BFT signatures: threshold, aggregation, and AggQC evidence

Start by identifying what every replica signed

A regular QC collects 2f + 1 votes over the same proposal. Message equality lets the protocol compress evidence without changing its semantic claim. An aggregate signature may combine signatures, but verification must still reconstruct signers, message, domain separation, and committee membership.

Threshold and aggregate are not interchangeable labels

In a threshold scheme, replicas produce compatible shares and the quorum interpolates one signature under a joint public key. With BLS aggregation, complete signatures may be combined, even over distinct messages, provided the verifier preserves the mapping among keys, messages, and proofs of possession.

• Compression saves bytes; it does not replace quorum or signed-domain validation.
• Operational security also depends on key distribution, rotation, and defenses against rogue-key attacks.

AggQC proves a view selection, not only a proposal

After a timeout, each replica may report a different highQC. AggQC proves that the new leader received evidence from 2f + 1 participants and selected the highest verifiable QC. It is therefore not merely a larger regular QC: it certifies a safe choice among distinct reports.