Post Syndicated from Jeremy Stieglitz original https://aws.amazon.com/blogs/security/the-curious-case-of-faster-aws-kms-symmetric-key-rotation/

Today, AWS Key Management Service (AWS KMS) is introducing faster options for automatic symmetric key rotation. We’re also introducing rotate on-demand, rotation visibility improvements, and a new limit on the price of all symmetric keys that have had two or more rotations (including existing keys). In this post, I discuss all those capabilities and changes. I also present a broader overview of how symmetric cryptographic key rotation came to be, and cover our recommendations on when you might need rotation and how often to rotate your keys. If you’ve ever been curious about AWS KMS automatic key rotation—why it exists, when to enable it, and when to use it on-demand—read on.

How we got here

There are longstanding reasons for cryptographic key rotation. If you were Caesar in Roman times and you needed to send messages with sensitive information to your regional commanders, you might use keys and ciphers to encrypt and protect your communications. There are well-documented examples of using cryptography to protect communications during this time, so much so that the standard substitution cipher, where you swap each letter for a different letter that is a set number of letters away in the alphabet, is referred to as Caesar’s cipher. The cipher is the substitution mechanism, and the key is the number of letters away from the intended letter you go to find the substituted letter for the ciphertext.

The challenge for Caesar in relying on this kind of symmetric key cipher is that both sides (Caesar and his field generals) needed to share keys and keep those keys safe from prying eyes. What happens to Caesar’s secret invasion plans if the key used to encipher his attack plan was secretly intercepted in transmission down the Appian Way? Caesar had no way to know. But if he rotated keys, he could limit the scope of which messages could be read, thus limiting his risk. Messages sent under a key created in the year 52 BCE wouldn’t automatically work for messages sent the following year, provided that Caesar rotated his keys yearly and the newer keys weren’t accessible to the adversary. Key rotation can reduce the scope of data exposure (what a threat actor can see) when some but not all keys are compromised. Of course, every time the key changed, Caesar had to send messengers to his field generals to communicate the new key. Those messengers had to ensure that no enemies intercepted the new keys without their knowledge – a daunting task.

Figure 1: The state of the art for secure key rotation and key distribution in 52 BC.

Fast forward to the 1970s–2000s

In modern times, cryptographic algorithms designed for digital computer systems mean that keys no longer travel down the Appian Way. Instead, they move around digital systems, are stored in unprotected memory, and sometimes are printed for convenience. The risk of key leakage still exists, therefore there is a need for key rotation. During this period, more significant security protections were developed that use both software and hardware technology to protect digital cryptographic keys and reduce the need for rotation. The highest-level protections offered by these techniques can limit keys to specific devices where they can never leave as plaintext. In fact, the US National Institute of Standards and Technologies (NIST) has published a specific security standard, FIPS 140, that addresses the security requirements for these cryptographic modules.

Modern cryptography also has the risk of cryptographic key wear-out

Besides addressing risks from key leakage, key rotation has a second important benefit that becomes more pronounced in the digital era of modern cryptography—cryptographic key wear-out. A key can become weaker, or “wear out,” over time just by being used too many times. If you encrypt enough data under one symmetric key, and if a threat actor acquires enough of the resulting ciphertext, they can perform analysis against your ciphertext that will leak information about the key. Current cryptographic recommendations to protect against key wear-out can vary depending on how you’re encrypting data, the cipher used, and the size of your key. However, even a well-designed AES-GCM implementation with robust initialization vectors (IVs) and large key size (256 bits) should be limited to encrypting no more than 4.3 billion messages (2³²), where each message is limited to about 64 GiB under a single key.

Figure 2: Used enough times, keys can wear out.

During the early 2000s, to help federal agencies and commercial enterprises navigate key rotation best practices, NIST formalized several of the best practices for cryptographic key rotation in the NIST SP 800-57 Recommendation for Key Management standard. It’s an excellent read overall and I encourage you to examine Section 5.3 in particular, which outlines ways to determine the appropriate length of time (the cryptoperiod) that a specific key should be relied on for the protection of data in various environments. According to the guidelines, the following are some of the benefits of setting cryptoperiods (and rotating keys within these periods):

5.3 Cryptoperiods

A cryptoperiod is the time span during which a specific key is authorized for use by legitimate entities or the keys for a given system will remain in effect. A suitably defined cryptoperiod:

1. Limits the amount of information that is available for cryptanalysis to reveal the key (e.g. the number of plaintext and ciphertext pairs encrypted with the key);
2. Limits the amount of exposure if a single key is compromised;
3. Limits the use of a particular algorithm (e.g., to its estimated effective lifetime);
4. Limits the time available for attempts to penetrate physical, procedural, and logical access mechanisms that protect a key from unauthorized disclosure;
5. Limits the period within which information may be compromised by inadvertent disclosure of a cryptographic key to unauthorized entities; and
6. Limits the time available for computationally intensive cryptanalysis.

Sometimes, cryptoperiods are defined by an arbitrary time period or maximum amount of data protected by the key. However, trade-offs associated with the determination of cryptoperiods involve the risk and consequences of exposure, which should be carefully considered when selecting the cryptoperiod (see Section 5.6.4).

(Source: NIST SP 800-57 Recommendation for Key Management, page 34).

One of the challenges in applying this guidance to your own use of cryptographic keys is that you need to understand the likelihood of each risk occurring in your key management system. This can be even harder to evaluate when you’re using a managed service to protect and use your keys.

Fast forward to the 2010s: Envisioning a key management system where you might not need automatic key rotation

When we set out to build a managed service in AWS in 2014 for cryptographic key management and help customers protect their AWS encryption workloads, we were mindful that our keys needed to be as hardened, resilient, and protected against external and internal threat actors as possible. We were also mindful that our keys needed to have long-term viability and use built-in protections to prevent key wear-out. These two design constructs—that our keys are strongly protected to minimize the risk of leakage and that our keys are safe from wear out—are the primary reasons we recommend you limit key rotation or consider disabling rotation if you don’t have compliance requirements to do so. Scheduled key rotation in AWS KMS offers limited security benefits to your workloads.

Specific to key leakage, AWS KMS keys in their unencrypted, plaintext form cannot be accessed by anyone, even AWS operators. Unlike Caesar’s keys, or even cryptographic keys in modern software applications, keys generated by AWS KMS never exist in plaintext outside of the NIST FIPS 140-2 Security Level 3 fleet of hardware security modules (HSMs) in which they are used. See the related post AWS KMS is now FIPS 140-2 Security Level 3. What does this mean for you? for more information about how AWS KMS HSMs help you prevent unauthorized use of your keys. Unlike many commercial HSM solutions, AWS KMS doesn’t even allow keys to be exported from the service in encrypted form. Why? Because an external actor with the proper decryption key could then expose the KMS key in plaintext outside the service.

This hardened protection of your key material is salient to the principal security reason customers want key rotation. Customers typically envision rotation as a way to mitigate a key leaking outside the system in which it was intended to be used. However, since KMS keys can be used only in our HSMs and cannot be exported, the possibility of key exposure becomes harder to envision. This means that rotating a key as protection against key exposure is of limited security value. The HSMs are still the boundary that protects your keys from unauthorized access, no matter how many times the keys are rotated.

If we decide the risk of plaintext keys leaking from AWS KMS is sufficiently low, don’t we still need to be concerned with key wear-out? AWS KMS mitigates the risk of key wear-out by using a key derivation function (KDF) that generates a unique, derived AES 256-bit key for each individual request to encrypt or decrypt under a 256-bit symmetric KMS key. Those derived encryption keys are different every time, even if you make an identical call for encrypt with the same message data under the same KMS key. The cryptographic details for our key derivation method are provided in the AWS KMS Cryptographic Details documentation, and KDF operations use the KDF in counter mode, using HMAC with SHA256. These KDF operations make cryptographic wear-out substantially different for KMS keys than for keys you would call and use directly for encrypt operations. A detailed analysis of KMS key protections for cryptographic wear-out is provided in the Key Management at the Cloud Scale whitepaper, but the important take-away is that a single KMS key can be used for more than a quadrillion (2⁵⁰) encryption requests without wear-out risk.

In fact, within the NIST 800-57 guidelines is consideration that when the KMS key (key-wrapping key in NIST language) is used with unique data keys, KMS keys can have longer cryptoperiods:

“In the case of these very short-term key-wrapping keys, an appropriate cryptoperiod (i.e., which includes both the originator and recipient-usage periods) is a single communication session. It is assumed that the wrapped keys will not be retained in their wrapped form, so the originator-usage period and recipient-usage period of a key-wrapping key is the same. In other cases, a key-wrapping key may be retained so that the files or messages encrypted by the wrapped keys may be recovered later. In such cases, the recipient-usage period may be significantly longer than the originator-usage period of the key-wrapping key, and cryptoperiods lasting for years may be employed.”

Source: NIST 800-57 Recommendations for Key Management, section 5.3.6.7.

So why did we build key rotation in AWS KMS in the first place?

Although we advise that key rotation for KMS keys is generally not necessary to improve the security of your keys, you must consider that guidance in the context of your own unique circumstances. You might be required by internal auditors, external compliance assessors, or even your own customers to provide evidence of regular rotation of all keys. A short list of regulatory and standards groups that recommend key rotation includes the aforementioned NIST 800-57, Center for Internet Security (CIS) benchmarks, ISO 27001, System and Organization Controls (SOC) 2, the Payment Card Industry Data Security Standard (PCI DSS), COBIT 5, HIPAA, and the Federal Financial Institutions Examination Council (FFIEC) Handbook, just to name a few.

Customers in regulated industries must consider the entirety of all the cryptographic systems used across their organizations. Taking inventory of which systems incorporate HSM protections, which systems do or don’t provide additional security against cryptographic wear-out, or which programs implement encryption in a robust and reliable way can be difficult for any organization. If a customer doesn’t have sufficient cryptographic expertise in the design and operation of each system, it becomes a safer choice to mandate a uniform scheduled key rotation.

That is why we offer an automatic, convenient method to rotate symmetric KMS keys. Rotation allows customers to demonstrate this key management best practice to their stakeholders instead of having to explain why they chose not to.

Figure 3 details how KMS appends new key material within an existing KMS key during each key rotation.

Figure 3: KMS key rotation process

We designed the rotation of symmetric KMS keys to have low operational impact to both key administrators and builders using those keys. As shown in Figure 3, a keyID configured to rotate will append new key material on each rotation while still retaining and keeping the existing key material of previous versions. This append method achieves rotation without having to decrypt and re-encrypt existing data that used a previous version of a key. New encryption requests under a given keyID will use the latest key version, while decrypt requests under that keyID will use the appropriate version. Callers don’t have to name the version of the key they want to use for encrypt/decrypt, AWS KMS manages this transparently.

Some customers assume that a key rotation event should forcibly re-encrypt any data that was ever encrypted under the previous key version. This is not necessary when AWS KMS automatically rotates to use a new key version for encrypt operations. The previous versions of keys required for decrypt operations are still safe within the service.

We’ve offered the ability to automatically schedule an annual key rotation event for many years now. Lately, we’ve heard from some of our customers that they need to rotate keys more frequently than the fixed period of one year. We will address our newly launched capabilities to help meet these needs in the final section of this blog post.

More options for key rotation in AWS KMS (with a price reduction)

After learning how we think about key rotation in AWS KMS, let’s get to the new options we’ve launched in this space:

• Configurable rotation periods: Previously, when using automatic key rotation, your only option was a fixed annual rotation period. You can now set a rotation period from 90 days to 2,560 days (just over seven years). You can adjust this period at any point to reset the time in the future when rotation will take effect. Existing keys set for rotation will continue to rotate every year.
• On-demand rotation for KMS keys: In addition to more flexible automatic key rotation, you can now invoke on-demand rotation through the AWS Management Console for AWS KMS, the AWS Command Line Interface (AWS CLI), or the AWS KMS API using the new RotateKeyOnDemand API. You might occasionally need to use on-demand rotation to test workloads, or to verify and prove key rotation events to internal or external stakeholders. Invoking an on-demand rotation won’t affect the timeline of any upcoming rotation scheduled for this key.
  
  

  Note: We’ve set a default quota of 10 on-demand rotations for a KMS key. Although the need for on-demand key rotation should be infrequent, you can ask to have this quota raised. If you have a repeated need for testing or validating instant key rotation, consider deleting the test keys and repeating this operation for RotateKeyOnDemand on new keys.

• Improved visibility: You can now use the AWS KMS console or the new ListKeyRotations API to view previous key rotation events. One of the challenges in the past is that it’s been hard to validate that your KMS keys have rotated. Now, every previous rotation for a KMS key that has had a scheduled or on-demand rotation is listed in the console and available via API.
   
  

  Figure 4: Key rotation history showing date and type of rotation

• Price cap for keys with more than two rotations: We’re also introducing a price cap for automatic key rotation. Previously, each annual rotation of a KMS key added $1 per month to the price of the key. Now, for KMS keys that you rotate automatically or on-demand, the first and second rotation of the key adds $1 per month in cost (prorated hourly), but this price increase is capped at the second rotation. Rotations after your second rotation aren’t billed. Existing customers that have keys with three or more annual rotations will see a price reduction for those keys to $3 per month (prorated) per key starting in the month of May, 2024.

Summary

In this post, I highlighted the more flexible options that are now available for key rotation in AWS KMS and took a broader look into why key rotation exists. We know that many customers have compliance needs to demonstrate key rotation everywhere, and increasingly, to demonstrate faster or immediate key rotation. With the new reduced pricing and more convenient ways to verify key rotation events, we hope these new capabilities make your job easier.

Flexible key rotation capabilities are now available in all AWS Regions, including the AWS GovCloud (US) Regions. To learn more about this new capability, see the Rotating AWS KMS keys topic in the AWS KMS Developer Guide.

 
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