Draft BIP to prevent a potential CPU exhaustion attack if a significantly
larger maximum blocksize is adopted:

  Title: Limit maximum transaction size
  Author: Gavin Andresen <gavinandresen@gmail.com>
  Status: Draft
  Type: Standards Track
  Created: 2015-07-17

==Abstract==

Mitigate a potential CPU exhaustion denial-of-service attack by limiting
the maximum size of a transaction included in a block.

==Motivation==

Sergio Demian Lerner reported that a maliciously constructed block could
take several minutes to validate, due to the way signature hashes are
computed for OP_CHECKSIG/OP_CHECKMULTISIG ([[
https://bitcointalk.org/?topic=140078|CVE-2013-2292]]).
Each signature validation can require hashing most of the transaction's
bytes, resulting in O(s*b) scaling (where n is the number of signature
operations and m is the number of bytes in the transaction, excluding
signatures). If there are no limits on n or m the result is O(n^2) scaling.

This potential attack was mitigated by changing the default relay and
mining policies so transactions larger than 100,000 bytes were not
relayed across the network or included in blocks. However, a miner
not following the default policy could choose to include a
transaction that filled the entire one-megaybte block and took
a long time to validate.

==Specification==

After deployment, the maximum serialized size of a transaction allowed
in a block shall be 100,000 bytes.

==Compatibility==

This change should be compatible with existing transaction-creation
software,
because transactions larger than 100,000 bytes have been considered
"non-standard"
(they are not relayed or mined by default) for years.

Software that assembles transactions into blocks and that validates blocks
must be
updated to reject oversize transactions.

==Deployment==

This change will be deployed with BIP 100 or BIP 101.

==Discussion==

Alternatives to this BIP:

1. A new consensus rule that limits the number of signature operations in a
single transaction instead of limiting size. This might be more compatible
with
future opcodes that require larger-than-100,000-byte transactions, although
any such future opcodes would likely require changes to the Script
validation
rules anyway (e.g. the 520-byte limit on data items).

2. Fix the SIG opcodes so they don't re-hash variations of the
transaction's data.
This is the "most correct" solution, but would require updating every
piece of transaction-creating and transaction-validating software to change
how
they compute the signature hash.

==References==

[[https://bitcointalk.org/?topic=140078|CVE-2013-2292]]: Sergio Demian
Lerner's original report