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+<pre>
+  BIP: x
+  Title: Segregated Witness (Consensus layer)
+  Author: 
+  Status: Draft
+  Type: Standards Track
+  Created: 2015-12
+</pre>
+
+==Abstract==
+
+This BIP defines a new structure called a "witness" that is committed to blocks separately from the transaction merkle tree. This structure contains data required to check transaction validity but not required to determine transaction effects. In particular, scripts and signatures are moved into this new structure.
+
+The witness is committed in a tree that is nested into the block's existing merkle root via the coinbase transaction for the purpose of making this BIP soft fork compatible. A future hard fork can place this tree in its own branch.
+
+==Motivation==
+
+The entirety of the transaction's effects are determined by output consumption (spends) and new output creation. Other transaction data, and signatures in particular, are only required to validate the blockchain state, not to determine it.
+
+By removing this data from the transaction structure committed to the transaction merkle tree, several problems are fixed:
+
+# '''Nonintentional malleability becomes impossible'''. Since signature data is no longer part of the transaction hash, changes to how the transaction was authorized is no longer relevant to transaction identification. As a solution of transaction malleability, this is superior to the canonical signature approach (BIP62):
+#* It prevents involuntary transaction malleability for any type of scripts, as long as all inputs are signed (with at least one CHECKSIG or MULTICHECKSIG operation)
+#* In the case of a n-of-m MULTICHECKSIG script, a transaction is malleable only with agreement of n private key holders (as opposed to only 1 private key holder with BIP62)
+#* It prevents involuntary transaction malleability due to unknown ECDSA signature malleability
+#* It allows creation of unconfirmed transaction dependency chains without counterparty risk
+# '''Transmission of signature data becomes optional'''. It is needed only if a peer is trying to validate a transaction, instead of just to prove its existence. This also improves prviacy of SPV clients as using the same bandwidth they could request for more transactions for obscuration.
+# '''Some constriants could be bypassed with a soft fork''' by moving part of the transaction data to a structure unknown to current protocol, for example:
+#* Size of witness could be ignored / discounted when calculating the block size, effectively increasing the block size to some extent
+#* Hard coded constants, such as maximum data push size (520 bytes) or sigops limit could be reevaluated or removed
+#* New script system could be introduced without any limitation from the existing script semantic
+
+==Specification==
+
+Maybe a seperate BIP
+<pre>
+* CTransaction gets, in addition to vin and vout, a vwit, which
+contains a CTxInWitness object for each input. A CTxInWitness contains a CScriptWitness object
+and can potentially be extended to contain other kinds of witness data.
+A CScriptWitness is a vector of byte vectors (nominally: the input stack to the program, no longer
+encoded as a CScript, but just the resulting stack directly).
+
+* A new serialization for CTransaction is defined: http://blockhawk.net/diagrams/witnesstx.png
+(int32 nVersion,
+0x00 marker, 0x01 flag, vector<CTxIn>, vector<CTxOut>,
+vector<CTxInWitness>, int32 nLockTime) instead of (int32 nVersion,
+vector<CTxIn>, vector<CTxOut>, int32 nLockTime). This will never parse
+as a valid transaction (even if parsing succeeds, it means it's
+interpreted as a transaction with no inputs and 1 output). If all
+witnesses are empty, the old serialization format is used.
+- Rationale for not having an independent CWitnessTransaction with
+its own serialization: this would require separate "tx" and "block"
+messages, and all RPC calls operating on raw transactions would need
+to be duplicated, or need inefficinent or nondeterministic guesswork
+to know which type is to be used.
+- Rationale for not using just a single 0x00 byte as marker: that
+would lead to empty transactions (no inputs, no outputs, which are
+used in some tests) to be interpreted as new serialized data.
+- Rationale for the 0x01 flag byte in between: this will allow us to
+easily add more extra non-committed data to transactions (like txouts
+being spent, ...). It can be interpreted as a bitvector.
+
+* A new message 'havewitness' is sent after receiving 'verack' to
+indicate that a node can provide witness if requested (similar to
+'sendheaders')
+
+* New inv types MSG_WITNESS_TX and MSG_WITNESS_BLOCK are added, only
+for use in getdata. Inv itself still use just MSG_TX and MSG_BLOCK,
+similar to MSG_FILTERED_BLOCK.
+- Rationale for not advertizing witnessness in invs: we don't always
+use invs anymore (with 'sendheaders' BIP 130), plus it's not useful:
+implicitly, every transaction and block have a witness, old ones just
+have empty ones.
+
+* Transactions' GetHash is always computed on the old non-witness
+serialization. A new CTransaction::GetWitnessHash is added which is
+computed from the witness-serialization (this means that transactions
+with an empty witness have witness hash equal to normal hash).
+</pre>
+
+=== Commitment structure ===
+
+UPDATE NEEDED
+A new block rule is added which requires a commitment (a merkle root
+computed similarly to the normal transaction one) to the witness
+hashes to be present as the last 32 bytes of
+block.vtx[0].vin[0].scriptSig (it doesn't need to be a push). This
+hopefully does not conflict with any other existing commitment
+schemes. To make it extensible, an extra merkle path can be provided
+(in the coinbase's "witness" field) so that coinbase commitment can be
+used for multiple things.
+<pre>
+    // Validation for witness commitments.
+    // * We compute the witness hash (which is the hash including witnesses) of all the block's transactions, except the
+    //   coinbase (where 0x0000....0000 is used instead).
+    // * We build a merkle tree with all those witness hashes as leaves (similar to the hashMerkleRoot in the block header).
+    // * The first coinbase scriptSig minimal push of 41 bytes for which the first 4 bytes are {0xaa, 0x21, 0xa9, 0xed} is
+    //   treated as a commitment header. If no such push is present, the block is invalid. If multiple are present, the first
+    //   is used.
+    //   * The first 4 bytes of the commitment header are just magic identifier bytes, and have no further meaning.
+    //   * The next 4 bytes describe a nonce.
+    //   * The next 1 byte describes the number of levels in a Merkle tree.
+    //   * locator = SHA256('WitnessV1\x00\x00\x00\x00\x00\x00\x00' || nonce). The first levels bits of locator, interpreted
+    //     in little endian, are assumed to be the position in the leaves of this Merkle tree where the witness commitment
+    //     goes.
+    //   * The last 32 bytes of the commitment header are its root hash.
+    //   * The coinbase's input's witness must consist of a single byte array of 32 * levels bytes, and are assumed to be
+    //     the Merkle path to connect the witness root hash to the commitment root hash.
+    
+    // No witness data is allowed in blocks that don't commit to witness data, as this would otherwise leave room from spam.
+</pre>
+
+=== Block size limit ===
+2-4-8? Discount for witness data?
+
+=== Witness program ===
+
+* A scriptPubKey (or redeemScript as defined in BIP16/P2SH) that consists of a single push of 2 to 41 bytes gets a new special meaning. The byte vector pushed by it is called the "witness program".
+** In case the scriptPubKey pushes a witness program directly, the scriptSig must be exactly empty.
+** In case the redeemScript pushes a witness program, the scriptSig must be exactly the single push of the redeemScript.
+
+* The first byte of a witness program is the "version byte", an unsigned integer.
+** If the version byte is 0, the rest of the witness program is the actual script.
+*** The script is executed after normal script evaluation but with data from the witness rather than the scriptSig.
+*** The program must not fail, and result in exactly a single TRUE on the stack.
+** If the version byte is 1, the rest of the witness program must be 32 bytes, as a SHA256 hash of the actual script.
+*** The witness must consist of an input stack to feed to the program, followed by the serialized program.
+*** The serialized program is popped off the initial witness stack. Hash of the serialized program must match the hash pushed in the witness program.
+*** The serialized program is deserialized, and executed after normal script evaluation with the remaining witness stack.
+*** The script must not fail, and result in exactly a single TRUE on the stack.
+** If the witness version byte is 2 or above, no further interpretation of the witness program or witness happens.
+
+=== Other consensus critical constraints ===
+
+== Block size analysis ==
+Definitions:
+<pre>
+ Core block size (CBS): The block size as seen by a non-upgrading full node
+ Witness size (WS): The total size of witness in a block
+ Total block size (TBS): CBS + WS
+ Witness discount (WD): A discount factor for witness for calculationg of VBS (1 = no discount)
+ Virtual block size (VBS): CBS + (WS * WD)
+ Witness adoption (WA): Proportion of new format transactions among all transactions
+ Prunable ratio (PR): Proportion of signature data size in a transaction
+</pre>
+
+With some transformation it could be shown that:
+<pre>
+ TBS = CBS / (1 - WA * PR)
+     = VBS / (1 - WA * PR * (1 - WD))
+</pre>
+
+In order to keep the proposal as a soft fork, the CBS must not have a upper limit higher than 1MB.
+
+TBS is a function of only CBS, PR, and WA.
+
+The PR heavily depends on the transaction script type and input-output ratio. For example, the PR of 1-in 2-out P2PKH and 1-in 1-out 2-of-2 multisig P2SH are about 47% and 72% respectively. According to the data presented by Pieter Wuille on 7 December 2015, the current average PR on the blockchain is about 60%.
+
+
+
+== Examples ==
+
+=== Version 0 witness program ===
+
+The following example is a version 0 witness program, equivalent to the existing Pay-to-Pubkey-Hash (P2PKH) output.
+
+    witness:      <signature> <pubkey>
+    scriptSig:    (empty)
+    scriptPubKey: <0x0076A914{20-byte-hash-value}88AC>
+
+The version byte 0x00 is removed. The rest of the witness program is deserialized and becomes:
+
+    DUP HASH160 <20byte-hash-value> EQUALVERIFY CHECKSIG
+
+The script is executed with the data from witness
+
+    <signature> <pubkey> DUP HASH160 <20byte-hash-value> EQUALVERIFY CHECKSIG
+
+Comparing with a P2PKH output, the witness program equivalent occupies 2 more bytes in the scriptPubKey, while moving the signature and public key from scriptSig to witness.
+
+=== Version 1 witness program ===
+
+The following example is an 1-of-2 multi-signature version 1 witness program.
+
+    witness:      0 <signature1> <0x5121{33-byte-pubkey1}21{33-byte-pubkey2}52AE>
+    scriptSig:    (empty)
+    scriptPubKey: <0x01{32-byte-hash-value}>
+
+The last item in the witness is popped off, hashed with SHA256, compared against the 32-byte-hash-value in scriptPubKey, and deserialized:
+
+    1 <33-byte-pubkey1> <33-byte-pubkey2> 2 CHECKMULTISIG
+
+The script is executed with the remaining data from witness:
+
+    0 <signature1> 1 <33-byte-pubkey1> <33-byte-pubkey2> 2 CHECKMULTISIG
+
+Since the actual program is larger than 40 bytes, it cannot be accommodated in a version 0 witness program. A version 1 witness program allows arbitrarily large script as the 520-byte push limit is bypassed.
+
+The scriptPubKey occupies 34 bytes, as opposed to 23 bytes of P2SH. The increased size improves security against possible collision attacks, as 2^80 work is not infeasible anymore (By the end of 2015, 2^84 hashes have been calculated in Bitcoin mining since the creation of Bitcoin). The spending script is same as the one for an equivalent P2SH output but is moved to witness.
+
+=== Witness program nested in Pay-to-Script-Hash ===
+
+The following example is the same 1-of-2 multi-signature version 1 witness program, but nested in a P2SH output.
+
+    witness:      0 <signature1> <0x5121{33-byte-pubkey1}21{33-byte-pubkey2}52AE>
+    scriptSig:    <0x2101{32-byte-hash-value}>
+    scriptPubKey: HASH160 <20-byte-hash-value> EQUAL
+
+The only item in scriptSig is hashed with HASH160, compared against the 20-byte-hash-value in scriptPubKey, and interpreted as a single push of:
+
+    <0x01{32-byte-hash-value}>
+
+The version 1 witness program is then executed as described in the last example
+
+Comparing with the last example, the scriptPubKey is 11 bytes smaller (with reduced security) while witness is the same. However, it also requires 35 bytes in scriptSig, which is not prunable in transmission. Although a nested witness program is less efficient in many ways, its payment address is fully transparent and backward compatible for all Bitcoin reference client since version 0.6.0.
+
+=== Trust-free unconfirmed transaction dependency chain ===
+Segregated witness fixes the problem of transaction malleability fundamentally, which enables the building of unconfirmed transaction dependency chains in a trust-free manner.
+
+Two parties, Alice and Bob, may agree to send certain amount of Bitcoin to a 2-of-2 multisig output (the "funding transaction"). Without signing the funding transaction, they may create another transaction, time-locked in the future, spending the 2-of-2 multisig output to third account(s) (the "spending transaction"). Alice and Bob will sign the spending transaction and exchange the signatures. After examining the signatures, they will sign and commit the funding transaction to the blockchain. Without further action, the spending transaction will be confirmed after the lock-time and release the funding according to the original contract. It also retains the flexibility of revoking the original contract before the lock-time, by another spending transaction with shorter lock-time, but only with mutual-agreement of both parties.
+
+Such setups is not possible with BIP62 as the malleability fix, since the spending transaction could not be created without both parties first signing the funding transaction. If Alice reveals the funding transaction signature before Bob does, Bob is able to lock up the funding indefinitely without ever signing the spending transaction.
+
+Unconfirmed transaction dependency chain is a fundamental building block of more sophisticated payment networks, such as duplex micropayment channel and the Lightning Network, which have the potential to greatly improve the scalability and efficiency of the Bitcoin system.
+
+== Future extensions ==
+=== Compact fraud proof for SPV nodes ===
+Bitcoin right now only has two real security models. A user either runs a full-node which validates every block with all rules in the system, or a SPV (Simple Payment Verification) client which only validates the headers as a proof of publication of some transactions. The Bitcoin whitepaper suggested that SPV nodes may accept alerts from full nodes when they detect an invalid block, prompting the SPV node to download the questioned blocks and transactions for validation. This approach, however, could become a DoS attack vector as there is virtually no cost to generate a false alarm. An alarm must come with a compact, yet deterministic fraud proof.
+
+In the current Bitcoin protocol, it is possible to generate compact fraud proof for almost all rules except a few:
+
+# It is not possible to proof a miner has introduced too many Bitcoins in the coinbase transaction outputs without showing the whole block itself and all input transactions.
+# It is not possible to prove the violation of any block specific constraints, such as size and sigop limits, without showing the whole block (and all input transactions in the case of sigop limit)
+# It is not possible to prove the spending of a non-existing input without showing all transaction IDs in the blockchain way back to the genesis block.
+
+It is possible to proof the first 2 types of fraud if a block is committed to a Merkle-sum-tree of the fee, size, and sigop count of each transaction. It is also possible to proof the last type of fraud if a block is committed to a Merkle tree with the originating block height and transaction index of all inputs. These commitments could be included in the extensible witness commitment through a soft fork and will be transparent to nodes that do not understand such new rules.
+
+=== New script system ===
+Since all witness programs begin with a version byte, and programs with unknown versions are always considered as anyone-can-spend script, it is possible to introduce any new script system with a soft fork. The witness as a structure is not restricted by any existing script semantics and constraints, the 520-byte push limit in particular, and therefore allows arbitrarily large scripts and signatures.
+
+Examples of new script system include Schnorr signatures which reduce the size of multisig transactions dramatically, Lamport signature which is quantum computing resistance, and Merklized abstract syntax trees which allow very compact witness for conditional scripts with extreme complexity.
+
+The 41-byte limitation for witness programme could be easily extended through a soft fork in case a stronger hash function is needed in the future. The version byte is also expandable by introducing a secondary version byte for some specific primary version values.
+
+== Backward compatibility ==
+Without lifting the core block size limit of 1MB at the beginning, this proposal is a soft fork which all existing full nodes and SPV nodes are compatible. Non-upgrading nodes, however, will not see nor validate the witness data, and will consider all witness programs as anyone-can-spend scripts (except a few edge cases in version 0 witness programs which are provably unspendable with original script semantics). Non-upgrading nodes are also unable to generate or sign for scriptPubKey templates (payment address). Nonetheless, they may still pay to a witness program if it is nested in a P2SH address, which has been defined since 2012.
+
+When the core block size limit is increased to over 1MB as scheduled, this proposal becomes a hard fork. Anybody running code that fully validates blocks must upgrade before the activation time or they will reject a chain containing blocks with core block size above 1MB. SPV software is not affected, unless it makes assumptions about the maximum depth of the Merkle tree based on the minimum size of a transaction and the maximum block size.
+
+== Deployment ==
+
+We reuse the double-threshold IsSuperMajority() switchover mechanism used in
+BIP65 with the same thresholds, but for nVersion = 5. The new rules are
+in effect for every block (at height H) with nVersion = 5 and at least
+750 out of 1000 blocks preceding it (with heights H-1000..H-1) also
+have nVersion >= 5. Furthermore, when 950 out of the 1000 blocks
+preceding a block do have nVersion >= 5, nVersion < 5 blocks become
+invalid, and all further blocks enforce the new rules.
+
+It should be noted that BIP9 involves permanently setting a high-order bit to
+1 which results in nVersion >= all prior IsSuperMajority() soft-forks and thus
+no bits in nVersion are permanently lost.
+
+
+=== SPV Clients ===
+
+While SPV clients are (currently) unable to validate blocks in general,
+trusting miners to do validation for them, they are able to validate block
+headers and thus can validate a subset of the deployment rules. SPV clients
+should reject nVersion < 5 blocks if 950 out of 1000 preceding blocks have
+nVersion >= 5 to prevent false confirmations from the remaining 5% of
+non-upgraded miners when the 95% threshold has been reached.
+
+== Credits ==
+
+== Reference Implementation ==
+https://github.com/sipa/bitcoin/commits/segwit