1.
Intro to Cryptography
Galin Dinkov

2.
Intro

3.
Definition
● Cryptography - κρυπτό (kryptós) "hidden, secret" + γράφειν (graphein)
"writing"
● Cryptology - κρυπτό (kryptós) "hidden, secret" + λογία (logia), "study"
● Study of techniques for secure communication in the presence of third parties
called adversaries
● Main fields
○ Data confidentiality - restricted access over the information
○ Data integrity - assurance of the accuracy and consistency of the data
○ Authentication - confirming the validity of sorm form of identification
○ Non-repudiation - assurance of data origin and authorship

4.
Terminology
● Plaintext - ordinary information, understandable by human
● Ciphertext - unintelligible text, encrypted text
● Encryption - encoding a message or information in such a way that only
authorized parties can access it
● Decryption - restoring the original plaintext from the ciphertext
● Cipher - algorithm for encryption/decryption
● Key - a secret parameter used in the encryption/decryption process
● Cryptosystem - definitions, key-generation, enc/dec algorithms
● Symmetric/Asymmetric - single common key / different keys
● Cryptanalysis - acquiring information from encrypted messages without having
the secret key

5.
Common Actors
● Alice and Bob - fictional characters using a cryptosystem
● Eve (eavesdropper) - listening to a private conversation
● Mallory - an active attacker

6.
History
● Classical cryptography - ancient, medieval
● WWI, WWII - Enigma …
● Modern - computers, public-key, hashing, quantum
computing …
● Politics - NSA, NIST ...

7.
Crypto How to...

8.
How to
Communicate
With Secret
Messages?

9.
Classic Ciphers
● Transposition cipher - positions of the letters
are shifted according to a regular system
● Substitution cipher - plaintext letters are
altered according to some rules (key)
○ Simple - per letter
○ Mono/Poly-alphabetic - single, or multiple substitutions
○ Polygraphic - substitute groups of letters
Route cipher
Caesar cipher
Vigenère cipher

10.
Breaking Classic Ciphers
● Brute-force
● Frequency analysis
● Demo

11.
Perfect Secrecy
● One-time pad - random key, long at least as the message
● Does not reveal any of the functions of the message, except its maximum
length
● Security depends on the entropy of the key
● Difficult to use for long messages
● If reused it is vulnerable

12.
Stream Cipher
● Generates one time pad using pseudo random number generator
● Then combines it with the message
● The pad is generated based on a key
● Security depends on the entropy of the key and the random function
● Example algorithms - RC4 and ChaCha20
● Used in hardware because is fast

13.
Block Cipher
● Divides the message into blocks and encrypt each block
● If the message length is not divisible by block size padding is applied
● Works in different modes: ECB, CBC, PCBC, CFB, OFB, CTR ...
● Examples: DES, 3DES, AES, Blowfish ...

14.
How to Hide
Message
Existence?

15.
Steganography
Practice of concealing a file, message,
image, or video within another file,
message, image, or video.
● Physical - invisible ink, microdots,
knits, …
● Digital - image, audio, video or other
file, text in blogs or forums
● Linguistic - acrostic, misspellings, ...
Tools

16.
How to Ensure
Data Integrity?

17.
Hash Functions
Function used to map data of arbitrary size to
data of fixed size.
● Cryptographic hash functions -
deterministic, one-Way, uniformly
distributed, infeasible collisions, quick to
compute
● MD 2/4/5/6 - Message-Digest Algorithm
● SHA 1/2/3 - Secure Hash Algorithms
● RIPEMD - RACE Integrity Primitives
Evaluation Message Digest
● BLAKE 1/2

18.
Hash Function Security
● Preimage resistance - hard to find message m from a given hash h = hash(m)
● Second preimage resistance - given a message m1 it should be difficult to find
another different message m2 such as hash(m1) = hash(m2)
● Collision resistance - it should be difficult to find to messages m1 and m2 such
as hash(m1) = hash(m2)
● Rainbow table - precomputed table for reversing cryptographic hash
functions, usually for cracking password hashes
● Birthday attack

19.
Message Authentication Code (MAC)
Short piece of information used to authenticate a message, to confirm that the
message came from the stated sender (its authenticity) and has not been changed.
● CBC-MAC
● HMAC
● …
●
● Length-extension attack

20.
How to Securely
Share Common
Secret?

21.
Public-Key Cryptography
● Digital signatures - content is digitally signed with an individual’s private key and is
verified by the individual’s public key
● Encryption - content is encrypted using an individual’s public key and can only be
decrypted with the individual’s private key
● Authentication – since the individual’s unique private key was used to apply the
signature, recipients can be confident that the individual was the one to actually apply
the signature
● Non-repudiation – since the individual is the only one with access to the private key
used to apply the signature, he/she cannot later claim that it wasn’t him/her who
applied the signature
● Integrity - when the signature is verified, it checks that the contents of the document or
message match what was in there when the signature was applied. Even the slightest
change to the original document would cause this check to fail.

22.
Key Exchange Theory

23.
Math
● One-way function - easy to compute on every input, but hard to invert
● Trapdoor function - easy to compute in one direction, yet difficult to compute
in the opposite unless having a special information, called the "trapdoor"
○ Modular arithmetics
○ Discrete logarithm
○ Prime factorization

24.
Diffie-Hellman Key Exchange
Video

25.
RSA - Rivest–Shamir–Adleman
● Invented in 1977
● Used for digital signatures and
securing sessions (SSL)
Explanation

26.
Elliptic curve cryptography (ECC)
Explanation:
https://youtu.be/dCvB-mhkT0w
https://youtu.be/F3zzNa42-tQ
ECC Domain Parameters:
p - prime field (modulo)
a,b - curve parameters
G - generator point
n - ord(G) (n x G = 0)
h - cofactor of n
Private Key (k) = Random Number < n - 1
Public Key (K) = k ∗ G
K = 1E99423A4ED27608A15A2616A2B0E9E52CED330AC530EDCC32C8FFC6A526AEDD * G
K = (x, y)
x = F028892BAD7ED57D2FB57BF33081D5CFCF6F9ED3D3D7F159C2E2FFF579DC341A
y = 07CF33DA18BD734C600B96A72BBC4749D5141C90EC8AC328AE52DDFE2E505BDB

27.
Breaking
● Find the private key from the public key
● Classic quantum computer
● Topological quantum computer

28.
Zero knowledge proofs
Method by which one party (the prover) can prove to another party (the verifier)
that a given statement is true, without conveying any information apart from the
fact that the statement is indeed true.

29.
How to Send
Secure Emails?

30.
Email Encryption
PGP:
https://en.wikipedia.org/wiki/Pretty_
Good_Privacy
S/MIME:
https://en.wikipedia.org/wiki/S/MIME
More...

31.
How to Surf the
Web Securely?

32.
SSL/TLS

33.
Public Key Infrastructure (PKI)
● Certificate Authorities (CA) - X.509
● Web of Trust
● Blockchain PKI - Emercoin

34.
How to Surf the
Web Privately?

35.
Tor

36.
Final
https://www.linkedin.com/in/galindinkov/
by Galin Dinkov