[#792] zcash_primitives: implement Seed Signature section of ZIP-32

Implements a type to enclose a Seed Fingerprint byte sequence
called `SeedFingerprint`

implements a public function to create a `SeedFingerprint` from
seed bytes

Closes #792

Credits for PR Suggestions by Daira and Kris

Edit: Added suggestions from Daira Emma and a test for returning `None`

Added test refactor suggestions as well

removed reference to test vectors

zcash_primitives/src/zip32.rs

@@ -6,7 +6,7 @@ use memuse::{self, DynamicUsage};
 use subtle::{Choice, ConditionallySelectable};
 
 use crate::sapling::{Diversifier, NullifierDerivingKey, PaymentAddress, ViewingKey};
-
+pub mod fingerprint;
 pub mod sapling;
 
 #[deprecated(note = "Please use the types exported from the `zip32::sapling` module instead.")]

zcash_primitives/src/zip32/fingerprint.rs

@@ -0,0 +1,80 @@
+//! Seed Fingerprints according to ZIP 32
+//!
+//! Implements section `Seed Fingerprints` of Shielded Hierarchical Deterministic Wallets (ZIP 32)
+//!
+//! [Section Seed Fingerprints]: https://zips.z.cash/zip-0032#seed-fingerprints
+use blake2b_simd::Params as Blake2bParams;
+
+pub const ZIP32_SEED_FP_PERSONALIZATION: &[u8; 16] = b"Zcash_HD_Seed_FP";
+pub struct SeedFingerprint([u8; 32]);
+
+impl SeedFingerprint {
+    /// Return the seed fingerprint of the wallet as defined in
+    /// <https://zips.z.cash/zip-0032#seed-fingerprints> or None
+    /// if the length of `seed_bytes` is less than 32 or
+    /// greater than 252.
+    pub fn from_seed(seed_bytes: &[u8]) -> Option<SeedFingerprint> {
+        let seed_len = seed_bytes.len();
+
+        if (32..=252).contains(&seed_len) {
+            let seed_len: u8 = seed_len.try_into().unwrap();
+            Some(SeedFingerprint(
+                Blake2bParams::new()
+                    .hash_length(32)
+                    .personal(ZIP32_SEED_FP_PERSONALIZATION)
+                    .to_state()
+                    .update(&[seed_len])
+                    .update(seed_bytes)
+                    .finalize()
+                    .as_bytes()
+                    .try_into()
+                    .expect("hash length should be 32 bytes"),
+            ))
+        } else {
+            None
+        }
+    }
+
+    /// Returns the fingerprint as a byte array.
+    pub fn to_bytes(&self) -> [u8; 32] {
+        self.0
+    }
+}
+
+#[test]
+fn test_seed_fingerprint() {
+    struct TestVector {
+        root_seed: Vec<u8>,
+        fingerprint: Vec<u8>,
+    }
+
+    let test_vectors = vec![TestVector {
+        root_seed: vec![
+            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d,
+            0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b,
+            0x1c, 0x1d, 0x1e, 0x1f,
+        ],
+        fingerprint: vec![
+            0xde, 0xff, 0x60, 0x4c, 0x24, 0x67, 0x10, 0xf7, 0x17, 0x6d, 0xea, 0xd0, 0x2a, 0xa7,
+            0x46, 0xf2, 0xfd, 0x8d, 0x53, 0x89, 0xf7, 0x7, 0x25, 0x56, 0xdc, 0xb5, 0x55, 0xfd,
+            0xbe, 0x5e, 0x3a, 0xe3,
+        ],
+    }];
+
+    for tv in test_vectors {
+        let fp = SeedFingerprint::from_seed(&tv.root_seed).expect("root_seed has valid length");
+        assert_eq!(&fp.to_bytes(), &tv.fingerprint[..]);
+    }
+}
+#[test]
+fn test_seed_fingerprint_is_none() {
+    let odd_seed = vec![
+        0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e,
+        0x0f,
+    ];
+
+    assert!(
+        SeedFingerprint::from_seed(&odd_seed).is_none(),
+        "fingerprint from short seed should be `None`"
+    );
+}