Draft BIP to prevent a potential CPU exhaustion attack if a significantly larger maximum blocksize is adopted:
Title: Limit maximum transaction size
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
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==