What it means
Every blockchain needs a fair way to choose who adds the next block. Proof of work uses a race of computing power, which wastes electricity, while proof of elapsed time uses a lottery in which each participant is assigned a random waiting period and the shortest wait wins.
For this to be fair, participants must not be able to fake their waiting time. The mechanism relies on a trusted execution environment, which is a protected area inside a processor that runs code in isolation and can prove to others that it ran correctly.
Intel's SGX technology is the best known example, and the approach was developed by Intel and used in the Hyperledger Sawtooth project. When a block is due, each node's protected area generates a random wait time and issues a certificate that proves the time was genuinely assigned.
The node with the shortest wait wakes up first, builds the block, and shares its certificate so that others can verify it before accepting the block. The big advantage is that it uses almost no energy and gives every node roughly an equal chance, regardless of wealth or computing power.
This suits consortium blockchains, where banks, suppliers or other businesses share a ledger and want a cheap, quick method of agreement. The weakness is that trust moves from the network to the hardware maker.
If the secure chips have a flaw, or if someone finds a way to manipulate the timer, the fairness of the system is damaged, and the design suits closed groups better than open public networks. For a business weighing up the options, the key question is how much it trusts the hardware supply chain and the other members of the group.
If the answer is a great deal, proof of elapsed time can deliver fast agreement at very low cost, and if the answer is not much, a method that does not rely on special chips may be safer.
In practice
Real-world examples.
Example
A group of eight logistics companies runs a shared ledger to track containers. They use proof of elapsed time so that every member has a fair chance of recording blocks without paying for expensive mining hardware.
Example
A bank consortium tests a trade finance network. The technology team chooses proof of elapsed time because it is fast and low in energy, and because all of the participants are identified and regulated.
Example
A security consultant warns a client that the system depends on the secure chips working as claimed. She recommends including a plan for updating or replacing the hardware if a vulnerability is discovered.
Formula
Calculation
If every node has the same hardware and the same random wait distribution, each has an equal chance of being first:
Probability of winning a block = 1 / Number of nodes
Expected blocks = Probability x Total blocks
Suppose a consortium of 25 nodes produces 100 blocks in a day.
Probability of winning each block = 1 / 25 = 4%.
Expected blocks per node = 4% x 100 = 4 blocks.
If each block carries fees worth $50, each node can expect about 4 x $50 = $200 per day. Unlike proof of work, adding more computing power would not increase this share.Case study
Seen in the real world.
Tidewater Freight Network is an illustrative, fictional alliance of 12 shipping and port companies that wanted a shared record of cargo handovers. They rejected proof of work because none of them wanted to buy mining equipment for what was essentially a record-keeping tool.
They adopted proof of elapsed time, with each member running a node on a server whose processor supported a trusted execution environment. Blocks were created every few seconds and no member could dominate, because every node had an equal one-in-twelve chance.
During an audit, the security team found that one member had not applied a firmware update that fixed a known weakness in its secure hardware. The alliance made updates compulsory and monitored compliance each month. The illustrative lesson is that the method is efficient and fair only if the hardware it depends on is properly maintained.
Watch out
Common mistakes.
- Assuming proof of elapsed time works for any public cryptocurrency, when it is designed mainly for permissioned networks where members are known.
- Forgetting that the fairness of the lottery depends on specific secure hardware, which becomes a point of trust and a possible weakness.
- Thinking it has no cost, when members still need compatible servers and ongoing security maintenance.
Questions
People also ask.
Why is it called proof of elapsed time?
Because the winning node proves, through a certificate from its secure hardware, that it genuinely waited the random time it was assigned.
Does proof of elapsed time use less energy than proof of work?
Yes, much less, because nodes mostly sit idle while waiting and do not compete in heavy computation.
Who would use proof of elapsed time?
Mostly businesses and consortia that want a fast, low-cost way to agree on shared records, rather than open public networks.
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