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refactor: rewrite JWT generation from python to bash

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Replaced generate_jwt.py with generate_jwt.sh using only openssl and
coreutils. Updated SKILL.md to remove the python fallback section and
use grep/cut for JSON parsing instead of python3.
This commit is contained in:
Chris Farhood
2026-03-25 21:46:02 -04:00
parent f425b5191e
commit dedb35953b
3 changed files with 49 additions and 109 deletions
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github-app-token/SKILL.md
+8 -25
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@@ -19,19 +19,18 @@ The following environment variables MUST be set before invoking this skill:
If any variable is missing, stop and tell the user which ones are required.
Requires `openssl`, `curl`, and `grep` (standard on macOS and Linux).
## Steps
### 1. Generate a JWT
Create a JWT signed with the GitHub App's private key. You MUST use the helper script bundled with this skill:
Run the helper script bundled with this skill:
```bash
# generates a JWT valid for 10 minutes
TOKEN=$(python3 "$(dirname "$0")/../skills/github-app-token/scripts/generate_jwt.py")
TOKEN=$(bash /path/to/skills/github-app-token/scripts/generate_jwt.sh)
```
If `python3` is not available, fall back to the inline openssl method described in the Fallback section below.
The JWT uses:
- **Algorithm**: RS256
- **Header**: `{"alg": "RS256", "typ": "JWT"}`
@@ -43,12 +42,13 @@ The JWT uses:
### 2. Exchange the JWT for an installation access token
```bash
INSTALL_TOKEN=$(curl -s -X POST \
RESPONSE=$(curl -s -X POST \
-H "Authorization: Bearer ${TOKEN}" \
-H "Accept: application/vnd.github+json" \
-H "X-GitHub-Api-Version: 2022-11-28" \
"https://api.github.com/app/installations/${GITHUB_APP_INSTALLATION_ID}/access_tokens" \
| python3 -c "import sys,json; print(json.load(sys.stdin)['token'])")
"https://api.github.com/app/installations/${GITHUB_APP_INSTALLATION_ID}/access_tokens")
INSTALL_TOKEN=$(echo "$RESPONSE" | grep -o '"token":"[^"]*"' | head -1 | cut -d'"' -f4)
```
If the response contains an error (e.g., `401 Unauthorized`), check:
@@ -81,23 +81,6 @@ curl -s -X DELETE \
"https://api.github.com/installation/token"
```
## Fallback: JWT generation without Python
If `python3` is unavailable, generate the JWT using `openssl` and `bash`:
```bash
header=$(printf '{"alg":"RS256","typ":"JWT"}' | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
now=$(date +%s)
iat=$((now - 60))
exp=$((now + 600))
payload=$(printf '{"iat":%d,"exp":%d,"iss":"%s"}' "$iat" "$exp" "$GITHUB_APP_ID" | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
unsigned="${header}.${payload}"
signature=$(printf '%s' "$unsigned" | openssl dgst -sha256 -sign "${GITHUB_APP_PEM_FILE}" -binary | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
TOKEN="${unsigned}.${signature}"
```
Then continue from Step 2.
## Security Notes
- Never log or echo the PEM key, JWT, or installation token to stdout in production.
github-app-token/scripts/generate_jwt.py
-84
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@@ -1,84 +0,0 @@
#!/usr/bin/env python3
"""Generate a JWT for GitHub App authentication.
Reads from environment variables:
GITHUB_APP_ID - The GitHub App's numeric ID
GITHUB_APP_PEM_FILE - Path to the PEM-encoded private key file
Prints the signed JWT to stdout.
"""
import json
import os
import sys
import time
import base64
try:
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import padding
USE_CRYPTOGRAPHY = True
except ImportError:
import subprocess
USE_CRYPTOGRAPHY = False
def b64url(data: bytes) -> str:
return base64.urlsafe_b64encode(data).rstrip(b"=").decode("ascii")
def sign_with_cryptography(unsigned: str, pem_contents: str) -> str:
"""Sign using the cryptography library. Handles both PKCS#1 and PKCS#8 PEM formats."""
private_key = serialization.load_pem_private_key(pem_contents.encode(), password=None)
signature = private_key.sign(unsigned.encode(), padding.PKCS1v15(), hashes.SHA256())
return b64url(signature)
def sign_with_openssl(unsigned: str, pem_file: str) -> str:
"""Sign using the openssl CLI. Reads the key from the file path directly."""
result = subprocess.run(
["openssl", "dgst", "-sha256", "-sign", pem_file, "-binary"],
input=unsigned.encode(),
capture_output=True,
)
if result.returncode != 0:
print(f"error: openssl signing failed: {result.stderr.decode()}", file=sys.stderr)
sys.exit(1)
return b64url(result.stdout)
def main():
app_id = os.environ.get("GITHUB_APP_ID")
pem_file = os.environ.get("GITHUB_APP_PEM_FILE")
if not app_id:
print("error: GITHUB_APP_ID is not set", file=sys.stderr)
sys.exit(1)
if not pem_file:
print("error: GITHUB_APP_PEM_FILE is not set", file=sys.stderr)
sys.exit(1)
if not os.path.isfile(pem_file):
print(f"error: PEM file not found: {pem_file}", file=sys.stderr)
sys.exit(1)
now = int(time.time())
header = b64url(json.dumps({"alg": "RS256", "typ": "JWT"}).encode())
payload = b64url(
json.dumps({"iat": now - 60, "exp": now + 600, "iss": app_id}).encode()
)
unsigned = f"{header}.{payload}"
if USE_CRYPTOGRAPHY:
with open(pem_file, "r") as f:
pem_contents = f.read()
signature = sign_with_cryptography(unsigned, pem_contents)
else:
signature = sign_with_openssl(unsigned, pem_file)
print(f"{unsigned}.{signature}")
if __name__ == "__main__":
main()
github-app-token/scripts/generate_jwt.sh
Executable
+41
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@@ -0,0 +1,41 @@
#!/usr/bin/env bash
# Generate a JWT for GitHub App authentication.
#
# Required environment variables:
# GITHUB_APP_ID - The GitHub App's numeric ID
# GITHUB_APP_PEM_FILE - Path to the PEM-encoded private key file
#
# Prints the signed JWT to stdout.
set -euo pipefail
if [[ -z "${GITHUB_APP_ID:-}" ]]; then
echo "error: GITHUB_APP_ID is not set" >&2
exit 1
fi
if [[ -z "${GITHUB_APP_PEM_FILE:-}" ]]; then
echo "error: GITHUB_APP_PEM_FILE is not set" >&2
exit 1
fi
if [[ ! -f "${GITHUB_APP_PEM_FILE}" ]]; then
echo "error: PEM file not found: ${GITHUB_APP_PEM_FILE}" >&2
exit 1
fi
## Build JWT
header=$(printf '{"alg":"RS256","typ":"JWT"}' | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
now=$(date +%s)
iat=$((now - 60))
exp=$((now + 600))
payload=$(printf '{"iat":%d,"exp":%d,"iss":"%s"}' "$iat" "$exp" "$GITHUB_APP_ID" | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
unsigned="${header}.${payload}"
signature=$(printf '%s' "$unsigned" | openssl dgst -sha256 -sign "${GITHUB_APP_PEM_FILE}" -binary | openssl base64 -e -A | tr '+/' '-_' | tr -d '=')
echo "${unsigned}.${signature}"