The Curious case of - PowerPoint Presentation

Slide1

The Curious case of Protobufs…

De-mystifying Google’s hottest binary protocol

Prasanna

Kanagasabai

Jovin LoboSlide2

About us :

Prasanna Kanagasabai :Security Engineer @

T

houghtWorks

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 NetworksMember of null- The Open Security Community.Author of GameOver – Linux distro for learning web security.Spoken at nullCon, GNUnify before. Slide3

Agenda

Introduction.Anatomy of ProtobufsDefining Message formats in .Proto files.Protobuf

compiler

Python API to read write messages.

Encoding SchemeProblem Statement. Decoding like-a-pro with IronWasp ‘Protobuf Decoder’.Slide4

Introduction:Protocol Buffers

a.k.a Protobufs :Protobufs

are Google's own way of serializing structured data .

Extensible, language-neutral and platform-neutral .

Smaller, faster and simpler to implement. Java, C++ and Python Slide5

Anatomy:Over view :Slide6

Defining a .Proto file.

#> less Example.protomessage 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. Slide7

Compiling Syntax:

protoc –I=$_input_Dir --python_out=

$_

out_Dir

$_Path_ProtoFileEg:protoc –I=. --python_out=. Example.protoThis will generate a Example_pb2.py file in the specified destination directory.Slide8

$ProtoFile_pb2.pyThe

Protobuf compiler generates special descriptors for all your messages, enums, and fields.It also generates empty classes, one for each message type:Eg:Slide9

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.Slide10

Demo: Protobuf… how it wrks

Slide11

Encoding.

example2.protomessage Ex1 { required int32 num = 1; // field tag

}

Code snippet:

obj = example2_pb2.Ex1();obj.num = 290; // field valueobj.SerializeToString();Output : 08 A2 02 #hex000010001010001000000010 #binarySlide12

Problem statement.Slide13

This is what freaked him out

08 A2 02

000010001010001000000010Slide14

Lets Decode it .. Step 1 :

Find the wire type . Step 2:Find the field number.Step 3:Find the field tag.Slide15

Step1: finding wire type.

0000 1000 1010 0010 0000 0010To 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.Slide16

Wire typesSlide17

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.00010000001

000

‘0001 ‘ i.e. ‘ 1’

So we get the field tag = 1Slide18

Step 3: Find the field value

0000 1000 1010 0010 0000 0010We drop the 1st byte

1010 0010

0000 0010

Drop the MSB’s from each of these bytes1010 0010 0000 0010010 0010 000 0010 Reverse these bytes to obtain the field value.000 0010 010 0010 000 0010 010 0010 i.e 256 + 32 + 2 = 290So we finally get the value of the field = 290. Slide19

So we successfully decoded

example2.protomessage Ex1 { required int32 num = 1; }

Code snippet:

obj

= example2_pb2.Ex1();obj.num = 290;obj.SerializeToString();Output : 08 A2 02 #hex000010001010001000000010 #binaryWe successfully Decoded Value : “290” Slide20

Demo : Lets do this live Slide21

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.orgSlide22

ProtoBuf DecoderSlide23

Road Map for Protobuf DecoderSlide24

01101000001111010000010110111001111001001000000101000101110101011001010111001101110100011010010110111101101110011100110010000000111111Slide25

01101000001111010000010110111001111001001000000101000101110101011001010111001101110100011010010110111101101110011100110010000000111111Slide26

01101000001111010000010110111001111001001000000101000101110101011001010111001101110100011010010110111101101110011100110010000000111111

Hmmm … Decoding ……Slide27

Any Questions ?

Done … It says ……Slide28

Any Questions ?

Done … It says ……Slide29

Thank You