utils.py: Add helper method to parse ePDG Identifier from hex string

The hex string consists of contains zero or more ePDG identifier data objects.
Each ePDG Identifier TLV data object consists of tag value of '80', length, address type, identifier.

TS 31.102 version 13.4.0 Release 13. The same parsing method applies for both EF.ePDGId and EF.ePDGIdEm

Change-Id: I96fb129d178cfd7ec037989526da77899ae8d344

pySim/utils.py

@@ -402,3 +402,37 @@ def TLV_parser(bytelist):
 		else:
 			bytelist = bytelist[ L+2 : ]
 	return ret
+
+def dec_epdgid(hexstr):
+	"""
+	Decode ePDG Id to get EF.ePDGId or EF.ePDGIdEm.
+	See 3GPP TS 31.102 version 13.4.0 Release 13, section 4.2.102 and 4.2.104.
+	"""
+
+	# Convert from hex str to int bytes list
+	epdgid_bytes = h2i(hexstr)
+
+	s = ""
+
+	# Get list of tuples containing parsed TLVs
+	tlvs = TLV_parser(epdgid_bytes)
+
+	for tlv in tlvs:
+		# tlv = (T, L, [V])
+		# T = Tag
+		# L = Length
+		# [V] = List of value
+
+		# Invalid Tag value scenario
+		if tlv[0] != 0x80:
+			continue
+
+		# First byte in the value has the address type
+		addr_type = tlv[2][0]
+		# TODO: Support parsing of IPv4 and IPv6
+		if addr_type == 0x00: #FQDN
+			# Skip address tye byte i.e. first byte in value list
+			content = tlv[2][1:]
+			s += "\t%s # %s\n" % (i2h(content), i2s(content))
+
+	return s