miden_crypto/merkle/smt/mod.rs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461
use alloc::{collections::BTreeMap, vec::Vec};
use super::{EmptySubtreeRoots, InnerNodeInfo, MerkleError, MerklePath, NodeIndex};
use crate::{
hash::rpo::{Rpo256, RpoDigest},
Felt, Word, EMPTY_WORD,
};
mod full;
pub use full::{Smt, SmtLeaf, SmtLeafError, SmtProof, SmtProofError, SMT_DEPTH};
mod simple;
pub use simple::SimpleSmt;
// CONSTANTS
// ================================================================================================
/// Minimum supported depth.
pub const SMT_MIN_DEPTH: u8 = 1;
/// Maximum supported depth.
pub const SMT_MAX_DEPTH: u8 = 64;
// SPARSE MERKLE TREE
// ================================================================================================
/// An abstract description of a sparse Merkle tree.
///
/// A sparse Merkle tree is a key-value map which also supports proving that a given value is indeed
/// stored at a given key in the tree. It is viewed as always being fully populated. If a leaf's
/// value was not explicitly set, then its value is the default value. Typically, the vast majority
/// of leaves will store the default value (hence it is "sparse"), and therefore the internal
/// representation of the tree will only keep track of the leaves that have a different value from
/// the default.
///
/// All leaves sit at the same depth. The deeper the tree, the more leaves it has; but also the
/// longer its proofs are - of exactly `log(depth)` size. A tree cannot have depth 0, since such a
/// tree is just a single value, and is probably a programming mistake.
///
/// Every key maps to one leaf. If there are as many keys as there are leaves, then
/// [Self::Leaf] should be the same type as [Self::Value], as is the case with
/// [crate::merkle::SimpleSmt]. However, if there are more keys than leaves, then [`Self::Leaf`]
/// must accomodate all keys that map to the same leaf.
///
/// [SparseMerkleTree] currently doesn't support optimizations that compress Merkle proofs.
pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
/// The type for a key
type Key: Clone + Ord;
/// The type for a value
type Value: Clone + PartialEq;
/// The type for a leaf
type Leaf: Clone;
/// The type for an opening (i.e. a "proof") of a leaf
type Opening;
/// The default value used to compute the hash of empty leaves
const EMPTY_VALUE: Self::Value;
/// The root of the empty tree with provided DEPTH
const EMPTY_ROOT: RpoDigest;
// PROVIDED METHODS
// ---------------------------------------------------------------------------------------------
/// Returns an opening of the leaf associated with `key`. Conceptually, an opening is a Merkle
/// path to the leaf, as well as the leaf itself.
fn open(&self, key: &Self::Key) -> Self::Opening {
let leaf = self.get_leaf(key);
let mut index: NodeIndex = {
let leaf_index: LeafIndex<DEPTH> = Self::key_to_leaf_index(key);
leaf_index.into()
};
let merkle_path = {
let mut path = Vec::with_capacity(index.depth() as usize);
for _ in 0..index.depth() {
let is_right = index.is_value_odd();
index.move_up();
let InnerNode { left, right } = self.get_inner_node(index);
let value = if is_right { left } else { right };
path.push(value);
}
MerklePath::new(path)
};
Self::path_and_leaf_to_opening(merkle_path, leaf)
}
/// Inserts a value at the specified key, returning the previous value associated with that key.
/// Recall that by definition, any key that hasn't been updated is associated with
/// [`Self::EMPTY_VALUE`].
///
/// This also recomputes all hashes between the leaf (associated with the key) and the root,
/// updating the root itself.
fn insert(&mut self, key: Self::Key, value: Self::Value) -> Self::Value {
let old_value = self.insert_value(key.clone(), value.clone()).unwrap_or(Self::EMPTY_VALUE);
// if the old value and new value are the same, there is nothing to update
if value == old_value {
return value;
}
let leaf = self.get_leaf(&key);
let node_index = {
let leaf_index: LeafIndex<DEPTH> = Self::key_to_leaf_index(&key);
leaf_index.into()
};
self.recompute_nodes_from_index_to_root(node_index, Self::hash_leaf(&leaf));
old_value
}
/// Recomputes the branch nodes (including the root) from `index` all the way to the root.
/// `node_hash_at_index` is the hash of the node stored at index.
fn recompute_nodes_from_index_to_root(
&mut self,
mut index: NodeIndex,
node_hash_at_index: RpoDigest,
) {
let mut node_hash = node_hash_at_index;
for node_depth in (0..index.depth()).rev() {
let is_right = index.is_value_odd();
index.move_up();
let InnerNode { left, right } = self.get_inner_node(index);
let (left, right) = if is_right {
(left, node_hash)
} else {
(node_hash, right)
};
node_hash = Rpo256::merge(&[left, right]);
if node_hash == *EmptySubtreeRoots::entry(DEPTH, node_depth) {
// If a subtree is empty, when can remove the inner node, since it's equal to the
// default value
self.remove_inner_node(index)
} else {
self.insert_inner_node(index, InnerNode { left, right });
}
}
self.set_root(node_hash);
}
/// Computes what changes are necessary to insert the specified key-value pairs into this Merkle
/// tree, allowing for validation before applying those changes.
///
/// This method returns a [`MutationSet`], which contains all the information for inserting
/// `kv_pairs` into this Merkle tree already calculated, including the new root hash, which can
/// be queried with [`MutationSet::root()`]. Once a mutation set is returned,
/// [`SparseMerkleTree::apply_mutations()`] can be called in order to commit these changes to
/// the Merkle tree, or [`drop()`] to discard them.
fn compute_mutations(
&self,
kv_pairs: impl IntoIterator<Item = (Self::Key, Self::Value)>,
) -> MutationSet<DEPTH, Self::Key, Self::Value> {
use NodeMutation::*;
let mut new_root = self.root();
let mut new_pairs: BTreeMap<Self::Key, Self::Value> = Default::default();
let mut node_mutations: BTreeMap<NodeIndex, NodeMutation> = Default::default();
for (key, value) in kv_pairs {
// If the old value and the new value are the same, there is nothing to update.
// For the unusual case that kv_pairs has multiple values at the same key, we'll have
// to check the key-value pairs we've already seen to get the "effective" old value.
let old_value = new_pairs.get(&key).cloned().unwrap_or_else(|| self.get_value(&key));
if value == old_value {
continue;
}
let leaf_index = Self::key_to_leaf_index(&key);
let mut node_index = NodeIndex::from(leaf_index);
// We need the current leaf's hash to calculate the new leaf, but in the rare case that
// `kv_pairs` has multiple pairs that go into the same leaf, then those pairs are also
// part of the "current leaf".
let old_leaf = {
let pairs_at_index = new_pairs
.iter()
.filter(|&(new_key, _)| Self::key_to_leaf_index(new_key) == leaf_index);
pairs_at_index.fold(self.get_leaf(&key), |acc, (k, v)| {
// Most of the time `pairs_at_index` should only contain a single entry (or
// none at all), as multi-leaves should be really rare.
let existing_leaf = acc.clone();
self.construct_prospective_leaf(existing_leaf, k, v)
})
};
let new_leaf = self.construct_prospective_leaf(old_leaf, &key, &value);
let mut new_child_hash = Self::hash_leaf(&new_leaf);
for node_depth in (0..node_index.depth()).rev() {
// Whether the node we're replacing is the right child or the left child.
let is_right = node_index.is_value_odd();
node_index.move_up();
let old_node = node_mutations
.get(&node_index)
.map(|mutation| match mutation {
Addition(node) => node.clone(),
Removal => EmptySubtreeRoots::get_inner_node(DEPTH, node_depth),
})
.unwrap_or_else(|| self.get_inner_node(node_index));
let new_node = if is_right {
InnerNode {
left: old_node.left,
right: new_child_hash,
}
} else {
InnerNode {
left: new_child_hash,
right: old_node.right,
}
};
// The next iteration will operate on this new node's hash.
new_child_hash = new_node.hash();
let &equivalent_empty_hash = EmptySubtreeRoots::entry(DEPTH, node_depth);
let is_removal = new_child_hash == equivalent_empty_hash;
let new_entry = if is_removal { Removal } else { Addition(new_node) };
node_mutations.insert(node_index, new_entry);
}
// Once we're at depth 0, the last node we made is the new root.
new_root = new_child_hash;
// And then we're done with this pair; on to the next one.
new_pairs.insert(key, value);
}
MutationSet {
old_root: self.root(),
new_root,
node_mutations,
new_pairs,
}
}
/// Apply the prospective mutations computed with [`SparseMerkleTree::compute_mutations()`] to
/// this tree.
///
/// # Errors
/// If `mutations` was computed on a tree with a different root than this one, returns
/// [`MerkleError::ConflictingRoots`] with a two-item [`Vec`]. The first item is the root hash
/// the `mutations` were computed against, and the second item is the actual current root of
/// this tree.
fn apply_mutations(
&mut self,
mutations: MutationSet<DEPTH, Self::Key, Self::Value>,
) -> Result<(), MerkleError>
where
Self: Sized,
{
use NodeMutation::*;
let MutationSet {
old_root,
node_mutations,
new_pairs,
new_root,
} = mutations;
// Guard against accidentally trying to apply mutations that were computed against a
// different tree, including a stale version of this tree.
if old_root != self.root() {
return Err(MerkleError::ConflictingRoots(vec![old_root, self.root()]));
}
for (index, mutation) in node_mutations {
match mutation {
Removal => self.remove_inner_node(index),
Addition(node) => self.insert_inner_node(index, node),
}
}
for (key, value) in new_pairs {
self.insert_value(key, value);
}
self.set_root(new_root);
Ok(())
}
// REQUIRED METHODS
// ---------------------------------------------------------------------------------------------
/// The root of the tree
fn root(&self) -> RpoDigest;
/// Sets the root of the tree
fn set_root(&mut self, root: RpoDigest);
/// Retrieves an inner node at the given index
fn get_inner_node(&self, index: NodeIndex) -> InnerNode;
/// Inserts an inner node at the given index
fn insert_inner_node(&mut self, index: NodeIndex, inner_node: InnerNode);
/// Removes an inner node at the given index
fn remove_inner_node(&mut self, index: NodeIndex);
/// Inserts a leaf node, and returns the value at the key if already exists
fn insert_value(&mut self, key: Self::Key, value: Self::Value) -> Option<Self::Value>;
/// Returns the value at the specified key. Recall that by definition, any key that hasn't been
/// updated is associated with [`Self::EMPTY_VALUE`].
fn get_value(&self, key: &Self::Key) -> Self::Value;
/// Returns the leaf at the specified index.
fn get_leaf(&self, key: &Self::Key) -> Self::Leaf;
/// Returns the hash of a leaf
fn hash_leaf(leaf: &Self::Leaf) -> RpoDigest;
/// Returns what a leaf would look like if a key-value pair were inserted into the tree, without
/// mutating the tree itself. The existing leaf can be empty.
///
/// To get a prospective leaf based on the current state of the tree, use `self.get_leaf(key)`
/// as the argument for `existing_leaf`. The return value from this function can be chained back
/// into this function as the first argument to continue making prospective changes.
///
/// # Invariants
/// Because this method is for a prospective key-value insertion into a specific leaf,
/// `existing_leaf` must have the same leaf index as `key` (as determined by
/// [`SparseMerkleTree::key_to_leaf_index()`]), or the result will be meaningless.
fn construct_prospective_leaf(
&self,
existing_leaf: Self::Leaf,
key: &Self::Key,
value: &Self::Value,
) -> Self::Leaf;
/// Maps a key to a leaf index
fn key_to_leaf_index(key: &Self::Key) -> LeafIndex<DEPTH>;
/// Maps a (MerklePath, Self::Leaf) to an opening.
///
/// The length `path` is guaranteed to be equal to `DEPTH`
fn path_and_leaf_to_opening(path: MerklePath, leaf: Self::Leaf) -> Self::Opening;
}
// INNER NODE
// ================================================================================================
#[derive(Debug, Default, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
pub(crate) struct InnerNode {
pub left: RpoDigest,
pub right: RpoDigest,
}
impl InnerNode {
pub fn hash(&self) -> RpoDigest {
Rpo256::merge(&[self.left, self.right])
}
}
// LEAF INDEX
// ================================================================================================
/// The index of a leaf, at a depth known at compile-time.
#[derive(Debug, Default, Copy, Clone, Eq, PartialEq, PartialOrd, Ord, Hash)]
#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
pub struct LeafIndex<const DEPTH: u8> {
index: NodeIndex,
}
impl<const DEPTH: u8> LeafIndex<DEPTH> {
pub fn new(value: u64) -> Result<Self, MerkleError> {
if DEPTH < SMT_MIN_DEPTH {
return Err(MerkleError::DepthTooSmall(DEPTH));
}
Ok(LeafIndex { index: NodeIndex::new(DEPTH, value)? })
}
pub fn value(&self) -> u64 {
self.index.value()
}
}
impl LeafIndex<SMT_MAX_DEPTH> {
pub const fn new_max_depth(value: u64) -> Self {
LeafIndex {
index: NodeIndex::new_unchecked(SMT_MAX_DEPTH, value),
}
}
}
impl<const DEPTH: u8> From<LeafIndex<DEPTH>> for NodeIndex {
fn from(value: LeafIndex<DEPTH>) -> Self {
value.index
}
}
impl<const DEPTH: u8> TryFrom<NodeIndex> for LeafIndex<DEPTH> {
type Error = MerkleError;
fn try_from(node_index: NodeIndex) -> Result<Self, Self::Error> {
if node_index.depth() != DEPTH {
return Err(MerkleError::InvalidDepth {
expected: DEPTH,
provided: node_index.depth(),
});
}
Self::new(node_index.value())
}
}
// MUTATIONS
// ================================================================================================
/// A change to an inner node of a [`SparseMerkleTree`] that hasn't yet been applied.
/// [`MutationSet`] stores this type in relation to a [`NodeIndex`] to keep track of what changes
/// need to occur at which node indices.
#[derive(Debug, Clone, PartialEq, Eq)]
pub(crate) enum NodeMutation {
/// Corresponds to [`SparseMerkleTree::remove_inner_node()`].
Removal,
/// Corresponds to [`SparseMerkleTree::insert_inner_node()`].
Addition(InnerNode),
}
/// Represents a group of prospective mutations to a `SparseMerkleTree`, created by
/// `SparseMerkleTree::compute_mutations()`, and that can be applied with
/// `SparseMerkleTree::apply_mutations()`.
#[derive(Debug, Clone, PartialEq, Eq, Default)]
pub struct MutationSet<const DEPTH: u8, K, V> {
/// The root of the Merkle tree this MutationSet is for, recorded at the time
/// [`SparseMerkleTree::compute_mutations()`] was called. Exists to guard against applying
/// mutations to the wrong tree or applying stale mutations to a tree that has since changed.
old_root: RpoDigest,
/// The set of nodes that need to be removed or added. The "effective" node at an index is the
/// Merkle tree's existing node at that index, with the [`NodeMutation`] in this map at that
/// index overlayed, if any. Each [`NodeMutation::Addition`] corresponds to a
/// [`SparseMerkleTree::insert_inner_node()`] call, and each [`NodeMutation::Removal`]
/// corresponds to a [`SparseMerkleTree::remove_inner_node()`] call.
node_mutations: BTreeMap<NodeIndex, NodeMutation>,
/// The set of top-level key-value pairs we're prospectively adding to the tree, including
/// adding empty values. The "effective" value for a key is the value in this BTreeMap, falling
/// back to the existing value in the Merkle tree. Each entry corresponds to a
/// [`SparseMerkleTree::insert_value()`] call.
new_pairs: BTreeMap<K, V>,
/// The calculated root for the Merkle tree, given these mutations. Publicly retrievable with
/// [`MutationSet::root()`]. Corresponds to a [`SparseMerkleTree::set_root()`]. call.
new_root: RpoDigest,
}
impl<const DEPTH: u8, K, V> MutationSet<DEPTH, K, V> {
/// Queries the root that was calculated during `SparseMerkleTree::compute_mutations()`. See
/// that method for more information.
pub fn root(&self) -> RpoDigest {
self.new_root
}
}