The Curious case of Protobufs*

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THE CURIOUS CASE OF
PROTOBUFS…
De-mystifying Google’s hottest
binary protocol
Prasanna Kanagasabai
Jovin Lobo
About us :
 Prasanna Kanagasabai :
 Security Engineer @ ThoughtWorks
 Member of null- The Open Security Community .
 Author of IronSAP a module over IronWASP.
 Speaker @ nullcon-Delhi, Clubhack, IIT Guwahati and various null
meetups.
 Jovin Lobo :
 Associate Consultant @ Aujas Networks
 Member of null- The Open Security Community.
 Author of GameOver – Linux distro for learning web security.
 Spoken at nullCon, GNUnify before.
Agenda
 Introduction.
 Anatomy of Protobufs
 Defining Message formats in .Proto files.
 Protobuf compiler
 Python API to read write messages.
 Encoding Scheme
 Problem Statement.
 Decoding like-a-pro with IronWasp ‘Protobuf
Decoder’.
Introduction:
 Protocol Buffers a.k.a Protobufs :
 Protobufs are Google's own way of serializing
structured data .
 Extensible, language-neutral and platformneutral .
 Smaller, faster and simpler to implement.
 Java, C++ and Python
Anatomy:
 Over view :
Defining a .Proto file.
 #> less Example.proto
message Conference {
required string conf_name = 1 ;
required int32 no_of_days = 2 ;
optional string email = 3 ;
}
// * 1,2,3 are unique tags. These are used by the
fields in binary encoding.
* For optimization use tags from 1-15 as
higher nos. will use one more byte to encode.
Compiling
 Syntax:
 protoc –I=$_input_Dir --python_out=$_out_Dir
$_Path_ProtoFile
 Eg:
 protoc –I=. --python_out=. Example.proto
This will generate a Example_pb2.py file in the
specified destination directory.
$ProtoFile_pb2.py
 The Protobuf compiler generates special
descriptors for all your messages, enums, and
fields.
 It also generates empty classes, one for each
message type:
 Eg:
Reading and writing messages
using the Protobuf binary
format :
 SerializeToString()
 serializes the message and returns it as a string.
 ParseFromString(data)
 parses a message from the given string.
Demo: Protobuf… how it wrks
Encoding.
 example2.proto
message Ex1 {
required int32 num = 1; // field tag
}
 Code snippet:
obj = example2_pb2.Ex1();
obj.num = 290;
// field value
obj.SerializeToString();
 Output :
08 A2 02 #hex
000010001010001000000010 #binary
Problem statement.
This is what freaked him out
08 A2 02
000010001010001000000010
Lets Decode it ..
 Step 1 :
 Find the wire type .
 Step 2:
 Find the field number.
 Step 3:
 Find the field tag.
Step1: finding wire type.
 0000 1000 1010 0010 0000 0010
 To find wire type take the first byte:
 0000 1000 1010 0010 0000 0010
 [0]000 1000 Drop MSB from First byte.
 0001 000 The last 3 bits give wire type.
 Wire type is 000
 type = 0 is Varint.
Wire types
Step 2: Field tag.
 What we already have is 0001000
 Now we right shift value by 3 bits and the
remaining bits will give us the field tag.
 0001000
 0001 000
 ‘0001 ‘ i.e. ‘ 1’
 So we get the field tag = 1
Step 3: Find the field value
 0000 1000 1010 0010 0000 0010
 We drop the 1st byte
 1010 0010 0000 0010
 Drop the MSB’s from each of these bytes
 1010 0010 0000 0010
 010 0010 000 0010
 Reverse these bytes to obtain the field value.
 000 0010 010 0010
 000 0010 010 0010 i.e 256 + 32 + 2 = 290
 So we finally get the value of the field = 290.
So we successfully decoded

example2.proto
message Ex1 {
required int32 num = 1;
}

Code snippet:
obj = example2_pb2.Ex1();
obj.num = 290;
obj.SerializeToString();

Output :
08 A2 02 #hex
000010001010001000000010 #binary
 We successfully Decoded Value : “290”
Demo : Lets do this live
Automating all this with
IronWasp Protobuf Decoder:
 About IronWasp :
 IronWasp is an open-source web security scanner.
 It is designed to be customizable to the extent
where users can create their own custom security
scanners using it.
 Author – Lavakumar Kuppan (@lavakumark)
 Website : www.ironwasp.org
ProtoBuf Decoder
Road Map for Protobuf
Decoder
0110100000111101000001011011
1001111001001000000101000101
1101010110010101110011011101
0001101001011011110110111001
1100110010000000111111
0110100000111101000001011011
1001111001001000000101000101
1101010110010101110011011101
0001101001011011110110111001
1100110010000000111111
0110100000111101000001011011
1001111001001000000101000101
1101010110010101110011011101
0001101001011011110110111001
1100110010000000111111
Hmmm … Decoding ……
Done … It says ……
Any Questions ?
Done … It says ……
Any Questions ?
Thank You
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