Merge rust-bitcoin/rust-bitcoin#3275: Enforce that `Hash::Bytes` is an array

be13397570 Make hmac & hkdf more robust against buggy `Hash` (Martin Habovstiak)
94c0614bda Enforce that `Hash::Bytes` is an array (Martin Habovstiak)

Pull request description:

  This makes sure `Hash::Bytes` is an array. We've discussed this somewhere but I don't remember where.

  I'm not sure if the second commit is actually valuable but hopefully shouldn't make things worse.

ACKs for top commit:
  apoelstra:
    ACK be13397570 successfully ran local tests; yep, this looks like an improvement. Agreed that the second commit has questionable value but doe not make things worse
  tcharding:
    ACK be13397570

Tree-SHA512: 0fed982084f0f98927c2b4a275cec81cb4bbc0efbf01551a0a4a8b6b39a4504830243ee8d55a5c0418d81b5d4babc7b22332dbacc0609ced8fada84d2961ae71

hashes/src/hkdf.rs

@@ -11,7 +11,7 @@ use alloc::vec;
 use alloc::vec::Vec;
 use core::fmt;
 
-use crate::{GeneralHash, HashEngine, Hmac, HmacEngine};
+use crate::{GeneralHash, HashEngine, Hmac, HmacEngine, IsByteArray};
 
 /// Output keying material max length multiple.
 const MAX_OUTPUT_BLOCKS: usize = 255;
@@ -54,14 +54,14 @@ where
     /// but the info must be independent from the ikm for security.
     pub fn expand(&self, info: &[u8], okm: &mut [u8]) -> Result<(), MaxLengthError> {
         // Length of output keying material in bytes must be less than 255 * hash length.
-        if okm.len() > (MAX_OUTPUT_BLOCKS * T::LEN) {
-            return Err(MaxLengthError { max: MAX_OUTPUT_BLOCKS * T::LEN });
+        if okm.len() > (MAX_OUTPUT_BLOCKS * T::Bytes::LEN) {
+            return Err(MaxLengthError { max: MAX_OUTPUT_BLOCKS * T::Bytes::LEN });
         }
 
         // Counter starts at "1" based on RFC5869 spec and is committed to in the hash.
         let mut counter = 1u8;
         // Ceiling calculation for the total number of blocks (iterations) required for the expand.
-        let total_blocks = (okm.len() + T::LEN - 1) / T::LEN;
+        let total_blocks = (okm.len() + T::Bytes::LEN - 1) / T::Bytes::LEN;
 
         while counter <= total_blocks as u8 {
             let mut hmac_engine: HmacEngine<T> = HmacEngine::new(self.prk.as_ref());
@@ -69,18 +69,18 @@ where
             // First block does not have a previous block,
             // all other blocks include last block in the HMAC input.
             if counter != 1u8 {
-                let previous_start_index = (counter as usize - 2) * T::LEN;
-                let previous_end_index = (counter as usize - 1) * T::LEN;
+                let previous_start_index = (counter as usize - 2) * T::Bytes::LEN;
+                let previous_end_index = (counter as usize - 1) * T::Bytes::LEN;
                 hmac_engine.input(&okm[previous_start_index..previous_end_index]);
             }
             hmac_engine.input(info);
             hmac_engine.input(&[counter]);
 
             let t = Hmac::from_engine(hmac_engine);
-            let start_index = (counter as usize - 1) * T::LEN;
+            let start_index = (counter as usize - 1) * T::Bytes::LEN;
             // Last block might not take full hash length.
             let end_index =
-                if counter == (total_blocks as u8) { okm.len() } else { counter as usize * T::LEN };
+                if counter == (total_blocks as u8) { okm.len() } else { counter as usize * T::Bytes::LEN };
 
             okm[start_index..end_index].copy_from_slice(&t.as_ref()[0..(end_index - start_index)]);
 

hashes/src/hmac.rs

@@ -72,10 +72,11 @@ impl<T: GeneralHash> HmacEngine<T> {
 
         if key.len() > T::Engine::BLOCK_SIZE {
             let hash = <T as GeneralHash>::hash(key);
-            for (b_i, b_h) in ipad.iter_mut().zip(hash.as_ref()) {
+            let hash = hash.as_byte_array().as_ref();
+            for (b_i, b_h) in ipad.iter_mut().zip(hash) {
                 *b_i ^= *b_h;
             }
-            for (b_o, b_h) in opad.iter_mut().zip(hash.as_ref()) {
+            for (b_o, b_h) in opad.iter_mut().zip(hash) {
                 *b_o ^= *b_h;
             }
         } else {
@@ -119,7 +120,8 @@ impl<T: GeneralHash> fmt::LowerHex for Hmac<T> {
 }
 
 impl<T: GeneralHash> convert::AsRef<[u8]> for Hmac<T> {
-    fn as_ref(&self) -> &[u8] { self.0.as_ref() }
+    // Calling as_byte_array is more reliable
+    fn as_ref(&self) -> &[u8] { self.0.as_byte_array().as_ref() }
 }
 
 impl<T: GeneralHash> GeneralHash for Hmac<T> {
@@ -127,7 +129,7 @@ impl<T: GeneralHash> GeneralHash for Hmac<T> {
 
     fn from_engine(mut e: HmacEngine<T>) -> Hmac<T> {
         let ihash = T::from_engine(e.iengine);
-        e.oengine.input(ihash.as_ref());
+        e.oengine.input(ihash.as_byte_array().as_ref());
         let ohash = T::from_engine(e.oengine);
         Hmac(ohash)
     }
@@ -135,7 +137,6 @@ impl<T: GeneralHash> GeneralHash for Hmac<T> {
 
 impl<T: GeneralHash> Hash for Hmac<T> {
     type Bytes = T::Bytes;
-    const LEN: usize = T::LEN;
 
     fn from_slice(sl: &[u8]) -> Result<Hmac<T>, FromSliceError> { T::from_slice(sl).map(Hmac) }
 

hashes/src/internal_macros.rs

@@ -108,7 +108,6 @@ macro_rules! hash_trait_impls {
         impl<$($gen: $gent),*> crate::Hash for Hash<$($gen),*> {
             type Bytes = [u8; $bits / 8];
 
-            const LEN: usize = $bits / 8;
             const DISPLAY_BACKWARD: bool = $reverse;
 
             fn from_slice(sl: &[u8]) -> $crate::_export::_core::result::Result<Hash<$($gen),*>, $crate::FromSliceError> {

hashes/src/lib.rs

@@ -274,10 +274,10 @@ pub trait Hash:
     + convert::AsRef<[u8]>
 {
     /// The byte array that represents the hash internally.
-    type Bytes: hex::FromHex + Copy;
+    type Bytes: hex::FromHex + Copy + IsByteArray /* <LEN={Self::LEN}> is still unsupported by Rust */;
 
     /// Length of the hash, in bytes.
-    const LEN: usize;
+    const LEN: usize = Self::Bytes::LEN;
 
     /// Copies a byte slice into a hash object.
     fn from_slice(sl: &[u8]) -> Result<Self, FromSliceError>;
@@ -297,6 +297,23 @@ pub trait Hash:
     fn from_byte_array(bytes: Self::Bytes) -> Self;
 }
 
+/// Ensures that a type is an array.
+pub trait IsByteArray: AsRef<[u8]> + sealed::IsByteArray {
+    /// The length of the array.
+    const LEN: usize;
+}
+
+impl<const N: usize> IsByteArray for [u8; N] {
+    const LEN: usize = N;
+}
+
+mod sealed {
+    #[doc(hidden)]
+    pub trait IsByteArray { }
+
+    impl<const N: usize> IsByteArray for [u8; N] { }
+}
+
 /// Attempted to create a hash from an invalid length slice.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct FromSliceError {

hashes/src/util.rs

@@ -231,7 +231,6 @@ macro_rules! hash_newtype {
         impl $crate::Hash for $newtype {
             type Bytes = <$hash as $crate::Hash>::Bytes;
 
-            const LEN: usize = <$hash as $crate::Hash>::LEN;
             const DISPLAY_BACKWARD: bool = $crate::hash_newtype_get_direction!($hash, $(#[$($type_attrs)*])*);
 
             #[inline]