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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972
// Rust Bitcoin Library
// Written in 2019 by
// The rust-bitcoin developers.
// To the extent possible under law, the author(s) have dedicated all
// copyright and related and neighboring rights to this software to
// the public domain worldwide. This software is distributed without
// any warranty.
//
// You should have received a copy of the CC0 Public Domain Dedication
// along with this software.
// If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
//
//! Taproot
//!
use std::cmp::Reverse;
use std::{error, io, fmt};
use crate::hashes::{sha256, sha256t_hash_newtype, Hash, HashEngine};
use crate::schnorr::{UntweakedPublicKey, TweakedPublicKey, TapTweak};
use crate::Script;
use std::collections::{BTreeMap, BTreeSet, BinaryHeap};
use secp256k1_zkp::{self, Secp256k1, Scalar};
use crate::encode::Encodable;
// Taproot test vectors from BIP-341 state the hashes without any reversing
sha256t_hash_newtype! {
pub struct TapLeafTag = hash_str("TapLeaf/elements");
/// Taproot-tagged hash for elements tapscript Merkle tree leafs.
#[hash_newtype(forward)]
pub struct TapLeafHash(_);
pub struct TapBranchTag = hash_str("TapBranch/elements");
/// Tagged hash used in taproot trees; see BIP-340 for tagging rules.
#[hash_newtype(forward)]
pub struct TapNodeHash(_);
pub struct TapTweakTag = hash_str("TapTweak/elements");
/// Taproot-tagged hash for elements public key tweaks.
#[hash_newtype(forward)]
pub struct TapTweakHash(_);
pub struct TapSighashTag = hash_str("TapSighash/elements");
/// Taproot-tagged hash for the elements taproot signature hash.
#[hash_newtype(forward)]
pub struct TapSighashHash(_);
}
impl TapTweakHash {
/// Create a new BIP341 [`TapTweakHash`] from key and tweak
/// Produces H_taptweak(P||R) where P is internal key and R is the merkle root
pub fn from_key_and_tweak(
internal_key: UntweakedPublicKey,
merkle_root: Option<TapNodeHash>,
) -> TapTweakHash {
let mut eng = TapTweakHash::engine();
// always hash the key
eng.input(&internal_key.serialize());
if let Some(h) = merkle_root {
eng.input(h.as_ref());
} else {
// nothing to hash
}
TapTweakHash::from_engine(eng)
}
/// Converts a `TapTweakHash` into a `Scalar` ready for use with key tweaking API.
pub fn to_scalar(self) -> Scalar {
// This is statistically extremely unlikely to panic.
Scalar::from_be_bytes(self.to_byte_array()).expect("hash value greater than curve order")
}
}
impl TapLeafHash {
/// function to compute leaf hash from components
pub fn from_script(script: &Script, ver: LeafVersion) -> TapLeafHash {
let mut eng = TapLeafHash::engine();
ver.as_u8()
.consensus_encode(&mut eng)
.expect("engines don't error");
script
.consensus_encode(&mut eng)
.expect("engines don't error");
TapLeafHash::from_engine(eng)
}
}
/// Maximum depth of a Taproot Tree Script spend path
pub const TAPROOT_CONTROL_MAX_NODE_COUNT: usize = 128;
/// Size of a taproot control node
pub const TAPROOT_CONTROL_NODE_SIZE: usize = 32;
/// Tapleaf mask for getting the leaf version from first byte of control block
pub const TAPROOT_LEAF_MASK: u8 = 0xfe;
/// Tapscript leaf version (Note that this is different from bitcoin's 0xc0)
pub const TAPROOT_LEAF_TAPSCRIPT: u8 = 0xc4;
/// Tapscript control base size
pub const TAPROOT_CONTROL_BASE_SIZE: usize = 33;
/// Tapscript control max size
pub const TAPROOT_CONTROL_MAX_SIZE: usize =
TAPROOT_CONTROL_BASE_SIZE + TAPROOT_CONTROL_NODE_SIZE * TAPROOT_CONTROL_MAX_NODE_COUNT;
// type alias for versioned tap script corresponding merkle proof
type ScriptMerkleProofMap = BTreeMap<(Script, LeafVersion), BTreeSet<TaprootMerkleBranch>>;
/// Data structure for representing Taproot spending information.
/// Taproot output corresponds to a combination of a
/// single public key condition (known the internal key), and zero or more
/// general conditions encoded in scripts organized in the form of a binary tree.
///
/// Taproot can be spent be either:
/// - Spending using the key path i.e., with secret key corresponding to the output_key
/// - By satisfying any of the scripts in the script spent path. Each script can be satisfied by providing
/// a witness stack consisting of the script's inputs, plus the script itself and the control block.
///
/// If one or more of the spending conditions consist of just a single key (after aggregation),
/// the most likely one should be made the internal key.
/// See [BIP341 for elements](https://github.com/ElementsProject/elements/blob/master/doc/taproot-sighash.mediawiki) for more details
/// on choosing internal keys for a taproot application
///
/// Note: This library currently does not support [annex](https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki#cite_note-5)
#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct TaprootSpendInfo {
/// The BIP341 internal key.
internal_key: UntweakedPublicKey,
/// The Merkle root of the script tree (None if there are no scripts)
merkle_root: Option<TapNodeHash>,
/// The sign final output pubkey as per BIP 341
output_key_parity: secp256k1_zkp::Parity,
/// The tweaked output key
output_key: TweakedPublicKey,
/// Map from (script, leaf_version) to (sets of) [`TaprootMerkleBranch`].
/// More than one control block for a given script is only possible if it
/// appears in multiple branches of the tree. In all cases, keeping one should
/// be enough for spending funds, but we keep all of the paths so that
/// a full tree can be constructed again from spending data if required.
script_map: ScriptMerkleProofMap,
}
impl TaprootSpendInfo {
/// Create a new [`TaprootSpendInfo`] from a list of script(with default script version) and
/// weights of satisfaction for that script. The weights represent the probability of
/// each branch being taken. If probabilities/weights for each condition are known,
/// constructing the tree as a Huffman tree is the optimal way to minimize average
/// case satisfaction cost. This function takes input an iterator of tuple(u64, &Script)
/// where usize represents the satisfaction weights of the branch.
/// For example, [(3, S1), (2, S2), (5, S3)] would construct a TapTree that has optimal
/// satisfaction weight when probability for S1 is 30%, S2 is 20% and S3 is 50%.
///
/// # Errors:
///
/// - When the optimal huffman tree has a depth more than 128
/// - If the provided list of script weights is empty
///
/// # Edge Cases:
/// - If the script weight calculations overflow, a sub-optimal tree may be generated. This
/// should not happen unless you are dealing with billions of branches with weights close to
/// 2^32.
pub fn with_huffman_tree<C, I>(
secp: &Secp256k1<C>,
internal_key: UntweakedPublicKey,
script_weights: I,
) -> Result<Self, TaprootBuilderError>
where
I: IntoIterator<Item = (u32, Script)>,
C: secp256k1_zkp::Verification,
{
let mut node_weights = BinaryHeap::<(Reverse<u64>, NodeInfo)>::new();
for (p, leaf) in script_weights {
node_weights.push((Reverse(p as u64), NodeInfo::new_leaf_with_ver(leaf, LeafVersion::default())));
}
if node_weights.is_empty() {
return Err(TaprootBuilderError::IncompleteTree);
}
while node_weights.len() > 1 {
// Combine the last two elements and insert a new node
let (p1, s1) = node_weights.pop().expect("len must be at least two");
let (p2, s2) = node_weights.pop().expect("len must be at least two");
// Insert the sum of first two in the tree as a new node
// N.B.: p1 + p2 can not practically saturate as you would need to have 2**32 max u32s
// from the input to overflow. However, saturating is a reasonable behavior here as
// huffman tree construction would treat all such elements as "very likely".
let p = Reverse(p1.0.saturating_add(p2.0));
node_weights.push((p, NodeInfo::combine(s1, s2)?));
}
// Every iteration of the loop reduces the node_weights.len() by exactly 1
// Therefore, the loop will eventually terminate with exactly 1 element
debug_assert!(node_weights.len() == 1);
let node = node_weights.pop().expect("huffman tree algorithm is broken").1;
Ok(Self::from_node_info(secp, internal_key, node))
}
/// Create a new key spend with internal key and proided merkle root.
/// Provide [`None`] for merkle_root if there is no script path.
///
/// *Note*: As per BIP341
///
/// When the merkle root is [`None`], the output key commits to an unspendable
/// script path instead of having no script path. This is achieved by computing
/// the output key point as Q = P + int(hashTapTweak(bytes(P)))G.
/// See also [`TaprootSpendInfo::tap_tweak`].
/// Refer to BIP 341 footnote (Why should the output key always have
/// a taproot commitment, even if there is no script path?) for more details
///
pub fn new_key_spend<C: secp256k1_zkp::Verification>(
secp: &Secp256k1<C>,
internal_key: UntweakedPublicKey,
merkle_root: Option<TapNodeHash>,
) -> Self {
let (output_key, parity) = internal_key.tap_tweak(secp, merkle_root);
Self {
internal_key,
merkle_root,
output_key_parity: parity,
output_key,
script_map: BTreeMap::new(),
}
}
/// Obtain the tweak and parity used to compute the output_key
pub fn tap_tweak(&self) -> TapTweakHash {
TapTweakHash::from_key_and_tweak(self.internal_key, self.merkle_root)
}
/// Obtain the internal key
pub fn internal_key(&self) -> UntweakedPublicKey {
self.internal_key
}
/// Obtain the merkle root
pub fn merkle_root(&self) -> Option<TapNodeHash> {
self.merkle_root
}
/// Output key(the key used in script pubkey) from Spend data. See also
/// [`TaprootSpendInfo::output_key_parity`]
pub fn output_key(&self) -> TweakedPublicKey {
self.output_key
}
/// Parity of the output key. See also [`TaprootSpendInfo::output_key`]
pub fn output_key_parity(&self) -> secp256k1_zkp::Parity {
self.output_key_parity
}
// Internal function to compute [`TaprootSpendInfo`] from NodeInfo
fn from_node_info<C: secp256k1_zkp::Verification>(
secp: &Secp256k1<C>,
internal_key: UntweakedPublicKey,
node: NodeInfo,
) -> TaprootSpendInfo {
// Create as if it is a key spend path with the given merkle root
let root_hash = Some(TapNodeHash::from_byte_array(node.hash.to_byte_array()));
let mut info = TaprootSpendInfo::new_key_spend(secp, internal_key, root_hash);
for leaves in node.leaves {
let key = (leaves.script, leaves.ver);
let value = leaves.merkle_branch;
if let Some(set) = info.script_map.get_mut(&key) {
set.insert(value);
} else {
let mut set = BTreeSet::new();
set.insert(value);
info.script_map.insert(key, set);
}
}
info
}
/// Access the internal script map
pub fn as_script_map(&self) -> &ScriptMerkleProofMap {
&self.script_map
}
/// Obtain a [`ControlBlock`] for particular script with the given version.
/// Returns [`None`] if the script is not contained in the [`TaprootSpendInfo`]
/// If there are multiple ControlBlocks possible, this returns the shortest one.
pub fn control_block(&self, script_ver: &(Script, LeafVersion)) -> Option<ControlBlock> {
let merkle_branch_set = self.script_map.get(script_ver)?;
// Choose the smallest one amongst the multiple script maps
let smallest = merkle_branch_set
.iter()
.min_by(|x, y| x.0.len().cmp(&y.0.len()))
.expect("Invariant: Script map key must contain non-empty set value");
Some(ControlBlock {
internal_key: self.internal_key,
output_key_parity: self.output_key_parity,
leaf_version: script_ver.1,
merkle_branch: smallest.clone(),
})
}
}
/// Builder for building taproot iteratively. Users can specify tap leaf or omitted/hidden
/// branches in a DFS(Depth first search) walk to construct this tree.
// Similar to Taproot Builder in bitcoin core
#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Default)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize), serde(crate = "actual_serde"))]
pub struct TaprootBuilder {
// The following doc-comment is from bitcoin core, but modified for rust
// The comment below describes the current state of the builder for a given tree.
//
// For each level in the tree, one NodeInfo object may be present. branch at index 0
// is information about the root; further values are for deeper subtrees being
// explored.
//
// During the construction of Taptree, for every right branch taken to
// reach the position we're currently working in, there will be a (Some(_))
// entry in branch corresponding to the left branch at that level.
//
// For example, imagine this tree: - N0 -
// / \
// N1 N2
// / \ / \
// A B C N3
// / \
// D E
//
// Initially, branch is empty. After processing leaf A, it would become
// {None, None, A}. When processing leaf B, an entry at level 2 already
// exists, and it would thus be combined with it to produce a level 1 one,
// resulting in {None, N1}. Adding C and D takes us to {None, N1, C}
// and {None, N1, C, D} respectively. When E is processed, it is combined
// with D, and then C, and then N1, to produce the root, resulting in {N0}.
//
// This structure allows processing with just O(log n) overhead if the leaves
// are computed on the fly.
//
// As an invariant, there can never be None entries at the end. There can
// also not be more than 128 entries (as that would mean more than 128 levels
// in the tree). The depth of newly added entries will always be at least
// equal to the current size of branch (otherwise it does not correspond
// to a depth-first traversal of a tree). branch is only empty if no entries
// have ever be processed. branch having length 1 corresponds to being done.
//
branch: Vec<Option<NodeInfo>>,
}
impl TaprootBuilder {
/// Create a new instance of [`TaprootBuilder`]
pub fn new() -> Self {
TaprootBuilder { branch: vec![] }
}
/// Check if the builder is a complete tree
pub fn is_complete(&self) -> bool {
self.branch.len() == 1 && self.branch[0].is_some()
}
pub(crate) fn branch(&self) -> &[Option<NodeInfo>]{
&self.branch
}
/// Just like [`TaprootBuilder::add_leaf`] but allows to specify script version
pub fn add_leaf_with_ver(
self,
depth: usize,
script: Script,
ver: LeafVersion,
) -> Result<Self, TaprootBuilderError> {
let leaf = NodeInfo::new_leaf_with_ver(script, ver);
self.insert(leaf, depth)
}
/// Add a leaf script at a depth `depth` to the builder with default script version.
/// This will error if the leave are not provided in a DFS walk order. The depth of the
/// root node is 0 and it's immediate child would be at depth 1.
/// See [`TaprootBuilder::add_leaf_with_ver`] for adding a leaf with specific version
/// See [Wikipedia](https://en.wikipedia.org/wiki/Depth-first_search) for more details
pub fn add_leaf(self, depth: usize, script: Script) -> Result<Self, TaprootBuilderError> {
self.add_leaf_with_ver(depth, script, LeafVersion::default())
}
/// Add a hidden/omitted node at a depth `depth` to the builder.
/// This will error if the node are not provided in a DFS walk order. The depth of the
/// root node is 0 and it's immediate child would be at depth 1.
pub fn add_hidden(self, depth: usize, hash: sha256::Hash) -> Result<Self, TaprootBuilderError> {
let node = NodeInfo::new_hidden(hash);
self.insert(node, depth)
}
/// Create [`TaprootSpendInfo`] with the given internal key
pub fn finalize<C: secp256k1_zkp::Verification>(
mut self,
secp: &Secp256k1<C>,
internal_key: UntweakedPublicKey,
) -> Result<TaprootSpendInfo, TaprootBuilderError> {
if self.branch.len() > 1 {
return Err(TaprootBuilderError::IncompleteTree);
}
let node = self
.branch
.pop()
.ok_or(TaprootBuilderError::EmptyTree)?
.expect("Builder invariant: last element of the branch must be some");
Ok(TaprootSpendInfo::from_node_info(secp, internal_key, node))
}
// Helper function to insert a leaf at a depth
fn insert(mut self, mut node: NodeInfo, mut depth: usize) -> Result<Self, TaprootBuilderError> {
// early error on invalid depth. Though this will be checked later
// while constructing TaprootMerkelBranch
if depth > TAPROOT_CONTROL_MAX_NODE_COUNT {
return Err(TaprootBuilderError::InvalidMerkleTreeDepth(depth));
}
// We cannot insert a leaf at a lower depth while a deeper branch is unfinished. Doing
// so would mean the add_leaf/add_hidden invocations do not correspond to a DFS traversal of a
// binary tree.
if depth + 1 < self.branch.len() {
return Err(TaprootBuilderError::NodeNotInDfsOrder);
}
while self.branch.len() == depth + 1 {
let child = match self.branch.pop() {
None => unreachable!("Len of branch checked to be >= 1"),
Some(Some(child)) => child,
// Needs an explicit push to add the None that we just popped.
// Cannot use .last() because of borrow checker issues.
Some(None) => {
self.branch.push(None);
break;
} // Cannot combine further
};
if depth == 0 {
// We are trying to combine two nodes at root level.
// Can't propagate further up than the root
return Err(TaprootBuilderError::OverCompleteTree);
}
node = NodeInfo::combine(node, child)?;
// Propagate to combine nodes at a lower depth
depth -= 1;
}
if self.branch.len() < depth + 1 {
// add enough nodes so that we can insert node at depth `depth`
let num_extra_nodes = depth + 1 - self.branch.len();
self.branch
.extend((0..num_extra_nodes).map(|_| None));
}
// Push the last node to the branch
self.branch[depth] = Some(node);
Ok(self)
}
}
/// Structure to represent the node information in taproot tree
#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize), serde(crate = "actual_serde"))]
pub struct NodeInfo {
/// Merkle Hash for this node
pub(crate) hash: sha256::Hash,
/// information about leaves inside this node
pub(crate) leaves: Vec<LeafInfo>,
}
impl NodeInfo {
/// Creates a new NodeInfo with omitted/hidden info
pub fn new_hidden(hash: sha256::Hash) -> Self {
Self {
hash,
leaves: vec![],
}
}
/// Creates a new leaf with NodeInfo
pub fn new_leaf_with_ver(script: Script, ver: LeafVersion) -> Self {
let leaf = LeafInfo::new(script, ver);
Self {
hash: leaf.hash(),
leaves: vec![leaf],
}
}
/// Combines two NodeInfo's to create a new parent
pub fn combine(a: Self, b: Self) -> Result<Self, TaprootBuilderError> {
let mut all_leaves = Vec::with_capacity(a.leaves.len() + b.leaves.len());
for mut a_leaf in a.leaves {
a_leaf.merkle_branch.push(b.hash)?; // add hashing partner
all_leaves.push(a_leaf);
}
for mut b_leaf in b.leaves {
b_leaf.merkle_branch.push(a.hash)?; // add hashing partner
all_leaves.push(b_leaf);
}
let mut eng = TapNodeHash::engine();
if a.hash < b.hash {
eng.input(a.hash.as_ref());
eng.input(b.hash.as_ref());
} else {
eng.input(b.hash.as_ref());
eng.input(a.hash.as_ref());
};
Ok(Self {
hash: sha256::Hash::from_engine(eng),
leaves: all_leaves,
})
}
}
/// Data Structure to store information about taproot leaf node
#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize), serde(crate = "actual_serde"))]
pub struct LeafInfo {
// The underlying script
pub(crate) script: Script,
// The leaf version
pub(crate) ver: LeafVersion,
// The merkle proof(hashing partners) to get this node
pub(crate) merkle_branch: TaprootMerkleBranch,
}
impl LeafInfo {
/// Creates an instance of Self from Script with default version and no merkle branch
pub fn new(script: Script, ver: LeafVersion) -> Self {
Self {
script,
ver,
merkle_branch: TaprootMerkleBranch(vec![]),
}
}
// Computes a leaf hash for the given leaf
fn hash(&self) -> sha256::Hash {
let leaf_hash = TapLeafHash::from_script(&self.script, self.ver);
sha256::Hash::from_byte_array(leaf_hash.to_byte_array())
}
}
/// The Merkle proof for inclusion of a tree in a taptree hash
// The type of hash is sha256::Hash because the vector might contain
// both TapNodeHash and TapLeafHash
#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Default)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
#[cfg_attr(feature = "serde", serde(crate = "actual_serde"))]
pub struct TaprootMerkleBranch(Vec<sha256::Hash>);
impl TaprootMerkleBranch {
/// Obtain a reference to inner
pub fn as_inner(&self) -> &[sha256::Hash] {
&self.0
}
/// Create a merkle proof from slice
pub fn from_slice(sl: &[u8]) -> Result<Self, TaprootError> {
if sl.len() % TAPROOT_CONTROL_NODE_SIZE != 0 {
Err(TaprootError::InvalidMerkleBranchSize(sl.len()))
} else if sl.len() > TAPROOT_CONTROL_NODE_SIZE * TAPROOT_CONTROL_MAX_NODE_COUNT {
Err(TaprootError::InvalidMerkleTreeDepth(
sl.len() / TAPROOT_CONTROL_NODE_SIZE,
))
} else {
let inner = sl
// TODO: Use chunks_exact after MSRV changes to 1.31
.chunks(TAPROOT_CONTROL_NODE_SIZE)
.map(|chunk| {
sha256::Hash::from_slice(chunk)
.expect("chunk exact always returns the correct size")
})
.collect();
Ok(TaprootMerkleBranch(inner))
}
}
/// Serialize to a writer. Returns the number of bytes written
pub fn encode<Write: io::Write>(&self, mut writer: Write) -> io::Result<usize> {
let mut written = 0;
for hash in self.0.iter() {
written += writer.write(hash.as_ref())?;
}
Ok(written)
}
/// Serialize self as bytes
pub fn serialize(&self) -> Vec<u8> {
self.0.iter().flat_map(|e| e.as_byte_array()).copied().collect::<Vec<u8>>()
}
// Internal function to append elements to proof
fn push(&mut self, h: sha256::Hash) -> Result<(), TaprootBuilderError> {
if self.0.len() >= TAPROOT_CONTROL_MAX_NODE_COUNT {
Err(TaprootBuilderError::InvalidMerkleTreeDepth(self.0.len()))
} else {
self.0.push(h);
Ok(())
}
}
/// Create a MerkleProof from Vec<[`sha256::Hash`]>. Returns an error when
/// inner proof len is more than TAPROOT_CONTROL_MAX_NODE_COUNT (128)
pub fn from_inner(inner: Vec<sha256::Hash>) -> Result<Self, TaprootError> {
if inner.len() > TAPROOT_CONTROL_MAX_NODE_COUNT {
Err(TaprootError::InvalidMerkleTreeDepth(inner.len()))
} else {
Ok(TaprootMerkleBranch(inner))
}
}
/// Consume Self to get Vec<[`sha256::Hash`]>
pub fn into_inner(self) -> Vec<sha256::Hash> {
self.0
}
}
/// Control Block data structure used in Tapscript satisfaction
#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize), serde(crate = "actual_serde"))]
pub struct ControlBlock {
/// The tapleaf version,
pub leaf_version: LeafVersion,
/// The parity of the output key (NOT THE INTERNAL KEY WHICH IS ALWAYS XONLY)
pub output_key_parity: secp256k1_zkp::Parity,
/// The internal key
pub internal_key: UntweakedPublicKey,
/// The merkle proof of a script associated with this leaf
pub merkle_branch: TaprootMerkleBranch,
}
impl ControlBlock {
/// Obtain a ControlBlock from slice. This is an extra witness element
/// that provides the proof that taproot script pubkey is correctly computed
/// with some specified leaf hash. This is the last element in
/// taproot witness when spending a output via script path.
///
/// # Errors:
/// - If the control block size is not of the form 33 + 32m where
/// 0 <= m <= 128, InvalidControlBlock is returned
pub fn from_slice(sl: &[u8]) -> Result<ControlBlock, TaprootError> {
if sl.len() < TAPROOT_CONTROL_BASE_SIZE
|| (sl.len() - TAPROOT_CONTROL_BASE_SIZE) % TAPROOT_CONTROL_NODE_SIZE != 0
{
return Err(TaprootError::InvalidControlBlockSize(sl.len()));
}
let output_key_parity = secp256k1_zkp::Parity::from_u8(sl[0] & 1)
.expect("Parity is a single bit because it is masked by 0x01");
let leaf_version = LeafVersion::from_u8(sl[0] & TAPROOT_LEAF_MASK)?;
let internal_key = UntweakedPublicKey::from_slice(&sl[1..TAPROOT_CONTROL_BASE_SIZE])
.map_err(TaprootError::InvalidInternalKey)?;
let merkle_branch = TaprootMerkleBranch::from_slice(&sl[TAPROOT_CONTROL_BASE_SIZE..])?;
Ok(ControlBlock {
leaf_version,
output_key_parity,
internal_key,
merkle_branch,
})
}
/// Obtain the size of control block. Faster and more efficient than calling
/// serialize() followed by len(). Can be handy for fee estimation
pub fn size(&self) -> usize {
TAPROOT_CONTROL_BASE_SIZE + TAPROOT_CONTROL_NODE_SIZE * self.merkle_branch.as_inner().len()
}
/// Serialize to a writer. Returns the number of bytes written
pub fn encode<Write: io::Write>(&self, mut writer: Write) -> io::Result<usize> {
let first_byte: u8 = self.output_key_parity.to_u8() | self.leaf_version.as_u8();
let mut bytes_written = 0;
bytes_written += writer.write(&[first_byte])?;
bytes_written += writer.write(&self.internal_key.serialize())?;
bytes_written += self.merkle_branch.encode(&mut writer)?;
Ok(bytes_written)
}
/// Serialize the control block. This would be required when
/// using ControlBlock as a witness element while spending an output via
/// script path. This serialization does not include the VarInt prefix that would be
/// applied when encoding this element as a witness.
pub fn serialize(&self) -> Vec<u8> {
let mut buf = Vec::with_capacity(self.size());
self.encode(&mut buf)
.expect("writers don't error");
buf
}
/// Verify that a control block is correct proof for a given output key and script
/// This only checks that script is contained inside the taptree described by
/// output key, full verification must also execute the script with witness data
pub fn verify_taproot_commitment<C: secp256k1_zkp::Verification>(
&self,
secp: &Secp256k1<C>,
output_key: &TweakedPublicKey,
script: &Script,
) -> bool {
// compute the script hash
// Initially the curr_hash is the leaf hash
let leaf_hash = TapLeafHash::from_script(script, self.leaf_version);
let mut curr_hash = TapNodeHash::from_byte_array(leaf_hash.to_byte_array());
// Verify the proof
for elem in self.merkle_branch.as_inner() {
let mut eng = TapNodeHash::engine();
if curr_hash.as_byte_array() < elem.as_byte_array() {
eng.input(curr_hash.as_ref());
eng.input(elem.as_ref());
} else {
eng.input(elem.as_ref());
eng.input(curr_hash.as_ref());
}
// Recalculate the curr hash as parent hash
curr_hash = TapNodeHash::from_engine(eng);
}
// compute the taptweak
let tweak = TapTweakHash::from_key_and_tweak(self.internal_key, Some(curr_hash));
let tweak = Scalar::from_be_bytes(tweak.to_byte_array()).expect("hash value greater than curve order");
self.internal_key.tweak_add_check(
secp,
output_key.as_inner(),
self.output_key_parity,
tweak,
)
}
}
/// The leaf version for tapleafs
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize), serde(crate = "actual_serde"))]
pub struct LeafVersion(u8);
impl Default for LeafVersion {
fn default() -> Self {
LeafVersion(TAPROOT_LEAF_TAPSCRIPT)
}
}
impl LeafVersion {
/// Obtain LeafVersion from u8, will error when last bit of ver is even or
/// when ver is 0x50 (ANNEX_TAG)
// Text from BIP341:
// In order to support some forms of static analysis that rely on
// being able to identify script spends without access to the output being
// spent, it is recommended to avoid using any leaf versions that would conflict
// with a valid first byte of either a valid P2WPKH pubkey or a valid P2WSH script
// (that is, both v and v | 1 should be an undefined, invalid or disabled opcode
// or an opcode that is not valid as the first opcode).
// The values that comply to this rule are the 32 even values between
// 0xc0 and 0xfe and also 0x66, 0x7e, 0x80, 0x84, 0x96, 0x98, 0xba, 0xbc, 0xbe
pub fn from_u8(ver: u8) -> Result<Self, TaprootError> {
if ver & TAPROOT_LEAF_MASK == ver && ver != 0x50 {
Ok(LeafVersion(ver))
} else {
Err(TaprootError::InvalidTaprootLeafVersion(ver))
}
}
/// Get the inner version from LeafVersion
pub fn as_u8(&self) -> u8 {
self.0
}
}
impl From<LeafVersion> for u8 {
fn from(lv: LeafVersion) -> u8 {
lv.0
}
}
/// Detailed error type for taproot builder
#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum TaprootBuilderError {
/// Merkle Tree depth must not be more than 128
InvalidMerkleTreeDepth(usize),
/// Nodes must be added specified in DFS order
NodeNotInDfsOrder,
/// Two nodes at depth 0 are not allowed
OverCompleteTree,
/// Invalid taproot internal key
InvalidInternalKey(secp256k1_zkp::UpstreamError),
/// Called finalize on an incomplete tree
IncompleteTree,
/// Called finalize on a empty tree
EmptyTree,
}
impl fmt::Display for TaprootBuilderError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match *self {
TaprootBuilderError::NodeNotInDfsOrder => {
write!(f, "add_leaf/add_hidden must be called in DFS walk order",)
}
TaprootBuilderError::OverCompleteTree => write!(
f,
"Attempted to create a tree with two nodes at depth 0. There must\
only be a exactly one node at depth 0",
),
TaprootBuilderError::InvalidMerkleTreeDepth(d) => write!(
f,
"Merkle Tree depth({}) must be less than {}",
d, TAPROOT_CONTROL_MAX_NODE_COUNT
),
TaprootBuilderError::InvalidInternalKey(e) => {
write!(f, "Invalid Internal XOnly key : {}", e)
}
TaprootBuilderError::IncompleteTree => {
write!(f, "Called finalize on an incomplete tree")
}
TaprootBuilderError::EmptyTree => {
write!(f, "Called finalize on an empty tree")
}
}
}
}
impl error::Error for TaprootBuilderError {}
/// Detailed error type for taproot utilities
#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum TaprootError {
/// Proof size must be a multiple of 32
InvalidMerkleBranchSize(usize),
/// Merkle Tree depth must not be more than 128
InvalidMerkleTreeDepth(usize),
/// The last bit of tapleaf version must be zero
InvalidTaprootLeafVersion(u8),
/// Invalid Control Block Size
InvalidControlBlockSize(usize),
/// Invalid taproot internal key
InvalidInternalKey(secp256k1_zkp::UpstreamError),
/// Empty TapTree
EmptyTree,
}
impl fmt::Display for TaprootError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match *self {
TaprootError::InvalidMerkleBranchSize(sz) => write!(
f,
"Merkle branch size({}) must be a multiple of {}",
sz, TAPROOT_CONTROL_NODE_SIZE
),
TaprootError::InvalidMerkleTreeDepth(d) => write!(
f,
"Merkle Tree depth({}) must be less than {}",
d, TAPROOT_CONTROL_MAX_NODE_COUNT
),
TaprootError::InvalidTaprootLeafVersion(v) => write!(
f,
"Leaf version({}) must have the least significant bit 0",
v
),
TaprootError::InvalidControlBlockSize(sz) => write!(
f,
"Control Block size({}) must be of the form 33 + 32*m where 0 <= m <= {} ",
sz, TAPROOT_CONTROL_MAX_NODE_COUNT
),
// TODO: add source when in MSRV
TaprootError::InvalidInternalKey(e) => write!(f, "Invalid Internal XOnly key : {}", e),
TaprootError::EmptyTree => write!(f, "Taproot Tree must contain at least one script"),
}
}
}
impl error::Error for TaprootError {}
#[cfg(test)]
mod tests{
use super::*;
use crate::hashes::HashEngine;
use crate::hashes::sha256t::Tag;
use crate::hex::FromHex;
use std::str::FromStr;
fn tag_engine(tag_name: &str) -> sha256::HashEngine {
let mut engine = sha256::Hash::engine();
let tag_hash = sha256::Hash::hash(tag_name.as_bytes());
engine.input(&tag_hash[..]);
engine.input(&tag_hash[..]);
engine
}
#[test]
fn test_midstates() {
// test that engine creation roundtrips
assert_eq!(tag_engine("TapLeaf/elements").midstate(), TapLeafTag::engine().midstate());
assert_eq!(tag_engine("TapBranch/elements").midstate(), TapBranchTag::engine().midstate());
assert_eq!(tag_engine("TapTweak/elements").midstate(), TapTweakTag::engine().midstate());
assert_eq!(tag_engine("TapSighash/elements").midstate(), TapSighashTag::engine().midstate());
// check that hash creation is the same as building into the same engine
fn empty_hash(tag_name: &str) -> [u8; 32] {
let mut e = tag_engine(tag_name);
e.input(&[]);
sha256::Hash::from_engine(e).to_byte_array()
}
assert_eq!(empty_hash("TapLeaf/elements"), TapLeafHash::hash(&[]).to_byte_array());
assert_eq!(empty_hash("TapBranch/elements"), TapNodeHash::hash(&[]).to_byte_array());
assert_eq!(empty_hash("TapTweak/elements"), TapTweakHash::hash(&[]).to_byte_array());
assert_eq!(empty_hash("TapSighash/elements"), TapSighashHash::hash(&[]).to_byte_array());
}
#[test]
fn build_huffman_tree() {
let secp = Secp256k1::verification_only();
let internal_key = UntweakedPublicKey::from_str("93c7378d96518a75448821c4f7c8f4bae7ce60f804d03d1f0628dd5dd0f5de51").unwrap();
let script_weights = vec![
(10, Script::from_hex("51").unwrap()), // semantics of script don't matter for this test
(20, Script::from_hex("52").unwrap()),
(20, Script::from_hex("53").unwrap()),
(30, Script::from_hex("54").unwrap()),
(19, Script::from_hex("55").unwrap()),
];
let tree_info = TaprootSpendInfo::with_huffman_tree(&secp, internal_key, script_weights.clone()).unwrap();
/* The resulting tree should put the scripts into a tree similar
* to the following:
*
* 1 __/\__
* / \
* /\ / \
* 2 54 52 53 /\
* 3 55 51
*/
for &(script, length) in &[("51", 3), ("52", 2), ("53", 2), ("54", 2), ("55", 3)] {
assert_eq!(
length,
tree_info
.script_map
.get(&(Script::from_hex(script).unwrap(), LeafVersion::default()))
.expect("Present Key")
.iter()
.next()
.expect("Present Path")
.0
.len()
);
}
// Obtain the output key
let output_key = tree_info.output_key();
// Try to create and verify a control block from each path
for (_weights, script) in script_weights {
let ver_script = (script, LeafVersion::default());
let ctrl_block = tree_info.control_block(&ver_script).unwrap();
assert!(ctrl_block.verify_taproot_commitment(&secp, &output_key, &ver_script.0))
}
}
#[test]
fn taptree_builder() {
let secp = Secp256k1::verification_only();
let internal_key = UntweakedPublicKey::from_str("93c7378d96518a75448821c4f7c8f4bae7ce60f804d03d1f0628dd5dd0f5de51").unwrap();
let builder = TaprootBuilder::new();
// Create a tree as shown below
// For example, imagine this tree:
// A, B , C are at depth 2 and D,E are at 3
// ....
// / \
// /\ /\
// / \ / \
// A B C / \
// D E
let a = Script::from_hex("51").unwrap();
let b = Script::from_hex("52").unwrap();
let c = Script::from_hex("53").unwrap();
let d = Script::from_hex("54").unwrap();
let e = Script::from_hex("55").unwrap();
let builder = builder.add_leaf(2, a.clone()).unwrap();
let builder = builder.add_leaf(2, b.clone()).unwrap();
let builder = builder.add_leaf(2, c.clone()).unwrap();
let builder = builder.add_leaf(3, d.clone()).unwrap();
let builder = builder.add_leaf(3, e.clone()).unwrap();
let tree_info = builder.finalize(&secp, internal_key).unwrap();
let output_key = tree_info.output_key();
for script in [a, b, c, d, e] {
let ver_script = (script, LeafVersion::default());
let ctrl_block = tree_info.control_block(&ver_script).unwrap();
assert!(ctrl_block.verify_taproot_commitment(&secp, &output_key, &ver_script.0))
}
}
}