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BIP114: MAST proposal v2

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Johnson Lau 2016-07-29 00:59:06 +08:00
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@ -24,71 +24,172 @@ The [[bip-0016.mediawiki|BIP16]] (Pay-to-script-hash, "P2SH") fixes the first 3
The [[bip-0141.mediawiki|BIP141]] defines 2 new types of scripts that support segregated witness. The pay-to-witness-script-hash (P2WSH) is similar to P2SH is many ways. By supplying the script in witness, P2WSH restores the original 10,000 byte script limit. However, it still requires publishing of unexecuted branches. The [[bip-0141.mediawiki|BIP141]] defines 2 new types of scripts that support segregated witness. The pay-to-witness-script-hash (P2WSH) is similar to P2SH is many ways. By supplying the script in witness, P2WSH restores the original 10,000 byte script limit. However, it still requires publishing of unexecuted branches.
===Merkelized Abstract Syntax Tree=== ===Merkelized Abstract Syntax Tree===
The idea of Merkelized Abstract Syntax Tree (MAST) is to use a Merkle tree to encode mutually exclusive branches in a script. When spending, the redeemer may provide only the branch they are executing, and hashes that connect the branch to the fixed size Merkel root. This reduces the size of redemption stack from O(n) to O(log n) (n as the number of mutually exclusive branches). This enables complicated redemption conditions that is currently not possible due to the script size and op code limit, improves privacy by hiding unexecuted branches, and allows inclusion of non-consensus enforced data with very low or no additional cost. The idea of Merkelized Abstract Syntax Tree (MAST) is to use a Merkle tree to encode branches in a script. When spending, users may provide only the branches they are executing, and hashes that connect the branches to the fixed size Merkel root. This reduces the size of redemption stack from O(n) to O(log n) (n as the number of branches). This enables complicated redemption conditions that is currently not possible due to the script size and opcode limit, improves privacy by hiding unexecuted branches, and allows inclusion of non-consensus enforced data with very low or no additional cost.
==Specification== ==Specification==
In [[bip-0141.mediawiki|BIP141]], witness programs with a version byte of 1 or larger are considered to be anyone-can-spend scripts. The following new validation rules are applied if the witness program version byte is 1 and the program size is 32 bytes. The witness program is the <code>MAST Root</code>. In [[bip-0141.mediawiki|BIP141]], witness programs with a version byte of 1 or larger are considered to be anyone-can-spend scripts. The following new validation rules are applied if the witness program version byte is 1 and the program size is 32 bytes.<ref>If the version byte is 1, but the witness program is not 32 bytes, no further interpretation of the witness program or witness stack happens. This is reserved for future extensions.</ref> The witness program is the <code>MAST Root</code>.
To redeem an output of this kind, the witness must consist of an input stack to feed to the script, followed by a <code>Postion</code> value, a serialized Merkle path (<code>Path</code>), and a serialized script (<code>MAST Script</code>). To redeem an output of this kind, the witness must consist of the following items:
The <code>Position</code>, <code>Path</code>, and <code>MAST Script</code> are popped off the initial witness stack.
The double-SHA256 of the MAST Script (≤ TBD bytes) must be correctly connected to the <code>MAST Root</code> with the <code>ComputeMerkleRootFromBranch</code> function, with the specified <code>Path</code> and <code>Position</code>. Script_stack_1
Script_stack_2
.
.
Script_stack_X (X ≥ 0)
Subscript_1
Subscript_2
.
.
Subscript_Y (1 ≤ Y ≤ 255)
Position
Path
Metadata (Y|MAST Version)
<code>Path</code> is the serialized Merkle path for the <code>MAST Script</code>. Size of <code>Path</code> must be a multiple of 32 bytes, and not more than 1024 bytes (which allows 32 levels). Each 32 byte word is a double-SHA256 merkle node in the merkle branch connecting to the <code> MAST Root</code>. If the size of <code>Path</code> is zero, the double-SHA256 of the <code>MAST Script</code> must match the <code>MAST Root</code>.
<code>Position</code> indicates the location of the <code>MAST Script</code> in the Merkle tree, with zero indicating the leftmost position. It is an unsigned little-endian integer with not more than 4 bytes. It must be encoded in the most parsimonious way possible, with no leading zero and not larger than the maximum number of items allowed by the depth of the tree (as implied by the size of <code>Path</code>). <code>Metadata</code> is the last witness item. It is a vector of 1 to 5 bytes. The first byte is an unsigned integer between 1 to 255 denoting the number of <code>Subscript</code> (defined hereinafter). The following 0 to 4 byte(s) is an unsigned little-endian integer denoting the <code>MAST version</code>. <code>MAST Version</code> must be minimally encoded (the most significant byte must not be 0).
The <code>MAST Script</code> is then deserialized, and executed after normal script evaluation with the remaining witness stack (≤ TBD bytes for each stack item). The script must not fail, and result in exactly a single TRUE on the stack. <code>Path</code> is the second last witness item. It is a serialized Merkle path of the <code>Script Hash</code> (defined hereinafter). Size of <code>Path</code> must be a multiple of 32 bytes, and not more than 1024 bytes. Each 32 byte word is a double-SHA256 merkle node in the merkle branch connecting to the <code>Script Root</code> (defined hereinafter). <code>Depth</code> of the tree (0 to 32) is the size of <code>Path</code> divided by 32.
Sigops in MAST program are counted to the block sigop limit in the same way as the version 0 witness program (see BIP141). <code>Position</code> is the third last witness item. It indicates the location of the <code>Script Hash</code> in the Merkle tree, with zero indicating the leftmost position. It is an unsigned little-endian integer with not more than 4 bytes. It must be minimally encoded: the value must not be larger than the maximum number of items allowed by the <code>Depth</code> of the tree, and the most significant byte must not be 0. For example, if <code>Depth</code> is 4, the valid range of <code>Position</code> is 0 to 15 (2<sup>4</sup>-1).
If the version byte is 1, but the witness program is not 32 bytes, the script must fail. Depends on the first byte of <code>Metadata</code>, there should be 1 to 255 <code>Subscript</code> witness item(s) before <code>Position</code>.
<code>Script Hash</code> is defined as:
Script Hash = H(Y|H(Subscript_1)|H(Subscript_2)|...|H(Subscript_Y))
H() = SHA256(SHA256())
where <code>Y</code> is a 1-byte value denoting number of <code>Subscript</code>, followed by the hash of each <code>Subscript</code>
<code>Script Root</code> is the Merkle root calculated by the <code>ComputeMerkleRootFromBranch</code> function, using <code>Script Hash</code>, <code>Path</code> and <code>Position</code>.
<code>MAST Root</code> is <code>H(MAST Version|Script Root)</code>. The pre-image has a fixed size of 36 bytes: 4 bytes for <code>MAST Version</code> (unsigned little-endian integer) and 32 bytes for <code>Script Root</code>.
The script evaluation fails if <code>MAST Root</code> does not match the witness program.
If the <code>MAST Root</code> matches the witness program and <code>MAST Version</code> is greater than 0, the script returns a success without further evaluation. <code>SigOpsCost</code> is counted as 0. This is reserved for future script upgrades.
If the <code>MAST Version</code> is 0, the <code>Subscript</code>(s) are serialized to form the final <code>MAST Script</code>, beginning with </code>Subscript_1</code>. The unused witness item(s) before the </code>Subscript_1</code> are used as <code>Input Stack</code> to feed to the <code>MASTScript</code>. (Similar to P2WSH in BIP141)
The script fails with one of the following conditions:
* <code>MAST Script</code> is malformed (i.e. not enough data provided for the last push operation). Individual <code>Subscript</code> might be malformed, as long as they are serialized into a valid <code>MAST Script</code>
* Size of <code>MAST Script</code> is larger than 10,000 bytes
* Size of any one of the <code>Input Stack</code> item is larger than 520 bytes
* Number of non-push operations (<code>nOpCount</code>) is more than 201. <code>nOpCount</code> is the sum of the number of non-push operations in <code>MAST Script</code> (counted in the same way as P2WSH <code>witnessScript</code>), number of <code>Subscript</code> (Y), and <code>Depth</code> of the Merkle tree.
The <code>MAST Script</code> is then evaluated with the <code>Input Stack</code> (with some new or redefined opcodes described in BIPXXX). The evaluation must not fail, and result in an exactly empty stack.
Counting of <code>SigOpsCost</code> is based on the <code>MAST Script</code>, described in BIPYYY.
== Rationale ==
=== MAST Structure ===
This proposal is a restricted case of more general MAST. In a general MAST design, users may freely assign one or more script branches for execution. In this proposal, only one branch is allowed for execution, and users are required to transform a complicated condition into several mutually exclusive branches. For example, if the desired redeem condition is:
(A or B) and (C or D or E) and (F or G)
In a general MAST design, the 7 branches (A to G) will form a 3-level Merkle tree, plus an "overall condition" describing the relationship of different branches. In redemption, the "overall condition", executed branches (e.g. B, D, F), and Merkle path data will be provided for validation.
In the current proposal, the user has to transform the redeem condition into 12 mutually exclusive branches and form a 4-level Merkle tree, and present only one branch in redemption:
A and C and F
B and C and F
A and D and F
.
.
B and E and G
One way to implement the general MAST design is using a combination of <code>OP_EVAL</code>, <code>OP_CAT</code>, and <code>OP_HASH256</code>. However, that will suffer from the problems of <code>OP_EVAL</code>, including risks of indefinite program loop and inability to do static program analysis. A complicated implementation is required to fix these problems and is difficult to review.
The advantages of the current proposal are:
* <code>Subscript</code> are located at a fixed position in the witness stack. This allows static program analysis, such as static <code>SigOpsCost</code> counting and early termination of scripts with disabled opcodes.
* If different parties in a contract do not want to expose their scripts to each other, they may provide only <code>H(Subscript)</code> and keep the <code>Subscript</code> private until redemption.
* If they are willing to share the actual scripts, they may combine them into one <code>Subscript</code> for each branch, saving some <code>nOpCount</code> and a few bytes of witness space.
The are some disadvantages, but only when the redemption condition is very complicated:
* It may require more branches than a general MAST design (as shown in the previous example) and take more witness space in redemption
* Creation and storage of the MAST structure may take more time and space. However, such additional costs affect only the related parties in the contract but not any other Bitcoin users.
=== MAST Version ===
This proposal allows users to indicate the version of scripting language in the witness, which is cheaper than doing that in <code>scriptPubKey</code> or <code>scriptSig</code>. Undefined versions remain anyone-can-spend and are reserved for future expansions. A new version could be used for relaxing constraints (e.g. the 10,000 bytes size limit of <code>MAST Script</code>), adding or redefining opcodes, or even introducing a completely novel scripting system.
=== nOpCount limit ===
In version 0 MAST, the extra hashing operations in calculating the <code>MAST Root</code> are counted towards the 201 <code>nOpCount</code> limit to prevent abusive use. This limitation is not applied to undefined <code>MAST Version</code> for flexibility, but it is constrained by the 255 <code>Subscript</code> and 32 <code>Depth</code> limits.
=== Script evaluation ===
This proposal requires script evaluation resulting in an empty stack, instead of a single <code>TRUE</code> value as in P2WSH. This allows each party in a contract to provide its own <code>Subscript</code>, and demonstrate the required <code>Input Stack</code> to clean up its own <code>Subscript</code>. In this case, order of the <code>Subscript</code> is not important since the overall objective is to clean up the stack after evaluation.
== Examples == == Examples ==
=== Calculation of MAST Root === === Calculation of MAST Root ===
To calculate the MAST Root for 4 branches, the double-SHA256 of each branch is first calculated:
<1> EQUAL (0x5187), HASH256=3b647cb856a965fa6feffb4621eb2a4f4c2453693b2f64e021383ccbd80a1abb
<2> EQUAL (0x5287), HASH256=37772654bdce9b3d59e1169ea16ddbaa8a2ae8ee265db64863d0b76f02c882fa
<3> EQUAL (0x5387), HASH256=2e972642436151cd96e4b0868077b6362ffb5eb30b420a6f1c5e1c6fff02bc33
<4> EQUAL (0x5487), HASH256=4c954fc1e635ce8417341465f85b59d700806f6e57bb96b2a25bec5ca3f9f154
Serialize the hashes of the first pair and calculate the hash. Same for the other pair: <img src=bip-0114/mastexample.png></img>
HASH256(HASH256(5187)|HASH256(5287)) = 64fbdfc0a82ecc3b33434bfea63440e9a5275fa5e533200d2eaf18281e8b28b6
HASH256(HASH256(5387)|HASH256(5487)) = aeadea837d5e640a1444208f7aca3be63bc8ab3c6b28a19878a00cc9c631ac31
Subscript:
SA = 1 EQUALVERIFY (0x5188)
SB = 2 EQUALVERIFY (0x5288)
SC = 3 EQUALVERIFY (0x5388)
SD = 4 EQUALVERIFY (0x5488)
SE = 5 EQUALVERIFY (0x5588)
SF = 6 EQUALVERIFY (0x5688)
SG = 7 EQUALVERIFY (0x5788)
SH = 8 EQUALVERIFY (0x5888)
M = RETURN "Hello" (0x6a0548656c6c6f)
Hash:
HA = H(0x01|H(SA)) = H(0x015acb54166e0db370cd1b05a29120373568dacea2abc3748459ec3da2106e4b4e) = 0xd385d7268ad7e1ec51660f833d54787d2d8d79b6b1809d9c1d06c9e71f7be204
HB = H(0x02|H(SB)|H(SC)) = 0x7cbfa08e44ea9f4f996873be95d9bffd97d4b91a5af32cc5f64efb8461727cdd
HF = H(0x03|H(SD)|H(SE)|H(SF)) = 0x4611414355945a7c2fcc62a53a0004821b87e68f93048ffba7a55a3cb1e9783b
HG = H(0x01|H(SG)) = 0xaa5fbdf58264650eadec33691ba1e7606d0a62f570eea348a465c55bc86ffc10
HC = H(0x01|H(M)) = 0x70426d480d5b28d93c5be54803681f99abf4e8df4eab4dc87aaa543f0d138159
HD = H(0x0x|H(SH)) = 0x8482f6c9c3fe90dd4d533b4efedb6a241b95ec9267d1bd5aaaee36d2ce2dd6da
HE = H(HA|HB) = 0x049b9f2f94f0a9bdea624e39cd7d6b27a365c6a0545bf0e9d88d86eff4894210
HH = H(HC|HD) = 0xc709fdc632f370f3367da45378d1cf430c5fda6805e731ad5761c213cf2d276e
HI = H(HE|HF) = 0xead5e1a1e7e41b77b794f091df9be3f0e9f41d47304eb43dece90688f69843b7
HJ = H(HG|HH) = 0xd00fc690c4700d0f983f9700740066531ea826b21a4cbc62f80317261723d477
Script Root = H(HI|HJ) = 0x26d5235d20daf1440a15a248f5b5b4f201392128072c55afa64a26ccc6f56bd9
MAST Root = H(MAST Version|Script Root) = H(0x0000000026d5235d20daf1440a15a248f5b5b4f201392128072c55afa64a26ccc6f56bd9) = 0xb4b706e0c02eab9aba58419eb7ea2a286fb1c01d7406105fc12742bf8a3f97c9
Serialize the 2 hashes from the previous step and calculate the hash, which is the <code>MAST Root</code>:
MAST Root = 6746003b5c9d342b2c210d406802c351e7eb5943412dcfc4718be625a8a59c0e
The scriptPubKey with native witness program is: The scriptPubKey with native witness program is:
<1> <0x6746003b5c9d342b2c210d406802c351e7eb5943412dcfc4718be625a8a59c0e>
(0x51206746003b5c9d342b2c210d406802c351e7eb5943412dcfc4718be625a8a59c0e)
To redeem with the <code><4> EQUAL</code> branch, the witness is 1 <0xb4b706e0c02eab9aba58419eb7ea2a286fb1c01d7406105fc12742bf8a3f97c9>
04 (Stack for evaluation) (0x5120b4b706e0c02eab9aba58419eb7ea2a286fb1c01d7406105fc12742bf8a3f97c9)
03 (Position)
2e972642436151cd96e4b0868077b6362ffb5eb30b420a6f1c5e1c6fff02bc3364fbdfc0a82ecc3b33434bfea63440e9a5275fa5e533200d2eaf18281e8b28b6 (Path)
5487 (MAST Script) To redeem with the <code>SD|SE|SF</code> branch, the witness is
Script_stack_1: 0x06
Script_stack_2: 0x05
Script_stack_3: 0x04
Subscript_1: 0x5488
Subscript_2: 0x5588
Subscript_3: 0x5688
Position: 0x01 (HF is the second hash in its level)
Path (HE|HJ): 0x049b9f2f94f0a9bdea624e39cd7d6b27a365c6a0545bf0e9d88d86eff4894210d00fc690c4700d0f983f9700740066531ea826b21a4cbc62f80317261723d477
Metadata: 0x03 (3 Subscript)
=== Imbalance MAST === === Imbalance MAST ===
When constructing a MAST, if the user believes that some of the branches are more likely to be executed, they may put them closer to the <code>MAST Root</code>. It will save some witness space when the preferred branches are actually executed. When constructing a MAST, if the user believes that some of the branches are more likely to be executed, they may put them closer to the <code>Script Root</code>. It will save some witness space when the preferred branches are actually executed.
=== Escrow with Timeout === === Escrow with Timeout ===
The following is the "Escrow with Timeout" example in [[bip-0112.mediawiki|BIP112]]: The following is the "Escrow with Timeout" example in [[bip-0112.mediawiki|BIP112]]:
IF IF
2 <Alice's pubkey> <Bob's pubkey> <Escrow's pubkey> 3 CHECKMULTISIGVERIFY 2 <Alice's pubkey> <Bob's pubkey> <Escrow's pubkey> 3 CHECKMULTISIG
ELSE ELSE
"30d" CHECKSEQUENCEVERIFY DROP "30d" CHECKSEQUENCEVERIFY DROP
<Alice's pubkey> CHECKSIGVERIFY <Alice's pubkey> CHECKSIG
ENDIF ENDIF
Using compressed public key, the size of this script is 150 bytes. Using compressed public key, the size of this script is 150 bytes.
With MAST, this script could be broken down into 2 mutually exclusive branches: With MAST, this script could be broken down into 2 mutually exclusive branches:<ref>In BIPXXX, it is proposed that CHECKLOCKTIMEVERIFY and CHECKSEQUENCEVERIFY will pop the top stack item</ref>
2 <Alice's pubkey> <Bob's pubkey> <Escrow's pubkey> 3 CHECKMULTISIGVERIFY (105 bytes) 2 <Alice's pubkey> <Bob's pubkey> <Escrow's pubkey> 3 CHECKMULTISIGVERIFY (105 bytes)
"30d" CHECKSEQUENCEVERIFY DROP <Alice's pubkey> CHECKSIGVERIFY (42 bytes) "30d" CHECKSEQUENCEVERIFY <Alice's pubkey> CHECKSIGVERIFY (42 bytes)
With 2 branches, the <code>Path</code> will be 32 bytes (as the hash of the unexecuted branch), and the <code>Position</code> will be 1 byte as either 0 or 1. Since only one branch will be published, it is more difficult for a blockchain analyst to determine the details of the escrow. Since only one branch will be published, it is more difficult for a blockchain analyst to determine the details of the escrow.
=== Hashed Time-Lock Contract === === Hashed Time-Lock Contract ===
The following is the "Hashed TIme-Lock Contract" example in [[bip-0112.mediawiki|BIP112]]: The following is the "Hashed TIme-Lock Contract" example in [[bip-0112.mediawiki|BIP112]]:
@ -107,16 +208,16 @@ The following is the "Hashed TIme-Lock Contract" example in [[bip-0112.mediawiki
CHECKSIG CHECKSIG
To create a MAST Root, it is flattened to 3 mutually exclusive branches: To create a MAST Root, it is flattened to 3 mutually exclusive branches:
HASH160 <R-HASH> EQUALVERIFY "24h" CHECKSEQUENCEVERIFY DROP <Alice's pubkey> CHECKSIG HASH160 <R-HASH> EQUALVERIFY "24h" CHECKSEQUENCEVERIFY <Alice's pubkey> CHECKSIGVERIFY
HASH160 <Commit-Revocation-Hash> EQUALVERIFY <Bob's pubkey> CHECKSIG HASH160 <Commit-Revocation-Hash> EQUALVERIFY <Bob's pubkey> CHECKSIGVERIFY
"Timestamp" CHECKLOCKTIMEVERIFY DROP <Bob's pubkey> CHECKSIG "Timestamp" CHECKLOCKTIMEVERIFY <Bob's pubkey> CHECKSIGVERIFY
which significantly improves readability and reduces the witness size when it is redeemed. which significantly improves readability and reduces the witness size when it is redeemed.
=== Large multi-signature constructs === === Large multi-signature constructs ===
The current CHECKMULTISIG supports up to 20 public keys. Although it is possible to extend it beyond 20 keys by using multiple CHECKSIGs and IF/ELSE conditions, the construction could be very complicated and soon use up the 10,000 bytes and 201 op codes limit. The current CHECKMULTISIG supports up to 20 public keys. Although it is possible to extend it beyond 20 keys by using multiple CHECKSIGs and IF/ELSE conditions, the construction could be very complicated and soon use up the 10,000 bytes and 201 <code>nOpCount</code> limit.
With MAST, large and complex multi-signature constructs could be flattened to many simple CHECKMULTISIG conditions. For example, a 3-of-2000 multi-signature scheme could be expressed as 1,331,334,000 3-of-3 CHECKMULTISIGs, which forms a 31-level MAST. The scriptPubKey still maintains a fixed size of 34 bytes, and the redemption witness will be very compact, with less than 1,500 bytes. With MAST, large and complex multi-signature constructs could be flattened to many simple CHECKMULTISIGVERIFY conditions. For example, a 3-of-2000 multi-signature scheme could be expressed as 1,331,334,000 3-of-3 CHECKMULTISIGVERIFY, which forms a 31-level MAST. The scriptPubKey still maintains a fixed size of 34 bytes, and the redemption witness will be very compact, with less than 1,500 bytes.
=== Commitment of non-consensus enforced data === === Commitment of non-consensus enforced data ===
Currently, committing non-consensus enforced data in the scriptPubKey requires the use of OP_RETURN which occupies additional block space. With MAST, users may commit such data as a branch. Depends on the number of executable branches, inclusion of such a commitment may incur no extra witness space, or 32 bytes at most. Currently, committing non-consensus enforced data in the scriptPubKey requires the use of OP_RETURN which occupies additional block space. With MAST, users may commit such data as a branch. Depends on the number of executable branches, inclusion of such a commitment may incur no extra witness space, or 32 bytes at most.
@ -133,76 +234,121 @@ This BIP depends on [[bip-0141.mediawiki|BIP141]] and will be deployed by versio
The idea of MAST originates from Russell O’Connor, Pieter Wuille, and [https://bitcointalk.org/index.php?topic=255145.msg2757327#msg2757327 Peter Todd]. The idea of MAST originates from Russell O’Connor, Pieter Wuille, and [https://bitcointalk.org/index.php?topic=255145.msg2757327#msg2757327 Peter Todd].
== Reference Implementation == == Reference Implementation ==
https://github.com/jl2012/bitcoin/tree/segwit_mast https://github.com/jl2012/bitcoin/tree/bip114v2 (WIP)
<source lang="cpp"> <source lang="cpp">
//New rules apply if version byte is 1 and witness program size is 32 bytes //New rules apply if version byte is 1 and witness program size is 32 bytes
if (witversion == 1) { if (witversion == 1 && program.size() == 32 && (flags & SCRIPT_VERIFY_MAST)) {
if (program.size() == 32) { CHashWriter sRoot(SER_GETHASH, 0);
CHashWriter sScriptHash(SER_GETHASH, 0);
uint32_t nMASTVersion = 0;
size_t stacksize = witness.stack.size();
if (stacksize < 4)
return set_error(serror, SCRIPT_ERR_INVALID_MAST_STACK);
std::vector<unsigned char> metadata = witness.stack.back(); // The last witness stack item is metadata
if (metadata.size() < 1 || metadata.size() > 5)
return set_error(serror, SCRIPT_ERR_INVALID_MAST_STACK);
//Witness stack must have at least 3 items // The first byte of metadata is the number of subscripts (1 to 255)
if (witness.stack.size() < 3) uint32_t nSubscript = static_cast<uint32_t>(metadata[0]);
return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH); if (nSubscript == 0 || stacksize < nSubscript + 3)
return set_error(serror, SCRIPT_ERR_INVALID_MAST_STACK);
int nOpCount = nSubscript; // Each condition consumes a nOpCount
sScriptHash << metadata[0];
//Script is the last witness stack item // The rest of metadata is MAST version in minimally-coded unsigned little endian int
scriptPubKey = CScript(witness.stack.back().begin(), witness.stack.back().end()); if (metadata.back() == 0)
uint256 hashScriptPubKey; return set_error(serror, SCRIPT_ERR_INVALID_MAST_STACK);
CHash256().Write(&scriptPubKey[0], scriptPubKey.size()).Finalize(hashScriptPubKey.begin()); if (metadata.size() > 1) {
for (size_t i = 1; i != metadata.size(); ++i)
//Path is the second last witness stack item nMASTVersion |= static_cast<uint32_t>(metadata[i]) << 8 * (i - 1);
std::vector<unsigned char> pathdata = witness.stack.at(witness.stack.size() - 2);
// Size of Path must be a multiple of 32 bytes (0 byte is allowed)
if (pathdata.size() & 0x1F)
return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
// Depth of the tree is size of Path divided by 32
unsigned int depth = pathdata.size() >> 5;
// Maximum allowed depth is 32
if (depth > 32)
return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
std::vector<uint256> path;
path.resize(depth);
for (unsigned int i = 0; i < depth; i++)
memcpy(path[i].begin(), &pathdata[32 * i], 32);
//Position is the third last witness stack item
std::vector<unsigned char> positiondata = witness.stack.at(witness.stack.size() - 3);
//Position may have 4 bytes at most
if (positiondata.size() > 4)
return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
uint32_t position = 0;
//Position is an unsigned little-endian integer with no leading zero byte
if (positiondata.size() > 0) {
if (positiondata.back() == 0x00)
return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
for (size_t i = 0; i != positiondata.size(); ++i)
position |= static_cast<uint32_t>(positiondata[i]) << 8 * i;
}
//Position must not be larger than the maximum number of items allowed by the depth of tree
if (depth < 32) {
if (position >= (1U << depth))
return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
}
//Calculate the Merkle Root and compare with the witness program
uint256 root = ComputeMerkleRootFromBranch(hashScriptPubKey, path, position);
if (memcmp(root.begin(), &program[0], 32))
return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
//Remaining stack items used for evaluation
stack = std::vector<std::vector<unsigned char> >(witness.stack.begin(), witness.stack.end() - 3);
} }
else { // Unknown MAST version is non-standard
//Invalid if version byte is 1 but witness program size is not 32 bytes if (nMASTVersion > 0 && flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM)
return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_WRONG_LENGTH); return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM);
sRoot << nMASTVersion;
// The second last witness stack item is the pathdata
// Size of pathdata must be divisible by 32 (0 is allowed)
// Depth of the Merkle tree is implied by the size of pathdata, and must not be greater than 32
std::vector<unsigned char> pathdata = witness.stack.at(stacksize - 2);
if (pathdata.size() & 0x1F)
return set_error(serror, SCRIPT_ERR_INVALID_MAST_STACK);
unsigned int depth = pathdata.size() >> 5;
if (depth > 32)
return set_error(serror, SCRIPT_ERR_INVALID_MAST_STACK);
// Each level of Merkle tree consumes a nOpCount
// Evaluation of version 0 MAST terminates early if there are too many nOpCount
// Not enforced in unknown MAST version for upgrade flexibility
nOpCount = nOpCount + depth;
if (nMASTVersion == 0 && nOpCount > MAX_OPS_PER_SCRIPT)
return set_error(serror, SCRIPT_ERR_OP_COUNT);
// path is a vector of 32-byte hashes
std::vector <uint256> path;
path.resize(depth);
for (unsigned int j = 0; j < depth; j++)
memcpy(path[j].begin(), &pathdata[32 * j], 32);
// The third last witness stack item is the positiondata
// Position is in minimally-coded unsigned little endian int
std::vector<unsigned char> positiondata = witness.stack.at(stacksize - 3);
if (positiondata.size() > 4)
return set_error(serror, SCRIPT_ERR_INVALID_MAST_STACK);
uint32_t position = 0;
if (positiondata.size() > 0) {
if (positiondata.back() == 0)
return set_error(serror, SCRIPT_ERR_INVALID_MAST_STACK);
for (size_t k = 0; k != positiondata.size(); ++k)
position |= static_cast<uint32_t>(positiondata[k]) << 8 * k;
} }
// Position value must not exceed the number of leaves at the depth
if (depth < 32) {
if (position >= (1U << depth))
return set_error(serror, SCRIPT_ERR_INVALID_MAST_STACK);
}
// Sub-scripts are located before positiondata
for (size_t i = stacksize - nSubscript - 3; i <= stacksize - 4; i++) {
CScript subscript(witness.stack.at(i).begin(), witness.stack.at(i).end());
// Evaluation of version 0 MAST terminates early if script is oversize
// Not enforced in unknown MAST version for upgrade flexibility
if (nMASTVersion == 0 && (scriptPubKey.size() + subscript.size()) > MAX_SCRIPT_SIZE)
return set_error(serror, SCRIPT_ERR_SCRIPT_SIZE);
uint256 hashSubScript;
CHash256().Write(&subscript[0], subscript.size()).Finalize(hashSubScript.begin());
sScriptHash << hashSubScript;
scriptPubKey = scriptPubKey + subscript; // Final scriptPubKey is a serialization of subscripts
}
uint256 hashScript = sScriptHash.GetHash();
// Calculate MAST Root and compare against witness program
uint256 rootScript = ComputeMerkleRootFromBranch(hashScript, path, position);
sRoot << rootScript;
uint256 rootMAST = sRoot.GetHash();
if (memcmp(rootMAST.begin(), &program[0], 32))
return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
if (nMASTVersion == 0) {
stack = std::vector<std::vector<unsigned char> >(witness.stack.begin(), witness.stack.end() - 3 - nSubscript);
for (unsigned int i = 0; i < stack.size(); i++) {
if (stack.at(i).size() > MAX_SCRIPT_ELEMENT_SIZE)
return set_error(serror, SCRIPT_ERR_PUSH_SIZE);
}
// Script evaluation must not fail, and return an empty stack
if (!EvalScript(stack, scriptPubKey, flags, checker, SIGVERSION_WITNESS_V1, nOpCount, serror))
return false;
if (stack.size() != 0)
return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
}
return set_success(serror);
} }
</source> </source>

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