Introduction to Cryptography Parts II and III

Presentation is based on the book
„Understanding Cryptography – A Textbook for
Students and Practitioners“
by Christof Paar and Jan Pelzl
www.crypto-textbook.com
Parts II and III– Introduction to Cryptography
These slides were prepared by Christof Paar, Jan Pelzl and Maksim Djackov

2/36 Understanding Cryptography by Christof Paar and Jan Pelzl
Some legal stuff: Terms of Use
• The slides can used free of charge. All copyrights for the slides remain with
Christof Paar and Jan Pelzl.
• The title of the accompanying book “Understanding Cryptography” by
Springer and the author’s names must remain on each slide.
• If the slides are modified, appropriate credits to the book authors and the
book title must remain within the slides.
• It is not permitted to reproduce parts or all of the slides in printed form
whatsoever without written consent by the authors.

 Further Reading and Information
Addition to Understanding Cryptography .
• A.Menezes, P. van Oorschot, S. Vanstone, Handbook of Applied Cryptography.
CRC Press, October 1996.
• H.v.Tilborg (ed.), Encyclopedia of Cryptography and Security, Springer, 2005
History of Cryptography (great bedtime reading)
• S. Singh, The Code Book: The Science of Secrecy from Ancient Egypt to
Quantum Cryptography, Anchor, 2000.
• D. Kahn, The Codebreakers: The Comprehensive History of Secret
Communication from Ancient Times to the Internet. 2nd edition, Scribner, 1996.
Software (excellent demonstration of many ancient and modern ciphers)
• Cryptool, http://www.cryptool.de

Part II

Content of this Chapter
• Random number generators (RNGs)

 Random number generators (RNGs)
RNG
Cryptographically
Secure RNG
Pseudorandom NGTrue RNG

 True Random Number Generators (TRNGs)
• Based on physical random processes: coin flipping, dice rolling, semiconductor
noise, radioactive decay, mouse movement, clock jitter of digital circuits
• Output stream si should have good statistical properties:
Pr(si = 0) = Pr(si = 1) = 50% (often achieved by post-processing)
• Output can neither be predicted nor be reproduced
Typically used for generation of keys, nonces (used only-once values) and for
many other purposes

 Pseudorandom Number Generator (PRNG)
• Generate sequences from initial seed value
• Typically, output stream has good statistical properties
• Output can be reproduced and can be predicted
Often computed in a recursive way:
),...,,( 11
0
tiiii sssfs
seeds
 

Example: rand() function in ANSI C:
31
1
0
2mod123451103515245
12345


 ii ss
s
Most PRNGs have bad cryptographic properties!

 Cryptographically Secure Pseudorandom Number
Generator (CSPRNG)
• Special PRNG with additional property:
• Output must be unpredictable
More precisely: Given n consecutive bits of output si , the following output bits sn+1
cannot be predicted (in polynomial time).
• Needed in cryptography, in particular for stream ciphers
• Remark: There are almost no other applications that need unpredictability,
whereas many, many (technical) systems need PRNGs.

 Cryptographically Secure Pseudorandom Number
Generator (CSPRNG) (taken from Wikipedia.org)
CSPRNG designs are divided into three classes:
1. Those based on cryptographic primitives such as ciphers and cryptographic
hashes
2. Those based upon mathematical problems thought to be hard
3. Special-purpose designs. The last often introduce additional entropy when
available and, strictly speaking, are not "pure" pseudorandom number
generators, as their output is not completely determined by their initial state.
This addition can prevent attacks even if the initial state is compromised.
Example: s(i+1) = SHA-1(s) is a PRNG or CSPRNG? What makes it different from
using one of the symmetric stream ciphers as a CSPRNG? Can we predict s(i+1)
when using SHA-1 and not knowing initial seed? Stream cipher?

Part III

12
• Symmetric Cryptography Revisited
• Principles of Asymmetric Cryptography
• Practical Aspects of Public-Key Cryptography
• Important Public-Key Algorithms
• The RSA Cryptosystem
• Diffie–Hellman Key Exchange
• The Principle of Digital Signatures
• Why We Need Hash Functions
• Hash Algorithms
• Asymmetric Key Distribution
• Certificates
Understanding Cryptography by Christof Paar and Jan Pelzl

13
Two properties of symmetric (secret-key) crypto-systems:
• The same secret key K is used for encryption and decryption
• Encryption and Decryption are very similar (or even identical) functions
 Symmetric Cryptography revisited
eK(x) dK(y)x
y
x
KK
Alice Bob

14
 Symmetric Cryptography: Analogy
K K
Safe with a strong lock, only Alice and Bob have a copy of the key
• Alice encrypts  locks message in the safe with her key
• Bob decrypts  uses his copy of the key to open the safe

• Symmetric algorithms, e.g., AES or 3DES, are very secure, fast & widespread but:
• Key distribution problem: The secret key must be transported securely
• Number of keys: In a network, each pair of users requires an individual key
 n users in the network require keys, each user stores (n-1) keys
• Alice or Bob can cheat each other, because they have identical keys.
Example: Alice can claim that she never ordered a TV on-line from Bob (he could
have fabricated her order). To prevent this: „non-repudiation“
15
 Symmetric Cryptography: Shortcomings
Example:
6 users (nodes)
keys (edges)
15
2
56



16
• Hash Algorithms
• Certificates

New Idea:
Use the „good old mailbox“ principle:
Everyone can drop a letter
But: Only the owner has the
correct key to open the box
17
 Idea behind Asymmetric Cryptography
1976: first publication of such an algorithm by Whitfield Diffie and Martin Hellman,and
also by Ralph Merkle.

18
 Asymmetric (Public-Key) Cryptography
Principle: “Split up” the key
K
Public Key (Kpub)
(Encrypt)
Secret Key (Kpr)
(Decrypt)
 During the key generation, a key pair Kpub and Kpr is computed

19
 Asymmetric Cryptography: Analogy
Safe with public lock and private lock:
• Alice deposits (encrypts) a message with the - not secret - public key Kpub
• Only Bob has the - secret - private key Kpr to retrieve (decrypt) the message
(Kpub) (Kpr)

20
• Hash Algorithms
• Certificates

21
 Basic Protocol for Public-Key Encryption
Alice Bob
(KpubB,KprB) = KKpubB
x
y=eKpubB
(x) y
x=dKprB
(y)
 Key Distribution Problem solved *
*) at least for now; public keys need to be authenticated, cf.Chptr. 13 of Understanding Cryptogr.

22
 Security Mechanisms of Public-Key Cryptography
Here are main mechanisms that can be realized with asymmetric cryptography:
• Key Distribution (e.g., Diffie-Hellman key exchange, RSA) without a pre-
shared secret (key)
• Nonrepudiation and Digital Signatures (e.g., RSA, DSA or ECDSA) to
provide message integrity
• Identification, using challenge-response protocols with digital signatures
• Encryption (e.g., RSA / Elgamal)
Disadvantage: Computationally very intensive
(1000 times slower than symmetric Algorithms!)

23
 Basic Key Transport Protocol 1/2
In practice: Hybrid systems, incorporating asymmetric and symmetric algorithms
1. Key exchange (for symmetric schemes) and digital signatures are performed
with (slow) asymmetric algorithms
2. Encryption of data is done using (fast) symmetric ciphers, e.g., block ciphers
or stream ciphers

 Basic Key Transport Protocol 2/2
Alice
24 Understanding Cryptography by Christof Paar and Jan Pelzl
Bob
y1 = eKpubB
(K) y1
K = dKprB
(y1)
Key Exchange
(asymmetric)
y2 = AESK (x) x = AES-1
K (y2)
y2
Data Encryption
(symmetric)
(KpubB,KprB) = K
KpubB
Choose random
symmetric key K
message x
Example: Hybrid protocol with AES as the symmetric cipher

 Key Transport Protocol Example

26
• Hash Algorithms
• Certificates

27
 How to build Public-Key Algorithms
Asymmetric schemes are based on a „one-way function“ f():
• Computing y = f(x) is computationally easy
• Computing x = f-1(y) is computationally infeasible
One way functions are based on mathematically hard problems.
Three main families:
• Factoring integers (RSA, ...):
Given a composite integer n, find its prime factors
(Multiply two primes: easy)
• Discrete Logarithm (Diffie-Hellman, Elgamal, DSA, …):
Given a, y and m, find x such that ax = y mod m
(Exponentiation ax : easy)
• Elliptic Curves (EC) (ECDH, ECDSA): Generalization of discrete logarithm
Note: The problems are considered mathematically hard, but no proof exists (so far).

28
 Key Lengths and Security Levels
Symmetric ECC RSA, DL Remark
64 Bit 128 Bit  700 Bit Only short term security
(a few hours or days)
80 Bit 160 Bit  1024 Bit Medium security
(except attacks from big
governmental institutions etc.)
128 Bit 256 Bit  3072 Bit Long term security
(without quantum computers)
• The exact complexity of RSA (factoring) and DL (Index-Calculus) is difficult to estimate
• The existence of quantum computers would probably be the end for ECC, RSA & DL
(at least 2-3 decades away, and some people doubt that QC will ever exist)

29
• Hash Algorithms
• Certificates

The RSA Cryptosystem
• Martin Hellman and Whitfield Diffie published their landmark public-
key paper in 1976
• Ronald Rivest, Adi Shamir and Leonard Adleman proposed the
asymmetric RSA cryptosystem in1977
• Until now, RSA is the most widely use asymmetric cryptosystem
although elliptic curve cryptography (ECC) becomes increasingly
popular
• RSA is mainly used for two applications
• Transport of (i.e., symmetric) keys (cf. Chptr 13 of Understanding
Cryptography)
• Digital signatures (cf. Chptr 10 of Understanding Cryptography)

31
• Hash Algorithms
• Certificates

• Proposed in 1976 by Whitfield Diffie and Martin Hellman
• Widely used, e.g. in Secure Shell (SSH), Transport Layer Security (TLS), and Internet
Protocol Security (IPSec)
• The Diffie–Hellman Key Exchange (DHKE) is a key exchange protocol and not used for
encryption
(For the purpose of encryption based on the DHKE, ElGamal can be used.)
 Diffie–Hellman Key Exchange: Overview

 Diffie–Hellman Key Exchange
•https://vimeo.com/67469421

34
• Hash Algorithms
• Certificates

 Motivation
• Alice orders a pink car from the car salesmen
Bob
• After seeing the pink car, Alice states that she
has never ordered it:
• How can Bob prove towards a judge that Alice
has ordered a pink car? (And that he did not
fabricate the order himself)
 Symmetric cryptography fails because both
Alice and Bob can be malicious
 Can be achieved with public-key cryptography

 Basic Principle of Digital Signatures

 Main idea
• For a given message x, a digital signature is
appended to the message (just like a conventional
signature).
• Only the person with the private key should be
able to generate the signature.
• The signature must change for every document.
The signature is realized as a function with the
message x and the private key as input.
The public key and the message x are the inputs
to the verification function.

38
• Hash Algorithms
• Certificates

Motivation
Problem:
Naive signing of long messages generates a signature of same length.
• Three Problems
• Computational overhead
• Message overhead
• Security limitations
• For more info see Section 11.1 in “Understanding Cryptography”.
Solution:
Instead of signing the whole message, sign only a digest (=hash)
Also secure, but much faster
Needed:
Hash Functions

Notes:
• x has fixed length
• z, y have fixed length
• z, x do not have equal length in general
• h(x) does not require a key.
• h(x) is public.
x
zi = h( xi || zi-1 )
sigkpr
z)
xi
z
y = sigkpr
(z)
 Digital Signature with a Hash Function

 Basic Protocol for Digital Signatures with a Hash Function:
Alice Bob
z = h(x)
s = sigKpr
(z)
(x, s)
z' = h(x)
verKpub
(s,z')=true/false
Kpub

 Principal input–output behavior of hash functions

43
• Hash Algorithms
• Certificates

 Hash Funktionen: Algorithms
• MD5 - family
• SHA-1: output - 160 Bit; input - 512 bit chunks of message x;
operations - bitwise AND, OR, XOR, complement und cyclic shifts.
• RIPE-MD 160: output - 160 Bit; input - 512 bit chunks of message x;
operations – like in SHA-1, but two in parallel and combinations of them
after each round.
Hash Algorithms
based on
block ciphers
Special Algorithms,
e.g. MD5 - family

45
• Hash Algorithms
• Certificates

 Certificates
 In order to authenticate public keys (and thus, prevent the MIM attack) , all public keys
are digitally signed by a central trusted authority.
 Such a construction is called certificate
certificate = public key + ID(user) + digital signature over public key and ID
 In its most basic form, a certificate for the key kpub of user Alice is:
Cert(Alice) = (kpub, ID(Alice), sigKCA
(kpub,ID(Alice) )
 Certificates bind the identity of user to her public key
 The trusted authority that issues the certificate is referred to as certifying authority (CA)
 „Issuing certificates“ means in particular that the CA computes the signature sigKCA
(kpub)
using its (super secret!) private key kCA
 The party who receives a certificate, e.g., Bob, verifies Alice‘s public key using the public
key of the CA

• In the wild certificates contain much more information than
just a public key and a signature.
• X509 is a popular signature standard. The main fields of
such a certificate are shown to the right.
• Note that the „Signature“ at the bottom is computed over all
other fields in the certifcate (after hashing of all those
fields).
• It is important to note that there are two public-key
schemes involved in every certificate:
1. The public-key that actually is protected by the signature
(„Subject‘s Public Key“ on the right). This was the public
Diffie-Hellman key in the earlier examples.
2. The digital signature algorithm used by the CA to sign the
certificate data.
• For more information on certificates, see Section 13.3 of
Understanding Cryptography
 Certificates in the Real World

There are many additional problems when certificates are to be used in systems with a
large number of participants. The more pressing ones are:
1.Users communicate which other whose certificates are issued by different CAs
• This requires cross-certification of CAs, e.g.. CA1 certifies the public-key of
CA2. If Alice trusts „her“ CA1, cross-certification ensures that she also
trusts CA2. This is called a „chain of trust“ and it is said that „trust is
delegated“.
2.Certificate Revocation Lists (CRLs)
• Another real-world problem is that certificates must be revoced, e.g., if a
smart card with certificate is lost or if a user leaves an organization. For
this, CRLs must be sent out periodically (e.g., daily) which is a burden on
the bandwidth of the system.
More information on PKIs and CAs can be found in Section 13.3 of Understanding
Cryptography
 Remaining Issues with PKIs

50
 Lessons Learned
• Public-key algorithms have capabilities that symmetric ciphers don’t have, in
particular digital signature and key establishment functions.
• Public-key algorithms are computationally intensive (a nice way of saying that they are
slow), and hence are poorly suited for bulk data encryption.
• Only three families of public-key schemes are widely used. This is considerably fewer
than in the case of symmetric algorithms.
• RSA is the most widely used public-key cryptosystem
• RSA is mainly used for key transport and digital signatures
• The Diffie–Hellman protocol is a widely used method for key exchange. It is based on
cyclic groups.
• Digital signatures provide message integrity, message authentication and non-
repudiation.
• RSA is currently the most widely used digital signature algorithm.
• Hash functions are keyless. The two most important applications of hash functions are
their use in digital signatures and in message authentication codes such as HMAC.

Introduction to Cryptography Parts II and III

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