Cross-Signing
Cross-signing refers to a process in which an already trusted authority confirms a new digital certificate with its signature – thereby immediately granting the new authority credibility. It solves the practical problem that new participants on the internet first need to become known before devices and browsers trust them.
On the internet, websites need a digital certificate, a kind of official ID that proves the site is genuine. This ID is issued by a certificate authority — an organization that browsers and operating systems fundamentally trust. Trust works here like a chain: because you trust the issuer, you also trust the ID. Cross-signing addresses exactly this chain. An already recognized certificate authority signs the certificate of a new or still unknown authority — thereby lending it its own trust. The new authority can then immediately issue certificates that are accepted by all devices, without first having to build up recognition over many years itself.
Why cross-signing made the internet accessible
Before around 2015, a website certificate often cost several hundred euros per year. Small websites, school projects, or nonprofit offerings could hardly afford that. The nonprofit organization Let’s Encrypt wanted to offer free certificates for everyone — but had a problem: its own certificate authority was brand new and not pre-installed on any device in the world.
The solution was cross-signing. The established certificate authority IdenTrust signed Let’s Encrypt’s root certificate. Suddenly, all browsers and smartphones recognized Let’s Encrypt as trustworthy — even though it itself was not yet on their trust lists. The result: within a few years, the share of encrypted websites rose from under 40% to over 80%. Cross-signing was a decisive mechanism for this.
The mechanics behind mutual confirmation
A digital certificate contains a public key and information about the holder. The issuing certificate authority places its digital signature underneath — comparable to a signature on a document. With cross-signing, there are now two signatures on the same certificate or the same key: that of the new authority itself and that of the already trusted authority. A browser that does not yet know the new authority can follow the second signature and thus arrive at a certificate authority it already trusts.
Technically, this creates two separate certificate chains that lead to the same endpoint. Older devices use the chain via the known authority, while newer devices — which by now know the new authority directly — use the shorter, more direct chain. This makes cross-signing particularly useful during transition periods in which new and old devices are in use simultaneously.
Important: cross-signing does not transfer unlimited trust. The new authority may only issue certificates in the areas that the confirming authority permits. Certificate authorities can technically embed these restrictions in the cross-certificate.
Cross-signing in products, systems, and headlines
Cross-signing makes headlines whenever certificate chains expire or break. In September 2021, exactly this happened with IdenTrust’s DST root certificate, which had cross-signed Let’s Encrypt. Older Android devices that did not know the newer Let’s Encrypt root certificate abruptly lost access to millions of websites. This was a global incident triggered by an expired cross-signing certificate.
However, cross-signing is not limited to websites. In software development, operating system manufacturers such as Microsoft or Apple sign third-party code — the principle is the same. Cross-signing also ensures in email encryption and software updates that new participants don’t have to start from zero.
A common misconception: cross-signing does not mean that two authorities confirm each other. The direction is clear — the established authority confirms the new one, not the other way around. Mutual confirmation would be a separate process and would require two independent cross-signings.