Adopting ASN1 3.x with much better native decoding support

appstoreserverlibrary/receipt_utility.py

@@ -13,6 +13,12 @@
 ORIGINAL_TRANSACTION_IDENTIFIER = 1705
 
 class ReceiptUtility:
+    def _decode_octet_string(self, octet_string: bytes):
+        decoder = asn1.Decoder()
+        decoder.start(octet_string)
+        _, value = decoder.read()
+        return value
+
     def extract_transaction_id_from_app_receipt(self, app_receipt: str) -> Optional[str]:
         """
         Extracts a transaction id from an encoded App Receipt. Throws if the receipt does not match the expected format.
@@ -21,68 +27,25 @@ def extract_transaction_id_from_app_receipt(self, app_receipt: str) -> Optional[
         :param appReceipt: The unmodified app receipt
         :return: A transaction id from the array of in-app purchases, null if the receipt contains no in-app purchases
         """
-        decoder = IndefiniteFormAwareDecoder()
-        decoder.start(b64decode(app_receipt, validate=True))
-        tag = decoder.peek()
-        if tag.typ != asn1.Types.Constructed or tag.nr != asn1.Numbers.Sequence:
-            raise ValueError()
-        decoder.enter()
-        # PKCS#7 object
-        tag, value = decoder.read()
-        if tag.typ != asn1.Types.Primitive or tag.nr != asn1.Numbers.ObjectIdentifier or value != PKCS7_OID:
-            raise ValueError()
-        # This is the PKCS#7 format, work our way into the inner content
-        decoder.enter()
-        decoder.enter()
-        decoder.read()
-        decoder.read()
-        decoder.enter()
-        decoder.read()
-        decoder.enter()
-        tag, value = decoder.read()
-        # Xcode uses nested OctetStrings, we extract the inner string in this case
-        if tag.typ == asn1.Types.Constructed and tag.nr == asn1.Numbers.OctetString:
-            inner_decoder = asn1.Decoder()
-            inner_decoder.start(value)
-            tag, value = inner_decoder.read()
-        if tag.typ != asn1.Types.Primitive or tag.nr != asn1.Numbers.OctetString:
-            raise ValueError()
-        decoder = asn1.Decoder()
-        decoder.start(value)
-        tag = decoder.peek()
-        if tag.typ != asn1.Types.Constructed or tag.nr != asn1.Numbers.Set:
-            raise ValueError()
-        decoder.enter()
-        # We are in the top-level sequence, work our way to the array of in-apps
-        while not decoder.eof():
-            decoder.enter()
-            tag, value = decoder.read()
-            if tag.typ == asn1.Types.Primitive and tag.nr == asn1.Numbers.Integer and value == IN_APP_ARRAY:
-                decoder.read()
-                tag, value = decoder.read()
-                if tag.typ != asn1.Types.Primitive or tag.nr != asn1.Numbers.OctetString:
-                    raise ValueError()
-                inapp_decoder = asn1.Decoder()
-                inapp_decoder.start(value)
-                inapp_decoder.enter()
-                # In-app array
-                while not inapp_decoder.eof():
-                    inapp_decoder.enter()
-                    tag, value = inapp_decoder.read()
-                    if (
-                        tag.typ == asn1.Types.Primitive
-                        and tag.nr == asn1.Numbers.Integer
-                        and (value == TRANSACTION_IDENTIFIER or value == ORIGINAL_TRANSACTION_IDENTIFIER)
-                    ):
-                        inapp_decoder.read()
-                        tag, value = inapp_decoder.read()
-                        singleton_decoder = asn1.Decoder()
-                        singleton_decoder.start(value)
-                        tag, value = singleton_decoder.read()
-                        return value
-                    inapp_decoder.leave()
-            decoder.leave()
-        return None
+        try:
+            val = self._decode_octet_string(b64decode(app_receipt, validate=True))
+            found_oid = val[0]
+            if found_oid != PKCS7_OID:
+                raise ValueError()
+            inner_value = val[1][0][2][1][0]
+            # Xcode uses nested OctetStrings, we extract the inner string in this case
+            value = self._decode_octet_string(inner_value)
+            # We are in the top-level sequence, work our way to the array of in-apps
+            for inner_value in value:
+                if inner_value[0] == IN_APP_ARRAY:
+                    array_values = self._decode_octet_string(inner_value[2])
+                    # In-app array
+                    for array_value in array_values:
+                        if array_value[0] == TRANSACTION_IDENTIFIER or array_value[0] == ORIGINAL_TRANSACTION_IDENTIFIER:
+                            return self._decode_octet_string(array_value[2])
+            return None
+        except Exception as e:
+            raise ValueError(e)
 
     def extract_transaction_id_from_transaction_receipt(self, transaction_receipt: str) -> Optional[str]:
         """
@@ -99,19 +62,3 @@ def extract_transaction_id_from_transaction_receipt(self, transaction_receipt: s
             if inner_matching_result:
                 return inner_matching_result.group(1)
         return None
-
-class IndefiniteFormAwareDecoder(asn1.Decoder):
-    def _read_length(self) -> int:
-        index, input_data = self.m_stack[-1]
-        try:
-            byte = input_data[index]
-        except IndexError:
-            raise asn1.Error('Premature end of input.')
-        if byte == 0x80:
-            # Xcode receipts use indefinite length encoding, not supported by all parsers
-            # Indefinite length encoding is only entered, but never left during parsing for receipts
-            # We therefore round up indefinite length encoding to be the remaining length
-            self._read_byte()
-            index, input_data = self.m_stack[-1]
-            return len(input_data) - index
-        return super()._read_length()

pyproject.toml

@@ -20,7 +20,7 @@ dependencies = [
     "requests>=2.28.0,<3",
     "cryptography>=40.0.0",
     "pyOpenSSL>=23.1.1",
-    "asn1==2.8.0",
+    "asn1==3.2.0",
     "cattrs>=23.1.2",
 ]
 

requirements.txt

@@ -3,6 +3,6 @@ PyJWT >= 2.6.0, < 3
 requests >= 2.28.0, < 3
 cryptography >= 40.0.0
 pyOpenSSL >= 23.1.1
-asn1==2.8.0
+asn1==3.2.0
 cattrs >= 23.1.2
 httpx==0.28.1