1. Informal Introduction

1. Informal Introduction

Certificate transparency aims to mitigate the problem of misissued certificates by providing publicly auditable, append-only, untrusted logs of all issued certificates. The logs are publicly auditable so that it is possible for anyone to verify the correctness of each log and to monitor when new certificates are added to it. The logs do not themselves prevent misissue, but they ensure that interested parties (particularly those named in certificates) can detect such misissuance. Note that this is a general mechanism, but in this document, we only describe its use for public TLS server certificates issued by public certificate authorities (CAs).

Each log consists of certificate chains, which can be submitted by anyone. It is expected that public CAs will contribute all their newly issued certificates to one or more logs; it is also expected that certificate holders will contribute their own certificate chains. In order to avoid logs being spammed into uselessness, it is required that each chain is rooted in a known CA certificate. When a chain is submitted to a log, a signed timestamp is returned, which can later be used to provide evidence to clients that the chain has been submitted. TLS clients can thus require that all certificates they see have been logged.

Those who are concerned about misissue can monitor the logs, asking them regularly for all new entries, and can thus check whether domains they are responsible for have had certificates issued that they did not expect. What they do with this information, particularly when they find that a misissuance has happened, is beyond the scope of this document, but broadly speaking, they can invoke existing business mechanisms for dealing with misissued certificates. Of course, anyone who wants can monitor the logs and, if they believe a certificate is incorrectly issued, take action as they see fit.

Similarly, those who have seen signed timestamps from a particular log can later demand a proof of inclusion from that log. If the log is unable to provide this (or, indeed, if the corresponding certificate is absent from monitors' copies of that log), that is evidence of the incorrect operation of the log. The checking operation is asynchronous to allow TLS connections to proceed without delay, despite network connectivity issues and the vagaries of firewalls.

The append-only property of each log is technically achieved using Merkle Trees, which can be used to show that any particular version of the log is a superset of any particular previous version. Likewise, Merkle Trees avoid the need to blindly trust logs: if a log attempts to show different things to different people, this can be efficiently detected by comparing tree roots and consistency proofs. Similarly, other misbehaviors of any log (e.g., issuing signed timestamps for certificates they then don't log) can be efficiently detected and proved to the world at large.