Telia: Certificates with RSA keys where modulus is not divisible by 8

Telia reported an incident involving certificates with RSA keys where the modulus was not divisible by 8. Telia said it became aware of the problem when its nightly zlint script produced “not-divisible-by-8” alarms on 2020-10-19 and 2020-10-20, after customers created problematic certificates. Telia stated that it revoked the two invalid certificates after analysis by its PKI team and that it performed a full scan of its database to check for older certificates with the same issue, finding none. Telia also described that it had incorrectly assumed its “nist-public-key-validation” checking would prevent this problem and that zlint scans had not previously detected it because requests had not used this kind of invalid key. In the thread, Telia later reported that it updated all Telia applications that receive CSRs to handle the divisible-by-8 requirement and that, from Telia’s perspective, the issue was closed. Mozilla indicated it would close the bug on or about 13-Nov-2020 unless other remaining issues were raised, and the bug is marked RESOLVED with resolution FIXED.

1. 2020-10-19 Telia customers created the first problematic certificate with an RSA modulus not divisible by 8.
2. 2020-10-20 Telia customers created a second problematic certificate with the same RSA modulus issue.
3. 2020-10-21 Telia revoked both invalid certificates after PKI analysis.
4. 2020-11-09 Telia updated all applications that receive CSRs to enforce the divisible-by-8 requirement.

1. 2020-10-31 Teliasonera representative — Telia reported that zlint alarms showed certificates were issued with RSA moduli not divisible by 8, provided CT links, and described revocation and remediation steps.
2. 2020-10-31 Fozzie representative — A participant questioned why it took Telia about 10–11 days to report the issue after revocation and asked whether Telia checks other CAs’ incident reports.
3. 2020-11-02 Teliasonera representative — Telia responded that it decided to report the issue earlier but delayed due to focusing on recovery steps and waiting for a software fix; Telia also said it checks other CAs’ incident reports and allocated new resources.
4. 2020-11-06 Teliasonera representative — Telia stated a software fix was done for two of three applications and the last one would be completed the next week, and that another team member was trained for incident reporting.
5. 2020-11-09 Teliasonera representative — Telia said it updated all Telia applications receiving CSRs to handle the divisible-by-8 requirement and that the issue was closed from Telia’s perspective.
6. 2020-11-10 Mozilla representative — Mozilla indicated it would close the bug on or about 13-Nov-2020 unless other remaining issues were raised.
7. 2021-02-01 Teliasonera representative — Telia noted that an incident report was accidentally added to comment 7 here and should have been in a different Bugzilla bug.