Sign and verify data with Cloud KMS (Asymmetric)

1. Overview

Cloud KMS is a cloud-hosted key management service that lets you manage cryptographic keys for your cloud services the same way you do on-premises. It includes support for encryption, decryption, signing, and verification using a variety of key types and sources including Cloud HSM for hardware-backed keys. This tutorial teaches you how to sign and verify data using asymmetric Cloud KMS keys.

You will learn

• How to enable the Cloud KMS API
• How to create a Key Ring
• How to create a Crypto Key for asymmetric signing/verification

2. Setup and Requirements

Self-paced environment setup

1. Sign in to Cloud Console and create a new project or reuse an existing one. (If you don't already have a Gmail or G Suite account, you must create one.)

Note: You can easily access Cloud Console by memorizing its URL, which is console.cloud.google.com.

Remember the project ID, a unique name across all Google Cloud projects (the name above has already been taken and will not work for you, sorry!). It will be referred to later in this codelab as PROJECT_ID.

Note: If you're using a Gmail account, you can leave the default location set to No organization. If you're using a G Suite account, then choose a location that makes sense for your organization.

2. Next, you'll need to enable billing in Cloud Console in order to use Google Cloud resources.

Running through this codelab shouldn't cost much, if anything at all. Be sure to to follow any instructions in the "Cleaning up" section which advises you how to shut down resources so you don't incur billing beyond this tutorial. New users of Google Cloud are eligible for the $300USD Free Trial program.

Start Cloud Shell

In this codelab you will use Cloud Shell, a free virtualized environment running on Google Cloud. From the GCP Console click the Cloud Shell icon on the top right toolbar:

It should only take a few moments to provision and connect to the environment. When it is finished, you should see something like this:

This virtual machine is loaded with all the development tools you'll need. It offers a persistent 5GB home directory, and runs on the Google Cloud, greatly enhancing network performance and authentication. Unless otherwise instructed, run all commands from this shell.

3. Enable Cloud KMS Service

Before you can use Cloud KMS, you must first enable the service in your project. This only needs to be done once per project. To enable the Cloud KMS service, run the following command:

$ gcloud services enable cloudkms.googleapis.com \
    --project "${GOOGLE_CLOUD_PROJECT}"

It can take up to a minute to enable. The command will report success when it finishes.

4. Create KMS Key

Create a Cloud KMS Key Ring. In Cloud KMS, a Key Ring is a logical collection of cryptographic keys. The Key Ring contains metadata about the keys such as their location. Create a Key Ring named my-keyring in the global region:

$ gcloud kms keyrings create "my-keyring" \
    --location "global"

Already Exists

If you have done this tutorial (or other Cloud KMS tutorials) previously, you may see an error that "my-keyring" already exists. You can ignore this error and continue to the next step as you have already created the Key Ring.

Now create a Crypto Key named my-asymmetric-signing-key with the purpose asymmetric-signing inside the Key Ring you just created.

$ gcloud kms keys create "my-asymmetric-signing-key" \
    --location "global" \
    --keyring "my-keyring" \
    --purpose "asymmetric-signing" \
    --default-algorithm "rsa-sign-pkcs1-4096-sha512"

Asymmetric Keys

This tutorial uses asymmetric encryption keys. With asymmetric keys, Cloud KMS generates a public key and private key. The public key is made available via the API, but the private key is never disclosed. Cloud KMS also supports symmetric keys, but symmetric keys cannot be used for signing and verification at this time.

5. Sign Data

Unlike encryption, decrypting data that was encrypted using an asymmetric Cloud KMS key does require online access to the Cloud KMS service. Decrypt the ciphertext from the file using the gcloud command line tool:

Create a file with data to sign and use the gcloud command line tool to sign the data with the Cloud KMS key:

$ echo "my-contents" > ./data.txt

$ gcloud kms asymmetric-sign \
    --location "global" \
    --keyring "my-keyring" \
    --key "my-asymmetric-signing-key" \
    --version "1" \
    --digest-algorithm "sha512" \
    --input-file ./data.txt \
    --signature-file ./data.txt.sig

The signature is saved in data.txt.sig on disk. If you open the data.txt.sig file, you will notice that it has strange, unprintable characters. That is because the resulting data is in binary format.

When storing the signature in a database or transmitting it as part of an HTTP request, you may need to encode the data. A common encoding mechanism is base64.

6. Verify Data

With asymmetric keys, Cloud KMS does not perform the verification directly. Instead, it provides access to a public key and you verify data using that public key via public key cryptography. With asymmetric keys, verification can be done completely offline and does not require access to Cloud KMS or any other Google Cloud APIs. Verification is performed using a cryptographic tool like openssl or with a programming language or library that supports public key cryptography.

Download the public key from Cloud KMS:

$ gcloud kms keys versions get-public-key "1" \
    --location "global" \
    --keyring "my-keyring" \
    --key "my-asymmetric-signing-key" \
    --output-file ./key.pub

Verify the signature against the public key using the openssl command line tool:

$ openssl dgst -sha256 \
    -verify ./key.pub \
    -signature ./data.txt.sig ./data.txt

The console will print a success message, indicating the digital signature is valid.

Verified OK

Matching Algorithms

The algorithm chosen upon key creation must match the algorithm used during verification. In this tutorial, we created a key that uses SHA512, therefore the verification step includes the same parameters. If you created a key with a different algorithm, you would need to adjust the verification step to match accordingly.

7. Congratulations!

You enabled the Cloud KMS API, created an asymmetric signing key, and signed and verified data! Cloud KMS is a powerful product and signing/verification just scratches the surface of its capabilities.

Clean up

If you are done exploring, please consider deleting your project.

• Go to the Cloud Platform Console
• Select the project you want to shut down, then click "Delete" at the top. This schedules the project for deletion.

Learn More

• Cloud KMS

License

This work is licensed under a Creative Commons Attribution 2.0 Generic License.