Monero activated its smaller CLSAG transaction format

Monero activated the first stage of its Oxygen Orion network upgrade on October 17, 2020, enabling the new Concise Linkable Spontaneous Anonymous Group, or CLSAG, transaction format at block 2,210,000.

The consensus change mattered because ring signatures are central to how Monero obscures which member of a set authorized a transaction. CLSAG was designed as a more compact replacement for the network’s existing multilayered linkable spontaneous anonymous group, or MLSAG, construction. The project expected the change to reduce the blockchain space and computing time required for ordinary transactions without removing the signer ambiguity provided by the earlier format.

Monero’s maintained protocol history identifies block 2,210,000 as consensus version 13 and dates it to October 17, 2020. That record specifically describes the change as introducing the new CLSAG transaction format. The activation was therefore a consensus event, not merely the publication of wallet software or a future development plan.

Smaller signatures, not a larger anonymity set

Monero’s August 18, 2020 upgrade notice estimated that CLSAG would reduce the size of a typical transaction by approximately 25% and improve typical verification performance by approximately 10%. A more detailed project explanation described a representative transaction with two inputs and two outputs shrinking from roughly 2.5 kilobytes to 1.9 kilobytes. It also reported testing that indicated a 20% improvement in signature verification and at least a 10% overall improvement for typical transactions.

Those figures were project benchmarks and estimates, not measurements of every transaction processed on October 17. Actual size depends on transaction structure, while verification time depends on hardware and software conditions. The upgrade did not enlarge Monero’s ring size, change its proof-of-work algorithm or establish that every transaction became 25% smaller.

The narrower achievement was still consequential. Smaller signatures reduce the amount of data nodes must store and relay. Faster verification can reduce the cost of checking historical transactions when a node synchronizes. For exchanges, custodians, merchants and wallet operators, however, the efficiency gain arrived with an operational obligation: Monero described the upgrade as a hard fork requiring participants to run compatible software or remain on incompatible consensus rules.

Oxygen Orion carried additional changes

Monero released version 0.17.0.0, named Oxygen Orion, on September 17, 2020. Its release record listed CLSAG support alongside deterministic unlock times, enforcement of the maximum claimable coinbase amount, serialization changes, updated transaction-proof formats and changes to transaction relay behavior. The project attributed the release to 30 contributors, 292 commits and 11,523 added lines of code.

CLSAG had also undergone an external review before activation. Cryptographers JP Aumasson and Antony Vennard examined both the construction and its implementation. Their July 29, 2020 report required substantial revisions to two security proofs but reported no security issue in the algorithms or implementation after the review process. That audit increased confidence in deployment; it did not guarantee that the surrounding network software contained no defects.

Later context

The project’s maintained upgrade table records a second consensus stage at block 2,210,720 on October 18, 2020. Version 14 prohibited the older MLSAG transaction format. That later entry clarifies the two-stage transition: October 17 enabled CLSAG at the consensus level, while October 18 completed the scheduled exclusion of the prior format. The October 18 event is not treated here as part of the October 17 development.