YubiKey TokenSource

2019-08-22

Google Cloud Credentials source for Service Account keys embedded within a Yubikey PIV.

WARNING: YubiKeyTokenSource is highly experimental. This repo is NOT supported by Google

google/oauth2/YubiKeyTokenSource is a variation of google/oauth2/JWTAccessTokenSourceFromJSON where the private key used to sign the JWT is embedded within a PIV-enabled YubiKey.

The private key in raw form not exportable or exposed to the filesystem or any process other than through the Yubikey interface. This token source uses the yubikey alone to sign the JWT which is then used to access a Google Cloud API.

This library uses go-piv which is a go-native interface to yubikey (vs wrapper around C). To use this library, install PSCS Lite libpcsclite-dev

The default installation steps here first creates a service account private key that will get imported into the yubikey. It is expected that you discard/destroy the private key at that point so that the only copy is on the Yubikey.

However, the better way is to only generate the private key on the Yubikey and then export its CSR. From there, use your own CSR to sign it to generate a cert. Import the cert back into the yubikey at slot 9c. Once all that is done, you can import the signed certificate back into GCP and associate that with a service account. For more information on the last part, see Uploading public keys for service accounts. This repo does not show these steps and just takes the lazy way out.


You can find the source here


Usage

  1. Prepare Yubikey for Key import

    First embed a GCP Service Account file as a combined x509 certificate within a YubiKey.

    You must have a YubiKey Neo or YubiKey 4 or 5 as thoese keys support embedded keys.

    You are free to provision the key by any means but for reference, the following uses:

    On any other system, install supporting libraries for both components listed above.

    Check Yubikey is plugged in

    $ lsusb  | grep -i yubikey
    Bus 001 Device 013: ID 1050:0111 Yubico.com Yubikey NEO(-N) OTP+CCID
    

    ``

    Launch the ykman-gui UI Application, it should also show the type of key you are using. We will use ykman-gui later to import the keys.

  2. Extract Service account certificate file

    Download a GCP Service Account **in .p12 format".

    Remove default passphrase (notasecret), generate an x509 file for importing into the YubiKey.

    openssl pkcs12 -in svc_account.p12  -nocerts -nodes -passin pass:notasecret | openssl rsa -out privkey.pem
    openssl rsa -in privkey.pem -outform PEM -pubout -out public.pem
    openssl req -new -x509  -key privkey.pem  -out public.crt
    		 Use CN=<your_service_account_email>
    openssl pkcs12 --export -in public.crt -inkey privkey.pem -outform PEM -out cert.pfx
    

    ``

  3. Embed Service Account within YubiKey

    Launch yubikey-manager-qt and navigate to the Digital Signature (9c). (see Certificate Slots) Import the certificate cert.pfx. If this is a new Yubikey, just use the default PIN and Management Keys provided.

    You should see a loaded certificate:

    images/imported.png

    You can verify certificate load Status by running the yubico-piv-tool:

    $ yubico-piv-tool -a status
    Version:	1.0.4
    Serial Number:	-1879017761
    CHUID:	3019d4e739da739ced39ce739d836858210842108421c84210c3eb34109acae3dbacb7f8b5295a1be28d916b2c350832303330303130313e00fe00
    CCC:	No data available
    Slot 9c:	
    	Algorithm:	RSA2048
    	Subject DN:	C=AU, ST=Some-State, O=Internet Widgits Pty Ltd, CN=svc-2-429@project.iam.gserviceaccount.com
    	Issuer DN:	C=AU, ST=Some-State, O=Internet Widgits Pty Ltd, CN=svc-2-429@project.iam.gserviceaccount.com
    	Fingerprint:	5726b54b6b50d737307a9ec09c0fc857258e23e7c05acf3f4d28cb3a2f37056b
    	Not Before:	Oct 16 05:09:20 2019 GMT
    	Not After:	Nov 15 05:09:20 2019 GMT
    PIN tries left:	3
    

    ``

    And also verify sign/verify steps:

    $ yubico-piv-tool -a read-certificate -s 9c
    -----BEGIN CERTIFICATE-----
    ...
    -----END CERTIFICATE-----
    
    $ yubico-piv-tool -a verify-pin -a test-signature -s 9c
    Enter PIN: 
    Successfully verified PIN.
    Please paste the certificate to verify against...
    -----BEGIN CERTIFICATE-----
    ...
    -----END CERTIFICATE-----
    Successful RSA verification.
    

    ``

Alternatively, you can generate a private on the Yubkiey, gen a CSR, sign it with your own CA, then associate it with the service account

images/gencsr.png

  1. Install libpcsclite-dev on target system

    On any system you wish to use this library, you must first install libpcsclite-dev:

    sudo apt-get install libpcsclite-dev
    

    ``

    Insert the YubiKey and verify its detected:

    $ lsusb  | grep -i yubikey
    Bus 001 Device 013: ID 1050:0111 Yubico.com Yubikey NEO(-N) OTP+CCID
    

    ``

  2. Use TokenSource

    After the key is embedded into the yubikey, you can DELETE any reference to private.pem or the .p12 file (the private key now exists protected by the physical access to the yubikey).

    The YubiKey based TokenSource can now be used to access a GCP resource using either a plain HTTPClient or native GCP library (google-cloud-pubsub)!!

    package main
    
    import (
    	"log"
    	"net/http"
    	"cloud.google.com/go/pubsub"
    
    	"golang.org/x/oauth2"
    	sal "github.com/salrashid123/yubikey/google"
    
    	"google.golang.org/api/iterator"
    	"google.golang.org/api/option"
    )
    
    func main() {
    	yubiKeyTokenSource, err := sal.YubiKeyTokenSource(
    		&sal.YubiKeyTokenConfig{
    			Email:    "svcAccount@project.iam.gserviceaccount.com",
    			Audience: "https://pubsub.googleapis.com/google.pubsub.v1.Publisher",
    			Pin:      "123456",
    		},
    	)
    
    	// tok, err := yubiKeyTokenSource.Token()
    	// if err != nil {
    	// 	 log.Fatal(err)
    	// }
    	// log.Printf("Token: %v", tok.AccessToken)
    	client := &http.Client{
    		Transport: &oauth2.Transport{
    			Source: yubiKeyTokenSource,
    		},
    	}
    
    	url := "https://pubsub.googleapis.com/v1/projects/YOURPROJECT/topics"
    	resp, err := client.Get(url)
    	if err != nil {
    		log.Fatalf("Unable to get Topics %v", err)
    	}
    	log.Printf("Response: %v", resp.Status)
    
    
    	// Using google-cloud library
    
    	ctx := context.Background()
    	pubsubClient, err := pubsub.NewClient(ctx, proj, option.WithTokenSource(yubiKeyTokenSource))
    	if err != nil {
    		log.Fatalf("Could not create pubsub Client: %v", err)
    	}
    
    	it := pubsubClient.Topics(ctx)
    	for {
    		topic, err := it.Next()
    		if err == iterator.Done {
    			break
    		}
    		if err != nil {
    			log.Fatalf("Unable to iterate topics %v", err)
    		}
    		log.Printf("Topic: %s", topic.ID())
    	}
    

    ``

    Note: by default the Yubikey allows for 3 PIN attempts before going into lockout. To unlock, see PIN and Management Key

    If you do not have the Yubikey Plugged in, you may see an error like this

    error: SCardListReaders failed, rc=8010002e
    2019/10/16 07:33:10 Unable to open yubikey ykpiv ykpiv_connect: PKCS Error (-2) - Error in PCSC call
    

    ``

Some notes:

  • Slot 9c: Digital Signature is reserved for Digital Signatures)
  • The default PIN for access is 123456. The default unlock code is 12345678.

See previous article about using the Yubikey NEO with GPG decryption Encrypting Google Application Default and gcloud credentials with GPG SmardCard. The distinction here is that the RSA signing happens all onboard.

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