// pin.go: certificate PINNING for the peer link, as opposed to CA trust.
//
// There's no certificate authority that can issue for a Tailscale/LAN IP,
// so the peer doesn't ask "was this signed by someone I trust?" — it asks
// "is this the exact certificate my invite code told me to expect?". That
// makes InsecureSkipVerify safe here: it turns off the check that doesn't
// apply (hostname/CA validation) and VerifyPeerCertificate replaces it
// with the one that does.
package main

import (
	"crypto/sha256"
	"crypto/tls"
	"crypto/x509"
	"encoding/hex"
	"fmt"
	"strings"
)

func certMatchesFingerprint(rawCert []byte, wantHex string) bool {
	sum := sha256.Sum256(rawCert)
	return hex.EncodeToString(sum[:]) == strings.ToLower(strings.TrimSpace(wantHex))
}

func validFingerprint(s string) bool {
	s = strings.TrimSpace(s)
	if len(s) != sha256.Size*2 {
		return false
	}
	_, err := hex.DecodeString(s)
	return err == nil
}

// pinnedClientTLSConfig builds a client TLS config that accepts exactly
// one certificate: the one whose SHA-256 fingerprint matches. Nothing
// else about the certificate (hostname, expiry chain, issuer) is checked.
func pinnedClientTLSConfig(fingerprint string) (*tls.Config, error) {
	if !validFingerprint(fingerprint) {
		return nil, fmt.Errorf("fingerprint %q doesn't look like a SHA-256 hex digest (want %d hex chars)", fingerprint, sha256.Size*2)
	}
	want := strings.ToLower(strings.TrimSpace(fingerprint))
	return &tls.Config{
		InsecureSkipVerify: true,
		MinVersion:         tls.VersionTLS13,
		VerifyPeerCertificate: func(rawCerts [][]byte, _ [][]*x509.Certificate) error {
			if len(rawCerts) == 0 {
				return fmt.Errorf("the server didn't present a certificate")
			}
			if !certMatchesFingerprint(rawCerts[0], want) {
				return fmt.Errorf("the server's certificate doesn't match the pinned fingerprint — wrong hub, or its certificate was regenerated (you'd need a fresh invite code)")
			}
			return nil
		},
	}, nil
}

// hubServerTLSConfig is the hub side: just present the certificate, no
// client-certificate verification (the peer proves itself at the
// application layer with the HMAC challenge/response, see auth.go).
func hubServerTLSConfig(cert tls.Certificate) *tls.Config {
	return &tls.Config{
		Certificates: []tls.Certificate{cert},
		MinVersion:   tls.VersionTLS13,
	}
}