aws-kms-sign-csr

Given an existing CSR (in PEM format) and a keypair in AWS KMS, this script:

• updates the public key to the public key of the asymmetric keypair
• signs the CSR with the private key of the asymmetric keypair

Why would I want to do this?

You may have a use-case where you're signing arbitrary data using KMS, but checking this signature against a certificate (or, by extension, checking that the certificate has been chained from a trusted root or intermediate).

This script allows you to generate a CSR which uses the private key in KMS, which can then be signed by your PKI. From here you can sign your arbitrary data using KMS and you've maintained the security of your private key, as it has never left KMS.

Note that this does NOT sign the CSR with a CA to make it into a bona fide certificate: a CSR is signed with the private key of the generator so that the CA can ensure that the public key is owned by the person who is requesting the certificate, and this script re-signs with the private key held in KMS.

Installation

# create a new virtualenv
python3 -m venv aws-kms-sign-csr
. aws-kms-sign-csr/bin/activate
# install prerequisite modules
pip3 install -r requirements.txt

Usage

RSA

# generate a PEM csr - the key doesn't matter as it will be replaced
openssl req -new -newkey rsa:2048 -keyout /dev/null -nodes -out test.csr
./aws-kms-sign-csr.py --region eu-west-1 --keyid alias/mykeyalias --hashalgo sha256 test.csr > new.csr

ECDSA

# Create a fake key
openssl ecparam -genkey -name secp256k1 -out fake.key -genkey
# Create CSR from fake key
openssl req -new -key fake.key -out test.csr
# Update CSR using KMS key
./aws-kms-sign-csr.py --region eu-west-1 --keyid alias/mykeyalias --hashalgo sha256 --signalgo ECDSA test.csr > new.csr

The script will use your existing AWS credentials: to override use environment variables per https://boto3.amazonaws.com/v1/documentation/api/latest/guide/configuration.html

The key ID can be a key ARN, an actual key ID, a key alias (prefixed with alias/), or a key alias ARN. See https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/kms.html#KMS.Client.sign for more info.

Limitations

• only supports RSA with sha256, sha384 and sha512 and ECDSA with sha224, sha256, sha384, sha512 at time of writing
• should have better error handling
• should have better handling of boto profiles