diff --git a/src/cerbero_mcp/exchanges/ibkr/oauth.py b/src/cerbero_mcp/exchanges/ibkr/oauth.py
index f8b462f..56245e5 100644
--- a/src/cerbero_mcp/exchanges/ibkr/oauth.py
+++ b/src/cerbero_mcp/exchanges/ibkr/oauth.py
@@ -5,8 +5,8 @@ Reference: https://www.interactivebrokers.com/api/doc.html (Self-Service OAuth)
 from __future__ import annotations
 
 import base64
-import hashlib  # noqa: F401
-import hmac  # noqa: F401
+import hashlib  # noqa: F401  -- used in Task 3 (DH live session token mint)
+import hmac  # noqa: F401  -- used in Task 3 (DH live session token mint)
 import secrets
 import time
 from dataclasses import dataclass, field
@@ -14,6 +14,7 @@ from urllib.parse import quote
 
 from cryptography.hazmat.primitives import hashes, serialization
 from cryptography.hazmat.primitives.asymmetric import padding
+from cryptography.hazmat.primitives.asymmetric.rsa import RSAPrivateKey
 
 
 def _percent_encode(value: str) -> str:
@@ -32,6 +33,7 @@ def build_signature_base_string(
         f"{_percent_encode(k)}%3D{_percent_encode(v)}"
         for k, v in sorted_params
     ]
+    # Manual %3D / %26 = double-encoding of '=' and '&' per RFC 5849 §3.4.1.3.2
     params_str = "%26".join(encoded_pairs)
     return f"{method.upper()}&{_percent_encode(url)}&{params_str}"
 
@@ -45,8 +47,8 @@ class OAuth1aSigner:
     encryption_key_path: str
     dh_prime: str  # hex string
 
-    _signature_key: object = field(default=None, init=False, repr=False)
-    _encryption_key: object = field(default=None, init=False, repr=False)
+    _signature_key: RSAPrivateKey | None = field(default=None, init=False, repr=False)
+    _encryption_key: RSAPrivateKey | None = field(default=None, init=False, repr=False)
     _live_session_token: str | None = field(default=None, init=False, repr=False)
     _lst_expires_at: float = field(default=0.0, init=False, repr=False)
 
@@ -62,8 +64,9 @@ class OAuth1aSigner:
 
     def sign(self, method: str, url: str, params: dict[str, str]) -> str:
         """Firma RSA-SHA256 della signature base string. Ritorna base64."""
+        assert self._signature_key is not None  # set in __post_init__
         base = build_signature_base_string(method, url, params)
-        signature = self._signature_key.sign(  # type: ignore[attr-defined]
+        signature = self._signature_key.sign(
             base.encode("utf-8"),
             padding.PKCS1v15(),
             hashes.SHA256(),
diff --git a/tests/unit/exchanges/ibkr/test_oauth.py b/tests/unit/exchanges/ibkr/test_oauth.py
index f652105..303309c 100644
--- a/tests/unit/exchanges/ibkr/test_oauth.py
+++ b/tests/unit/exchanges/ibkr/test_oauth.py
@@ -1,8 +1,13 @@
 from __future__ import annotations
 
-from cerbero_mcp.exchanges.ibkr.oauth import OAuth1aSigner, build_signature_base_string
-from cryptography.hazmat.primitives import serialization
-from cryptography.hazmat.primitives.asymmetric import rsa  # noqa: I001
+import base64 as _b64
+
+from cerbero_mcp.exchanges.ibkr.oauth import (
+    OAuth1aSigner,
+    build_signature_base_string,
+)
+from cryptography.hazmat.primitives import hashes, serialization
+from cryptography.hazmat.primitives.asymmetric import padding, rsa
 
 
 def test_signature_base_string_canonical_order():
@@ -58,5 +63,23 @@ def test_oauth_signer_signs_with_rsa(tmp_path):
             "oauth_version": "1.0",
         },
     )
-    assert isinstance(sig, str)
-    assert len(sig) > 100
+
+    # Verify signature against the public key — proves correctness, not just shape
+    base = build_signature_base_string(
+        method="GET",
+        url="https://api.ibkr.com/v1/api/iserver/auth/status",
+        params={
+            "oauth_consumer_key": "TEST_CONSUMER",
+            "oauth_token": "TEST_TOKEN",
+            "oauth_nonce": "abc",
+            "oauth_timestamp": "1700000000",
+            "oauth_signature_method": "RSA-SHA256",
+            "oauth_version": "1.0",
+        },
+    )
+    key.public_key().verify(
+        _b64.b64decode(sig),
+        base.encode("utf-8"),
+        padding.PKCS1v15(),
+        hashes.SHA256(),
+    )