diff --git a/eth2deposit/credentials.py b/eth2deposit/credentials.py
index e25d3c5..bb1f09d 100644
--- a/eth2deposit/credentials.py
+++ b/eth2deposit/credentials.py
@@ -21,9 +21,17 @@ from eth2deposit.utils.ssz import (
 
 class ValidatorCredentials:
     def __init__(self, *, mnemonic: str, index: int, amount: int):
-        self.signing_key_path = 'm/12381/3600/%s/0' % index
-        self.signing_sk = mnemonic_and_path_to_key(mnemonic=mnemonic, path=self.signing_key_path)
-        self.withdrawal_sk = mnemonic_and_path_to_key(mnemonic=mnemonic, path=self.signing_key_path + '/0')
+        # Set path as EIP-2334 format
+        # https://eips.ethereum.org/EIPS/eip-2334
+        purpose = '12381'
+        coin_type = '3600'
+        account = str(index)
+        withdrawal_key_path = f'm/{purpose}/{coin_type}/{account}/0'
+        self.signing_key_path = f'{withdrawal_key_path}/0'
+
+        # Do NOT use password for seed generation.
+        self.withdrawal_sk = mnemonic_and_path_to_key(mnemonic=mnemonic, path=withdrawal_key_path, password='')
+        self.signing_sk = mnemonic_and_path_to_key(mnemonic=mnemonic, path=self.signing_key_path, password='')
         self.amount = amount
 
     @property
diff --git a/eth2deposit/key_handling/key_derivation/mnemonic.py b/eth2deposit/key_handling/key_derivation/mnemonic.py
index ef8e93c..e9f32c5 100644
--- a/eth2deposit/key_handling/key_derivation/mnemonic.py
+++ b/eth2deposit/key_handling/key_derivation/mnemonic.py
@@ -2,9 +2,9 @@ import os
 from unicodedata import normalize
 from secrets import randbits
 from typing import (
-    List,
     Optional,
     Sequence,
+    Tuple,
 )
 
 from eth2deposit.utils.crypto import (
@@ -22,27 +22,29 @@ def _get_word(*, word_list: Sequence[str], index: int) -> str:
     return word_list[index][:-1]
 
 
-def get_seed(*, mnemonic: str, password: str='') -> bytes:
+def get_seed(*, mnemonic: str, password: str) -> bytes:
     """
-    Derives the seed for the pre-image root of the tree.
+    Derive the seed for the pre-image root of the tree.
+
+    Ref: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki#from-mnemonic-to-seed
     """
     mnemonic = normalize('NFKD', mnemonic)
     salt = normalize('NFKD', 'mnemonic' + password).encode('utf-8')
     return PBKDF2(password=mnemonic, salt=salt, dklen=64, c=2048, prf='sha512')
 
 
-def get_languages(path: str) -> List[str]:
+def get_languages(path: str) -> Tuple[str, ...]:
     """
     Walk the `path` and list all the languages with word-lists available.
     """
     (_, _, filenames) = next(os.walk(path))
-    filenames = [name[:-4] for name in filenames]
-    return filenames
+    languages = tuple([name[:-4] for name in filenames])
+    return languages
 
 
 def get_mnemonic(*, language: str, words_path: str, entropy: Optional[bytes]=None) -> str:
     """
-    Returns a mnemonic string in a given `language` based on `entropy`.
+    Return a mnemonic string in a given `language` based on `entropy`.
     """
     if entropy is None:
         entropy = randbits(256).to_bytes(32, 'big')
diff --git a/eth2deposit/key_handling/key_derivation/path.py b/eth2deposit/key_handling/key_derivation/path.py
index 101b6d6..3a7e713 100644
--- a/eth2deposit/key_handling/key_derivation/path.py
+++ b/eth2deposit/key_handling/key_derivation/path.py
@@ -18,7 +18,7 @@ def path_to_nodes(path: str) -> List[int]:
     return [int(index) for index in indices]
 
 
-def mnemonic_and_path_to_key(*, mnemonic: str, path: str, password: str='') -> int:
+def mnemonic_and_path_to_key(*, mnemonic: str, path: str, password: str) -> int:
     """
     Returns the SK at position `path` secures with `password` derived from `mnemonic`.
     """
diff --git a/eth2deposit/utils/crypto.py b/eth2deposit/utils/crypto.py
index b66befb..7dda986 100644
--- a/eth2deposit/utils/crypto.py
+++ b/eth2deposit/utils/crypto.py
@@ -38,4 +38,5 @@ def HKDF(*, salt: bytes, IKM: bytes, L: int) -> bytes:
 
 
 def AES_128_CTR(*, key: bytes, iv: bytes) -> Any:
+    assert len(key) == 16
     return _AES.new(key=key, mode=_AES.MODE_CTR, initial_value=iv, nonce=b'')