What a Proof Proves

A proof with an ETC attestation shows that your digest existed before the ETC block holding the anchor transaction was mined. The block's time is an upper bound on when the digest was made, and the proof does not name who made it.

A verifier reads only the bytes at that position in that block. The calendar can be offline. Who sent the transaction makes no difference.

The round commitment carries the second your digest arrived, as the calendar recorded it, and only the calendar can vouch for that second, the same as with any OpenTimestamps calendar. The beacon gives each tick a lower bound, because a tick can only name blocks that already existed. That bound is for the tick. Your digest could have been made at any time before it.

An anchored proof shows your digest existed before the anchor block, and before a Bitcoin block hours later. The chain heads in a tick bound when the tick was built. They say nothing about when your digest was made.

Why a Proof-of-Work Anchor

An Etchd proof is a chain of SHA-256 hashes that ends in an Ethereum Classic transaction and, through the hourly bridge, in a Bitcoin block as well. No step in it is a signature. Large quantum computers are expected to break the elliptic-curve signatures most blockchains use, while hash functions hold up better, and the best known quantum attack on SHA-256 halves its effective strength to about 128 bits.

Whoever forges enough old validator keys on a proof-of-stake chain can sign a different past. No key helps on Ethereum Classic. Rewriting one of its blocks means redoing all the work piled on top of it, and a proof that your file existed before such a block rests on hashing alone.

The anchor transactions are still signed. A broken signing key would let someone send anchors that look like ours, and for that case we publish our signer keys and their dates. We do not know when quantum computers that large will exist.

Precision

The upper bound comes from the anchor block. These are the waits between your stamp and a proof you can verify.

Step	How long
The rest of the current minute, until the tick closes	0 to 60 s
Signing and broadcasting the batch, then mining it into an ETC block	The calendar alerts when the last mined anchor is more than five minutes old, and refuses new stamps once it is 15 minutes old
Any further confirmations the verifier asks for	About 13 s a block, so 400 take about 90 minutes. The proof still dates from the anchor block

The time in a block header is set by the miner who found the block, within the limits ETC consensus allows. The Bitcoin branch adds a second, looser upper bound from a later Bitcoin block.