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/secure_enclave.m

@@ -47,7 +47,6 @@ static SecKeyRef lookup_ctk_private_key(const char *label, char *error_out, int
 }
 
 int se_create_key(const char *label,
-                  uint8_t *pub_key_out, int *pub_key_len,
                   char *hash_out, int hash_out_len,
                   char *error_out, int error_out_len) {
     @autoreleasepool {
@@ -87,7 +86,56 @@ int se_create_key(const char *label,
             return -1;
         }
 
-        // Retrieve the public key from the created identity
+        // Get the identity hash, which deleting the key needs, by parsing
+        // sc_auth list output
+        hash_out[0] = '\0';
+        NSTask *listTask = [[NSTask alloc] init];
+        listTask.executableURL = [NSURL fileURLWithPath:@"/usr/sbin/sc_auth"];
+        listTask.arguments = @[@"list-ctk-identities"];
+
+        NSPipe *listPipe = [NSPipe pipe];
+        listTask.standardOutput = listPipe;
+        listTask.standardError = [NSPipe pipe];
+
+        if ([listTask launchAndReturnError:&nsError]) {
+            [listTask waitUntilExit];
+            NSData *listData = [listPipe.fileHandleForReading readDataToEndOfFile];
+            NSString *listStr = [[NSString alloc] initWithData:listData
+                                                      encoding:NSUTF8StringEncoding];
+
+            for (NSString *line in [listStr componentsSeparatedByString:@"\n"]) {
+                if ([line containsString:labelStr]) {
+                    NSMutableArray *tokens = [NSMutableArray array];
+                    for (NSString *part in [line componentsSeparatedByCharactersInSet:
+                                            [NSCharacterSet whitespaceCharacterSet]]) {
+                        if (part.length > 0) {
+                            [tokens addObject:part];
+                        }
+                    }
+                    if (tokens.count > 1) {
+                        snprintf(hash_out, hash_out_len, "%s", [tokens[1] UTF8String]);
+                    }
+                    break;
+                }
+            }
+        }
+
+        if (hash_out[0] == '\0') {
+            NSString *msg = [NSString stringWithFormat:
+                             @"created key '%s' but found no hash for it in sc_auth list-ctk-identities",
+                             label];
+            snprintf_error(error_out, error_out_len, msg);
+            return -1;
+        }
+
+        return 0;
+    }
+}
+
+int se_copy_public_key(const char *label,
+                       uint8_t *pub_key_out, int *pub_key_len,
+                       char *error_out, int error_out_len) {
+    @autoreleasepool {
         SecKeyRef privateKey = lookup_ctk_private_key(label, error_out, error_out_len);
         if (!privateKey) {
             return -1;
@@ -126,39 +174,6 @@ int se_create_key(const char *label,
         *pub_key_len = (int)length;
         CFRelease(pubKeyData);
 
-        // Get the identity hash by parsing sc_auth list output
-        hash_out[0] = '\0';
-        NSTask *listTask = [[NSTask alloc] init];
-        listTask.executableURL = [NSURL fileURLWithPath:@"/usr/sbin/sc_auth"];
-        listTask.arguments = @[@"list-ctk-identities"];
-
-        NSPipe *listPipe = [NSPipe pipe];
-        listTask.standardOutput = listPipe;
-        listTask.standardError = [NSPipe pipe];
-
-        if ([listTask launchAndReturnError:&nsError]) {
-            [listTask waitUntilExit];
-            NSData *listData = [listPipe.fileHandleForReading readDataToEndOfFile];
-            NSString *listStr = [[NSString alloc] initWithData:listData
-                                                      encoding:NSUTF8StringEncoding];
-
-            for (NSString *line in [listStr componentsSeparatedByString:@"\n"]) {
-                if ([line containsString:labelStr]) {
-                    NSMutableArray *tokens = [NSMutableArray array];
-                    for (NSString *part in [line componentsSeparatedByCharactersInSet:
-                                            [NSCharacterSet whitespaceCharacterSet]]) {
-                        if (part.length > 0) {
-                            [tokens addObject:part];
-                        }
-                    }
-                    if (tokens.count > 1) {
-                        snprintf(hash_out, hash_out_len, "%s", [tokens[1] UTF8String]);
-                    }
-                    break;
-                }
-            }
-        }
-
         return 0;
     }
 }