BIP 441: Restoration of disabled script (Tapleaf 0xC2)

In plain English: Proposes a new Taproot script version that restores many of Bitcoin's original script instructions disabled in 2010 and greatly loosens limits on data size. To avoid the denial-of-service risks that prompted the original restrictions, it relies on the computational budget defined in BIP 440.

Status

Draft

Type

Specification

Layer

Consensus (soft fork)

Authors

Rusty Russell, Julian Moik

Assigned

2026-03-25

Last changed

2026-09-15

Requires

BIP 440

Topics

Consensus

Source

bip-0441.mediawiki on GitHub · Markdown

Introduction

Abstract

This BIP introduces a new tapleaf version (0xc2) which restores Bitcoin script to its pre-0.3.1 capability, relying on the Varops Budget in BIP440 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 1,000 to 32,768.

• Removes the 32-bit size restriction on numerical values.

• Treats all numerical values as unsigned.

All opcodes are described in exact (painstaking) byte-by-byte operations, so that their varops budget can be easily derived. Note that this level of detail is unnecessary to users of script, only being of interest to implementers.

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 0xC2

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 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 32,768.

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 41 bytes (allowing almost 24,390 inputs), and the smallest possible output is 9 bytes (allowing almost 111,111 outputs). However, empty outputs are rare and not economically interesting. Thus we consider the smallest non-OP_RETURN standard output script, which is P2WPKH at 22 bytes, giving a minimum output size of 31 bytes, allowing 32,258 outputs in a maximally-sized transaction.

This makes 32,768 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_SUCCESS144Anthony Towns suggested this could become an

opcode which normalized the value on the top of the stack by truncating any trailing zeroes.

Rationale

Negative numbers are not natively supported in 0xC2 Tapscript. Arbitrary precision makes them difficult to manipulate and negative values are not used meaningfully in bitcoin transactions.

Arbitrary-length Values, Endianness, and Normalization of Results

The restoration of bit operations means that the little-endianness of stack values is once more exposed to the Script author, if they mix them with arithmetic operations. The restoration of arbitrary-length values additionally exposes the endianness to the implementation authors (who cannot simply load stack entries into registers), and requires explicit consideration when considering varops costs of operations.^[1]

Note that only arithmetic operations (those which treat operands as numbers) normalize their results: bit and byte operations do not.^[2] 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.^[3]

To be explicit, the following operations are defined as 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

Non-Arithmetic Opcodes Dealing With Stack Numbers

The following opcodes are redefined in 0xC2 Tapscript to read numbers from the stack as arbitrary-length little-endian values (instead of CScriptNum):

1. OP_CHECKLOCKTIMEVERIFY

1. OP_CHECKSEQUENCEVERIFY

1. OP_VERIFY

1. OP_PICK

1. OP_ROLL

1. OP_IFDUP

1. OP_CHECKSIGADD

For OP_CHECKLOCKTIMEVERIFY and OP_CHECKSEQUENCEVERIFY, the operand is decoded and costed as an arbitrary-length unsigned integer. However, the decoded value MUST be less than 2³², because it is compared against the 32-bit nLockTime or nSequence transaction field. OP_CHECKSEQUENCEVERIFY performs this range check before disable-flag handling or BIP68 masking, so any non-zero bits above bit 31 cause failure.

These opcodes are redefined in 0xC2 Tapscript to write numbers to the stack as minimal-length little-endian values (instead of CScriptNum):

1. OP_CHECKSIGADD

1. OP_DEPTH

1. OP_SIZE

In addition, the BIP-342 success requirement is modified to require a non-zero variable-length unsigned integer value (not CastToBool()):

Previously:

``4. (ii) If the execution results in anything but exactly one element on the stack which evaluates to true with CastToBool(), fail.``

Now:

``4. (ii) If the execution results in anything but exactly one element on the stack which contains one or more non-zero bytes, fail.``

This final success check consumes varops budget as wordspan(A) * 2 (COMPARINGZERO), where A is the remaining stack element. If this cost exceeds the remaining transaction varops budget, fail before performing the non-zero-byte check.

Enabled Opcodes

Fifteen opcodes that were removed in v0.3.1 are re-enabled in 0xC2 Tapscript.

If there are fewer than the required number of stack elements, these opcodes fail validation. These are popped off the stack in right-to-left order, i.e. [A B] means pop B off the stack, then pop A off the stack.

See BIP440 for the meaning of the annotations in the varops cost field.

Byte Lengths and Word Spans

This BIP uses the length(...) and wordspan(...) convention from BIP440: length(X) is the script-visible byte length of stack element X, while wordspan(X) is that length rounded up to the 64-bit word span used for numeric and bit-vector work. Some formulas intentionally mix the two when an opcode performs both word-rounded interpretation and exact byte movement.

Splice Opcodes

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 rightmost bytes, which depends on the bounded OFFSET value.

Bit Operation Opcodes

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 4 per byte), but OP_AND needs to do this until one operand is exhausted, and then zero the rest (ZEROING, cost 2 per byte). OP_OR and OP_XOR can stop processing the operands 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 (née OP_LSHIFT) and OP_DOWNSHIFT (née OP_RSHIFT).

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

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.^[4]

Extended Opcodes

The opcodes OP_ADD, OP_SUB, OP_1ADD and OP_1SUB are redefined in 0xC2 Tapscript to operate on variable-length unsigned integers. These always produce minimal values (no trailing zero bytes).

Rationale

Note that the basic cost for ADD is six times the maximum operand length (ARITH), but then considers the case where a reallocation and copy needs to occur to append the final carry byte (COPYING, which costs 3 units 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:

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 BIP440:

• 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

Any opcodes not mentioned in this document or the preceding list have a cost of 0 (they do not operate on variable-length stack objects).

Backwards compatibility

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

Reference Implementation

Work in progress:

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

Changelog

• 0.2.2: 2026-06-15: clarify wordspan costs, OP_RIGHT semantics, OP_UPSHIFT unaligned cost, CLTV/CSV bounds, and final success-check cost.

• 0.2.1: 2026-03-27: fix OP_MUL cost to round length(B) up

• 0.2.0: 2025-02-21: change costs to match those in varops budget

• 0.1.0: 2025-09-27: first public posting

Footnotes

1. For example, removing trailing bytes from a stack element is almost free, whereas removing bytes from the front involves copying all remaining bytes. ↩
2. Such non-arithmetic operations can be used to operate on values such as preimages or (with introspection) parts of transactions, where truncation of zeros would be unexpected. One could argue that even arithmetic operators should not normalize, but that would be a gratuitous and surprising change. Note that "0 OP_ADD" can always be used to cheaply normalize the top stack element. ↩
3. The original Bitcoin implementation had a similar operational split, but OP_LSHIFT and OP_RSHIFT did normalize, which was almost a requirement given that they also preserved the sign of the shifted operand ↩
4. 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. ↩

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

• Mark "Murch" Erhardt

• 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)) * 2 (ZEROING)

1. For each word in A:
   • Multiply by each word in B, into a scratch vector: cost = 6 * wordspan(B) (ARITH)
   • Sum scratch vector at the word offset into the result: cost = 6 * wordspan(B) (ARITH)

We increase the operand lengths to the next word boundary for the word-span loops, using wordspan(n) from BIP440, as the multiplication below makes the difference from the simple byte length significant.

Note: we do not assume Karatsuba, Toom-Cook or other optimizations.

The theoretical cost is: (length(A) + length(B)) * 2 + wordspan(A) / 8 * wordspan(B) * 12.

However, benchmarking reveals that the inner loop overhead (branch misprediction, cache effects on small elements) is undercosted by the theoretical model. A 2.25× multiplier on the quadratic term accounts for this, giving a cost of: (length(A) + length(B)) * 3 + wordspan(A) / 8 * wordspan(B) * 27.

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 = wordspan(A) * 6 + wordspan(B) * 4

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

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

1. Compare: cost = wordspan(A) * 2 (COMPARING)

1. Subtract: cost = wordspan(A) * 6 (ARITH)

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

1. OP_MOD: shift A down, trim trailing zeroes: cost = wordspan(A) * 4

1. OP_DIV: trim trailing zeros: cost = wordspan(A) * 4

Note that the loop at step 6 shrinks A every time, so the *average* cost of each iteration is (NormalizedLength(B) * 6 + wordspan(A) * 12) / 2. The cost of step 6 is:

(wordspan(A) - NormalizedLength(B)) / 8 * (NormalizedLength(B) * 6 + wordspan(A) * 12) / 2

The worst case is when NormalizedLength(B) is 0: wordspan(A) * wordspan(A) * 2 / 3.

The cost for all the steps is: wordspan(A) * 18 + wordspan(B) * 4 + wordspan(A) * wordspan(A) * 2 / 3.