How the interviewer opens it
“Design a public, append-only log of issued TLS certificates with efficient inclusion and consistency proofs. Detection of conflicting histories depends on independent observers who exchange evidence. The service must tolerate server failures.”
The prompt is vague on purpose. The first minutes are for you. Reduce the prompt to the core features and agree on the numbers before you draw.
What to clarify first
- Accepted certificates
- Scale
- Read and write mix
- Latency
- Consistency and availability
- Durability and retention
- Receipts and publication
- Proof meaning
- Trust model
- Access patterns
The interviewer has the answers. In a SysRound round, Alex gives these answers, as a real interviewer does.
Scope of a strong answer
Core requirements
- Accept certificates and return signed receipts
- Publish ordered entries and signed checkpoints
- Provide inclusion and consistency proofs
- Support independent checks of history and publication deadlines
Usually out of scope
- Issue or revoke certificates
- Change browser trust rules
- Support precertificates or exact CT wire formats
What interviewers ask about
- Proofs as the log grows
- The log grows while clients refer to older checkpoints. Each proof must match the exact tree size that its checkpoint signs.
- Receipts and publication deadlines
- A receipt promises publication before a deadline. A quorum outage can break that promise after acceptance.
- History during failures
- Servers can publish conflicting roots after a leader change. Read availability must not permit competing histories.
- Independent audits
- A valid signature identifies the log, not an honest history. An isolated observer cannot detect every internally consistent split view.
Common mistakes
- Treat a signed receipt as an inclusion proof.
- Assume signed checkpoints alone prove that every client sees one history.
- Generate every proof from the latest tree and ignore the requested tree size.
- Acknowledge a certificate before durable commit.
- Promise a fixed publication deadline during an arbitrarily long quorum outage.
- Treat a failed proof request as proof of omission or a missed deadline.
The bar by level
- Senior (L5/E5)
- Preserves receipts through retries and failover. Serves historical proofs without full scans. Defines safe partition behavior and the publication failure envelope.
- Staff (L6+/E6+)
- Defines evidence limits for conflicting histories and missed deadlines. Separates crash safety from operator trust. Plans independent observation, signer control, and long-term capacity.
Sources · 1 reference
These records explain where the question appears. A company tag or candidate report does not guarantee a future interview question.
SysRound · 1 record
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Design a certificate transparency log
Reported date not specified
Source snapshot: Nov 12, 2026
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