All posts by Jeremy Stieglitz

The curious case of faster AWS KMS symmetric key rotation

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.

Illustration of Roman solider on horseback riding through countryside on cobblestone trail.

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.

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

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

    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.

 
If you have feedback about this post, submit comments in the Comments section below. If you have questions about this post, contact AWS Support.

Author

Jeremy Stieglitz

Jeremy is the Principal Product Manager for AWS KMS, where he drives global product strategy and roadmap. Jeremy has more than 25 years of experience defining security products and platforms across large companies (RSA, Entrust, Cisco, and Imperva) and start-up environments (Dataguise, Voltage, and Centrify). Jeremy is the author or co-author of 23 patents in network security, user authentication, and network automation and control.

How to protect HMACs inside AWS KMS

Post Syndicated from Jeremy Stieglitz original https://aws.amazon.com/blogs/security/how-to-protect-hmacs-inside-aws-kms/

Today AWS Key Management Service (AWS KMS) is introducing new APIs to generate and verify hash-based message authentication codes (HMACs) using the Federal Information Processing Standard (FIPS) 140-2 validated hardware security modules (HSMs) in AWS KMS. HMACs are a powerful cryptographic building block that incorporate secret key material in a hash function to create a unique, keyed message authentication code.

In this post, you will learn the basics of the HMAC algorithm as a cryptographic building block, including how HMACs are used. In the second part of this post, you will see a few real-world use cases that show an application builder’s perspective on using the AWS KMS HMAC APIs.

HMACs provide a fast way to tokenize or sign data such as web API requests, credit cards, bank routing information, or personally identifiable information (PII).They are commonly used in several internet standards and communication protocols such as JSON Web Tokens (JWT), and are even an important security component for how you sign AWS API requests.

HMAC as a cryptographic building block

You can consider an HMAC, sometimes referred to as a keyed hash, to be a combination function that fuses the following elements:

  • A standard hash function such as SHA-256 to produce a message authentication code (MAC).
  • A secret key that binds this MAC to that key’s unique value.

Combining these two elements creates a unique, authenticated version of the digest of a message. Because the HMAC construction allows interchangeable hash functions as well as different secret key sizes, one of the benefits of HMACs is the easy replaceability of the underlying hash function (in case faster or more secure hash functions are required), as well as the ability to add more security by lengthening the size of the secret key used in the HMAC over time. The AWS KMS HMAC API is launching with support for SHA-224, SHA-256, SHA-384, and SHA-512 algorithms to provide a good balance of key sizes and performance trade-offs in the implementation. For more information about HMAC algorithms supported by AWS KMS, see HMAC keys in AWS KMS in the AWS KMS Developer Guide.

HMACs offer two distinct benefits:

  1. Message integrity: As with all hash functions, the output of an HMAC will result in precisely one unique digest of the message’s content. If there is any change to the data object (for example you modify the purchase price in a contract by just one digit: from “$350,000” to “$950,000”), then the verification of the original digest will fail.
  2. Message authenticity: What distinguishes HMAC from other hash methods is the use of a secret key to provide message authenticity. Only message hashes that were created with the specific secret key material will produce the same HMAC output. This dependence on secret key material ensures that no third party can substitute their own message content and create a valid HMAC without the intended verifier detecting the change.

HMAC in the real world

HMACs have widespread applications and industry adoption because they are fast, high performance, and simple to use. HMACs are particularly popular in the JSON Web Token (JWT) open standard as a means of securing web applications, and have replaced older technologies such as cookies and sessions. In fact, Amazon implements a custom authentication scheme, Signature Version 4 (SigV4), to sign AWS API requests based on a keyed-HMAC. To authenticate a request, you first concatenate selected elements of the request to form a string. You then use your AWS secret key material to calculate the HMAC of that string. Informally, this process is called signing the request, and the output of the HMAC algorithm is informally known as the signature, because it simulates the security properties of a real signature in that it represents your identity and your intent.

Advantages of using HMACs in AWS KMS

AWS KMS HMAC APIs provide several advantages over implementing HMACs in application software because the key material for the HMACs is generated in AWS KMS hardware security modules (HSMs) that are certified under the FIPS 140-2 program and never leave AWS KMS unencrypted. In addition, the HMAC keys in AWS KMS can be managed with the same access control mechanisms and auditing features that AWS KMS provides on all AWS KMS keys. These security controls ensure that any HMAC created in AWS KMS can only ever be verified in AWS KMS using the same KMS key. Lastly, the HMAC keys and the HMAC algorithms that AWS KMS uses conform to industry standards defined in RFC 2104 HMAC: Keyed-Hashing for Message Authentication.

Use HMAC keys in AWS KMS to create JSON Web Tokens

The JSON Web Token (JWT) open standard is a common use of HMAC. The standard defines a portable and secure means to communicate a set of statements, known as claims, between parties. HMAC is useful for applications that need an authorization mechanism, in which claims are validated to determine whether an identity has permission to perform some action. Such an application can only work if a validator can trust the integrity of claims in a JWT. Signing JWTs with an HMAC is one way to assert their integrity. Verifiers with access to an HMAC key can cryptographically assert that the claims and signature of a JWT were produced by an issuer using the same key.

This section will walk you through an example of how you can use HMAC keys from AWS KMS to sign JWTs. The example uses the AWS SDK for Python (Boto3) and implements simple JWT encoding and decoding operations. This example shows the ease with which you can integrate HMAC keys in AWS KMS into your JWT application, even if your application is in another language or uses a more formal JWT library.

Create an HMAC key in AWS KMS

Begin by creating an HMAC key in AWS KMS. You can use the AWS KMS console or call the CreateKey API action. The following example shows creation of a 256-bit HMAC key:

import boto3

kms = boto3.client('kms')

# Use CreateKey API to create a 256-bit key for HMAC
key_id = kms.create_key(
	KeySpec='HMAC_256',
	KeyUsage='GENERATE_VERIFY_MAC'
)['KeyMetadata']['KeyId']

Use the HMAC key to encode a signed JWT

Next, you use the HMAC key to encode a signed JWT. There are three components to a JWT token: the set of claims, header, and signature. The claims are the very application-specific statements to be authenticated. The header describes how the JWT is signed. Lastly, the MAC (signature) is the output of applying the header’s described operation to the message (the combination of the claims and header). All these are packed into a URL-safe string according to the JWT standard.

The following example uses the previously created HMAC key in AWS KMS within the construction of a JWT. The example’s claims simply consist of a small claim and an issuance timestamp. The header contains key ID of the HMAC key and the name of the HMAC algorithm used. Note that HS256 is the JWT convention used to represent HMAC with SHA-256 digest. You can generate the MAC using the new GenerateMac API action in AWS KMS.

import base64
import json
import time

def base64_url_encode(data):
	return base64.b64encode(data, b'-_').rstrip(b'=')

# Payload contains simple claim and an issuance timestamp
payload = json.dumps({
	"does_kms_support_hmac": "yes",
	"iat": int(time.time())
}).encode("utf8")

# Header describes the algorithm and AWS KMS key ID to be used for signing
header = json.dumps({
	"typ": "JWT",
	"alg": "HS256",
	"kid": key_id #This key_id is from the “Create an HMAC key in AWS KMS” #example. The “Verify the signed JWT” example will later #assert that the input header has the same value of the #key_id 
}).encode("utf8")

# Message to sign is of form <header_b64>.<payload_b64>
message = base64_url_encode(header) + b'.' + base64_url_encode(payload)

# Generate MAC using GenerateMac API of AWS KMS
MAC = kms.generate_mac(
	KeyId=key_id, #This key_id is from the “Create an HMAC key in AWS KMS” 
				 #example
	MacAlgorithm='HMAC_SHA_256',
	Message=message
)['Mac']

# Form JWT token of form <header_b64>.<payload_b64>.<mac_b64>
jwt_token = message + b'.' + base64_url_encode(mac)

Verify the signed JWT

Now that you have a signed JWT, you can verify it using the same KMS HMAC key. The example below uses the new VerifyMac API action to validate the MAC (signature) of the JWT. If the MAC is invalid, AWS KMS returns an error response and the AWS SDK throws an exception. If the MAC is valid, the request succeeds and the application can continue to do further processing on the token and its claims.

def base64_url_decode(data):
	return base64.b64decode(data + b'=' * (4 - len(data) % 4), b'-_')

# Parse out encoded header, payload, and MAC from the token
message, mac_b64 = jwt_token.rsplit(b'.', 1)
header_b64, payload_b64 = message.rsplit(b'.', 1)

# Decode header and verify its contents match expectations
header_map = json.loads(base64_url_decode(header_b64).decode("utf8"))
assert header_map == {
	"typ": "JWT",
	"alg": "HS256",
	"kid": key_id #This key_id is from the “Create an HMAC key in AWS KMS” 
				 #example
}

# Verify the MAC using VerifyMac API of AWS KMS. # If the verification fails, this will throw an error.
kms.verify_mac(
	KeyId=key_id, #This key_id is from the “Create an HMAC key in AWS KMS” 
				 #example
	MacAlgorithm='HMAC_SHA_256',
	Message=message,
	Mac=base64_url_decode(mac_b64)
)

# Decode payload for use application-specific validation/processing
payload_map = json.loads(base64_url_decode(payload_b64).decode("utf8"))

Create separate roles to control who has access to generate HMACs and who has access to validate HMACs

It’s often helpful to have separate JWT creators and validators so that you can distinguish between the roles that are allowed to create tokens and the roles that are allowed to verify tokens. HMAC signatures performed outside of AWS-KMS don’t work well for this because you can’t isolate creators and verifiers if they both must have a copy of the same key. However, this is not an issue for HMAC keys in AWS KMS. You can use key policies to separate out who has permission to ask AWS KMS to generate HMACs and who has permission to ask AWS KMS to validate. Each party uses their own unique access keys to access the HMAC key in AWS KMS. Only HSMs in AWS KMS will ever have access to the actual key material. See the following example key policy statements that separate out GenerateMac and VerifyMac permissions:

{
	"Id": "example-jwt-policy",
	"Version": "2012-10-17",
	"Statement": [
		{
			"Sid": "Allow use of the key for creating JWTs",
			"Effect": "Allow",
			"Principal": {
				"AWS": "arn:aws:iam::111122223333:role/JwtProducer"
			},
			"Action": [
				"kms:GenerateMac"
			],
			"Resource": "*"
		},
		{
			"Sid": "Allow use of the key for validating JWTs",
			"Effect": "Allow",
			"Principal": {
				"AWS": "arn:aws:iam::111122223333:role/JwtConsumer"
			},
			"Action": [
				"kms:VerifyMac"
			],
			"Resource": "*"
		}
	]
}

Conclusion

In this post, you learned about the new HMAC APIs in AWS KMS (GenerateMac and VerifyMac). These APIs complement existing AWS KMS cryptographic operations: symmetric key encryption, asymmetric key encryption and signing, and data key creation and key enveloping. You can use HMACs for JWTs, tokenization, URL and API signing, as a key derivation function (KDF), as well as in new designs that we haven’t even thought of yet. To learn more about HMAC functionality and design, see HMAC keys in AWS KMS in the AWS KMS Developer Guide.

If you have feedback about this post, submit comments in the Comments section below. If you have questions about this post, start a new thread on the KMS re:Post or contact AWS Support.
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Author

Jeremy Stieglitz

Jeremy is the Principal Product Manager for AWS Key Management Service (KMS) where he drives global product strategy and roadmap for AWS KMS. Jeremy has more than 20 years of experience defining new products and platforms, launching and scaling cryptography solutions, and driving end-to-end product strategies. Jeremy is the author or co-author of 23 patents in network security, user authentication and network automation and control.

Author

Peter Zieske

Peter is a Senior Software Developer on the AWS Key Management Service team, where he works on developing features on the service-side front-end. Outside of work, he enjoys building with LEGO, gaming, and spending time with family.

Encrypt global data client-side with AWS KMS multi-Region keys

Post Syndicated from Jeremy Stieglitz original https://aws.amazon.com/blogs/security/encrypt-global-data-client-side-with-aws-kms-multi-region-keys/

Today, AWS Key Management Service (AWS KMS) is introducing multi-Region keys, a new capability that lets you replicate keys from one Amazon Web Services (AWS) Region into another. Multi-Region keys are designed to simplify management of client-side encryption when your encrypted data has to be copied into other Regions for disaster recovery or is replicated in Amazon DynamoDB global tables.

In this blog post, we give an overview of how we got here and how to get started using multi-Region keys. We include a code example for multi-Region encryption of data in DynamoDB global tables.

How we got here

From its inception, AWS KMS has been strictly isolated to a single AWS Region for each implementation, with no sharing of keys, policies, or audit information across Regions. Region isolation can help you comply with security standards and data residency requirements. However, not sharing keys across Regions creates challenges when you need to move data that depends on those keys across Regions. AWS services that use your KMS keys for server-side encryption address this challenge by transparently re-encrypting data on your behalf using the KMS keys you designate in the destination Region. If you use client-side encryption, this work adds extra complexity and latency of re-encrypting between regionally isolated KMS keys.

Multi-Region keys are a new feature from AWS KMS for client-side applications that makes KMS-encrypted ciphertext portable across Regions. Multi-Region keys are a set of interoperable KMS keys that have the same key ID and key material, and that you can replicate to different Regions within the same partition. With symmetric multi-Region keys, you can encrypt data in one Region and decrypt it in a different Region. With asymmetric multi-Region keys, you encrypt, decrypt, sign, and verify messages in multiple Regions.

Multi-Region keys are supported in the AWS KMS console, the AWS KMS API, the AWS Encryption SDK, Amazon DynamoDB Encryption Client, and Amazon S3 Encryption Client. AWS services also let you configure multi-Region keys for server-side encryption in case you want the same key to protect data that needs both server-side and client-side encryption.

Getting started with multi-Region keys

To use multi-Region keys, you create a primary multi-Region key with a new key ID and key material. Then, you use the primary key to create a related multi-Region replica key in a different Region of the same AWS partition. Replica keys are KMS keys that can be used independently; they aren’t a pointer to the primary key. The primary and replica keys share only certain properties, including their key ID, key rotation, and key origin. In all other aspects, every multi-Region key, whether primary or replica, is a fully functional, independent KMS key resource with its own key policy, aliases, grants, key description, lifecycle, and other attributes. The key Amazon Resource Names (ARN) of related multi-Region keys differ only in the Region portion, as shown in the following figure (Figure 1).

Figure 1: Multi-Region keys have unique ARNs but identical key IDs

Figure 1: Multi-Region keys have unique ARNs but identical key IDs

You cannot convert an existing single-Region key to a multi-Region key. This design ensures that all data protected with existing single-Region keys maintain the same data residency and data sovereignty properties.

When to use multi-Region keys

You can use multi-Region keys in any client-side application. Since multi-Region keys avoid cross-Region calls, they’re especially useful for scenarios where you don’t want to depend on another Region or incur the latency of a cross-Region call. For example, disaster recovery, global data management, distributed signing applications, and active-active applications that span multiple Regions can all benefit from using multi-Region keys. You can also create and use multi-Region keys in a single Region and choose to replicate those keys at some later date when you need to move protected data to additional Regions.

Note: If your application will run in only one Region, you should continue to use single-Region keys to benefit from their data isolation properties.

One significant benefit of multi-Region keys is using them with DynamoDB global tables. Let’s explore that interaction in detail.

Using multi-Region keys with DynamoDB global tables

AWS KMS multi-Region keys (MRKs) can be used with the DynamoDB Encryption Client to protect data in DynamoDB global tables. You can configure the DynamoDB Encryption Client to call AWS KMS for decryption in a different Region than the one in which the data was encrypted, as shown in the following figure (Figure 2). This is useful for disaster recovery, or simply to improve performance when using DynamoDB in a globally distributed application.

Figure 2: Using multi-Region keys with DynamoDB global tables

Figure 2: Using multi-Region keys with DynamoDB global tables

The steps described in Figure 2 are:

  1. Encrypt record with primary MRK
  2. Put encrypted record
  3. Global table replication
  4. Get encrypted record
  5. Decrypt record with replica MRK

Create a multi-Region primary key

Begin by creating a multi-Region primary key and replicating it into your backup Regions. We’ll assume that you’ve created a DynamoDB global table that’s replicated to the same Regions.

Configure the DynamoDB Encryption Client to encrypt records

To use AWS KMS multi-Region keys, you need to configure the DynamoDB Encryption Client with the Region you want to call, which is typically the Region where the application is running. Then, you need to configure the ARN of the KMS key you want to use in that Region.

This example encrypts records in us-east-1 (US East (N. Virginia)) and decrypts records in us-west-2 (US West (Oregon)). If you use the following example configuration code, be sure to replace the example key ARNs with valid key ARNs for your multi-Region keys.

// Specify the multi-Region key in the us-east-1 Region
String encryptRegion = "us-east-1";
String cmkArnEncrypt = "arn:aws:kms:us-east-1:<111122223333>:key/<mrk-1234abcd12ab34cd56ef12345678990ab>";

// Set up SDK clients for KMS and DDB in us-east-1
AWSKMS kmsEncrypt = AWSKMSClientBuilder.standard().withRegion(encryptRegion).build();
AmazonDynamoDB ddbEncrypt = AmazonDynamoDBClientBuilder.standard().withRegion(encryptRegion).build();

// Configure the example global table
String tableName = "global-table-example";
String employeeIdAttribute = "employeeId";
String nameAttribute = "name";

// Configure attribute actions for the Dynamo DB Encryption Client
//   Sign the employee ID field
//   Encrypt and sign the name field
Map<String, Set<EncryptionFlags>> actions = new HashMap<>();
actions.put(employeeIdAttribute, EnumSet.of(EncryptionFlags.SIGN));
actions.put(nameAttribute, EnumSet.of(EncryptionFlags.ENCRYPT, EncryptionFlags.SIGN));

// Set an encryption context. This is an optional best practice.
final EncryptionContext encryptionContext = new EncryptionContext.Builder()
        .withTableName(tableName)
        .withHashKeyName(employeeIdAttribute)
        .build();

// Use the Direct KMS materials provider and the DynamoDB encryptor
// Specify the key ARN of the multi-Region key in us-east-1
DirectKmsMaterialProvider cmpEncrypt = new DirectKmsMaterialProvider(kmsEncrypt, cmkArnEncrypt);
DynamoDBEncryptor encryptor = DynamoDBEncryptor.getInstance(cmpEncrypt);

// Create a record, encrypt it, 
// and put it in the DynamoDB global table
Map<String, AttributeValue> rec = new HashMap<>();
rec.put(nameAttribute, new AttributeValue().withS("Andy"));
rec.put(employeeIdAttribute, new AttributeValue().withS("1234"));

final Map<String, AttributeValue> encryptedRecord = encryptor.encryptRecord(rec, actions, encryptionContext);
ddbEncrypt.putItem(tableName, encryptedRecord);

When you save the newly encrypted record, DynamoDB global tables automatically replicates this encrypted record to the replica tables in the us-west-2 Region.

Configure the DynamoDB Encryption Client to decrypt data

Now you’re ready to configure a DynamoDB client to decrypt the record in us-west-2 where both the replica table and the replica multi-Region key exist.

// Specify the Region and key ARN to use when decrypting          
String decryptRegion = "us-west-2";
String cmkArnDecrypt = "arn:aws:kms:us-west-2:<111122223333>:key/<mrk-1234abcd12ab34cd56ef12345678990ab>";

// Set up SDK clients for KMS and DDB in us-west-2
AWSKMS kmsDecrypt = AWSKMSClientBuilder.standard()
    .withRegion(decryptRegion)
    .build();

AmazonDynamoDB ddbDecrypt = AmazonDynamoDBClientBuilder.standard()
    .withRegion(decryptRegion)
    .build();

// Configure the DynamoDB Encryption Client
// Use the Direct KMS materials provider and the DynamoDB encryptor
// Specify the key ARN of the multi-Region key in us-west-2
final DirectKmsMaterialProvider cmpDecrypt = new DirectKmsMaterialProvider(kmsDecrypt, cmkArnDecrypt);
final DynamoDBEncryptor decryptor = DynamoDBEncryptor.getInstance(cmpDecrypt);

// Set up your query
Map<String, AttributeValue> query = new HashMap<>();
query.put(employeeIdAttribute, new AttributeValue().withS("1234"));

// Get a record from DDB and decrypt it
final Map<String, AttributeValue> retrievedRecord = ddbDecrypt.getItem(tableName, query).getItem();
final Map<String, AttributeValue> decryptedRecord = decryptor.decryptRecord(retrievedRecord, actions, encryptionContext);

Note: This example encrypts with the primary multi-Region key and then decrypts with a replica multi-Region key. The process could also be reversed—every multi-Region key can be used in the encryption or decryption of data.

Summary

In this blog post, we showed you how to use AWS KMS multi-Region keys with client-side encryption to help secure data in global applications without sacrificing high availability or low latency. We also showed you how you can start working with a global application with a brief example of using multi-Region keys with the DynamoDB Encryption Client and DynamoDB global tables.

This blog post is a brief introduction to the ways you can use multi-Region keys. We encourage you to read through the Using multi-Region keys topic to learn more about their functionality and design. You’ll learn about:

If you have feedback about this post, submit comments in the Comments section below. If you have questions about this post, start a new thread on the AWS KMS forum.

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Author

Jeremy Stieglitz

Jeremy is the Principal Product Manager for AWS Key Management Service (KMS) where he drives global product strategy and roadmap for AWS KMS. Jeremy has more than 20 years of experience defining new products and platforms, launching and scaling cryptography solutions, and driving end-to-end product strategies. Jeremy is the author or co-author of 23 patents in network security, user authentication and network automation and control.

Author

Peter Zieske

Peter is a Senior Software Developer on the AWS Key Management Service team, where he works on developing features on the service-side front-end. Outside of work, he enjoys building with LEGO, gaming, and spending time with family.

Author

Ben Farley

Ben is a Senior Software Developer on the AWS Crypto Tools team, where he works on client-side encryption libraries that help customers protect their data. Before that, he spent time focusing on the scalability and availability of services like AWS Identity and Access Management and AWS Key Management Service. Outside of work, he likes to explore the mountains with his fiancée and dog.