diff --git a/bitwarden_client.py b/bitwarden_client.py
index dc19461..d26321a 100644
--- a/bitwarden_client.py
+++ b/bitwarden_client.py
@@ -252,10 +252,20 @@ class BitwardenClient:
 
         # Get encrypted user key
         enc_user_key = result.get("Key")
+        _debug(f"[LOGIN] enc_user_key present: {enc_user_key is not None}")
         if enc_user_key:
-            self.enc_key, self.mac_key = self.crypto.decrypt_user_key(
-                enc_user_key, master_key
-            )
+            _debug(f"[LOGIN] enc_user_key (first 40): {enc_user_key[:40]}...")
+            _debug(f"[LOGIN] attempting to decrypt user key with master_key...")
+            try:
+                self.enc_key, self.mac_key = self.crypto.decrypt_user_key(
+                    enc_user_key, master_key
+                )
+                _debug(f"[LOGIN] user key decrypted successfully!")
+                _debug(f"[LOGIN] enc_key (first 8 b64): {b64encode(self.enc_key[:8]).decode()}")
+                _debug(f"[LOGIN] mac_key (first 8 b64): {b64encode(self.mac_key[:8]).decode()}")
+            except Exception as e:
+                _debug(f"[LOGIN] user key decryption FAILED: {e}")
+                raise
             return {
                 "success": True,
                 "master_key": master_key,
diff --git a/crypto.py b/crypto.py
index 158bd58..eff4ba3 100644
--- a/crypto.py
+++ b/crypto.py
@@ -4,11 +4,24 @@ Handles KDF (PBKDF2, Argon2), AES-CBC-256, HMAC, HKDF, and vault decryption.
 """
 import hashlib
 import hmac
+import logging
 import os
 import struct
 from base64 import b64decode, b64encode
 from typing import Optional, Tuple
 
+_debug_log = "/tmp/decky-vaultwarden-debug.log"
+
+def _debug(msg: str):
+    import datetime
+    ts = datetime.datetime.now().isoformat()
+    try:
+        with open(_debug_log, "a") as f:
+            f.write(f"[{ts}] {msg}\n")
+            f.flush()
+    except Exception:
+        pass
+
 from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
 from cryptography.hazmat.primitives import hashes, padding
 from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC
@@ -17,8 +30,8 @@ from cryptography.hazmat.backends import default_backend
 # Bitwarden-specific constants
 ENCRYPTION_KEY_LENGTH = 32  # 256 bits
 MAC_KEY_LENGTH = 32
-HKDF_INFO_EMAIL = b"enc"
-HKDF_INFO_MASTER_PASSWORD = b"enc"
+HKDF_INFO_ENC = b"enc"
+HKDF_INFO_MAC = b"mac"
 
 # Cipher type markers (ST arrays in Bitwarden)
 CIPHER_TYPE_AES_CBC_256_B64 = "2."
@@ -73,19 +86,16 @@ class BitwardenCrypto:
             )
 
     def stretch_master_key(self, master_key: bytes) -> Tuple[bytes, bytes]:
-        """Stretch master key into encryption key + MAC key using HKDF."""
-        # Bitwarden uses HKDF with empty salt and specific info strings
-        prk = hmac.new(b"", master_key, hashlib.sha256).digest()
+        """Stretch master key into encryption key + MAC key using HKDF-expand."""
+        _debug(f"[CRYPTO] stretch_master_key: master_key (first 8 b64)={b64encode(master_key[:8]).decode()}")
+        _debug(f"[CRYPTO] stretch_master_key: master_key length={len(master_key)}")
 
-        # Encryption key
-        enc_key = hmac.new(
-            prk, b"\x01" + HKDF_INFO_EMAIL + b"\x00", hashlib.sha256
-        ).digest()
+        # Bitwarden SDK: HKDF-expand(key, info) = HMAC-SHA256(key, info || 0x01)
+        enc_key = hmac.new(master_key, b"enc\x01", hashlib.sha256).digest()
+        mac_key = hmac.new(master_key, b"mac\x01", hashlib.sha256).digest()
 
-        # MAC key
-        mac_key = hmac.new(
-            prk, b"\x02" + HKDF_INFO_EMAIL + b"\x00", hashlib.sha256
-        ).digest()
+        _debug(f"[CRYPTO] stretch: enc_key (first 8 b64)={b64encode(enc_key[:8]).decode()}")
+        _debug(f"[CRYPTO] stretch: mac_key (first 8 b64)={b64encode(mac_key[:8]).decode()}")
 
         return enc_key, mac_key
 
@@ -121,9 +131,11 @@ class BitwardenCrypto:
         Format: TYPE.BASE64(IV) | BASE64(CT) | BASE64(MAC)
         or: TYPE.BASE64(IV).BASE64(CT).BASE64(MAC)
         """
+        _debug(f"[CRYPTO] decrypt_cipher_string: type={enc_string[:2]}, full={enc_string[:50]}...")
         # Determine cipher type
         if enc_string.startswith(CIPHER_TYPE_AES_CBC_256_B64):
             parts = enc_string[2:].split("|")
+            _debug(f"[CRYPTO] decrypt_cipher_string: type 2, parts count={len(parts)}")
             if len(parts) == 2:
                 # Format: TYPE.BASE64(IV)|BASE64(CT)
                 iv_b64, ct_b64 = parts
@@ -136,6 +148,13 @@ class BitwardenCrypto:
                 iv = b64decode(iv_b64)
                 ct = b64decode(ct_b64)
                 mac = b64decode(mac_b64)
+                _debug(f"[CRYPTO] decrypt_cipher_string: verifying MAC...")
+                computed_mac = self.hmac_sha256(mac_key, iv + ct)
+                _debug(f"[CRYPTO] decrypt_cipher_string: computed_mac (b64)={b64encode(computed_mac).decode()}")
+                _debug(f"[CRYPTO] decrypt_cipher_string: expected_mac (b64)={mac_b64}")
+                _debug(f"[CRYPTO] decrypt_cipher_string: mac_keys_equal={computed_mac == mac}")
+                _debug(f"[CRYPTO] decrypt_cipher_string: enc_key (first 8 b64)={b64encode(enc_key[:8]).decode()}")
+                _debug(f"[CRYPTO] decrypt_cipher_string: mac_key (first 8 b64)={b64encode(mac_key[:8]).decode()}")
                 if not self.verify_mac(mac_key, iv + ct, mac):
                     raise ValueError("MAC verification failed")
                 return self.decrypt_aes_cbc_256(enc_key, iv, ct)
@@ -166,10 +185,13 @@ class BitwardenCrypto:
         
         The user key is encrypted with the stretched master key.
         """
+        _debug(f"[CRYPTO] decrypt_user_key: encrypted_user_key (first 30)={encrypted_user_key[:30]}...")
         stretched_enc, stretched_mac = self.stretch_master_key(master_key)
+        _debug(f"[CRYPTO] decrypt_user_key: calling decrypt_cipher_string...")
         user_key = self.decrypt_cipher_string(
             encrypted_user_key, stretched_enc, stretched_mac
         )
+        _debug(f"[CRYPTO] decrypt_user_key: decrypted user_key length={len(user_key)}")
 
         if len(user_key) == 64:
             # Has separate MAC key