Extend transactions with memo sections

core/src/proto/types.rs

@@ -4,6 +4,7 @@ use std::collections::{BTreeMap, HashMap, HashSet};
 use std::convert::TryFrom;
 use std::hash::{Hash, Hasher};
 use std::marker::PhantomData;
+use std::ops::Deref;
 
 use borsh::schema::{add_definition, Declaration, Definition};
 use borsh::{BorshDeserialize, BorshSchema, BorshSerialize};
@@ -805,6 +806,47 @@ impl borsh::BorshSchema for MaspBuilder {
     }
 }
 
+/// Memo field that may be populated with arbitrary data.
+#[derive(
+    Clone,
+    Debug,
+    BorshSerialize,
+    BorshDeserialize,
+    BorshSchema,
+    Serialize,
+    Deserialize,
+)]
+pub struct Memo {
+    inner: Vec<u8>,
+}
+
+impl Memo {
+    /// Up to 1 KiB of data.
+    pub const MAX_CAPACITY: usize = 1024;
+
+    /// Build a new memo field.
+    pub fn new(data: &[u8]) -> Option<Self> {
+        (data.len() <= Self::MAX_CAPACITY).then(|| Memo {
+            inner: data.to_vec(),
+        })
+    }
+
+    /// Hash this memo section.
+    pub fn hash<'a>(&self, hasher: &'a mut Sha256) -> &'a mut Sha256 {
+        hasher.update(&self.inner);
+        hasher
+    }
+}
+
+impl Deref for Memo {
+    type Target = [u8];
+
+    #[inline]
+    fn deref(&self) -> &[u8] {
+        &self.inner
+    }
+}
+
 /// A section of a transaction. Carries an independent piece of information
 /// necessary for the processing of a transaction.
 #[derive(
@@ -836,6 +878,8 @@ pub enum Section {
     /// A section providing the auxiliary inputs used to construct a MASP
     /// transaction. Only send to wallet, never send to protocol.
     MaspBuilder(MaspBuilder),
+    /// Memo containing arbitrary data.
+    Memo(Memo),
     /// Wrap a header with a section for the purposes of computing hashes
     Header(Header),
 }
@@ -859,6 +903,7 @@ impl Section {
                 hasher.update(tx.txid().as_ref());
                 hasher
             }
+            Self::Memo(memo) => memo.hash(hasher),
             Self::Header(header) => header.hash(hasher),
         }
     }
@@ -1486,6 +1531,15 @@ impl Tx {
         (self, sechash)
     }
 
+    /// Add a memo section to the transaction
+    pub fn add_memo(
+        &mut self,
+        memo: Memo,
+    ) -> (&mut Self, crate::types::hash::Hash) {
+        let sechash = self.add_section(Section::Memo(memo)).get_hash();
+        (self, sechash)
+    }
+
     /// Add a masp tx section to the tx builder
     pub fn add_masp_tx_section(
         &mut self,