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Certificate revocation
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Certificate revocation
In public key cryptography, a certificate may be revoked before it expires, which signals that it is no longer valid. Without revocation, an attacker could exploit such a compromised or misissued certificate until expiry. Hence, revocation is an important part of a public key infrastructure. Revocation is performed by the issuing certificate authority, which produces a cryptographically authenticated statement of revocation.
For distributing revocation information to clients, the timeliness of the discovery of revocation (and hence the window for an attacker to exploit a compromised certificate) trades off against resource usage in querying revocation statuses and privacy concerns. If revocation information is unavailable (either due to an accident or an attack), clients must decide whether to fail-hard and treat a certificate as if it is revoked (and so degrade availability) or to fail-soft and treat it as unrevoked (and allow attackers to sidestep revocation).
Due to the cost of revocation checks and the availability impact from potentially-unreliable remote services, Web browsers limit the revocation checks they will perform, and will fail soft where they do. Certificate revocation lists are too bandwidth-costly for routine use, and the Online Certificate Status Protocol presents connection latency and privacy issues. Other schemes have been proposed but have not yet been successfully deployed to enable fail-hard checking.
The Heartbleed vulnerability, which was disclosed in 2014, triggered a mass revocation of certificates, as their private keys may have been leaked. GlobalSign revoked over 50% of their issued certificates. StartCom was criticised for issuing free certificates but then charging for revocation.
A 2015 study found an overall revocation rate of 8% for certificates used on the Web, though this may have been elevated due to Heartbleed.
Despite Web security being a priority for most browsers, due to the latency and bandwidth requirements associated with OCSP and CRLs, browsers place limits on checking certificate status. In 2015, Google Chrome only actively checked Extended Validation certificates, no mobile browser performed any validity checks, and no browser fully checked all certificates. Chrome and Firefox perform push-based checks for a small set of domains deemed critical; Chrome calls these CRLsets, and, in 2025, they cover approximately 1% of all revocations for a daily download cost of 600 kB. Browsers show little agreement in corner cases around certificate validity, potentially confusing even experienced users.
The number of certificates in the Web PKI increased massively during the last portion of the 2010s, from 30 million in January 2017 to 434 million in January 2020. A significant factor in this growth is Let's Encrypt providing free domain-validated certificates. The size of the potentially-revocable set of certificates places requirements on the scalability of the revocation mechanism.
Chuat et al. (2020) call revocation "notoriously challenging". In 2022, RFC 9325 characterised certificate revocation as an important problem with "no complete and efficient solution". OCSP and OCSP stapling are recommended as the "foundation for a possible solution".
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Certificate revocation
In public key cryptography, a certificate may be revoked before it expires, which signals that it is no longer valid. Without revocation, an attacker could exploit such a compromised or misissued certificate until expiry. Hence, revocation is an important part of a public key infrastructure. Revocation is performed by the issuing certificate authority, which produces a cryptographically authenticated statement of revocation.
For distributing revocation information to clients, the timeliness of the discovery of revocation (and hence the window for an attacker to exploit a compromised certificate) trades off against resource usage in querying revocation statuses and privacy concerns. If revocation information is unavailable (either due to an accident or an attack), clients must decide whether to fail-hard and treat a certificate as if it is revoked (and so degrade availability) or to fail-soft and treat it as unrevoked (and allow attackers to sidestep revocation).
Due to the cost of revocation checks and the availability impact from potentially-unreliable remote services, Web browsers limit the revocation checks they will perform, and will fail soft where they do. Certificate revocation lists are too bandwidth-costly for routine use, and the Online Certificate Status Protocol presents connection latency and privacy issues. Other schemes have been proposed but have not yet been successfully deployed to enable fail-hard checking.
The Heartbleed vulnerability, which was disclosed in 2014, triggered a mass revocation of certificates, as their private keys may have been leaked. GlobalSign revoked over 50% of their issued certificates. StartCom was criticised for issuing free certificates but then charging for revocation.
A 2015 study found an overall revocation rate of 8% for certificates used on the Web, though this may have been elevated due to Heartbleed.
Despite Web security being a priority for most browsers, due to the latency and bandwidth requirements associated with OCSP and CRLs, browsers place limits on checking certificate status. In 2015, Google Chrome only actively checked Extended Validation certificates, no mobile browser performed any validity checks, and no browser fully checked all certificates. Chrome and Firefox perform push-based checks for a small set of domains deemed critical; Chrome calls these CRLsets, and, in 2025, they cover approximately 1% of all revocations for a daily download cost of 600 kB. Browsers show little agreement in corner cases around certificate validity, potentially confusing even experienced users.
The number of certificates in the Web PKI increased massively during the last portion of the 2010s, from 30 million in January 2017 to 434 million in January 2020. A significant factor in this growth is Let's Encrypt providing free domain-validated certificates. The size of the potentially-revocable set of certificates places requirements on the scalability of the revocation mechanism.
Chuat et al. (2020) call revocation "notoriously challenging". In 2022, RFC 9325 characterised certificate revocation as an important problem with "no complete and efficient solution". OCSP and OCSP stapling are recommended as the "foundation for a possible solution".