With golang and tls, how to convert rawCerts [][]byte (passed to VerifyPeerCertificate) to an x509.Certificate?

Question

Context: I want to establish a TLS Connection to a server, which has a certificate with a different (but known!) domain.

So I want to use tls.Dial('tcp', 'real-domain', conf), but verify the domain of the certificate as if it would be the other domain (lets call it wrong-domain), of which I know the server should return it.

So I think the way to do this is override VerifyPeerCertificate in the clients tls.Config.

VerifyPeerCertificate gets rawCerts [][]byte as parameter, but to use x509.Verify I need the certificate as a x509.Certificate type.

The question is: How do I cenvert the rawCerts [][]byte, which are passed as a parameter to VerifyPeerCertificate converted to x509.Certificate?

This is how I want to use it:

 conf := &tls.Config{ VerifyPeerCertificate: func(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error { verifyOptions := x509.VerifyOptions{ DNSName: "wrong-domain", Roots: serverCertPool, } cert := &x509.Certificate{???} // How do I get the x509.Certificate out of the rawCerts [][]byte parameter? _, err := cert.Verify(verifyOptions) return err }, InsecureSkipVerify: true, // We verify in VerifyPeerCertificate }

blackgreen · Accepted Answer · 2021-07-08 20:24:49Z

Use x509.ParseCertificate.

ParseCertificate parses a single certificate from the given ASN.1 DER data.

Example: cert, err := x509.ParseCertificate(rawCerts[0])

By the way this is also how the function that calls VerifyPeerCertificate does:

// crypto/tls/handshake/handshake_client.go func (c *Conn) verifyServerCertificate(certificates [][]byte) error { for i, asn1Data := range certificates { cert, err := x509.ParseCertificate(asn1Data) if err != nil { c.sendAlert(alertBadCertificate) return errors.New("tls: failed to parse certificate from server: " + err.Error()) } certs[i] = cert } // later on... if c.config.VerifyPeerCertificate != nil { if err := c.config.VerifyPeerCertificate(certificates, c.verifiedChains); err != nil { c.sendAlert(alertBadCertificate) return err } }

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