Getting GCP access token from a service account key JSON file

README.md

Getting GCP access token from a service account key

Use your service account's key JSON file to get an access token to call Google APIs.

Good for seeing how things work, including the creation of JWT token.

To create a JWT token, you can replace create-jwt-token.sh script with tools like step.

If you just want to get an access token for a service account, you can do the same thing with just gcloud command. cf Authorization and authentication.

Prerequisites

• Have a GCP project and a service account.
• The service account has a permission for the request.
  • e.g. For listing buckets, "https://www.googleapis.com/auth/devstorage.read_only"
• The service account's key JSON file is downloaded (here, key.json).
• Commands available: jq, openssl

Usage

Get an access token.

$ ./get-access-token.sh /path/to/key.json "https://www.googleapis.com/auth/devstorage.read_only"
ya29...

You can call a Google API with the token. Here since we've requested storage readonly, we list buckets.

$ curl "https://www.googleapis.com/storage/v1/b?project=<your_project_id>" \
    -H "Authorization: Bearer ya29..."

References

• Using OAuth 2.0 for Server to Server Applications
• Generate JWT With Bash

create-jwt-token.sh

#!/bin/bash

set -euo pipefail

base64var() {
    printf "$1" | base64stream
}

base64stream() {
    base64 | tr '/+' '_-' | tr -d '=\n'
}

key_json_file="$1"
scope="$2"
valid_for_sec="${3:-3600}"
private_key=$(jq -r .private_key $key_json_file)
sa_email=$(jq -r .client_email $key_json_file)

header='{"alg":"RS256","typ":"JWT"}'
claim=$(cat <<EOF | jq -c .
  {
    "iss": "$sa_email",
    "scope": "$scope",
    "aud": "https://www.googleapis.com/oauth2/v4/token",
    "exp": $(($(date +%s) + $valid_for_sec)),
    "iat": $(date +%s)
  }
EOF
)
request_body="$(base64var "$header").$(base64var "$claim")"
signature=$(openssl dgst -sha256 -sign <(echo "$private_key") <(printf "$request_body") | base64stream)

printf "$request_body.$signature"

get-access-token.sh

#!/bin/bash

set -euo pipefail

key_json_file="$1"
scope="$2"

jwt_token=$(./create-jwt-token.sh "$key_json_file" "$scope")

curl -s -X POST https://www.googleapis.com/oauth2/v4/token \
    --data-urlencode 'grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer' \
    --data-urlencode "assertion=$jwt_token" \
    | jq -r .access_token