1. Symmetric cryptography
There are various flavors of secret writing. We focus on ciphers.
Monoalphabetic substitution (1 to 1)
 Simple case: shift alphabet by $x\in\{1,…,25\}$ positions [Caesar Cipher]
 More advanced: arbitrary letter mapping.
Easy to crack since only 25 different alphabets.
There are 26! different alphabets — much harder to decipher. Was cracked using frequency analysis of symbols!
Improved monoalphabetic substitution (1 to N)
 Use symbols that delete preceding symbol (to mess around with frequencies)
 Homophone encryption: use multiple symbols to encrypt one letter (1 to N)
 Intentional misspelling words
 Replace single words with one symbol (ie mix cipher with code)
Polyalphabetic substitution (N to N)
 Mapping of symbols depends on the position of that symbol in the text! Hard to break.
 An example, with the shared secret code word “CIF”, and Vigenère Cipher:
Code word  C  I  F  C  I  F  C  I  F  C 
Plain text  b  l  o  c  k  c  h  a  i  n 
Cipher text  D  T  T  E  S  H  J  I  N  P 
Breaking Polyalphabetic Substitution
 Ample frequency analysis not possible
 In mid 19th century vulnerability was discovered: use repetition of code word as a starting point
 Looking for patterns (like the word “the”), try to guess how long the code word is (n), and use frequency analysis on every nth cipher letter
Unbreakable Cipher
 Weakness of Vigenère Cipher: repetition of the code word (= key)
 Solution
 length of key = length of text
 random key (don’t use words or lists)
 use each key only once
 It’s called “onetime pad cipher” → theoretically unbreakable! [Used between USA and USSR]
 Still need exchanging keys (or, more precisely, lists of keys, in order to use them more than once)
Encryption in the age of computers
 Electronics are much faster than mechanical parts
 Cipher machines become possible
 Key difference: Letters are translated into Numbers (eg ASCII encoding below)
 Nowadays people use utf standard, which has different language letters, and even emojis :)
DES: Data Encryption Standard (block cipher)
 Companies need a standardized approach
 Encryption method “Lucifer” by Horst Feistel in early 1970s (this is a block cipher, see the prev leccture for the definition)
 Later becomes “DES”
 Advantage: standartization and security
 Disadvantage: key distribution problem persists, need a secure channel of communication “in the past”
2. Asymmetric Cryptography
Two ways to solve the “key distribution problem”:
 Come up with a secure key exchange algorithm
 Asymmetric cryptography: public and private keys
[DiffieHellman ‘76] key exchange
Gives a solution to the problem of generating a secure key for symmetric encryption on a potentially compromised channel.
The key exchangeAlice and Bob publicly predefine a oneway function $G^x ~(mod~P)$, where $P$ is prime and both $G$ and $P$ are large numbers.
Next they implement the following strategy, where all operations happen $mod~P$:
Alice
 Chooses privately a=3
 Sends Bob $x=G^a$, receives $y$
 Computes $y^a=G^{ab}$
Bob
 Chooses privately b=6
 Sends Alice $y=G^b$, receives $x$
 Computes $x^b=G^{ab}$
Result: the key $G^{ab}$ is now securely shared!
Problem in DiffieHellman key exchange is the separate setup for all “pairs” of actors — cumbersome process.
[RivestShamirAdleman (RSA), ‘77]
(Inspired by Diffie’s idea)
Properties of RSA Alice keeps priv_key as a secret, and publishes to everyone pub_key. Two scenarios now:
 Secrecy application Bob can encrypt a message by pub_key, and this message can be decrypted only by Alice using priv_key. [Bob]———msg’=encrypt(msg, pub_key)———>[Alice] [Alice]———decrypt(msg’, priv_key)———>msg
 Integrity and Authenticity application Alice can sign a message by priv_key, and anyone can check using pub_key that the message was signed by Alice (authenticity) and noone else (integrity). [Alice]———sig=sign(msg, priv_key)———>[Bob] [Bob]———verify(sig, pub_key)———>True/False
Note that public key in this scheme is a perfect thing to equate with “identity”, an idea that is used evreywhere in blockchains.
Key facts and concepts behind RSA:
 Euclidean algorithm ⇒ [gcd(a,b)=d ⇒ ∃x,y s.t. ax+by=d]
 Euler’s totient function ϕ(N) is the amount of numbers 0<a<N s.t. gcd(a,N)=1 [E.g. if p,q are prime, then ϕ(pq) = pq(p1)(q1)1 = (p1)(q1)]
 Euler’s theorem: [gcd(a,n)=1 ⇒ $a^{\phi(n)}$ ≡ 1 (mod n)]
The RSA setup: 1) Alice chooses two primes, p and q; computes N = pq 2) Chooses an additional integer e, such that gcd[e, ϕ(N)]=1, where ϕ(N)=(p1)(q1) is the number of primerelativetoN numbers smaller than N. 3) Computes the private key k by (extended) Euclidean algorithm, according to equation e·k = 1 mod ϕ(N) [Only Alice can do it, since only she knows p and q, and also gcd[e, ϕ(N)]=1 guarantees that k exists.]
Rmk: k is the inverse of e in $\mathbb Z^*_{N}$, and is called the trapdoor for the oneway function $\mathbf{x^e~(mod~N)}$, since it allows to invert it.
The RSA algorithm  
Alice  Bob  
• public key: (N, e)
• private key: p, q, k
Conditions:
 p & q are prime, N=pq
 gcd[e, ϕ(N)] = 1
 e·k = 1 mod ϕ(N)
[note: ϕ(N)=(p1)(q1)]  Wants to securely send an integer m to Alice  
Encrypts the message by computing $\mathbf{c=m^e}$ (mod N)  
$\longleftarrow$ c  
Decrypts the message:
$\mathbf{c^k=m^{ek}=m^{\ell\cdot \phi(N)+1} = }$ $\mathbf{=(m^{\phi(N)})^\ell m=1^\ell m = m ~(mod~N)}$

Remark: gcd(m, N)=1 is needed as a condition of the Euler’s theorem, and it is true probabilistically.
Definition: a Digital Signature Scheme (DSS) consists of 3 algorithms:
 Gen(): outputs a key pair (priv_key, pub_key)
 Sign(priv_key, msg): outputs a signture σ
 Verify(pub_key, msg, sig): outputs True or False
Note: unlike signatures in RL, digital signatures depend on the message.
HW: build a DSS using the RSA scheme.
Definition: (informal) DSS is secure if adversary seeing many signatures of Alice (on messages of her choice) cannot forge a signature on a new message.
Families of DSS: (none of these are quantumresistant)
 RSA signatures (not used in blockchain)  long signatures and public keys (≥256 bytes), fast to verify  might not be resistant longterm (the Quadratic Sieve and the General Number Field Sieve get more efficient as the numbers get larger)
 ECDSA and Schnorr (Ethereum before the merge, Bitcoin)  short signatures (64 and 48 bytes) and public keys (32 bytes)  better resistance
 BLS signatures (Ethereum 2.0) 48 bytes, aggregatable (gamechanger for blockchains), easy threshold building (3 out of 5, say)
Note: postquantum signatures are long (≥768 bytes). The signatures form the bulk of tx data on a blockchain.
Elliptic cryptography (ECDSA)
(screenshot taken from this lecture)
 Elliptic curve is a graph of Weierstrass equation: $y^2=x^3+ax+b$ Nonsingularity conditions: $4a^3+27b^2\neq0$
 Symmetry along the xaxis.
 Addition of points on Elliptic curve:
 Point doubling
Q. On elliptic curve, why geometrical addition of points coincides with algebraic addition? What about point doubling?
 Bitcoin (before Taproot upgrade) and Ethereum (before The Merge upgrade) use secp256k1: $y^2=x^3+7 ~ (mod~ p)$ where $p=2^{256}2^{32}2^92^62^41$.
 ECDSA cyclical subgroup:
This elliptic curve consists of 39 points!
the whole subgroup:
 ECDSA private and public keys
 $k_{prv}=9$ is the private key = number of steps in cyclic subgroup starting from the generator
 $G=\langle(8,1)\rangle$ is publicly known subgroup, G=13 in 39 element elliptic group $y^2=x^3+7$ over $\mathbb F_{37}$.
 $K_{pub}=9G(=G^9)$ = (23,1) is the public key. Is computed efficiently!
 This difference is the key for why its very easy to derive public key from private (log amount of steps), but not the other way around (n amount of steps, where n=<G>=13 in our case)
ECDSA signature scheme explained theoretically
Example of a signature: (simplified)
 $y^2=x^3+7$
 G=(8,1)
 $k_{prv}=9$
 $K_{pub} = (23,1)$
 $t=4$ (message)
 Choose a random number, e.g. $i=7$ (chosen differently for every signature)
 Compute (r and s comprise signature)
 $P=i\cdot G=7G=(18,20)$
 $r=x_P ~(mod~ 13)= 18~(mod~ 13)=5$
 $s= \{i^{1}(t+rk_{prv}\}~ (mod ~ n)$ $=\{2+(4+5\cdot 9)\} ~( mod ~ 13)$ $=7$
 Send
 (r, s) = (5, 7)
 t=4 (this is what you are signing)
 $K_{pub}= (23,1)$
(Inverses are computed easily by mirroring)
Example of verification: (simplified)
 $y^2=x^3+7$
 G=(8,1)
 $K_{pub} = (23,1)$
 $t=4$
 (r, s) = (5, 7)
 Compute
 $u_1 = (s^{1}t) ~ (mod ~ n)=8$
 $u_2 = (s^{1}t) ~ (mod ~ n)=10$
 $P = G^{u_1} + K_{pub}^{u_2}= 8 G+10 (23,1)= (32,20)+(8,36)=(18,20)$
 Check authenticity: $x_P ~ (mod ~ n)=r$ (in our case $18 ~ (mod ~ 13) = 5$)