Derive the NIST SP 800-57 cryptoperiod for a key type, add the retention tail and check the strength holds that long.
NIST splits a key's life into an originator-usage period, when you may encrypt or sign with it, and a longer recipient-usage period that must cover every day the protected data has to be readable. The total lifetime is the sum, and the algorithm's strength has to remain adequate across all of it. A key protecting seven years of archived records is exposed to seven years of cryptanalytic and quantum progress, so its strength must be chosen for the retention tail rather than the usage window.
Key Lifetime
recipient-usage period = originator cryptoperiod + ciphertext retention; NIST guidance is ~2 years for data keys, ~3 for private signing keys.
recipient-usage period = originator cryptoperiod + ciphertext retention; NIST guidance is ~2 years for data keys, ~3 for private signing keys. NIST splits a key's life into an originator-usage period, when you may encrypt or sign with it, and a longer recipient-usage period that must cover every day the protected data has to be readable. The total lifetime is the sum, and the algorithm's strength has to remain adequate across all of it.
A key protecting seven years of archived records is exposed to seven years of cryptanalytic and quantum progress, so its strength must be chosen for the retention tail rather than the usage window.
This calculator takes 4 inputs: Key type, Planned originator-usage period, Ciphertext retention after last use, Algorithm security strength. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
You stop encrypting with a key long before you stop needing to decrypt what it protected. The key must survive in escrow for the whole retention tail, which is also why that tail — not the usage window — drives the strength you choose.