feat(security): SSHFingerprintSHA256 helper (T02.1, REQ-058)

---ci---
project: orca
phase: 2
milestone: v0.8
status: execute
---/ci---

internal/security/sshkey.go

@@ -26,6 +26,17 @@ const (
 	sshPubFile = "orca_ssh_key.pub"
 )
 
+// SSHFingerprintSHA256 returns the canonical SSH public-key fingerprint
+// in the form `SHA256:base64` (no trailing padding), as produced by
+// `ssh-keygen -lf` and OpenSSH's host-key verification prompts. This is
+// a thin wrapper over ssh.FingerprintSHA256 (AD-027) for use by the
+// proxmox bootstrap pinned-host-key callback (REQ-058) and any other
+// SSH-domain identity checks. Do NOT reuse security.Fingerprint — that
+// returns an X.509 DER hex digest (different domain; RESEARCH §2.2).
+func SSHFingerprintSHA256(pubKey ssh.PublicKey) string {
+	return ssh.FingerprintSHA256(pubKey)
+}
+
 // GenerateOrLoadSSHKey returns the orca SSH keypair, generating it
 // lazily on first call (D-037). The key is Ed25519 (smaller, faster,
 // more secure than RSA for SSH auth), persisted as PKCS8 PEM to

internal/security/sshkey_test.go

@@ -1,6 +1,8 @@
 package security
 
 import (
+	"crypto/ed25519"
+	"crypto/rand"
 	"os"
 	"path/filepath"
 	"strings"
@@ -91,3 +93,43 @@ func TestGenerateOrLoadSSHKey_CreatesDir(t *testing.T) {
 		t.Errorf("nested dir not created: %v", err)
 	}
 }
+
+func TestSSHFingerprintSHA256_Ed25519(t *testing.T) {
+	pub, _, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatalf("ed25519 gen: %v", err)
+	}
+	sshPub, err := ssh.NewPublicKey(pub)
+	if err != nil {
+		t.Fatalf("new pubkey: %v", err)
+	}
+
+	got := SSHFingerprintSHA256(sshPub)
+
+	// Canonical form: SHA256: followed by unpadded base64.
+	if !strings.HasPrefix(got, "SHA256:") {
+		t.Fatalf("fingerprint = %q, want SHA256: prefix", got)
+	}
+	// Must match the reference implementation exactly.
+	want := ssh.FingerprintSHA256(sshPub)
+	if got != want {
+		t.Errorf("SSHFingerprintSHA256 = %q, want %q", got, want)
+	}
+}
+
+func TestSSHFingerprintSHA256_StableAcrossCalls(t *testing.T) {
+	pub, _, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatalf("ed25519 gen: %v", err)
+	}
+	sshPub, err := ssh.NewPublicKey(pub)
+	if err != nil {
+		t.Fatalf("new pubkey: %v", err)
+	}
+
+	a := SSHFingerprintSHA256(sshPub)
+	b := SSHFingerprintSHA256(sshPub)
+	if a != b {
+		t.Errorf("fingerprint not stable: %q vs %q", a, b)
+	}
+}