How to protect HMACs inside AWS KMS
April 20, 2022: In the section “Use the HMAC key to encode a signed JWT,” we fixed an error in the code sample.
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:
- 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.
- 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
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.
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.
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:
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.
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