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Support tagged ZIP 32 child derivation #7

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merged 4 commits into from
Feb 20, 2025
Merged

Support tagged ZIP 32 child derivation #7

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0d3f591
Add `PrfExpand<([u8; 32], [u8; 4])::{REGISTERED_ZIP32_CHILD, with_tag}`
daira Feb 12, 2025
a0d5af6
Adopt str4d's suggestion for a more typesafe `PrfExpand` API.
daira Feb 17, 2025
295ed47
Add `VariableLengthSlice` to work around a lifetime problem in users
daira Feb 18, 2025
997bf72
Cleaner API.
daira Feb 19, 2025
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14 changes: 14 additions & 0 deletions CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,20 @@ and this library adheres to Rust's notion of
[Semantic Versioning](https://semver.org/spec/v2.0.0.html).

## [Unreleased]
### Added
- `zcash_spec::PrfExpand::REGISTERED_ZIP32_CHILD` (for tagged ZIP 32 child
derivation).
- `zcash_spec::PrfExpand::ADHOC_ZIP32_CHILD` (renamed and retyped per ZIP 32).
- `zcash_spec::VariableLengthSlice`

### Changed
- `zcash_spec::PrfExpand::ORCHARD_ZIP32_CHILD` now has type
`PrfExpand<([u8; 32], [u8; 4], Option<(u8, VariableLengthSlice)>)>` due to
ZIP 32 changes.

### Removed
- `zcash_spec::PrfExpand::ARBITRARY_ZIP32_CHILD` (use `ADHOC_ZIP32_CHILD`
instead).

## [0.1.2] - 2024-10-01
### Added
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2 changes: 1 addition & 1 deletion src/lib.rs
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Expand Up @@ -13,4 +13,4 @@
#![deny(rustdoc::broken_intra_doc_links)]

mod prf_expand;
pub use prf_expand::PrfExpand;
pub use prf_expand::{PrfExpand, VariableLengthSlice};
20 changes: 18 additions & 2 deletions src/prf_expand.rs
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Expand Up @@ -48,6 +48,9 @@ impl<T> PrfExpand<T> {
}
}

/// Marker type used for `PrfExpand` instantiations that permit a variable-length input.
pub struct VariableLengthSlice;

macro_rules! with_inputs {
($($arr:ident, $arrlen:ident),*) => {
#[allow(unused_parens)]
Expand Down Expand Up @@ -94,8 +97,6 @@ with_inputs!(a, A);

impl PrfExpand<([u8; 32], [u8; 4])> {
pub const SPROUT_ZIP32_CHILD: Self = Self::new(0x80);
pub const ORCHARD_ZIP32_CHILD: Self = Self::new(0x81);
pub const ARBITRARY_ZIP32_CHILD: Self = Self::new(0xAB);
}
impl PrfExpand<([u8; 32], [u8; 32])> {
pub const ORCHARD_DK_OVK: Self = Self::new(0x82);
Expand All @@ -108,3 +109,18 @@ impl PrfExpand<([u8; 96], [u8; 32], [u8; 4])> {
pub const SAPLING_ZIP32_CHILD_NON_HARDENED: Self = Self::new(0x12);
}
with_inputs!(a, A, b, B, c, C);

impl PrfExpand<([u8; 32], [u8; 4], Option<(u8, VariableLengthSlice)>)> {
pub const ORCHARD_ZIP32_CHILD: Self = Self::new(0x81);
pub const ADHOC_ZIP32_CHILD: Self = Self::new(0xAB);
pub const REGISTERED_ZIP32_CHILD: Self = Self::new(0xAC);

/// Expands the given secret key in this domain.
pub fn with(self, sk: &[u8], a: &[u8; 32], b: &[u8; 4], c: Option<(u8, &[u8])>) -> [u8; 64] {
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@nuttycom nuttycom Feb 19, 2025 •
edited
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Is it intended that Some((0, &[])) be a valid input to this function?

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These APIs are not intended to exclude bad encodings in general. For example, many of the values are field elements for which non-canonical encodings are invalid. They're supposed to be low-level APIs taking already-encoded values that are assumed to be correct.

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Same question for Some((0, &[0xab, 0xcd]))

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Agreed. That being said, if we wanted to instead encode here that for these three specific PrfExpand domain separators that (0, &[]) is the canonical None then we could do that instead (in which case, usage in e.g orchard would pass that instead of None).

If we do this, then we should also remove the tuple and switch back to [u8; 1] in the API (so that future auto-derived arguments can be done correctly).

if let Some((d, e)) = c {
self.apply(sk, &[a, b, &[d], e])
} else {
self.apply(sk, &[a, b])
}
}
}
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