Certificate Validity Calculator

This calculator estimates the real-world cryptographic strength of an RSA certificate and its signing hash, then reports whichever one is weaker as the certificate's overall security margin. Certificate Security is not simply half the RSA key length -- RSA's best known attack is the sub-exponential General Number Field Sieve, not brute force, so its strength doesn't scale linearly with key size the way a symmetric cipher's does. Instead this figure interpolates between the equivalence points published in Table 2 of NIST SP 800-57 Part 1 Revision 5, the federal government's own reference table for comparable cryptographic strength (nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-57pt1r5.pdf): a 2048-bit RSA key is treated as roughly 112-bit strength, 3072-bit as 128-bit, 7680-bit as 192-bit, and 15360-bit as 256-bit, with values outside that tabulated range extrapolated from the nearest segment's slope rather than a separately invented constant. Hash Collision Resistance, by contrast, genuinely is half the hash's output length -- that's the standard birthday-bound result for collision resistance, distinct from the (larger) preimage-resistance figure, and it scales directly with the SHA-256/384/512 selection.

Overall Security Level always takes the smaller of the two, since a certificate's real-world strength is bounded by its weakest cryptographic component, and Recommended Renewal Period is tiered off that same Overall Security Level figure rather than off the raw key length. It is then capped at the CA/Browser Forum's maximum public TLS certificate lifetime, which Ballot SC-081v3 stepped down to 200 days in March 2026 and steps down again to 100 days in 2027 and 47 days in 2029 -- so for a publicly-trusted certificate the calendar ceiling, not the key strength, is what actually sets your renewal cadence. Key Strength Status flags what the bit figure does not: RSA below 2,048 bits is below the NIST SP 800-131A and CA/Browser Forum minimum and no public CA will issue it, and below 1,024 bits it is factorable with commodity hardware regardless of what the extrapolated bit count says.