106 lines
3.4 KiB
Markdown
106 lines
3.4 KiB
Markdown
---
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name: github-app-token
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description: Generate a GitHub installation access token from a GitHub App PEM key, App ID, and Installation ID, then authenticate the gh CLI with it.
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---
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# GitHub App Token Skill
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Generate a short-lived GitHub installation access token from a GitHub App's credentials and use it to authenticate the `gh` CLI.
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## Prerequisites
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The following environment variables MUST be set before invoking this skill:
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| Variable | Description |
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|---|---|
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| `GITHUB_APP_ID` | The numeric App ID from the GitHub App settings page |
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| `GITHUB_APP_INSTALLATION_ID` | The numeric Installation ID for the target org/user |
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| `GITHUB_APP_PEM_FILE` | Absolute path to the GitHub App's PEM private key file |
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If any variable is missing, stop and tell the user which ones are required.
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## Steps
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### 1. Generate a JWT
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Create a JWT signed with the GitHub App's private key. You MUST use the helper script bundled with this skill:
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```bash
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# generates a JWT valid for 10 minutes
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TOKEN=$(python3 "$(dirname "$0")/../skills/github-app-token/scripts/generate_jwt.py")
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```
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If `python3` is not available, fall back to the inline openssl method described in the Fallback section below.
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The JWT uses:
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- **Algorithm**: RS256
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- **Header**: `{"alg": "RS256", "typ": "JWT"}`
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- **Payload**:
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- `iat`: current time minus 60 seconds (clock drift buffer)
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- `exp`: current time plus 600 seconds (10 minute max)
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- `iss`: the `GITHUB_APP_ID`
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### 2. Exchange the JWT for an installation access token
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```bash
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INSTALL_TOKEN=$(curl -s -X POST \
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-H "Authorization: Bearer ${TOKEN}" \
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-H "Accept: application/vnd.github+json" \
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-H "X-GitHub-Api-Version: 2022-11-28" \
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"https://api.github.com/app/installations/${GITHUB_APP_INSTALLATION_ID}/access_tokens" \
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| python3 -c "import sys,json; print(json.load(sys.stdin)['token'])")
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```
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If the response contains an error (e.g., `401 Unauthorized`), check:
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1. The PEM key matches the App ID
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2. The Installation ID is valid for this App
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3. The system clock is accurate (JWT `iat`/`exp` are time-sensitive)
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### 3. Authenticate the gh CLI
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```bash
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echo "${INSTALL_TOKEN}" | gh auth login --with-token
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```
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Verify it worked:
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```bash
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gh auth status
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```
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You should see authentication via `token` for `github.com`.
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### 4. Cleanup
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The installation access token expires after 1 hour. There is nothing to revoke unless you want to explicitly invalidate it early:
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```bash
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curl -s -X DELETE \
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-H "Authorization: Bearer ${INSTALL_TOKEN}" \
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-H "Accept: application/vnd.github+json" \
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"https://api.github.com/installation/token"
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```
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## Fallback: JWT generation without Python
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If `python3` is unavailable, generate the JWT using `openssl` and `bash`:
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```bash
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header=$(printf '{"alg":"RS256","typ":"JWT"}' | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
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now=$(date +%s)
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iat=$((now - 60))
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exp=$((now + 600))
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payload=$(printf '{"iat":%d,"exp":%d,"iss":"%s"}' "$iat" "$exp" "$GITHUB_APP_ID" | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
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unsigned="${header}.${payload}"
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signature=$(printf '%s' "$unsigned" | openssl dgst -sha256 -sign "${GITHUB_APP_PEM_FILE}" -binary | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
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TOKEN="${unsigned}.${signature}"
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```
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Then continue from Step 2.
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## Security Notes
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- Never log or echo the PEM key, JWT, or installation token to stdout in production.
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- The JWT is valid for at most 10 minutes. The installation token is valid for 1 hour.
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- Store the PEM file with restrictive permissions (`chmod 600`) and never check it into git.
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