# Design a certificate transparency log: system design interview question

> Public certificate log: efficient proofs, publication promises, failure tolerance, and independent evidence of conflicting histories.

Source: https://sysround.com/questions/design-certificate-transparency-log
Updated: 2026-11-05


Difficulty: hard. Levels: Senior, Staff+.

## 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."

## 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

## 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 probe

### 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:** Preserves receipts through retries and failover. Serves historical proofs without full scans. Defines safe partition behavior and the publication failure envelope.
- **Staff+:** Defines evidence limits for conflicting histories and missed deadlines. Separates crash safety from operator trust. Plans independent observation, signer control, and long-term capacity.

Practice this question with an AI interviewer: https://sysround.com/practice/new?problem=certificate-transparency


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