Delete the keychain item or Secure Enclave key of a failed unlocker add (closes #89)

A Secure Enclave unlocker add gets the long-term key before it creates
the Secure Enclave key, so a wrong passphrase creates none, and deletes
the key if a later step fails. macse.CreateKey finds the new key's hash
right after sc_auth creates it, failing with an error naming the label
if it cannot, and deletes the key if getting its public key then fails.
A keychain unlocker add writes all of the unlocker's files before it
stores the keychain item, and deletes the item if moving the unlocker
into place then fails. A failure to delete is reported along with the
original error.

The Objective-C and macse_darwin.go were only read, never compiled or
run; the new tests run only on a Mac.

Model: opus-5-5

internal/macse/macse_darwin.go

@@ -15,6 +15,7 @@ package macse
 import "C"
 
 import (
+	"errors"
 	"fmt"
 	"unsafe"
 )
@@ -39,10 +40,9 @@ const (
 
 // CreateKey creates a new P-256 non-exportable key in the Secure Enclave via sc_auth.
 // Returns the uncompressed public key bytes (65 bytes) and the identity hash
-// (for deletion).
+// (for deletion). If getting the public key fails, CreateKey deletes the key
+// again; a failure to delete is returned along with the first error.
 func CreateKey(label string) (publicKey []byte, hash string, err error) {
-	pubKeyBuf := make([]C.uint8_t, p256UncompressedKeySize)
-	pubKeyLen := C.int(p256UncompressedKeySize)
 	var hashBuf [hashBufferSize]C.char
 	var errBuf [errorBufferSize]C.char
 
@@ -50,7 +50,6 @@ func CreateKey(label string) (publicKey []byte, hash string, err error) {
 	defer C.free(unsafe.Pointer(cLabel)) //nolint:nlreturn // CGo free pattern
 
 	result := C.se_create_key(cLabel,
-		&pubKeyBuf[0], &pubKeyLen,
 		&hashBuf[0], C.int(hashBufferSize),
 		&errBuf[0], C.int(errorBufferSize))
 
@@ -58,9 +57,29 @@ func CreateKey(label string) (publicKey []byte, hash string, err error) {
 		return nil, "", fmt.Errorf("secure enclave: %s", C.GoString(&errBuf[0]))
 	}
 
+	h := C.GoString(&hashBuf[0])
+
+	pubKeyBuf := make([]C.uint8_t, p256UncompressedKeySize)
+	pubKeyLen := C.int(p256UncompressedKeySize)
+
+	result = C.se_copy_public_key(cLabel,
+		&pubKeyBuf[0], &pubKeyLen,
+		&errBuf[0], C.int(errorBufferSize))
+
+	if result != 0 {
+		err = fmt.Errorf("secure enclave: %s", C.GoString(&errBuf[0]))
+
+		deleteErr := DeleteKey(h)
+		if deleteErr != nil {
+			err = errors.Join(err,
+				fmt.Errorf("failed to delete key %s: %w", label, deleteErr))
+		}
+
+		return nil, "", err
+	}
+
 	//nolint:nlreturn // CGo result extraction
 	pk := C.GoBytes(unsafe.Pointer(&pubKeyBuf[0]), pubKeyLen)
-	h := C.GoString(&hashBuf[0])
 
 	return pk, h, nil
 }