Cryptography

encryptioncrypto101

Cryptography has three main objectives:

  • 🔐 protect confidentiality (encryption)
  • 🧬 ensure integrity (checksum)
  • ✅ ensure authenticity (certificate, signatures)

There are three categories of cryptographic algorithms:

Cryptanalysts are those with the duty to analyze and find weaknesses in the underlying math of cryptographic algorithms.

➡️ Python is quite used in cryptography, because reasons like there are many libraries with cryptographic algorithms, integers are not limited in size, and it's popular in mathematical fields...

Where to learn?


Terminology

In a nutshell, we use these terms:

  • The input is a plaintext message 💬.

  • It is encrypted 🔒 (a.k.a. encipher) using a cryptographic algorithm and a key 🔑. The output is called a ciphertext 🔐.

  • It is decrypted 🔓 using a cryptographic algorithm and a key 🗝️.

Algorithms can be

  • Weak 🔥: vulnerableto known attacks/easily broken
  • Acceptable 🤔: meet the minimum security requirements
  • Secure 🎩: secure against known attacks, resistant to cryptanalysis
  • Strong 👑: both secure and efficient

Cryptographic attacks

Brute force attack 🧨: trying all possible values until the correct one is found.

Frequency analysis 🪶: some letters/combinations of letters occur more frequently in a language. For instance, e is the most frequent letter in English/French.

Known-plaintext attack (KPA) 🔮: an attacker knows both the plaintext and the ciphertext, and tries to deduce the key.

Cribbing attack🧞‍♂️(a KPA attack): use information to guess parts of the plaintext and try to deduce the key.

Entropy attacks 🦄: find a weakness in the random number generation process used to generate cryptographic keys.

Side-channel attacks 🦐: exploit the implementation of the algorithm rather than the algorithm itself.

Timing attacks 🕰️: analyze the time it takes to perform different operations to guess a potential weakness in an algorithm


Mathematical fundaments

Congruence

Let a and b be two numbers. We are saying that $a$ is congruent (congru, $\equiv$) with $b$ modulus (modulo) $m$, if we can find a $q$ giving us

@ a = b + m * q @

This is a **Euclidean division with $b = r$ in **. We are using one of the notations below

\[ \begin{align} a \equiv b\ (m)\\ a \equiv b\ (mod\ m)\\ a \equiv [b]\\ \end{align} \]

Ex: $27 \equiv 3\ (mod\ 12)$ as we have $12*2 + 3$ (b=3)


Euclidean division (Division euclidienne)

Dividing $a$ by $b$, means solving $a = b * q + r$. You need to find the quotient $q$ and the remainder (reste) $r$, with $r \lt b$. Both are unique.

We are saying that $a$ is a divisor of $b$ if $r = 0$, written $a\ |\ b$. It would also mean that $a$ is a multiple of $b$.

Is 5 a divisor of 25?
$25 = 5 * 5 + 0$, so yes $5|25$

Is 4 a divisor of 25?
$25 = 4 * 6 + 1$, so no

Greatest common divisor (Plus grand diviseur commun)

$D(a,b)$ is the set of common divisors between $a$ and $b$. We are calling the "greatest common divisor" (GCD or PGCD), the greatest value of $D(a,b)$, written $GCD(a,b)$ or $a \wedge b$.

Example: what's the GCD of $(27, 15)$

\begin{split} D(27, 15) = D(27-15{\color{grey}*1}, 15)\\ = D(12, 15) = D(12, 15-12{\color{grey}*1})\\ = D(12, 3) = D(12-3{\color{grey}*4}, 3)\\ = D(0, 3) = 3 = 27 \wedge 15 \end{split}

Formula: $a \wedge b = (a - b * q) \wedge b$
Pro tip: $a \wedge b = c \wedge (\frac{b}{c} \wedge \frac{a}{c})$, so $27 \wedge 15 = 3 \wedge (9 \wedge 5) = 3 \wedge 0 = 3 $.


Modular inverse

Bézout's identity (Thèorème de Bezout)

The formula is $a \wedge b = a * u + b * v$. Bézout coefficients $u$ and $v$ are not unique. Sometimes, this is easy to find the coefficients, but if this isn't, use a table (tip: read the example first)

k $r_k$ $q_k$ $u_k$ $v_k$ Bézout
0 $a$ ❌ 1 0 a = a * 1 + b * 0
1 $b$ $q_k= \lfloor \frac{r_{k-1}}{r_k} \rfloor$ 0 1 b = a * 0 + b * 1
... $r_k=r_{k-2}\ mod\ r_{k-1}$ ... $u_{k-2}-q_{k-1}*u_{k-1}$ $v_{k-2}-q_{k-1}*v_{k-1}$ $r_k=a*u_k+b*v_k$
Example: $a=98$ and $b=77$.
k $r_k$ $q_k$ $x_k$ $y_k$ Bézout
0 98 ❌ ${\color{red}1}$ 0 $98 = 98 * 1 + 77 * 0$
1 77 ${\color{green}1}$ ${\color{blue}0}$ 1 $77 = 98 * 0 + 77 * 1$
2 21 3 ${\color{red}1}-{\color{green}1}*{\color{blue}0}=1$ $0 - 1*1=-1$ $21 = 98 * 1 + 77 * -1$
3 14 1 $0 - 3 * 1 = -3$ $1 - 3 * -1=4$ $14 = 98 * -3 + 77 * 4$
3 7 2 $1 - 1 * -3 = 4$ $-1 - 1 * 4=-5$ $7 = 98 * 4 + 77 * -5$ (solution ✅)
3 0 ❌ $-3 - 2 * 4 = -11$ $4 - 2 * -5=14$ $0 = 98 * -11 + 77 * 14$

We are using the table because this one was hard. If you got $5 * a + 7 * b = 5 \wedge 7 = 1$, then you could find almost immediately that you can use $a=3$ and $b=-2$ giving us $15 - 14 = 1$...

Invertible numbers

A number $a$ is invertible modulus $m$ if $\exists a^{-1}$ giving us $a * a^{-1} \equiv 1\ (mod\ m)$. To find $a^{-1}$, you can use Bézout: $a u + m v = 1 \Leftrightarrow a u = 1\ (mod\ m) \Leftrightarrow a^{-1} \equiv u\ (mod\ m)$ with $u \gt 0$

  • Condition: $a \wedge m\ |\ 1$
  • Ex: in Bézout example, we had $98 * 4 + 77 * 5 = 7$
    • ❌: not invertible
  • Ex: we found that $5 * 3 + 7 * -2 = 1$
    • $m = 7$, and $a = 5$
    • $5^{-1} = 3\ (\text{mod}\ 7)$
    • check: $5 * 3 = 15 = 1\ (\text{mod}\ 7)$
    • $7^{-1} = -2 = 3\ (\text{mod}\ 5)$
    • check: $3 * 7 = 21 = 1\ (\text{mod}\ 5)$

Prime numbers (Nombres premiers)

A prime number is a number greater than or equal to 2, which is only divisible by 1 and itself.

  • ✅: 2, 3, 5, 7, 11, 13, etc.
  • ❌: 4 (→2), 6 (→2, →3), 9 (→3), 10 (→2, →5), etc.

Note: If $p$ and $q$ are prime numbers, and $p \neq q$, then $p \wedge q = 1$.
Note: $\phi(n)$ is called Euler's totient function (indicatrice d'Euler) and is the number of prime numbers with $n$.

We are calling $\xi_p(n)$, the exponent of $p$ in the factorization of a number $n$ with prime numbers (puissance de p dans n, ex: $\xi_3(27) = 3$ or $\xi_5(60)=1$).

GCP with prime numbers

You can calculate the GCD easily. Simply express each number with prime numbers. Then, take each unique number in both factorizations: they will be in the GCD. Their exponent is the lowest exponent that we have for each number in the two factorizations.

\begin{split} GCP(a, b) = \prod_{i\ \in\ unique\ prime\ numbers} i^{\min(\xi_i(a),\ \xi_i(b))} \\ 98 = 2 * 49 = 2 * 7^2 \\ 77 = 7 * 11 \\ GCP(98, 77) = 2^{min(1, 0)} * 7^{min(2, 1)} * 11^{min(0, 1)} = 1 * 7 * 1 = 7\\ \end{split}

Note: if you replace min with max, you will have the least common multiple (Plus petit commun multiple/PPCM).


Calculation shortcut

If you need to manually calculate a Euclidean division with big numbers, then you may want to use the prime Factorization shortcut.

Prime Factorization (Décomposition en produit de facteurs premiers)

Every number ($\ge 2$) can be expressed as a product of prime numbers.

  • $27 = 3 * 9 = 3^3$
  • $60 = 6 * 10 = 2 * 3 * 5 * 2 = 2^2 * 3 * 5$

Euclidean division shortcut

Rewrite $a$ as a product of factors, and evaluate each one.

  • Ex: $256\ \text{mod}\ 7 = 2^8 = 2^3 * 2^3 * 2^2$
    • $2^3 = 8 \equiv 1\ (\text{mod}\ 7)$
    • $2^2 = 4 \equiv 4\ (\text{mod}\ 7)$
    • Giving us $256 \equiv 1 * 1 * 4 \equiv 4 \ (\text{mod}\ 7)$
  • Ex: $2021\ \text{mod}\ 3 = 2000 + 20 + 1 = 2 * 10^3 + 2 * 10^2 + 1$
    • $10 \equiv 1\ \text{mod}\ 3$
    • $10^2 = 10 * 10 \equiv 1 \ (\text{mod}\ 3)$
    • $10^3 = 10 * 10 * 10 \equiv 1 \ (\text{mod}\ 3)$
    • $2021= 2 * 1 + 2 * 1 + 1 = 5 \equiv 2 \ (\text{mod}\ 3)$

This is called the Euler theorem (wiki).


👻 To-do 👻

Stuff that I found, but never read/used yet.

  • SRM (error-correcting code)
  • RsaCtfTool (4.2k ⭐)
  • rsatool (0.9k ⭐)
  • message authentication codes (MACs)
  • HMAC
  • certificates (chain of trust), digital signatures
  • cryptobook
  • keylength