Overview of Assured OSS artifact signature

Assured Open Source Software provides you with packages, package metadata, and utility scripts. Each Assured OSS package is signed using a separate Google-issued certificate which uses ECDSA P256 algorithm for key generation. Each package metadata and utility script is also signed using a single Google-issued certificate which uses ECDSA P256 algorithm for key generation. These certificates are issued using Certificate Authority Service and the corresponding public root certificate can be found in a Cloud Storage bucket.

This document applies to the free tier. For information about signatures in the premium tier, see Access security metadata and verify packages.

Overview of signatures

Signatures are stored in a signature.zip file. The signature.zip file contains the following files:

digest.txt: This file stores the hashing algorithm along with the package digest in a hex-encoded format. See the following example:
```
SHA-256:c5feab6f4de0878e94cf2a3074039b4f16a0c93a03501f047ee6eea29a8e33e0`.
```

signature.txt: This file stores the signature algorithm along with the hex-encoded signature. See the following example:

ECDSAP256_DER:30450220585d2a01f20de98dfe6cfab2c01a8f11787dbafbc6541304d23cc582e61be016022100f05a19f5ce473144579dfefc47905fd650584a1c7a31bd9d5bf93ecce739a7cb`

cert.pem: This file stores the public certificate.
certChain.pem: This file stores the certificate chain for the public certificate.

Verification of signatures

You can verify signatures using either of the following:

aoss-verifier tool
manually
verification script

Verify signatures using `aoss-verifier` tool

To verify the authenticity and integrity of open source software packages, install the aoss-verifier tool.
To verify a package, pass the LANGUAGE, PACKAGE_ID, and VERSION values and the PATH_TO_DATA_FILE that you want to verify using the --language, --package_id, --version, and --artifact_path flags.
```
aoss-verifier verify-package \
   --language LANGUAGE \
   --package_id PACKAGE_ID \
   --version VERSION \
   --artifact_path ARTIFACT_PATH
```
Replace the following:
- LANGUAGE: The package language. The value must be in lowercase.
- PACKAGE_ID: For Java, the format is groupId:artifactId. For Python, the format is packageName. The value must be in lowercase.
- VERSION: The version of the package.
- ARTIFACT_PATH: The path to the data file in your local directory that you want to verify. Use the following filename extensions:
  - jar file extension for a Java package
  - whl file extension for a Python package

Verify signatures manually

To verify the signatures manually, do the following:

Install Google Cloud CLI and the OpenSSL software, version 3.0.1 or higher.
Verify the digest.
1. Identify the hashing algorithm. The digest.txt file contains the hashing algorithm used to calculate the digest. Identify the hashing algorithm using the following command:
```
cut -d ':' -f1 digest.txt
```
  Make sure that the output is SHA256.
2. Calculate digest of the data file. You can calculate digest of the data file by using various command line tools depending on the hash algorithm used. For Assured OSS data files and scripts, use the SHA-256 hash algorithm to generate the digest.
  
  On Linux, use the following command to calculate the hash:
```
sha256sum PATH_TO_DATA_FILE | cut -d ' ' -f1 > datadigest.txt
```
  For package verification, replace PATH_TO_DATA_FILE with the path to the package in your local directory for package verification.
  
  For metadata verification, replace PATH_TO_DATA_FILE with the metadata filename. For example, buildInfo.json, vexInfo.json or healthInfo.json.
  
  On macOS, use the following command:
```
shasum -a 256 PATH_TO_DATA_FILE | cut -d ' ' -f1 > datadigest.txt
```
3. Extract hash from the digest.txt file using the following command:
```
cut -d ':' -f2 digest.txt > signaturedigest.txt
```
4. Compare the two digests using the following command:
```
diff datadigest.txt signaturedigest.txt
```
  If there is no difference, then there will be no output.
Verify the signature
1. Obtain a public key from cert.pem using the following command:
```
openssl x509 -pubkey -noout -in cert.pem  > pubkey.pem
```
2. Extract the signature in binary format using the following command (install xxd command if it isn't present):
```
cut -d ':' -f2 signature.txt | xxd -r -p > sig.sig
```
3. Extract the digest in binary format using the following command:
```
cut -d ':' -f2 digest.txt | xxd -r -p > digest.bin
```
4. Verify the signature using one of the following commands:
  
  Option 1
```
openssl pkeyutl -in digest.bin -inkey pubkey.pem -pubin -verify -sigfile sig.sig
```
  The command returns the following output:
  
  Signature Verified Successfully
  
  Option 2
```
openssl dgst -sha256 -verify pubkey.pem -signature sig.sig PATH_TO_DATA_FILE
```
  Replace PATH_TO_DATA_FILE with the path to the data file in your local directory.
  
  The command returns the following output:
  
  Verified OK

Verify the certificate

To verify the certificate, run the following command:

openssl verify -verbose -CAfile ca.crt -untrusted certChain.pem cert.pem

The ca.cert is the root certificate that can be download using this command:

curl -o PATH_TO_LOCAL_STORE https://privateca-content-6333d504-0000-2df7-afd6-30fd38154590.storage.googleapis.com/a2c725a592f1d586f1f8/ca.crt

The following output is returned upon success:

cert.pem: OK

Verify signatures using a script

To use a verification script to verify signatures, follow these steps:

Download the verification script.
1. Set up authentication using Application Default Credentials (ADC).
2. Download the signatureverification.sh, README.md and the signature.zip for the verification script using the following command:
```
gcloud storage cp "gs://cloud-aoss/utils/signature-verification/v1.0" PATH_TO_LOCAL_STORE --recursive
```
  Replace PATH_TO_LOCAL_STORE with the local path where you want to download the file.
  
  You can also perform utility script verification using the verification script.
Execute the script using the following steps:
1. Install Google Cloud CLI and the OpenSSL software, version 3.0.1 or higher.
2. Authenticate with the service account that has permission to access Assured OSS.
3. Grant the required permissions to execute the script using the following command:
```
chmod +x signatureverification.sh
```
4. Execute the script using the following command if you haven't downloaded the signature.zip file:
```
./signatureverification.sh --data_file_path PATH_TO_DATA_FILE \
--signature_url SIGNATURE_ZIP_URL \
--root_cert_url ROOT_CERT_URL
```
  Replace the following:
  - PATH_TO_DATA_FILE: the path to the data file in your local directory that you want to verify
  - SIGNATURE_ZIP_URL: the URL of the signature zip file
  - ROOT_CERT_URL: the public root certificate URL

Sample output

The following output indicates that the verification script has run successfully.

Signature verification successful

Certificate verification successful