Merge #148: Add all_zeros method to Hash trait

51be9bd Add all_zeros method to Hash trait (Tobin Harding)

Pull request description:

  Currently we use `Default::default()` to get an all zeros hash. This is confusing because an all zeroes hash does not have a real meaning since there is no known input to create it. The all zeros hash is, however used by the Bitcoin network to signify certain things (e.g. the previous blockhash in the genesis block).

  Remove the `Default` implementation from the `Hash` trait and add a `all_zeros` method instead. Remove the `Default` derive from any hash type that has it.

  To see the patch to `rust-bitcoin` that this PR imposes, check out [this branch](https://github.com/tcharding/rust-bitcoin/tree/bitcoin_hashes-all-zeros).

  Resolves #139

ACKs for top commit:
  apoelstra:
    ACK 51be9bd

Tree-SHA512: e5f56362fec5ef8f32e87f0949b2a28fd9a5a85bbbf1e0295d535b6e79582e16cb6c165becf6ed00051ab8ac42be40ba23427d553213e320b38366684ef9c1af

Author: apoelstra

src/hash160.rs

@@ -27,7 +27,7 @@ use core::slice::SliceIndex;
 use crate::{Error, hex, ripemd160, sha256};
 
 /// Output of the Bitcoin HASH160 hash function.
-#[derive(Copy, Clone, PartialEq, Eq, Default, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 #[repr(transparent)]
 pub struct Hash(
@@ -97,6 +97,10 @@ impl crate::Hash for Hash {
     fn from_inner(inner: Self::Inner) -> Self {
         Hash(inner)
     }
+
+    fn all_zeros() -> Self {
+        Hash([0x00; 20])
+    }
 }
 
 #[cfg(test)]

src/hmac.rs

@@ -27,7 +27,7 @@ use serde::{Serialize, Serializer, Deserialize, Deserializer};
 use crate::{Error, Hash, HashEngine};
 
 /// A hash computed from a RFC 2104 HMAC. Parameterized by the underlying hash function.
-#[derive(Copy, Clone, PartialEq, Eq, Default, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 #[cfg_attr(feature = "schemars", schemars(transparent))]
 #[repr(transparent)]
@@ -218,6 +218,11 @@ impl<T: Hash> Hash for Hmac<T> {
     fn from_inner(inner: T::Inner) -> Self {
         Hmac(T::from_inner(inner))
     }
+
+    fn all_zeros() -> Self {
+        let zeros = T::all_zeros();
+        Hmac(zeros)
+    }
 }
 
 #[cfg(feature = "serde")]

src/lib.rs

@@ -98,7 +98,7 @@ pub trait HashEngine: Clone + Default {
 }
 
 /// Trait which applies to hashes of all types.
-pub trait Hash: Copy + Clone + PartialEq + Eq + Default + PartialOrd + Ord +
+pub trait Hash: Copy + Clone + PartialEq + Eq + PartialOrd + Ord +
     hash::Hash + fmt::Debug + fmt::Display + fmt::LowerHex +
     ops::Index<ops::RangeFull, Output = [u8]> +
     ops::Index<ops::RangeFrom<usize>, Output = [u8]> +
@@ -149,6 +149,13 @@ pub trait Hash: Copy + Clone + PartialEq + Eq + Default + PartialOrd + Ord +
 
     /// Constructs a hash from the underlying byte array.
     fn from_inner(inner: Self::Inner) -> Self;
+
+    /// Returns an all zero hash.
+    ///
+    /// An all zeros hash is a made up construct because there is not a known input that can create
+    /// it, however it is used in various places in Bitcoin e.g., the Bitcoin genesis block's
+    /// previous blockhash and the coinbase transaction's outpoint txid.
+    fn all_zeros() -> Self;
 }
 
 #[cfg(test)]

src/ripemd160.rs

@@ -75,7 +75,7 @@ impl crate::HashEngine for HashEngine {
 }
 
 /// Output of the RIPEMD160 hash function.
-#[derive(Copy, Clone, PartialEq, Eq, Default, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 #[repr(transparent)]
 pub struct Hash(
@@ -159,6 +159,10 @@ impl crate::Hash for Hash {
     fn from_inner(inner: Self::Inner) -> Self {
         Hash(inner)
     }
+
+    fn all_zeros() -> Self {
+        Hash([0x00; 20])
+    }
 }
 
 macro_rules! round(

src/sha1.rs

@@ -70,7 +70,7 @@ impl crate::HashEngine for HashEngine {
 }
 
 /// Output of the SHA1 hash function.
-#[derive(Copy, Clone, PartialEq, Eq, Default, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 #[repr(transparent)]
 pub struct Hash(
@@ -146,6 +146,10 @@ impl crate::Hash for Hash {
     fn from_inner(inner: Self::Inner) -> Self {
         Hash(inner)
     }
+
+    fn all_zeros() -> Self {
+        Hash([0x00; 20])
+    }
 }
 
 impl HashEngine {

src/sha256.rs

@@ -70,7 +70,7 @@ impl crate::HashEngine for HashEngine {
 }
 
 /// Output of the SHA256 hash function.
-#[derive(Copy, Clone, PartialEq, Eq, Default, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 #[repr(transparent)]
 pub struct Hash(
@@ -158,6 +158,10 @@ impl crate::Hash for Hash {
     fn from_inner(inner: Self::Inner) -> Self {
         Hash(inner)
     }
+
+    fn all_zeros() -> Self {
+        Hash([0x00; 32])
+    }
 }
 
 /// Output of the SHA256 hash function.

src/sha256d.rs

@@ -22,7 +22,7 @@ use core::slice::SliceIndex;
 use crate::{Error, hex, sha256};
 
 /// Output of the SHA256d hash function.
-#[derive(Copy, Clone, PartialEq, Eq, Default, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 #[repr(transparent)]
 pub struct Hash(
@@ -94,6 +94,10 @@ impl crate::Hash for Hash {
     fn from_inner(inner: Self::Inner) -> Self {
         Hash(inner)
     }
+
+    fn all_zeros() -> Self {
+        Hash([0x00; 32])
+    }
 }
 
 #[cfg(test)]

src/sha256t.rs

@@ -132,6 +132,10 @@ impl<T: Tag> crate::Hash for Hash<T> {
     fn from_inner(inner: Self::Inner) -> Self {
         Hash(inner, PhantomData)
     }
+
+    fn all_zeros() -> Self {
+        Hash([0x00; 32], PhantomData)
+    }
 }
 
 /// Macro used to define a newtype tagged hash.

src/sha512.rs

@@ -204,6 +204,10 @@ impl crate::Hash for Hash {
     fn from_inner(inner: Self::Inner) -> Self {
         Hash(inner)
     }
+
+    fn all_zeros() -> Self {
+        Hash([0x00; 64])
+    }
 }
 
 macro_rules! Ch( ($x:expr, $y:expr, $z:expr) => ($z ^ ($x & ($y ^ $z))) );

src/siphash24.rs

@@ -195,7 +195,7 @@ impl crate::HashEngine for HashEngine {
 }
 
 /// Output of the SipHash24 hash function.
-#[derive(Copy, Clone, PartialEq, Eq, Default, PartialOrd, Ord, Hash)]
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 #[repr(transparent)]
 pub struct Hash(
@@ -306,6 +306,10 @@ impl crate::Hash for Hash {
     fn from_inner(inner: Self::Inner) -> Self {
         Hash(inner)
     }
+
+    fn all_zeros() -> Self {
+        Hash([0x00; 8])
+    }
 }
 
 /// Load an u64 using up to 7 bytes of a byte slice.