ssh: fix panic when authority callbacks are nil

Previously, if CertChecker.IsHostAuthority or CertChecker.IsUserAuthority
were left unset, calling CheckHostKey or Authenticate would result in a
nil pointer dereference panic.

This change adds checks to ensure these callbacks are defined before
invocation, returning an error instead of panicking.

This issue was found during a security audit by NCC Group Cryptography
Services, sponsored by Teleport.

Fixes golang/go#79563
Fixes CVE-2026-39835

Change-Id: I2bd9c8d76646232e49f6aedc7b5334f3825918be
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/781660
Commit-Queue: Neal Patel <nealpatel@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>

ssh/certs.go

diff --git a/ssh/certs.go b/ssh/certs.go
index 139fa31..b003234 100644
--- a/ssh/certs.go
+++ b/ssh/certs.go
@@ -348,6 +348,9 @@
 	if cert.CertType != HostCert {
 		return fmt.Errorf("ssh: certificate presented as a host key has type %d", cert.CertType)
 	}
+	if c.IsHostAuthority == nil {
+		return errors.New("ssh: cannot verify certificate, IsHostAuthority not set")
+	}
 	if !c.IsHostAuthority(cert.SignatureKey, addr) {
 		return fmt.Errorf("ssh: no authorities for hostname: %v", addr)
 	}
@@ -375,6 +378,9 @@
 	if cert.CertType != UserCert {
 		return nil, fmt.Errorf("ssh: cert has type %d", cert.CertType)
 	}
+	if c.IsUserAuthority == nil {
+		return nil, errors.New("ssh: cannot verify certificate, IsUserAuthority not set")
+	}
 	if !c.IsUserAuthority(cert.SignatureKey) {
 		return nil, fmt.Errorf("ssh: certificate signed by unrecognized authority")
 	}