fix(auth): decoy encryption keys are keys a device could have made

An address nobody has got, as the recovery phrase's first step, an
X25519 public key of 32 HMAC bytes, which is a point of Curve25519's
prime-order subgroup one time in sixteen, as every real key is: whoever
checked could tell nearly always whether an address has an account (the
second reading's M1). The decoy's key is now made from a seed, as its
Ed25519 key already was. The tests hold twenty decoys to the curve check
the review used.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_base_server/test/unit/password/key_proof_test.dart

@@ -6,6 +6,8 @@ import 'package:halkyon_base_server/src/password/key_proof.dart';
 import 'package:serverpod/serverpod.dart';
 import 'package:test/test.dart';
 
+import '../../integration/support/curve25519.dart';
+
 Uint8List _hex(String hex) => Uint8List.fromList([
   for (var i = 0; i < hex.length; i += 2)
     int.parse(hex.substring(i, i + 2), radix: 16),
@@ -147,5 +149,29 @@ void main() {
         )).extractPublicKey()).bytes,
       );
     });
+
+    test(
+      'decoy X25519 keys from a seed are points of the prime-order '
+      "subgroup, as devices' keys are, and random bytes mostly aren't",
+      () async {
+        for (var i = 0; i < 20; i++) {
+          final seed = List.generate(32, (j) => (i * 31 + j * 17) % 256);
+          final decoy = await x25519PublicKeyFromSeed(seed);
+          expect(await x25519PublicKeyFromSeed(seed), decoy);
+          expect(isX25519PublicKey(decoy), isTrue);
+          final made = await (await X25519().newKeyPair()).extractPublicKey();
+          expect(isX25519PublicKey(made.bytes), isTrue);
+        }
+        // The check itself: of byte strings that aren't keys, few pass.
+        var passing = 0;
+        for (var i = 0; i < 64; i++) {
+          final bytes = List.generate(32, (j) => (i * 131 + j * 29 + 7) % 256);
+          if (isX25519PublicKey(bytes)) {
+            passing++;
+          }
+        }
+        expect(passing, lessThan(16));
+      },
+    );
   });
 }