diff --git a/pywidevine/cdm.py b/pywidevine/cdm.py
index 927136e..177da4f 100644
--- a/pywidevine/cdm.py
+++ b/pywidevine/cdm.py
@@ -60,7 +60,7 @@ class Cdm:
     NUM_OF_SESSIONS = 0
     MAX_NUM_OF_SESSIONS = 50  # most common limit
 
-    def __init__(self, device: Device, pssh: Union[Container, bytes, str], raw: bool = False):
+    def __init__(self, device: Device, pssh: Union[Container, bytes, str]):
         """
         Open a Widevine Content Decryption Module (CDM) session.
 
@@ -69,9 +69,6 @@ class Cdm:
                 more device-specific information.
             pssh: Protection System Specific Header Box or Init Data. This should be a
                 compliant mp4 pssh box, or just the init data (Widevine Cenc Header).
-            raw: This should be set to True if the PSSH data provided is arbitrary data.
-                E.g., a PSSH Box where the init data is not a Widevine Cenc Header, or
-                is simply arbitrary data.
 
         Devices have a limit on how many sessions can be open and active concurrently.
         The limit is different for each device and security level, most commonly 50.
@@ -92,11 +89,7 @@ class Cdm:
         self.NUM_OF_SESSIONS += 1
 
         self.device = device
-        self.init_data = pssh
-
-        if not raw:
-            # we only want the init_data of the pssh box
-            self.init_data = PSSH.get_as_box(pssh).init_data
+        self.init_data = PSSH.get_as_box(pssh).init_data
 
         self.session_id = get_random_bytes(16)
         self.service_certificate: Optional[DrmCertificate] = None
diff --git a/pywidevine/pssh.py b/pywidevine/pssh.py
index 2ef4db9..fa261ab 100644
--- a/pywidevine/pssh.py
+++ b/pywidevine/pssh.py
@@ -1,9 +1,11 @@
 from __future__ import annotations
 
 import base64
+import binascii
 from typing import Union
 from uuid import UUID
 
+import construct
 from construct import Container
 from google.protobuf.message import DecodeError
 from lxml import etree
@@ -78,36 +80,59 @@ class PSSH:
         return box
 
     @staticmethod
-    def get_as_box(data: Union[Container, bytes, str]) -> Container:
+    def get_as_box(data: Union[Container, bytes, str], strict: bool = False) -> Container:
         """
-        Get the possibly arbitrary data as a parsed PSSH mp4 box.
-        If the data is just Widevine PSSH Data (init data) then it will be crafted
-        into a new PSSH mp4 box.
-        If the data could not be recognized as a PSSH box of some form of encoding
-        it will raise a ValueError.
+        Get possibly arbitrary data as a parsed PSSH mp4 box.
+
+        Parameters:
+            data: PSSH mp4 box, Widevine Cenc Header (init data), or arbitrary data to
+                parse or craft into a PSSH mp4 box.
+            strict: Do not return a PSSH box for arbitrary data. Require the data to be
+                at least a PSSH mp4 box, or a Widevine Cenc Header.
+
+        Raises:
+            ValueError: If the data is empty, or an unexpected type.
+            binascii.Error: If the data could not be decoded as Base64 if provided
+                as a string.
+            construct.ConstructError: If the data could not be parsed as a PSSH mp4 box
+                nor a Widevine Cenc Header while strict=True.
         """
-        if isinstance(data, str):
-            data = base64.b64decode(data)
-        if isinstance(data, bytes):
-            if base64.b64encode(data).startswith(b"CAES"):  # likely widevine pssh data
-                try:
-                    cenc_header = WidevinePsshData()
-                    cenc_header.ParseFromString(data)
-                except DecodeError:
-                    # not actually init data after all
-                    pass
-                else:
-                    data = Box.parse(Box.build(dict(
-                        type=b"pssh",
-                        version=0,
-                        flags=0,
-                        system_ID=PSSH.SystemId.Widevine,
-                        init_data=cenc_header.SerializeToString()
-                    )))
-            data = Box.parse(data)
+        if not data:
+            raise ValueError("Data must not be empty.")
+
         if isinstance(data, Container):
             return data
-        raise ValueError(f"Unrecognized PSSH data: {data!r}")
+
+        if isinstance(data, str):
+            try:
+                data = base64.b64decode(data)
+            except (binascii.Error, binascii.Incomplete) as e:
+                raise binascii.Error(f"Could not decode data as Base64, {e}")
+
+        if isinstance(data, bytes):
+            try:
+                data = Box.parse(data)
+            except construct.ConstructError:
+                if strict:
+                    try:
+                        cenc_header = WidevinePsshData()
+                        if cenc_header.MergeFromString(data) < len(data):
+                            raise DecodeError()
+                    except DecodeError:
+                        raise DecodeError(f"Could not parse data as a PSSH mp4 box nor a Widevine Cenc Header.")
+                    else:
+                        data = cenc_header.SerializeToString()
+                data = Box.parse(Box.build(dict(
+                    type=b"pssh",
+                    version=0,
+                    flags=0,
+                    system_ID=PSSH.SystemId.Widevine,
+                    init_data=data
+                )))
+        else:
+            raise ValueError(f"Data is an unexpected type, expected bytes got {data!r}.")
+
+        return data
 
     @staticmethod
     def get_key_ids(box: Container) -> list[UUID]: