utils: Introduce BER-TLV parsers that return raw tag or even raw TLV

In the eSIM RSP univers there are some rather ugly layering violatoins
where ASN.1 cannot be parsed but we have to mess with raw TLVs and the
details of DER encoding.  Let's add two funtions that make it more
convenient to work with this: They return the raw tag as integer, or
even the entire encoded TLV rather than the value part only.

Change-Id: I1e68a4003b833e86e9282c77325afa86ce144b98

pySim/esim/rsp.py

@@ -24,7 +24,7 @@ from cryptography.hazmat.primitives.asymmetric import ec
 from cryptography.hazmat.primitives.serialization import Encoding
 from cryptography import x509
 
-from pySim.utils import bertlv_parse_one, bertlv_encode_tag, bertlv_encode_len, b2h
+from pySim.utils import bertlv_parse_one_rawtag, bertlv_return_one_rawtlv, b2h
 from pySim.esim import compile_asn1_subdir
 
 asn1 = compile_asn1_subdir('rsp')
@@ -101,37 +101,31 @@ class RspSessionStore(shelve.DbfilenameShelf):
 def extract_euiccSigned1(authenticateServerResponse: bytes) -> bytes:
     """Extract the raw, DER-encoded binary euiccSigned1 field from the given AuthenticateServerResponse. This
     is needed due to the very peculiar SGP.22 notion of signing sections of DER-encoded ASN.1 objects."""
-    tdict, l, v, remainder = bertlv_parse_one(authenticateServerResponse)
-    rawtag = bertlv_encode_tag(tdict)
+    rawtag, l, v, remainder = bertlv_parse_one_rawtag(authenticateServerResponse)
     if len(remainder):
         raise ValueError('Excess data at end of TLV')
-    if b2h(rawtag) != 'bf38':
+    if rawtag != 0xbf38:
         raise ValueError('Unexpected outer tag: %s' % b2h(rawtag))
-    tdict, l, v1, remainder = bertlv_parse_one(v)
-    rawtag = bertlv_encode_tag(tdict)
-    if b2h(rawtag) != 'a0':
+    rawtag, l, v1, remainder = bertlv_parse_one_rawtag(v)
+    if rawtag != 0xa0:
         raise ValueError('Unexpected tag where CHOICE was expected')
-    tdict, l, v2, remainder = bertlv_parse_one(v1)
-    rawtag = bertlv_encode_tag(tdict)
-    if b2h(rawtag) != '30':
+    rawtag, l, tlv2, remainder = bertlv_return_one_rawtlv(v1)
+    if rawtag != 0x30:
         raise ValueError('Unexpected tag where SEQUENCE was expected')
-    return rawtag + bertlv_encode_len(l) + v2
+    return tlv2
 
 def extract_euiccSigned2(prepareDownloadResponse: bytes) -> bytes:
     """Extract the raw, DER-encoded binary euiccSigned2 field from the given prepareDownloadrResponse. This is
     needed due to the very peculiar SGP.22 notion of signing sections of DER-encoded ASN.1 objects."""
-    tdict, l, v, remainder = bertlv_parse_one(prepareDownloadResponse)
-    rawtag = bertlv_encode_tag(tdict)
+    rawtag, l, v, remainder = bertlv_parse_one_rawtag(prepareDownloadResponse)
     if len(remainder):
         raise ValueError('Excess data at end of TLV')
-    if b2h(rawtag) != 'bf21':
+    if rawtag != 0xbf21:
         raise ValueError('Unexpected outer tag: %s' % b2h(rawtag))
-    tdict, l, v1, remainder = bertlv_parse_one(v)
-    rawtag = bertlv_encode_tag(tdict)
-    if b2h(rawtag) != 'a0':
+    rawtag, l, v1, remainder = bertlv_parse_one_rawtag(v)
+    if rawtag != 0xa0:
         raise ValueError('Unexpected tag where CHOICE was expected')
-    tdict, l, v2, remainder = bertlv_parse_one(v1)
-    rawtag = bertlv_encode_tag(tdict)
-    if b2h(rawtag) != '30':
+    rawtag, l, tlv2, remainder = bertlv_return_one_rawtlv(v1)
+    if rawtag != 0x30:
         raise ValueError('Unexpected tag where SEQUENCE was expected')
-    return rawtag + bertlv_encode_len(l) + v2
+    return tlv2

pySim/utils.py

@@ -359,6 +359,32 @@ def bertlv_parse_one(binary: bytes) -> Tuple[dict, int, bytes, bytes]:
     remainder = remainder[length:]
     return (tagdict, length, value, remainder)
 
+def bertlv_parse_one_rawtag(binary: bytes) -> Tuple[int, int, bytes, bytes]:
+    """Parse a single TLV IE at the start of the given binary data; return tag as raw integer.
+    Args:
+            binary : binary input data of BER-TLV length field
+    Returns:
+            Tuple of (tag:int, len:int, remainder:bytes)
+    """
+    (tag, remainder) = bertlv_parse_tag_raw(binary)
+    (length, remainder) = bertlv_parse_len(remainder)
+    value = remainder[:length]
+    remainder = remainder[length:]
+    return (tag, length, value, remainder)
+
+def bertlv_return_one_rawtlv(binary: bytes) -> Tuple[int, int, bytes, bytes]:
+    """Return one single [encoded] TLV IE at the start of the given binary data.
+    Args:
+            binary : binary input data of BER-TLV length field
+    Returns:
+            Tuple of (tag:int, len:int, tlv:bytes, remainder:bytes)
+    """
+    (tag, remainder) = bertlv_parse_tag_raw(binary)
+    (length, remainder) = bertlv_parse_len(remainder)
+    tl_length = len(binary) - len(remainder)
+    value = binary[:tl_length] + remainder[:length]
+    remainder = remainder[length:]
+    return (tag, length, value, remainder)
 
 def dgi_parse_tag_raw(binary: bytes) -> Tuple[int, bytes]:
     # In absence of any clear spec guidance we assume it's always 16 bit