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| -rw-r--r-- | bip-taproot.mediawiki | 52 | ||||
| -rw-r--r-- | bip-tapscript.mediawiki | 18 |
2 files changed, 37 insertions, 33 deletions
diff --git a/bip-taproot.mediawiki b/bip-taproot.mediawiki index f100e6e..a553791 100644 --- a/bip-taproot.mediawiki +++ b/bip-taproot.mediawiki @@ -62,24 +62,23 @@ In the text below, ''hash<sub>tag</sub>(m)'' is a shorthand for ''SHA256(SHA256( === Script validation rules === -A Taproot output is a SegWit output (native or P2SH-nested, see [https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki BIP141]) with version number 1, and a 33-byte witness program whose first byte is 0 or 1. -The following rules only apply when such an output is being spent. Any other outputs, including version 1 outputs with lengths other than 33 bytes, or with a first byte different from 0 or 1, remain unencumbered. +A Taproot output is a SegWit output (native or P2SH-nested, see [https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki BIP141]) with version number 1, and a 32-byte witness program. +The following rules only apply when such an output is being spent. Any other outputs, including version 1 outputs with lengths other than 32 bytes, remain unencumbered. -* Let ''u'' be the 33-byte array containing the witness program (second push in scriptPubKey or P2SH redeemScript). -* Let ''Q = point(byte(2 + u[0]) || u[1:33])''<ref>'''Why is the public key directly included in the output?''' While typical earlier constructions store a hash of a script or a public key in the output, this is rather wasteful when a public key is always involved. To guarantee batch verifiability, ''Q'' must be known to every verifier, and thus only revealing its hash as an output would imply adding an additional 33 bytes to the witness. Furthermore, to maintain [https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-January/012198.html 128-bit collision security] for outputs, a 256-bit hash would be required anyway, which is comparable in size (and thus in cost for senders) to revealing the public key directly. While the usage of public key hashes is often said to protect against ECDLP breaks or quantum computers, this protection is very weak at best: transactions are not protected while being confirmed, and a very [https://twitter.com/pwuille/status/1108097835365339136 large portion] of the currency's supply is not under such protection regardless. Actual resistance to such systems can be introduced by relying on different cryptographic assumptions, but this proposal focuses on improvements that do not change the security model. Note that using P2SH-wrapped outputs only have 80-bit collision security. This is considered low, and is relevant whenever the output includes data from more than a single party (public keys, hashes, ...). </ref> If this is not a valid point on the curve, fail. +* Let ''q'' be the 32-byte array containing the witness program (second push in scriptPubKey or P2SH redeemScript) which represents a public key according to bip-schnorr.<ref>'''Why is the public key directly included in the output?''' While typical earlier constructions store a hash of a script or a public key in the output, this is rather wasteful when a public key is always involved. To guarantee batch verifiability, ''q'' must be known to every verifier, and thus only revealing its hash as an output would imply adding an additional 32 bytes to the witness. Furthermore, to maintain [https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-January/012198.html 128-bit collision security] for outputs, a 256-bit hash would be required anyway, which is comparable in size (and thus in cost for senders) to revealing the public key directly. While the usage of public key hashes is often said to protect against ECDLP breaks or quantum computers, this protection is very weak at best: transactions are not protected while being confirmed, and a very [https://twitter.com/pwuille/status/1108097835365339136 large portion] of the currency's supply is not under such protection regardless. Actual resistance to such systems can be introduced by relying on different cryptographic assumptions, but this proposal focuses on improvements that do not change the security model. Note that using P2SH-wrapped outputs only have 80-bit collision security. This is considered low, and is relevant whenever the output includes data from more than a single party (public keys, hashes, ...). </ref>. * Fail if the witness stack has 0 elements. -* If there are at least two witness elements, and the first byte of the last element is 0x50<ref>'''Why is the first byte of the annex <code>0x50</code>?''' Like the <code>0xc0</code>-<code>0xc1</code> constants, <code>0x50</code> is chosen as it could not be confused with a valid P2WPKH or P2WSH spending. As the control block's initial byte's lowest bit is used to indicate the public key's Y oddness, each script version needs two subsequence byte values that are both not yet used in P2WPKH or P2WSH spending. To indicate the annex, only an "unpaired" available byte is necessary like <code>0x50</code>. This choice maximizes the available options for future script versions.</ref>, this last element is called ''annex'' ''a''<ref>'''What is the purpose of the annex?''' The annex is a reserved space for future extensions, such as indicating the validation costs of computationally expensive new opcodes in a way that is recognizable without knowing the outputs being spent. Until the meaning of this field is defined by another softfork, users SHOULD NOT include <code>annex</code> in transactions, or it may lead to PERMANENT FUND LOSS.</ref> and is removed from the witness stack. The annex (or the lack of thereof) is always covered by the transaction digest and contributes to transaction weight, but is otherwise ignored during taproot validation. +* If there are at least two witness elements, and the first byte of the last element is 0x50<ref>'''Why is the first byte of the annex <code>0x50</code>?''' Like the <code>0xc0</code>-<code>0xc1</code> constants, <code>0x50</code> is chosen as it could not be confused with a valid P2WPKH or P2WSH spending. As the control block's initial byte's lowest bit is used to indicate the public key's Y quadratic residuosity, each script version needs two subsequence byte values that are both not yet used in P2WPKH or P2WSH spending. To indicate the annex, only an "unpaired" available byte is necessary like <code>0x50</code>. This choice maximizes the available options for future script versions.</ref>, this last element is called ''annex'' ''a''<ref>'''What is the purpose of the annex?''' The annex is a reserved space for future extensions, such as indicating the validation costs of computationally expensive new opcodes in a way that is recognizable without knowing the outputs being spent. Until the meaning of this field is defined by another softfork, users SHOULD NOT include <code>annex</code> in transactions, or it may lead to PERMANENT FUND LOSS.</ref> and is removed from the witness stack. The annex (or the lack of thereof) is always covered by the transaction digest and contributes to transaction weight, but is otherwise ignored during taproot validation. * If there is exactly one element left in the witness stack, key path spending is used: -** The single witness stack element is interpreted as the signature and must be valid (see the next section) for the public key ''Q'' and taproot transaction digest (to be defined hereinafter) as message. Fail if it is not. Otherwise pass. +** The single witness stack element is interpreted as the signature and must be valid (see the next section) for the public key ''q'' and taproot transaction digest (to be defined hereinafter) as message. Fail if it is not. Otherwise pass. * If there are at least two witness elements left, script path spending is used: ** Call the second-to-last stack element ''s'', the script. ** The last stack element is called the control block ''c'', and must have length ''33 + 32m'', for a value of ''m'' that is an integer between 0 and 32, inclusive. Fail if it does not have such a length. -** Let ''P = point(byte(2 + (c[0] & 1)) || c[1:33])''<ref>'''What is the purpose of the first byte of the control block?''' The first byte of the control block has three distinct functions: -* The low bit is used to denote the oddness of the Y coordinate of the ''P'' point. +** Let ''P = point(c[1:33])'' where ''point'' is defined as in bip-schnorr. Fail if this point is not on the curve. +** Let ''l = c[0] & 0xfe'', the leaf version<ref>'''What is the purpose of the first byte of the control block?''' The first byte of the control block has three distinct functions: +* The low bit is used to denote whether the ''Q'' point's Y coordinate is a quadratic residue.<ref>'''Why is the quadratic residuosity of the output public key's Y coordinate required in a script path spend?''' The ''point'' function always constructs a point with Y coordinate having that property, but because ''Q'' is constructed by adding the taproot tweak to the internal public key ''P'', it cannot easily be guaranteed that ''Q'' in fact has such a Y coordinate. We can not ignore the Y coordinate because it would prevent batch verification. Trying out multiple internal keys until there's such a ''Q'' is possible but undesirable and unnecessary since this information about the Y coordinate only consumes an unused bit.</ref> * By keeping the top two bits set to true, it can be guaranteed that scripts can be recognized without knowledge of the UTXO being spent, simplifying analysis. This is because such values cannot occur as first byte of the final stack element in either P2WPKH or P2WSH spends. * The remaining five bits are used for introducing new script versions that are not observable unless actually executed. -</ref>. Fail if this point is not on the curve. -** Let ''l = c[0] & 0xfe'', the leaf version. +</ref>. ** Let ''k<sub>0</sub> = hash<sub>TapLeaf</sub>(l || compact_size(size of s) || s)''; also call it the ''tapleaf hash''. ** For ''j'' in ''[0,1,...,m-1]'': *** Let ''e<sub>j</sub> = c[33+32j:65+32j]''. @@ -88,10 +87,11 @@ The following rules only apply when such an output is being spent. Any other out **** If ''k<sub>j</sub> ≥ e<sub>j</sub>'': ''k<sub>j+1</sub> = hash<sub>TapBranch</sub>(e<sub>j</sub> || k<sub>j</sub>)''. ** Let ''t = hash<sub>TapTweak</sub>(bytes(P) || k<sub>m</sub>) = hash<sub>TapTweak</sub>(2 + (c[0] & 1) || c[1:33] || k<sub>m</sub>)''. ** If ''t ≥ 0xFFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFE BAAEDCE6 AF48A03B BFD25E8C D0364141'' (order of secp256k1), fail. +** Let ''Q = point(q) if (c[0] & 1) = 1 and -point(q) otherwise'' ** If ''Q ≠ P + int(t)G'', fail. ** Execute the script, according to the applicable script rules<ref>'''What are the applicable script rules in script path spends?''' Bip-tapscript specifies validity rules that apply if the leaf version is ''0xc0'', but future proposals can introduce rules for other leaf versions.</ref>, using the witness stack elements excluding the script ''s'', the control block ''c'', and the annex ''a'' if present, as initial stack. -''Q'' is referred to as ''taproot output key'' and ''P'' as ''taproot internal key''. +''q'' is referred to as ''taproot output key'' and ''c[1:33]'' as ''taproot internal key''. === Signature validation rules === @@ -137,7 +137,7 @@ As the message for signature verification, transaction digest is ''hash<sub>TapS *** Bit-0 is set if the <code>scriptPubKey</code> being spent is P2SH (opposed to "native segwit"). *** Bit-1 is set if an annex is present (the original witness stack has two or more witness elements, and the first byte of the last element is <code>0x50</code>). *** The other bits are unset. -** <code>scriptPubKey</code> (24 or 36): <code>scriptPubKey</code> of the previous output spent by this input, serialized as script inside <code>CTxOut</code>. The size is 24-byte for P2SH-embedded segwit, or 36-byte for native segwit. +** <code>scriptPubKey</code> (24 or 35): <code>scriptPubKey</code> of the previous output spent by this input, serialized as script inside <code>CTxOut</code>. The size is 24-byte for P2SH-embedded segwit, or 35-byte for native segwit. ** If the <code>SIGHASH_ANYONECANPAY</code> flag is set: *** <code>outpoint</code> (36): the <code>COutPoint</code> of this input (32-byte hash + 4-byte little-endian). *** <code>amount</code> (8): value of the previous output spent by this input. @@ -150,7 +150,7 @@ As the message for signature verification, transaction digest is ''hash<sub>TapS ** If the <code>SIGHASH_SINGLE</code> flag is set: *** <code>sha_single_output</code> (32): the SHA256 of the corresponding output in <code>CTxOut</code> format. -The total number of bytes hashed is at most ''209''<ref>'''What is the number of bytes hashed for the signature hash?''' The total size of the input to ''hash<sub>TapSighash</sub>'' (excluding the initial 64-byte hash tag) can be computed using the following formula: ''177 - is_anyonecanpay * 50 - is_none * 32 - is_p2sh_spending * 12 + has_annex * 32''.</ref>. +The total number of bytes hashed is at most ''209''<ref>'''What is the number of bytes hashed for the signature hash?''' The total size of the input to ''hash<sub>TapSighash</sub>'' (excluding the initial 64-byte hash tag) can be computed using the following formula: ''176 - is_anyonecanpay * 50 - is_none * 32 - is_p2sh_spending * 11 + has_annex * 32''.</ref>. In summary, the semantics of the BIP143 sighash types remain unchanged, except the following: # The way and order of serialization is changed.<ref>'''Why is the serialization in the transaction digest changed?''' Hashes that go into the digest and the digest itself are now computed with a single SHA256 invocation instead of double SHA256. There is no expected security improvement by doubling SHA256 because this only protects against length-extension attacks against SHA256 which are not a concern for transaction digests because there is no secret data. Therefore doubling SHA256 is a waste of resources. The digest computation now follows a logical order with transaction level data first, then input data and output data. This allows to efficiently cache the transaction part of the digest across different inputs using the SHA256 midstate. Additionally, digest computation avoids unnecessary hashing as opposed to BIP143 digests in which parts may be set zero and before hashing them. Despite that, collisions are made impossible by committing to the length of the data (implicit in <code>hash_type</code> and <code>spend_type</code>) before the variable length data.</ref> @@ -165,7 +165,7 @@ This section discusses how to construct and spend Taproot outputs. It only affec and is not consensus critical in any way. Conceptually, every Taproot output corresponds to a combination of a single public key condition (the internal key), and zero or more general conditions encoded in scripts organized in a tree. -Satisfying any of these conditions is sufficient to spend the output. +Satisfying any of these conditions is sufficient to spend the output. '''Initial steps''' The first step is determining what the internal key and the organization of the rest of the scripts should be. The specifics are likely application dependent, but here are some general guidelines: * When deciding between scripts with conditionals (<code>OP_IF</code> etc.) and splitting them up into multiple scripts (each corresponding to one execution path through the original script), it is generally preferable to pick the latter. @@ -173,7 +173,7 @@ Satisfying any of these conditions is sufficient to spend the output. * 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. If no such condition exists, it may be worthwhile adding one that consists of an aggregation of all keys participating in all scripts combined; effectively adding an "everyone agrees" branch. If that is inacceptable, pick as internal key a point with unknown discrete logarithm (TODO). * The remaining scripts should be organized into the leaves of a binary tree. This can be a balanced tree if each of the conditions these scripts correspond to are equally likely. If probabilities for each condition are known, consider constructing the tree as a Huffman tree. -'''Computing the output script''' Once the spending conditions are split into an internal key <code>internal_pubkey</code> and a binary tree whose leaves are (leaf_version, script) tuples, the following Python3 algorithm can be used to compute the output script. In the code below, <code>ser_script</code> prefixes its input with a CCompactSize-encoded length, and public key objects have methods <code>get_bytes</code> to get their compressed encoding (see bip-schnorr) and <code>tweak_add</code> to add a multiple of the secp256k1 generator to it (similar to BIP32's derivation). +'''Computing the output script''' Once the spending conditions are split into an internal key <code>internal_pubkey</code> and a binary tree whose leaves are (leaf_version, script) tuples, the following Python3 algorithm can be used to compute the output script. In the code below, <code>ser_script</code> prefixes its input with a CCompactSize-encoded length. Public key objects hold 32-byte public keys according to bip-schnorr, have a method <code>get_bytes</code> to get the byte array and a method <code>tweak_add</code> which returns a new public key corresponding to the sum of the public key point and a multiple of the secp256k1 generator (similar to BIP32's derivation). The second return value of <code>tweak_add</code> is a boolean indicating the quadratic residuosity of the Y coordinate of the resulting point. <source lang="python"> import hashlib @@ -202,22 +202,26 @@ def taproot_output_script(internal_pubkey, script_tree): _, h = taproot_tree_helper(script_tree) t = tagged_hash("TapTweak", internal_pubkey.get_bytes() + h) assert int.from_bytes(t, 'big') < SECP256K1_ORDER - output_pubkey = internal_pubkey.tweak_add(t).get_bytes() - return bytes([0x51, 0x21, output_pubkey[0] & 1]) + output_pubkey[1:] + output_pubkey, _ = internal_pubkey.tweak_add(t) + return bytes([0x51, 0x20]) + output_pubkey.get_bytes() </source> The function <code>taproot_output_script</code> returns a byte array with the scriptPubKey. It can be P2SH wrapped if desired (see BIP141). [[File:bip-taproot/tree.png|frame|This diagram shows the hashing structure to obtain the tweak from an internal key ''P'' and a Merkle tree consisting of 5 script leaves. ''A'', ''B'', ''C'' and ''E'' are ''TapLeaf'' hashes similar to ''D'' and ''AB'' is a ''TapBranch'' hash. Note that when ''CDE'' is computed ''E'' is hashed first because ''E'' is less than ''CD''.]] -'''Spending using the internal key''' A Taproot output can be spent with the private key corresponding to the <code>internal_pubkey</code>. To do so, a witness stack consisting of a single element, a bip-schnorr signature on the signature hash as defined above, with the private key tweaked by the same <code>t</code> in the above snippet. See the code below: +'''Spending using the internal key''' A Taproot output can be spent with the private key corresponding to the <code>internal_pubkey</code>. To do so, a witness stack consists of a single element: a bip-schnorr signature on the signature hash as defined above, with the private key tweaked by the same <code>t</code> in the above snippet. In the code below, <code>internal_privkey</code> has a method <code>pubkey_gen</code> that returns a public key according to bip-schnorr and a boolean indicating the quadratic residuosity of the Y coordinate of the underlying point. +See the code below: <source lang="python"> -def taproot_sign_internal_key(internal_pubkey, script_tree, internal_privkey, hash_type): +def taproot_sign_internal_key(script_tree, internal_privkey, hash_type): + internal_pubkey, is_y_qresidue = internal_privkey.pubkey_gen() + if is_y_qresidue: + internal_privkey = internal_privkey.negate() _, h = taproot_tree_helper(script_tree) t = tagged_hash("TapTweak", internal_pubkey.get_bytes() + h) output_privkey = internal_privkey.tweak_add(t) - sig = output_privkey.sign_schnorr(sighash(hash_type)) + sig = output_privkey.schnorr_sign(sighash(hash_type)) if hash_type != 0: sig += bytes([hash_type]) return [sig] @@ -229,10 +233,12 @@ This function returns the witness stack necessary, and assumes a <code>tweak_add <source lang="python"> def taproot_sign_script(internal_pubkey, script_tree, script_num, inputs): - info, _ = taproot_tree_helper(script_tree) + info, h = taproot_tree_helper(script_tree) (leaf_version, script), path = info[script_num] - pubkey_bytes = internal_pubkey.get_bytes() - pubkey_data = bytes([(pubkey_bytes[0] & 1) + leaf_version]) + pubkey_bytes[1:] + t = tagged_hash("TapTweak", internal_pubkey.get_bytes() + h) + _, is_y_qresidue = internal_pubkey.tweak_add(t) + output_pubkey_tag = 0 if is_y_qresidue else 1 + pubkey_data = bytes([output_pubkey_tag + leaf_version]) + internal_pubkey.get_bytes() return inputs + [script, pubkey_data + path] </source> diff --git a/bip-tapscript.mediawiki b/bip-tapscript.mediawiki index ce42098..af46afd 100644 --- a/bip-tapscript.mediawiki +++ b/bip-tapscript.mediawiki @@ -45,7 +45,7 @@ Additionally, the new tapscript <code>OP_SUCCESS</code> opcodes allow introducin The rules below only apply when validating a transaction input for which all of the conditions below are true: * The transaction output is a '''segregated witness spend''' (i.e., either the scriptPubKey or BIP16 redeemScript is a witness program as defined in BIP141). -* It is a '''taproot spend''' as defined in bip-taproot (i.e., the witness version is 1, the witness program is 33 bytes, and the first of those is 0x00 or 0x01). +* It is a '''taproot spend''' as defined in bip-taproot (i.e., the witness version is 1, the witness program is 32 bytes). * It is a '''script path spend''' as defined in bip-taproot (i.e., after removing the optional annex from the witness stack, two or more stack elements remain). * The leaf version is ''0xc0'' (i.e. the first byte of the last witness element after removing the optional annex is ''0xc0'' or ''0xc1'')<ref>'''How is the ''0xc0'' constant chosen?''' Following the guidelines in bip-taproot, by choosing a value having the two top bits set, tapscript spends are identifiable even without access to the UTXO being spent.</ref>, marking it as a '''tapscript spend'''. @@ -71,7 +71,7 @@ The execution rules for tapscript are based on those for P2WSH according to BIP1 * '''Disabled script opcodes''' The following script opcodes are disabled in tapscript: <code>OP_CHECKMULTISIG</code> and <code>OP_CHECKMULTISIGVERIFY</code>. The disabled opcodes behave in the same way as <code>OP_RETURN</code>, by failing and terminating the script immediately when executed, and being ignored when found in unexecuted branch. While being ignored, they are still counted towards the 201 non-push opcodes limit. * '''Consensus-enforced MINIMALIF''' The MINIMALIF rules, which are only a standardness rule in P2WSH, are consensus enforced in tapscript. This means that the input argument to the <code>OP_IF</code> and <code>OP_NOTIF</code> opcodes must be either exactly 0 (the empty vector) or exactly 1 (the one-byte vector with value 1)<ref>'''Why make MINIMALIF consensus?''' This makes it considerably easier to write non-malleable scripts that take branch information from the stack.</ref>. * '''OP_SUCCESSx opcodes''' As listed above, some opcodes are renamed to <code>OP_SUCCESSx</code>, and make the script unconditionally valid. -* '''Signature opcodes'''. The <code>OP_CHECKSIG</code> and <code>OP_CHECKSIGVERIFY</code> are modified to operate on Schnorr signatures (see bip-schnorr) instead of ECDSA, and a new opcode <code>OP_CHECKSIGADD</code> is added. +* '''Signature opcodes'''. The <code>OP_CHECKSIG</code> and <code>OP_CHECKSIGVERIFY</code> are modified to operate on Schnorr public keys and signatures (see bip-schnorr) instead of ECDSA, and a new opcode <code>OP_CHECKSIGADD</code> is added. ** The opcode 186 (<code>0xba</code>) is named as <code>OP_CHECKSIGADD</code>. <ref>'''<code>OP_CHECKSIGADD</code>''' This opcode is added to compensate for the loss of <code>OP_CHECKMULTISIG</code>-like opcodes, which are incompatible with batch verification. <code>OP_CHECKSIGADD</code> is functionally equivalent to <code>OP_ROT OP_SWAP OP_CHECKSIG OP_ADD</code>, but is only counted as one opcode towards the 201 non-push opcodes limit. All <code>CScriptNum</code>-related behaviours of <code>OP_ADD</code> are also applicable to <code>OP_CHECKSIGADD</code>.</ref><ref>'''Comparison of <code>CHECKMULTISIG</code> and <code>CHECKSIG</code>''' A <code>CHECKMULTISIG</code> script <code>m <pubkey_1> ... <pubkey_n> n CHECKMULTISIG</code> with witness <code>0 <signature_1> ... <signature_m></code> can be rewritten as script <code><pubkey_1> CHECKSIG ... <pubkey_n> CHECKSIGADD m NUMEQUAL</code> with witness <code><w_1> ... <w_n></code>. Every witness element <code>w_i</code> is either a signature corresponding to the public key with the same index or an empty vector. A similar <code>CHECKMULTISIGVERIFY</code> script can be translated to bip-tapscript by replacing <code>NUMEQUAL</code> with <code>NUMEQUALVERIFY</code>. Alternatively, an m-of-n multisig policy can be implemented by splitting the script into several leaves of the Merkle tree, each implementing an m-of-m policy using <code><pubkey_1> CHECKSIGVERIFY ... <pubkey_(n-1)> CHECKSIGVERIFY <pubkey_n> CHECKSIG</code>. If the setting allows the participants to interactively collaborate while signing, multisig policies can be realized with [https://eprint.iacr.org/2018/068 MuSig] for m-of-m and with [http://cacr.uwaterloo.ca/techreports/2001/corr2001-13.ps threshold signatures] using verifiable secret sharing for m-of-n.</ref> ===Rules for signature opcodes=== @@ -84,11 +84,9 @@ The following rules apply to <code>OP_CHECKSIG</code>, <code>OP_CHECKSIGVERIFY</ ** If fewer than 3 elements are on the stack, the script MUST fail and terminate immediately. ** If <code>n</code> is larger than 4 bytes, the script MUST fail and terminate immediately. * If the public key size is zero, the script MUST fail and terminate immediately. -* If the first byte of the public key is <code>0x04</code>, <code>0x06</code>, or <code>0x07</code>, the script MUST fail and terminate immediately regardless of the public key size. -* If the first byte of the public key is <code>0x02</code> or <code>0x03</code>, it is considered to be a public key as described in bip-schnorr: -** If the public key is not 33 bytes, the script MUST fail and terminate immediately. +* If the public key size is 32 bytes, it is considered to be a public key as described in bip-schnorr: ** If the signature is not the empty vector, the signature is validated according to the bip-taproot signing validation rules against the public key and the tapscript transaction digest (to be defined hereinafter) as message. Validation failure MUST cause the script to fail and terminate immediately. -* If the first byte of the public key is not <code>0x02</code>, <code>0x03</code>, <code>0x04</code>, <code>0x06</code>, or <code>0x07</code>, the public key is of an ''unknown public key type''<ref>'''Unknown public key types''' allow adding new signature validation rules through softforks. A softfork could add actual signature validation which either passes or makes the script fail and terminate immediately. This way, new <code>SIGHASH</code> modes can be added, as well as [https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-December/016549.html NOINPUT-tagged public keys] and a public key constant which is replaced by the taproot internal key for signature validation.</ref> and no actual signature verification is applied. During script execution of signature opcodes they behave exactly as known public key types except that signature validation is considered to be successful. +* If the public key size is not zero and not 32 bytes, the public key is of an ''unknown public key type''<ref>'''Unknown public key types''' allow adding new signature validation rules through softforks. A softfork could add actual signature validation which either passes or makes the script fail and terminate immediately. This way, new <code>SIGHASH</code> modes can be added, as well as [https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-December/016549.html NOINPUT-tagged public keys] and a public key constant which is replaced by the taproot internal key for signature validation.</ref> and no actual signature verification is applied. During script execution of signature opcodes they behave exactly as known public key types except that signature validation is considered to be successful. * If the script did not fail and terminate before this step, regardless of the public key type: ** If the signature is the empty vector: *** For <code>OP_CHECKSIGVERIFY</code>, the script MUST fail and terminate immediately. @@ -113,14 +111,14 @@ The one-byte <code>spend_type</code> has a different value, specificially at bit As additional pieces of data, added at the end of the input to the ''hash<sub>TapSighash</sub>'' function: * <code>tapleaf_hash</code> (32): the tapleaf hash as defined in bip-taproot -* <code>key_version</code> (1): a constant value <code>0x02</code> representing the current version of public keys in the tapscript signature opcode execution. +* <code>key_version</code> (1): a constant value <code>0x00</code> representing the current version of public keys in the tapscript signature opcode execution. * <code>codeseparator_position</code> (2): the opcode position of the last executed <code>OP_CODESEPARATOR</code> before the currently executed signature opcode, with the value in little endian (or <code>0xffff</code> if none executed). The first opcode in a script has a position of 0. A multi-byte push opcode is counted as one opcode, regardless of the size of data being pushed. -The total number of bytes hashed is at most ''244''<ref>'''What is the number of bytes hashed for the signature hash?''' The total size of the input to ''hash<sub>TapSighash</sub>'' (excluding the initial 64-byte hash tag) can be computed using the following formula: ''212 - is_anyonecanpay * 50 - is_none * 32 - is_p2sh_spending * 12 + has_annex * 32''.</ref>. +The total number of bytes hashed is at most ''244''<ref>'''What is the number of bytes hashed for the signature hash?''' The total size of the input to ''hash<sub>TapSighash</sub>'' (excluding the initial 64-byte hash tag) can be computed using the following formula: ''211 - is_anyonecanpay * 50 - is_none * 32 - is_p2sh_spending * 11 + has_annex * 32''.</ref>. In summary, the semantics of the BIP143 sighash types remain unchanged, except the following: # The exceptions mentioned in bip-taproot. -# The digest commits to taproot-specific data <code>key_version</code>.<ref>'''Why does the transaction digest commit to the <code>key_version</code>?''' This is for future extensions that define unknown public key types, making sure signatures can't be moved from one key type to another. This value is intended to be set equal to the first byte of the public key, after masking out flags like the oddness of the Y coordinate.</ref> +# The digest commits to taproot-specific data <code>key_version</code>.<ref>'''Why does the transaction digest commit to the <code>key_version</code>?''' This is for future extensions that define unknown public key types, making sure signatures can't be moved from one key type to another.</ref> # The digest commits to the executed script through the <code>tapleaf_hash</code> which includes the leaf version and script instead of <code>scriptCode</code>. This implies that this commitment is unaffected by <code>OP_CODESEPARATOR</code>. # The digest commits to the opcode position of the last executed <code>OP_CODESEPARATOR</code>.<ref>'''Why does the transaction digest commit to the position of the last executed <code>OP_CODESEPARATOR</code>?''' This allows continuing to use <code>OP_CODESEPARATOR</code> to sign the executed path of the script. Because the <code>codeseparator_position</code> is the last input to the digest, the SHA256 midstate can be efficiently cached for multiple <code>OP_CODESEPARATOR</code>s in a single script. In contrast, the BIP143 handling of <code>OP_CODESEPARATOR</code> is to commit to the executed script only from the last executed <code>OP_CODESEPARATOR</code> onwards which requires unnecessary rehashing of the script. It should be noted that the one known <code>OP_CODESEPARATOR</code> use case of saving a second public key push in a script by sharing the first one between two code branches can be most likely expressed even cheaper by moving each branch into a separate taproot leaf.</ref> @@ -133,7 +131,7 @@ In addition to the 201 non-push opcodes limit, the use of signature opcodes is s * If <code>50 * (sigops_passed - 1)</code> is greater than <code>input_witness_weight</code>, the script MUST fail and terminate immediately. This rule limits worst-case validation costs in tapscript similar to the ''sigops limit'' that only applies to legacy and P2WSH scripts<ref>'''The tapscript sigop limit''' The signature opcode limit protects against scripts which are slow to verify due to excessively many signature operations. In tapscript the number of signature opcodes does not count towards the BIP141 or legacy sigop limit. The old sigop limit makes transaction selection in block construction unnecessarily difficult because it is a second constraint in addition to weight. Instead, the number of tapscript signature opcodes is limited by witness weight. Additionally, the limit applies to the transaction input instead of the block and only actually executed signature opcodes are counted. Tapscript execution allows one signature opcode per 50 witness weight units plus one free signature opcode. The tapscript signature opcode limit allows to add new signature opcodes like <code>CHECKSIGFROMSTACK</code> to count towards the limit through a soft fork. Even if in the future new opcodes are introduced which change normal script cost there is need to stuff the witness with meaningless data. In that case the taproot annex can be used to add weight to the witness without increasing the actual witness size.</ref> -<ref>'''Parameter choice of the sigop limit''' Regular witnesses are unaffected by the limit as their weight is composed of public key and (<code>SIGHASH_ALL</code>) signature pairs with ''34 + 65'' weight units each (which includes a 1 weight unit <code>CCompactSize</code> tag). This is also the case if public keys are reused in the script because a signature's weight alone is 65 or 66 weight units. However, the limit increases the fees of abnormal scripts with duplicate signatures (and public keys) by requiring additional weight. The weight per sigop factor 50 corresponds to the ratio of BIP141 block limits: 4 mega weight units divided by 80,000 sigops. The "free" signature opcode permitted by the limit exists to account for the weight of the non-witness parts of the transaction input.</ref>. +<ref>'''Parameter choice of the sigop limit''' Regular witnesses are unaffected by the limit as their weight is composed of public key and (<code>SIGHASH_ALL</code>) signature pairs with ''33 + 65'' weight units each (which includes a 1 weight unit <code>CCompactSize</code> tag). This is also the case if public keys are reused in the script because a signature's weight alone is 65 or 66 weight units. However, the limit increases the fees of abnormal scripts with duplicate signatures (and public keys) by requiring additional weight. The weight per sigop factor 50 corresponds to the ratio of BIP141 block limits: 4 mega weight units divided by 80,000 sigops. The "free" signature opcode permitted by the limit exists to account for the weight of the non-witness parts of the transaction input.</ref>. ==Rationale== |