The commitment is in the prime order subgroup, so mapping the
    // commitment to the u-coordinate is an injective encoding.
    Into::<&jubjub::ExtendedPoint>::into(point)
        .to_affine()
        .get_u()
}