Today, AWS Key Management Service (AWS KMS) unveils enhanced options for automatic symmetric key rotation. This update includes on-demand rotation, improved visibility into rotation processes, and a new pricing cap for all symmetric keys that have undergone two or more rotations, including those already in use. In this post, I will delve into these features and changes, provide a historical perspective on symmetric cryptographic key rotation, and share our recommendations on when and how often to rotate your keys. If you’ve ever wondered about the automatic key rotation in AWS KMS—its purpose, when to activate it, and the scenarios for on-demand use—keep reading.
Historical Context
The concept of cryptographic key rotation has deep roots. Imagine yourself as Julius Caesar in ancient Rome, needing to send encrypted messages to your generals. You would employ keys and ciphers to safeguard your communications. Notably, Caesar’s cipher—where letters are swapped based on a fixed number in the alphabet—illustrates this. The key determines how far to shift letters in the message.
The challenge for Caesar was ensuring that both he and his generals kept their keys secure from enemies. If a rival intercepted the key used for his plans, the secrecy would be compromised. However, by rotating keys, Caesar could mitigate risks by limiting the messages that could be deciphered. If he changed keys yearly, messages sent under an older key would not be readable with newer keys, assuming the adversary didn’t have access to the latest.
Advancements from the 1970s to 2000s
Fast forward to the late 20th century, and the landscape of cryptography had transformed. Keys no longer traveled physically but were stored digitally, sometimes even in unprotected memory. This shift increased the risk of key leakage, necessitating regular rotation. During this period, significant advancements emerged in both software and hardware to safeguard keys, with standards like NIST’s FIPS 140 establishing security requirements for cryptographic modules.
Additionally, the digital era introduced the risk of cryptographic key wear-out. Over time, excessive use of a single symmetric key could weaken it. If a threat actor accumulates enough ciphertext, they could potentially analyze it to expose the key. Current practices suggest limiting the number of messages encrypted under a single key to mitigate this risk.
NIST Recommendations
In the early 2000s, NIST formalized best practices for key rotation through its SP 800-57 Recommendation for Key Management. This essential document emphasizes the importance of defining a cryptoperiod—the time frame a key is deemed secure. A well-defined cryptoperiod can:
- Restrict the amount of data available for cryptanalysis.
- Limit exposure if a key is compromised.
- Ensure the longevity of algorithms in use.
However, establishing an appropriate cryptoperiod can be challenging, especially in managed services, where risks need to be evaluated carefully.
Looking Ahead to the 2010s
When AWS embarked on creating a managed service for cryptographic key management in 2014, our focus was on ensuring that keys were resilient and protected against both external and internal threats. We recognized the importance of designing keys that minimize leakage risks and resist wear-out. These priorities guide our recommendation to limit or even disable key rotation unless compliance mandates otherwise. In AWS KMS, scheduled key rotation offers limited security benefits for many users.
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