diff --git a/client/src/telethon/_impl/crypto/auth_key.py b/client/src/telethon/_impl/crypto/auth_key.py
index 90afc306..efdc0155 100644
--- a/client/src/telethon/_impl/crypto/auth_key.py
+++ b/client/src/telethon/_impl/crypto/auth_key.py
@@ -6,15 +6,26 @@ from typing_extensions import Self
 
 @dataclass
 class AuthKey:
+    """Represents a Telegram's authorization key.
+
+    To generate a new, valid authorization key, one should use the methods
+    provided by the generation module.
+
+    Authorization key: https://core.telegram.org/mtproto/auth_key
+    """
+
     data: bytes
     aux_hash: bytes
     key_id: bytes
 
     @classmethod
     def from_bytes(cls, data: bytes) -> Self:
+        if len(data) != 256:
+            raise ValueError("Auth key data must be exactly 256 bytes")
+
         sha = sha1(data).digest()
         aux_hash = sha[:8]
-        key_id = sha[12:]
+        key_id = sha[12:20]
         return cls(data=data, aux_hash=aux_hash, key_id=key_id)
 
     def __bytes__(self) -> bytes: