Transcript Cryptography Training Day II

CRYPTOGRAPHY II & SECURE COMMUNICATION
Hakan Tolgay
[email protected]
Agenda

Rivest-Shamir-Adleman (RSA)

Hashing and hash functions (MD5/SHA1/SHA256)

Networking in basics

Digital Signatures and Security Services

Certificate and trust mechanism

SSL/TLS
RSA – Why not Diffie-Hellman
Bob
Lora
Alice
Julie
Mark

Because of need a key for every client
RSA

In 1970 James Ellis was working on an idea, Non-Secret Encryption

Lock and unlock would be the inverse operations
Bob
Lora
Alice
Julie
Mark
RSA

The idea was basically spliting the key into two part

Encryption key

Decryption key
RSA

How could Bob send a color to Alice without Eve to intercepting it
Eve
Alice
Bob
RSA


The inverse of some color is called the complementary color

When added together produces white

Undo the effect of the first color
Mixing color is a one way function

Easy and fast to mix

Hard and slow to seperate
RSA

Lets think that Bob want to send secret color ‘Yellow’ to Alice
Eve
Alice
Public Key
Private Key
Bob
RSA – Mathematical Solution

Prime factorization is the key point

It is always easy to multiple numbers

How about finding foctorization of primes?

589 ?

437231?
RSA – Mathematical Solution

Let P1 and P2 both would be prime numbers which has150 digits

N = P1 x P2 (which is more than 300 digits long)

It would take less than a second to multiply them

Now hide P1 and P2.
RSA - Euler's Totient/Phi Function

the positive integers less than or equal to n that have no common multipliers (relatively prime to n)

ɸ [8] = 4

It is always hard to calculate Phi function except one case

ɸ [P] = P-1

ɸ [21377] = 21376

Phi function is also multiplicative



>>>> 1, 2, 3, 4, 5, 6, 7, 8
ɸ [A x B] = ɸ [A] x ɸ [B]
So
 N = P1 x P2
 ɸ[N] = ɸ [P1-1] x ɸ [P2-1]
If you know the foctorization for N then it is easy to find ɸ[N]
RSA – Eurlers Theorem

m^ɸ[n] = 1 mod n

Pick any two numbers. Let m=5 n=8

5^4 = 1mod 8 >> 625 =1 mod 8
RSA – Eurlers Theorem
RSA
Eve
Alice
P1=53
P2=59
n=53 x 59 = 3127
Bob
m=89
n=3127
e=3
ɸ(n) = 3016
e=3
D=(2 x (3016) +1) / 3 = 2011
1394^2011 = 89 mod 3127
m^e mod n
89^3 mod 3127 = 1394
c=1394
Hash functions

is any function that can be used to map digital data of arbitrary size to digital data of fixed size

slight differences in input data produce very big differences in output data

Used in:

Digital signing

Message authentication

PRNG

Password security
Cryptographic Hash functions

Hash functions vs Cryptographic hash function

Transformation should be efficient, fast

Hide information about the input

Should be well distrubuted

MD5

SHA1, SHA2, SHA3

Blake
fff5bbf073c6add1c635e9d43dcba5b3
Collision resistance

A hash function is collision resistant if it is hard to find two inputs that hash to the same output

Every hash function with more inputs than outputs will necessarily have collisions


if a hash function produces N bits of output, an attacker who computes "only" 2N/2 hash operations
on random input is likely to find two matching outputs
Cryptographic hash functions are usually designed to be collision resistant. MD5 and SHA-1 in
particular both have published techniques more efficient than brute force for finding collisions
Collision resistance

DEMO
Network – OSI model
Network – Point-to-Point communication
X
Y
Network – Basics about TCP

Connections are reliable

A sequence number is added to packets (32bit)

Data can be split
Network – TCP segment structure
Offsets
Octet
0
4
8
Octet
Bit
12
96
16
20
...
128
160
...
0
32
64
TCP Header
1
0
0
1
2
3
4
5
6
7
8
Source port
Sequence number
Acknowledgment number (if ACK set)
Reserved N C E
000
S W C
Data offset
R

2
3
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
E
Destination port
U A
R C
G K
P
S
H
R
S
T
S
Y
N
F
I
N
Window Size
Checksum
Urgent pointer (if URG set)
Options (if data offset > 5. Padded at the end with "0" bytes if necessary.)
...
TCP header can be a minimum 20 and a maximum of 60 bytes
Network – TCP segment structure

Wireshark Demo
Network - 3 way TCP handshake
Network – TCP flags

URG: urgent: is used to identify incoming data as 'urgent'.






Such incoming segments do not have to wait until the previous segments are consumed by the receiving end but
are sent directly and processed immediately.
ACK: acknowledge: is used to acknowledge the successful receipt of packets.
PSH: push: like the Urgent flag, exists to ensure that the data is given the priority and is
processed at the sending or receiving end.
RST: reset: is used when a segment arrives that is not intended for the current connection.
SYN: sync: is initialy sent when establishing the classical 3-way handshake between two
hosts
FIN: finished: is used to tear down the connection
Digital Signatures and Security Services

Goal: signature like function for electronic world
a signed email, .......
.
.
.
.
.
Signk(x)=y
------------------------------------------------1001010011.....
......101110
Digital Signatures
Message space
x
Signk(x)
(x,y)
Signature space
y

-
verk(x,y)
True if signature is valid
False if signature is invalid
Securtiy Services

The objectives of security system are called «security services»

4 important points

1. Confidentiality: Information is kept secret from all but the authorized parties

2. Authentication: The send of the message is authentic

3. Integrity: Message has not been modified during transmission

4. Non-repudiation: The send of the message can not deny the creation of message
Certificate and trust mechanism

Man-In-the-Middle (MITM) attack in asymmetric key approach

What if attacker is not only sniffing but also actively intercepting the traffic
Certificates

Need of centrally trusted authority

Certifiying Authority «CA»

Certificate is only a public key of Allice digitally signed by a CA
Certification process

Creating private key

Prepare a Certification Signing Request (CSR) with you Private key

Send CSR to CA

CA sends you your certificate
SSL/TLS

Secure Sockets Layer (SSL)

Transport Layer Security (TLS)

They use X.509 certificates and hence asymmetric cryptography to
authenticate the counterparty with whom they are communicating

This session key is then used to encrypt data flowing between the parties

Uses hash fuctions for message integrity
Defined
Protocol
Year
SSL 1.0 n/a
SSL 2.0
1995
SSL 3.0
1996
TLS 1.0
1999
TLS 1.1
2006
TLS 1.2
2008
TLS 1.3 TBD
SSL

The SSL protocol was originally developed by Netscape.

Version 1.0 was never publicly released because of serious security flaws in the protocol

version 2.0 was released in February 1995

SSL version 3.0, released in 1996, was a complete redesign of the protocol produced by Paul
Kocher
TLS

TLS 1.0 was first defined in RFC 2246 in January 1999 as an upgrade of SSL Version 3.0

As stated in the RFC, "the differences between this protocol and SSL 3.0 are not dramatic


TLS 1.0 does include a means by which a TLS implementation can downgrade the connection to SSL
3.0, thus weakening security
TLS 1.1 was defined in RFC 4346 in April 2006

Added protection against cipher-block chaining (CBC) attacks.

TLS 1.2 was defined in RFC 5246 in August 2008.

As of October 2014, TLS 1.3 is a draft, and details have not fixed yet
SSL/TLS - Authentication and key exchange/agreement
SSL/TLS - Cipher security against publicly known feasible attacks
SSL/TLS - Data integrity
SSL/TLS Handshake
Thank you