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@ryu1kn
Last active June 7, 2024 09:40
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Getting GCP access token from a service account key JSON file

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

#!/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"
#!/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
@bechampion
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another improvement:

#!/bin/bash
set -euo pipefail
key_json_file="$1"
scope="$2"
valid_for_sec="${3:-3600}"
IFS="," read -r private_key sa_email <<<$(cat $1 | jq '(.private_key|@base64)+","+.client_email' -r)
header='{"alg":"RS256","typ":"JWT"}'
claim=$(cat <<EOF | jq -c .
  {
    "iss": "$sa_email",
    "scope": "$scope",
    "aud": "https://www.googleapis.com/oauth2/v4/token",
    "exp": $(($(date +%s) + $valid_for_sec)),
    "iat": $(date +%s)
  }
EOF
     )
request_body="$(echo "$header" | base64 -w0).$(echo "$claim" | base64 -w0)"
signature=$(openssl dgst -sha256 -sign <(echo "$private_key"| base64 -d ) <(printf "$request_body") | base64 -w0)
printf "$request_body.$signature"

@apinter
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apinter commented Feb 24, 2022

Good Lord, you have no idea how much time this script would've saved me if I pick my search parameters wiser ehehe
Huge thanks for sharing this!

@MaikkiaM
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What is the max Expiry Date for it? Is there an endless version ? or just
comment it out?

@rekire
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rekire commented Apr 7, 2023

Based on the output of https://jwt.io/ the resulting JWT has syntax errors. There should be no = chars. However this helps me a lot to understand how I can build now myself the JWT without the huge google libs

@DinoChiesa
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DinoChiesa commented Nov 7, 2023

This is good stuff. And also, if you don't mind delegating to a tool, you can get a GCP Access token using the gcloud command line tool, with a couple of lines:

gcloud auth activate-service-account SERVICE_ACCOUNT@DOMAIN.COM --key-file=/path/key.json
gcloud auth print-access-token

Also, more on this theme: https://github.com/DinoChiesa/get-gcp-access-token

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