Accessing Google’s Private Key

Some time ago, I gained access to Google’s webserver certificate’s private key…

To prove this, I used the private key to sign the SHA-256 hash of a message:

Hash

1af3ebca01d642dc6f69a786710a79f76e79b8be46ce879049de77ae7c3c620f

Signature

MEYCIQDCKIiTYVoKVbWN67jx2WvO455Iks/B7KgfW0xVWaheKQIhANwHWLNGeAoQeYIIwhjvXJ23I1L+bzZRzEPpb/QdJfaz

The message is something similar to what @BenLaurie suggested at the mozilla.dev.security.policy mailing list, a message plus a random string/nonce. The reason this is done is that the only entities that can sign messages are the holders of the private key. By signing a specific message one shows that they can control what is signed, thus the certificate’s key, and according to the CA / Browser Forum (section 4.9.1.1.3) that key would require revocation.

The Signing

You can see the message to be signed by using the xxd command on the base64 encoded message :

xxd -l 64 <(base64 -d <<< cHJvb2Ygb2YgcHJpdmF0ZSBrZXkgY3RybDtub25jZS1aub2puDlP6jhTHgCx27YMFWKhQUVXFIDdxnFkjPGKKAMAF0EEVlO1Afskc+N/82BMOEvWUIaJNvyvAeniaqtA6oZGHG0sBXIioShZl0V94BZQ2u5/cF3rBoyEyZINfD6AgwWQ9A==)

Message to be signed

The message itself is hashed using SHA–256:

Message Hash

And that hash is then signed using the key (openssl creates the hash itself, so you still have to pass in the full message rather than just the hash):

The Signing

Verification:

Here are the steps to verify the signature from a Linux command line console:
1) Download cert:
wget -O 349531041.cer https://crt.sh/?d=349531041
2) Run the OpenSSL dgst command to show that the signature is valid
openssl dgst -sha256 -verify <pubkey> -signature <signature> <message>

Paste the following into a terminal, and the message that returns out will say that the signature is verified!

wget -O 349531041.cer https://crt.sh/?d=349531041

openssl dgst -sha256 -verify <(openssl x509 -in 349531041.cer -pubkey -noout) -signature <(base64 -d <<< MEYCIQDCKIiTYVoKVbWN67jx2WvO455Iks/B7KgfW0xVWaheKQIhANwHWLNGeAoQeYIIwhjvXJ23I1L+bzZRzEPpb/QdJfaz) <(base64 -d <<< cHJvb2Ygb2YgcHJpdmF0ZSBrZXkgY3RybDtub25jZS1aub2puDlP6jhTHgCx27YMFWKhQUVXFIDdxnFkjPGKKAMAF0EEVlO1Afskc+N/82BMOEvWUIaJNvyvAeniaqtA6oZGHG0sBXIioShZl0V94BZQ2u5/cF3rBoyEyZINfD6AgwWQ9A==)

Verified Signature

And there you have it, proof.

An update to this here:

https://medium.com/@ECCTLS/how-to-sign-with-googles-private-key-5b8e99abcdb3