Digital signing with the new asymmetric keys feature of AWS KMS
October 29, 2021: AWS KMS is replacing the term customer master key (CMK) with AWS KMS key and KMS key. The concept has not changed. To prevent breaking changes, AWS KMS is keeping some variations of this term. More info.
AWS Key Management Service (AWS KMS) now supports asymmetric keys. You can create, manage, and use public/private key pairs to protect your application data using the new APIs via the AWS SDK. Similar to the symmetric key features we’ve been offering, asymmetric keys can be generated as AWS KMS keys (KMS keys) where the private portion never leaves the service, or as a data key where the private portion is returned to your calling application encrypted under a KMS key. The private portion of asymmetric KMS keys are used in AWS KMS hardware security modules (HSMs) designed so that no one, including AWS employees, can access the plaintext key material. AWS KMS supports the following asymmetric key types – RSA 2048, RSA 3072, RSA 4096, ECC NIST P-256, ECC NIST P-384, ECC NIST-521, and ECC SECG P-256k1.
We’ve talked with customers and know that one popular use case for asymmetric keys is digital signing. In this post, I will walk you through an example of signing and verifying files using some of the new APIs in AWS KMS.
A common way to ensure the integrity of a digital message as it passes between systems is to use a digital signature. A sender uses a secret along with cryptographic algorithms to create a data structure that is appended to the original message. A recipient with access to that secret can cryptographically verify that the message hasn’t been modified since the sender signed it. In cases where the recipient doesn’t have access to the same secret used by the sender for verification, a digital signing scheme that uses asymmetric keys is useful. The sender can make the public portion of the key available to any recipient to verify the signature, but the sender retains control over creating signatures using the private portion of the key. Asymmetric keys are used for digital signature applications such as trusted source code, authentication/authorization tokens, document e-signing, e-commerce transactions, and secure messaging. AWS KMS supports what are known as raw digital signatures, where there is no identity information about the signer embedded in the signature object. A common way to attach identity information to a digital signature is to use digital certificates. If your application relies on digital certificates for signing and signature verification, we recommend you look at AWS Certificate Manager and Private Certificate Authority. These services allow you to programmatically create and deploy certificates with keys to your applications for digital signing operations. A common application of digital certificates is TLS termination on a web server to secure data in transit.
Signing and verifying files with AWS KMS
Assume that you have an application A that sends a file to application B in your AWS account. You want the file to be digitally signed so that the receiving application B can verify it hasn’t been tampered with in transit. You also want to make sure only application A can digitally sign files using the key because you don’t want application B to receive a file thinking it’s from application A when it was really from a different sender that had access to the signing key. Because AWS KMS is designed so that the private portion of the asymmetric key pair used for signing cannot be used outside the service or by unauthenticated users, you’re able to define and enforce policies so that only application A can sign with the key.
To start, application A will submit either the file itself or a digest of the file to the AWS KMS Sign API under an asymmetric KMS key. If the file is less than 4KB, AWS KMS will compute a digest for you as a part of the signing operation. If the file is greater than 4KB, you must send only the digest you created locally and you must tell AWS KMS that you’re passing a digest in the MessageType parameter of the request. You can use any of several hashing functions in your local environment to create a digest of the file, but be aware that the receiving application in account B will need to be able to compute the digest using the same hash function in order to verify the integrity of the file. In my example, I’m using SHA256 as the hash function. Once the digest is created, AWS KMS uses the private portion of the asymmetric KMS key to encrypt the digest using the signing algorithm specified in the API request. The result is a binary data object, which we’ll refer to as “the signature” throughout this post.
Once application B receives the file with the signature, it must create a digest of the file. It then passes this newly generated digest, the signature object, the signing algorithm used, and the KMS key keyId to the Verify API. AWS KMS uses the corresponding public key of the KMS key with the signing algorithm specified in the request to verify the signature. Instead of submitting the signature to the Verify API, application B could verify the signature locally by acquiring the public key. This might be an attractive option if application B didn’t have a way to acquire valid AWS credentials to make a request of AWS KMS. However, this method requires application B to have access to the necessary cryptographic algorithms and to have previously received the public portion of the asymmetric KMS key. In my example, application B is running in the same account as application A, so it can acquire AWS credentials to make the Verify API request. I’ll describe how to verify signatures using both methods in a bit more detail later in the post.
Creating signing keys and setting up key policy permissions
To start, you need to create an asymmetric KMS key. When calling the CreateKey API, you’ll pass one of the asymmetric values for the CustomerMasterKeySpec parameter. In my example, I’m choosing a key spec of ECC_NIST_P384 because keys used with elliptic curve algorithms tend to be more efficient than those used with RSA-based algorithms.
As a part of creating your asymmetric KMS key, you need to attach a resource policy to the key to control which cryptographic operations the AWS principals representing applications A and B can use. A best practice is to use a different IAM principal for each application in order to scope down permissions. In this case, you want application A to only be able to sign files, and application B to only be able to verify them. I will assume each of these applications are running in Amazon EC2, and so I’ll create a couple of IAM roles.
- The IAM role for application A (SignRole) will be given kms:Sign permission in the KMS key key policy
- The IAM role for application B (VerifyRole) will be given kms:Verify permission in the KMS key key policy
The stanza in the KMS key key policy document to allow signing should look like this (replace the account ID value of <111122223333> with your own):
The stanza in the KMS key key policy document to allow verification should look like this (replace the account ID value of <111122223333> with your own):
Once you have created the asymmetric KMS key and IAM roles, you’re ready to sign your file. Application A will create a message digest of the file and make a sign request to AWS KMS with the asymmetric KMS key keyId, and signing algorithm. The CLI command to do this is shown below. Replace the key-id parameter with your KMS key’s specific keyId.
I chose the ECDSA_SHA_256 signing algorithm for this example. See the Sign API specification for a complete list of supported signing algorithms.
After validating that the API call is authorized by the credentials available to SignRole, KMS generates a signature around the digest and returns the KMS key keyId, signature, and the signing algorithm.
Verify Workflow 1 — Calling the verify API
Once application B receives the file and the signature, it computes the SHA 256 digest over the copy of the file it received. It then makes a verify request to AWS KMS, passing this new digest, the signature it received from application A, signing algorithm, and the KMS key keyId. The CLI command to do this is shown below. Replace the key-id parameter with your KMS key’s specific keyId.
After validating that the verify request is authorized, AWS KMS verifies the signature by first decrypting the signature using the public portion of the KMS key. It then compares the decrypted result to the digest received in the verify request. If they match, it returns a SignatureValid boolean of True, indicating that the original digest created by the sender matches the digest created by the recipient. Because the original digest is unique to the original file, the recipient can know that the file was not tampered with during transit.
One advantage of using the AWS KMS verify API is that the caller doesn’t have to keep track of the specific public key matching the private key used to create the signature; the caller only has to know the KMS key keyId and signing algorithm used. Also, because all request to AWS KMS are logged to AWS CloudTrail, you can audit that the signature and verification operations were both executed as expected. See the Verify API specification for more detail on available parameters.
Verify Workflow 2 — Verifying locally using the public key
Apart from using the Verify API directly, you can choose to retrieve the public key in the KMS key using the AWS KMS GetPublicKey API and verify the signature locally. You might want to do this if application B needs to verify multiple signatures at a high rate and you don’t want to make a network call to the Verify API each time. In this method, application B makes a GetPublicKey request to AWS KMS to retrieve the public key. The CLI command to do this is below. Replace the key-id parameter with your KMS keyK’s specific keyId.
aws kms get-public-key \
Note that the application B will need permissions to make a GetPublicKey request to AWS KMS. The stanza in the KMS key key policy document to allow the VerifyRole identity to download the public key should look like this (replace the account ID value of <111122223333> with your own):
Once application B has the public key, it can use your preferred cryptographic provider to perform the signature verification locally. Application B needs to keep track of the public key and signing algorithm used for each signature object it will verify locally. Using the wrong public key will fail to decrypt the signature from application A, making the signature verification operation unsuccessful.
Availability and pricing
Asymmetric keys and operations in AWS KMS are available now in the Northern Virginia, Oregon, Sydney, Ireland, and Tokyo AWS Regions with support for other regions planned. Pricing information for the new feature can be found at the AWS KMS pricing page.
I showed you a simple example of how you can use the new AWS KMS APIs to digitally sign and verify an arbitrary file. By having AWS KMS generate and store the private portion of the asymmetric key, you can limit use of the key for signing only to IAM principals you define. OIDC ID tokens, OAuth 2.0 access tokens, documents, configuration files, system update messages, and audit logs are but a few of the types of objects you might want to sign and verify using this feature.
You can also perform encrypt and decrypt operations under asymmetric KMS keys in AWS KMS as an alternative to using the symmetric KMS keys available since the service launched. Similar to how you can ask AWS KMS to generate symmetric keys for local use in your application, you can ask AWS KMS to generate and return asymmetric key pairs for local use to encrypt and decrypt data. Look for a future AWS Security Blog post describing these use cases. For more information about asymmetric key support, see the AWS KMS documentation page.
If you have feedback about this blog post, submit comments in the Comments section below. If you have questions about the asymmetric key feature, please start a new thread on the AWS KMS Discussion Forum.
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