Subject: [bitcoindev] [2/4] Restoration of disabled script functionality (Tapscript v2)

From: Rusty Russell <[email-masked]>
Date: Sat, 27 Sep 2025 20:58:26 +0930

Hi all!

    This BIP is the core restoration.  All the operations come back, and
stack limits are increased (though not infinitely), and integers are not
limited to 31 bits.  The main difference is that all integers are now
unsigned: the combination of variable length and sign bit was a
side-effect of the SSL bignum representation and has proven awkward to
deal with.

    A word on the example implementation which was used to benchmark: it
treats numerical stack objects 64 bits at a time, but does not go so far
as using assembly code.  It also had to implement a proper stack class,
because the time to enforce the new "total stack size limit" naively was
measurable.  I consider it a fair benchmark when considering how a
straightforward implementation would perform.

Thank you for your consideration,
Rusty.

<pre>
  BIP: ?
  Layer: Consensus (soft fork)
  Title: Restoration of disabled script functionality (Tapscript v2)
  Author: Rusty Russell <[email-masked]>
          Julian Moik <[email-masked]>
  Comments-URI: TBA
  Status: Draft
  Type: Standards Track
  Created: 2025-05-16
  License: BSD-3-Clause
</pre>

==Introduction==

===Abstract===

This new BIP introduces a new tapleaf version (0xc2) which restores Bitcoin script to its pre-0.3.1 capability, relying on the Varops Budget in [[bip-unknown-varops-budget.mediawiki|BIP-varops]] to prevent the excessive computational time which caused CVE-2010-5137.

In particular, this BIP:
- Reenables disabled opcodes.
- Increases the maximum stack object size from 520 bytes to 4,000,000 bytes.
- Introduces a total stack byte limit of 8,000,000 bytes.
- Increases the maximum total number of stack objects from 1000 to 32768.
- Removes the 32-bit size restriction on numerical values.
- Treats all numerical values as unsigned.

===Copyright===

This document is licensed under the 3-clause BSD license.

===Motivation===

Since Bitcoin v0.3.1 (addressing CVE-2010-5137), Bitcoin's scripting capabilities have been significantly restricted to mitigate known vulnerabilities related to excessive computational time and memory usage.  These early safeguards were necessary to prevent denial-of-service attacks and ensure the stability and reliability of the Bitcoin network.

Unfortunately, these restrictions removed much of the ability for users to control the exact spending conditions of their outputs, which has frustrated the long-held ideal of programmable money without third-party trust.

==Execution of Tapscript v2==

If a taproot leaf has a version of 0xc2, execution of opcodes is as defined below.  All opcodes not explicitly defined here are treated exactly as defined by [[bip-0342.mediawiki|BIP342]].

Validation of a script fails if:
- It exceeds the remaining varops budget for the transaction.
- Any stack element exceeds 4,000,000 bytes.
- The total size of all stack (and altstack) elements exceeds 8,000,000 bytes.
- The number of stack elements (including altstack elements) exceeds 32768.

===Rationale===

There needs to be some limit on memory usage, to avoid a memory-based denial of service.

Putting the entire transaction on the stack is a foreseeable use case, hence using the block size (4MB) as a limit makes sense.  However, allowing 4MB stack elements is a significant increase in memory requirements, so a total limit of twice that many bytes (8MB) is introduced.  Many stack operations require making at least one copy, so this allows such use.

Putting all outputs or inputs from the transaction on the stack as separate elements requires as much stack capacity as there are inputs or outputs.  The smallest possible input is 34 bytes (allowing almost 26411 inputs), and the smallest possible output is 9 bytes (allowing almost 111111 inputs).  However, empty outputs are rare and not economically interesting.  Thus we consider smallest non-OP_RETURN standard output script, which is P2WPKH at 22 bytes, giving a minimum output size of 31 bytes, allowing 32258 outputs in a maximally-sized transaction.

This makes 32768 a reasonable upper limit for stack elements.

===SUCCESS Opcodes===

The following opcodes are renamed OP_SUCCESSx, and cause validation to immediately succeed:

* OP_1NEGATE = OP_SUCCESS79
* OP_NEGATE = OP_SUCCESS143
* OP_ABS = OP_SUCCESS144<ref>Anthony Towns suggested this could become an opcode which normalized the value on the top of the stack by truncating any trailing zeroes.</ref>

====Rationale====

Negative numbers are not natively supported in v2 Tapscript.  Arbitrary precision makes them difficult to manipulate and negative values are not used meaningfully in bitcoin transactions.

===Non-Arithmetic Opcodes Dealing With Stack Numbers===

The following opcodes are redefined in v2 Tapscript to read numbers from the stack as arbitrary-length little-endian values (instead of CScriptNum):

1. OP_CHECKLOCKTIMEVERIFY
2. OP_CHECKSEQUENCEVERIFY
3. OP_VERIFY
4. OP_PICK
5. OP_ROLL
6. OP_IFDUP
7. OP_CHECKSIGADD

These opcodes are redefined in v2 Tapscript to write numbers to the stack as minimal-length little-endian values (instead of CScriptNum):

1. OP_CHECKSIGADD
2. OP_DEPTH
3. OP_SIZE

In addition, the [[bip-0342.mediawiki#specification|BIP-342 success requirement]] is modified to require a non-zero variable-length unsigned integer value (not <code>CastToBool()</code>):

Previously:

    ## If the execution results in anything but exactly one element on the stack which evaluates to true with <code>CastToBool()</code>, fail.

Now:

    ## If the execution results in anything but exactly one element on the stack which contains one or more non-zero bytes, fail.

===Enabled Opcodes===

Fifteen opcodes which were removed in v0.3.1 are re-enabled in v2 Tapscript.

If there are less than the required number of stack elements, these opcodes fail validation.  Equivalently, a requirement to pop off the stack which cannot be satisfied causes the opcode to fail validation.

See [[bip-unknown-varops-budget.mediawiki|BIP-varops]] for the meaning of the annotations in the varops cost field.

====Splice Opcodes====

{|
! Opcode
! Value
! Required Stack Elements
! Varops Cost
! Varops Reason
! Definition
|-
|OP_CAT
|126
|2
|length(A) + length(B)
|COPYING
|
# Pop B off the stack.
# Pop A off the stack.
# Append B to A.
# Push A onto the stack.
|-
|OP_SUBSTR
|127
|3
|length(LEN) + length(BEGIN) + MIN(Value of LEN, length(A) - Value of BEGIN, 0)
|LENGTHCONV + COPYING
|
# Pop LEN off the stack.
# Pop BEGIN off the stack.
# Pop A off the stack.
# Remove BEGIN bytes from the front of A (all bytes if BEGIN greater than length of A).
# If length(A) is greater than value(LEN), truncate A to length value(LEN).
# Push A onto the stack.
|-
|OP_LEFT
|128
|2
|length(OFFSET)
|LENGTHCONV
|
# Pop OFFSET off the stack.
# Pop A off the stack.
# If length(A) is greater than value(OFFSET), truncate A to length value(OFFSET).
# Push A onto the stack.
|-
|OP_RIGHT
|129
|2
|length(OFFSET) + value of OFFSET
|LENGTHCONV + COPYING
|
# Pop OFFSET off the stack.
# Pop A off the stack.
# Copy value(OFFSET) bytes from offset length(A) - value(OFFSET) to offset 0, if value(OFFSET) is less than length(A).
# Push A onto the stack.
|}

=====Rationale=====

OP_CAT may require a reallocation of A (hence, COPYING A) before appending B.

OP_SUBSTR may have to copy LEN bytes, but also needs to read its two numeric operands.  LEN is limited to the length of the operand minus BEGIN.

OP_LEFT only needs to read its OFFSET operand (truncation is free), whereas OP_RIGHT must copy the bytes, which depends on the OFFSET value.

====Bit Operation Opcodes====

{|
! Opcode
! Value
! Required Stack Elements
! Varops Cost
! Varops Reason
! Definition
|-
|OP_INVERT
|131
|1
|length(A) * 2
|OTHER
|
# Pop A off the stack.
# For each byte in A, replace it with that byte bitwise XOR 0xFF (i.e. invert the bits)
# Push A onto the stack.
|-
|OP_AND
|132
|2
|length(A) + length(B)
|OTHER + ZEROING
|
# Pop B off the stack.
# Pop A off the stack.
# If B is longer than A, swap B and A.
# For each byte in A (the longer operand): bitwise AND it with the equivalent byte in B (or 0 if past end of B)
# Push A onto the stack.
|-
|OP_OR
|133
|2
|MIN(length(A), length(B)) * 2
|OTHER
|
# Pop B off the stack.
# Pop A off the stack.
# If B is longer than A, swap B and A.
# For each byte in B (the shorter operand): bitwise OR it with the equivalent byte in A.
# Push A onto the stack.
|-
|OP_XOR
|134
|2
|MIN(length(A), length(B)) * 2
|OTHER
|
# Pop B off the stack.
# Pop A off the stack.
# If B is longer than A, swap B and A.
# For each byte in B (the shorter operand): exclusive OR it with the equivalent byte in A.
# Push A onto the stack.
|}

=====Rationale=====

OP_AND, OP_OR and OP_XOR are assumed to fold the results into the longer of the two operands.  This is an OTHER operation (i.e. cost is 2 per byte), but OP_AND needs to do this until one operand is exhausted, and then zero the rest (ZEROING, cost 1 per byte).   OP_OR and OP_XOR can stop as soon as the shorter operand is exhausted.

====Bitshift Opcodes====

Note that these are raw bitshifts, unlike the sign-preserving arithmetic shifts in Bitcoin v0.3.0, and as such they also do not truncate trailing zeroes from results: they are renamed OP_UPSHIFT (nee OP_LSHIFT) and OP_DOWNSHIFT (nee OP_RSHIFT).

{|
! Opcode
! Value
! Required Stack Elements
! Varops Cost
! Varops Reason
! Definition
|-
|OP_UPSHIFT
|152
|2
|length(BITS) + (Value of BITS) / 8 + length(A).  If BITS % 8 != 0, add length(A) * 2
|LENGTHCONV + ZEROING + COPYING. If BITS % 8 != 0, + OTHER.
|
# Pop BITS off the stack.
# Pop A off the stack.
# If A shifted by value(BITS) would exceed the individual stack limit, fail.
# If value(BITS) % 8 == 0: simply prepend value(BITS) / 8 zeroes to A.
# Otherwise: prepend (value(BITS) / 8) + 1 zeroes to A, then shift A *down* (8 - (value(BITS) % 8)) bits.
# Push A onto the stack.
|-
|OP_DOWNSHIFT
|153
|2
|length(BITS) + MAX((length(A) - (Value of BITS) / 8), 0) * 2
|LENGTHCONV + OTHER
|
# Pop BITS off the stack.
# Pop A off the stack.
# For BITOFF from 0 to (length(A)-1) * 8 - value(BITS):
    # Copy each bit in A from BITOFF + value(BITS) to BITOFF.
# Truncate A to remove value(OFF) bytes (or all, if value(OFF) > length(A)).
# Push A onto the stack.
|}

=====Rationale=====

DOWNSHIFT needs to read the value of the second operand BITS.  It then needs to move the remainder of A (the part after offset BITS/8 bytes).  In practice this should be implemented in word-size chunks, not bit-by-bit!

UPSHIFT also needs to read BITS.  In general, it may need to reallocate (copying A and zeroing out remaining words).  If not moving an exact number of bytes (BITS % 8 != 0), another pass is needed to perform the bitshift.

OP_UPSHIFT can produce huge results, and so must be checked for limits prior to evaluation.  It is also carefully defined to avoid reallocating twice (reallocating to prepend bytes, then again to append a single byte) which has the practical advantage of being able to share the same downward bitshift routine as OP_DOWNSHIFT.

====Multiply and Divide Opcodes===

{|
! Opcode
! Value
! Required Stack Elements
! Varops Cost
! Varops Reason
! Definition
|-
|OP_2MUL
|141
|1
|length(A) * 3
|OTHER + COPYING
|
# Pop A off the stack.
# Shift each byte in A 1 bit to the left (increasing values, equivalent to C's << operator), tracking the last non-zero value.
# If the final byte overflows, append a single 1 byte.
# Otherwise, truncate A at the last non-zero byte.
# Push A onto the stack.
|-
|OP_2DIV
|142
|1
|length(A) * 2
|OTHER
|
# Pop A off the stack.
# Shift each byte 1 bit to the right (decreasing values, equivalent to C's >> operator), tracking the last non-zero value.
# Truncate A at the last non-zero byte.
# Push A onto the stack.
|--
|OP_MUL
|149
|2
|length(A) + length(B) + (length(A) + 7) / 8 * length(B) * 6  (BEWARE OVERFLOW)
|See Appendix
|
# Pop B off the stack.
# Pop A off the stack.
# Calculate the varops cost of the operation: if it exceeds the remaining budget, fail.
# Allocate an all-zero vector R of length equal to length(A) + length(B).
# For each word in A, multiply it by B and add it into the vector R, offset by the word offset in A.
# Truncate R at the last non-zero byte.
# Push R onto the stack.
|-
|OP_DIV
|150
|2
|length(A) * 9 + length(B) * 2 + length(A)^2 / 3  (BEWARE OVERFLOW)
|See Appendix
|
# Pop B off the stack.
# Pop A off the stack.
# Calculate the varops cost of the operation: if it exceeds the remaining budget, fail.
# If B is empty or all zeroes, fail.
# Perform division as per Knuth's The Art of Computer Programming v2 page 272, Algorithm D "Division of non-negative integers".
# Trim trailing zeroes off the quotient.
# Push the quotient onto the stack.
|-
|OP_MOD
|151
|2
|length(A) * 9 + length(B) * 2 + length(A)^2 / 3  (BEWARE OVERFLOW)
|See Appendix
|
# Calculate the varops cost of the operation: if it exceeds the remaining budget, fail.
# If B is empty or all zeroes, fail.
# Perform division as per Knuth's The Art of Computer Programming v2 page 272, Algorithm D "Division of non-negative integers".
# Trim trailing zeroes off the remainder.
# Push the remainder onto the stack.
|}

=====Rationale=====

These opcodes can be computationally intensive, which is why the varops budget must be checked before operations.  OP_2MUL and OP_2DIV are far simpler, equivalent to OP_UPSHIFT and OP_DOWNSHIFT by 1 bit, except truncating the most-significant zero bytes.

The detailed rationale for these costs can be found in Appendix A.

===Limited Hashing Opcodes===

OP_RIPEMD160 and OP_SHA1 are now defined to FAIL validation if their operands exceed 520 bytes.<ref>There seems little reason to allow large hashing with SHA1 and RIPEMD, and they are not as optimized as SHA256, so we restrict their usage to the older byte limit.</ref>

===Extended Opcodes===

The opcodes OP_ADD, OP_SUB, OP_1ADD and OP_1SUB are redefined in v2 Tapscript to operate on variable-length unsigned integers.  These always produce minimal values (no trailing zero bytes).

{|
! Opcode
! Value
! Required Stack Elements
! Varops Cost
! Varops Reason
! Definition
|-
|OP_ADD
|147
|2
|MAX(length(A), length(B)) * 4
|ARITH + COPYING
|
# Pop B off the stack.
# Pop A off the stack.
# Option 1: trim trailing zeroes off A and B.
# If B is longer than A, swap A and B.
# For each byte in B, add it and previous overflow into the equivalent byte in A, remembering next overflow.
# If there was final overflow, append a 1 byte to A.
# Option 2: If there was no final overflow, remember last non-zero byte written into A, and truncate A after that point.
# Either Option 1 or Option 2 MUST be implemented.
|-
|OP_1ADD
|139
|1
|MAX(1, length(A)) * 4
|ARITH + COPYING
|
# Pop A off the stack.
# Let B = 1, and continue as OP_ADD.
|-
|OP_SUB
|148
|2
|MAX(length(A), length(B)) * 3
|ARITH
|
# Pop B off the stack.
# Pop A off the stack.
# For each byte in B, subtract it and previous underflow from the equivalent byte in A, remembering next underflow.
# If there was final overflow, fail validation.
# Remember last non-zero byte written into A, and truncate A after that point.
|-
|OP_1SUB
|140	
|1
|MAX(1, length(A)) * 3
|ARITH
|
# Pop A off the stack.
# Let B = 1, and continue as OP_SUB.
|}

====Rationale====

Note the basic cost for ADD is three times the maximum operand length, but then considers the case where a reallocation and copy needs to occur to append the final carry byte (COPYING, which costs 1 unit per byte).

Subtraction is cheaper because underflow does not occur: that is a validation failure, as mathematicians agree the result would not be natural.

===Misc Operators===

The following opcodes have costs below:

{|
! Opcode
! Varops Budget Cost
! Varops Reason
|-
| OP_CHECKLOCKTIMEVERIFY
| Length of operand
| LENGTHCONV
|-
| OP_CHECKSEQUENCEVERIFY
| Length of operand
| LENGTHCONV
|-
| OP_CHECKSIGADD
| MAX(1, length(number operand)) * 4 + 26000
| ARITH + COPYING + SIGCHECK
|-
| OP_CHECKSIG
| 26000
| SIGCHECK
|-
| OP_CHECKSIGVERIFY
| 26000
| SIGCHECK
|}

====Rationale====

OP_CHECKSIGADD does an OP_1ADD on success, so we use the same cost as that.  For simplicity, this is charged whether the OP_CHECKSIGADD succeeds or not.

===Other Operators===

The varops costs of the following opcodes are defined in [[bip-unknown-varops-budget.mediawiki|BIP-varops]]:

* OP_VERIFY
* OP_NOT
* OP_0NOTEQUAL
* OP_EQUAL
* OP_EQUALVERIFY
* OP_2DUP
* OP_3DUP
* OP_2OVER
* OP_IFDUP
* OP_DUP
* OP_OVER
* OP_PICK
* OP_TUCK
* OP_ROLL
* OP_BOOLOR
* OP_NUMEQUAL
* OP_NUMEQUALVERIFY
* OP_NUMNOTEQUAL
* OP_LESSTHAN
* OP_GREATERTHAN
* OP_LESSTHANOREQUAL
* OP_GREATERTHANOREQUAL
* OP_MIN
* OP_MAX
* OP_WITHIN
* OP_SHA256
* OP_HASH160
* OP_HASH256

Those with costs not defined here have a cost of 0 (they do not operate on variable-length stack objects).

===Normalization of Results===

Note that only arithmetic operations (those which treat operands as numbers) normalize their results: bit operations do not.  Thus operations such as "0 OP_ADD" and "2 OP_MUL" will never result in a top stack entry with a trailing zero byte, but "0 OP_OR" and "1 OP_UPSHIFT" may.

To be clear, the following operations are arithmetic and will normalize their results:

* OP_1ADD
* OP_1SUB
* OP_2MUL
* OP_2DIV
* OP_ADD
* OP_SUB
* OP_MUL
* OP_DIV
* OP_MOD
* OP_MIN
* OP_MAX

==Backwards compatibility==

This BIP defines a previous unused (and thus, always-successful) tapscript version, for backwards compatibility.

==Reference Implementation==

Work in progress:

	https://github.com/jmoik/bitcoin/tree/gsr

==Thanks==

This BIP would not exist without the thoughtful contributions of coders who considered all the facets carefully and thoroughly, and also my inspirational wife Alex and my kids who have been tirelessly supportive of my esoteric-seeming endeavors such as this!

In alphabetical order:
- Anthony Towns
- Brandon Black (aka Reardencode)
- John Light
- Jonas Nick
- Rijndael (aka rot13maxi)
- Steven Roose
- FIXME: your name here!

==Appendix A: Cost Model Calculations for Multiply and Divide==

Multiplication and division require multiple passes over the operands, meaning a cost proportional to the square of the lengths involved, and the word size used for that iteration makes a difference.  We assume 8 bytes (64 bits) at a time are evaluated, and the ability to multiply two 64-bit numbers and receive a 128-bit result, and divide a 128-bit number by a 64 bit number to receive a 128 bit quotient and remainder.  This is true on modern 64-bit CPUs (sometimes using multiple instructions).

===Multiplication Cost====

For multiplication, the steps break down like so:
1. Allocate and zero the result: cost = length(A) + length(B) (ZEROING)
2. For each word in A:
  * Multiply by each word in B, into a scratch vector: cost = 3 * length(B) (ARITH)
  * Sum scratch vector at the word offset into the result: cost = 3 * length(B) (ARITH)

Note: we do not assume Karatsuba, Tom-Cooke or other optimizations.

This results in a cost of: length(A) + length(B) + (length(A) + 7) / 8 * length(B) * 6.

This is slightly asymmetric: in practice an implementation usually finds that CPU pipelining means choosing B as the larger operand is optimal.

===Division Cost====

For division, the steps break down like so:

1. Bit shift both operands to set top bit of B (OP_UPSHIFT, without overflow for B): cost = length(A) * 3 + length(B) * 2

2. Trim trailing bytes.  This costs according to the number of byte removed, but since that is subtractive on future costs, we ignore it.

3. If B is longer, the answer is 0 already.  So assume A is longer from now on (or equal length).

4. Compare: cost = length(A) (COMPARING)

5. Subtract: cost = length(A) * 3 (ARITH)

6. for (length(A) - NormalizedLength(B)) in words:
   1. Multiply word by B -> scratch: cost = NormalizedLength(B) * 3 (ARITH)
   2. Subtract scratch from A: cost = length(A) * 3 (ARITH)
   3. Add B into A (no overflow): cost = length(A) * 3 (ARITH)
   4. Shrink A by 1 word.

7. OP_MOD: shift A down, trim trailing zeros: cost = length(A) * 2

8. OP_DIV: trim trailing zeros: cost = length(A) * 2

Note that the loop at step 6 shrinks A every time, so the *average* cost of each iteration is (NormalizedLength(B) * 3 + length(A) * 6) / 2.  The cost of step 6 is:

	(length(A) - NormalizedLength(B)) / 8 * (NormalizedLength(B) * 3 + length(A) * 6) / 2

The worst case here is when NormalizedLength(B) is 0: length(A) * length(A) / 3.

The cost for all the steps in either case is: length(A) * 9 + length(B) * 2 + length(A) * length(A) / 3.

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