Cheat sheet

Some important notions

name	description
Identification	Who are you ?
Authentication	Prove it
Authorization	Do you have the right to do something ?
Audit	What did you do ?
FootPrint	Recognition
Integrity	Ensuring that information cannot be changed
Confidentiality	Ensuring that information can be accessed (read) only by authorized persons
DLP	Discret Logarithm Problem : finding the number y where x = g^y mod, g is given and p is a large prime number

Frequently used CMDs

gpg : is the cmd for encryption

Get Hostname

hostname -I

Connect remotely via a private network

ssh <username>@<hostname>

Secure copy :

scp <filename> <username>@<hostname>:<path>

Attacks

Attack	Description	Example
Birthday Attack	It is made against the Hash algorithms. (Belongs to a class of brute force)

All hashed messages have a fixed length (independent from the input length) and they unique for that message. this attack refers to the probability of finding two random messages m1 and m2 that has the same Hash h(m1) = h(m2) so the attacker can safely replace the message by his own one|What is the relation with the birthday:

Since we have a finite number of days in one year (365) there is a big chance to have 2 persons with the same birthday in a finite number of persons.| |Password attack|Trying to guess the password or having a databse of passwords called dictionnary|- Brute Force : using a random approach by trying different passwords and hoping that one work Some logic can be applied by trying passwords related to the person’s name, job title, hobbies or similar items.

Dictionary attack: dictionary of common passwords is used to attempt to gain access to a user’s computer and network. One approach is to copy an encrypted file that contains the passwords, apply the same encryption to a dictionary of commonly used passwords, and compare the results.| |Man in the middle|occurs when a hacker inserts itself between the communications of a client and a server|Session hijacking

mitm1

mitm2

|SQL Injection|SQL commands are inserted into data-plane input (for example, instead of the login or password) in order to run predefined SQL commands|“SELECT * FROM users WHERE account = ‘’ or ‘1’ = ‘1’;”

Because ‘1’ = ‘1’ always evaluates to TRUE, the database will return the data for all users instead of just a single user.| |Cross-site scripting (XSS)|Use third-party web resources to run scripts in the victim’s web browser or scriptable application|

xss |Ransomware|Ransomware is a type of malware from cryptovirology that threatens to publish the victim's personal data or perpetually block access to it unless a ransom is paid. While some simple ransomware may lock the system without damaging any files, more advanced malware uses a technique called cryptoviral extortion.|

Firewalls

The major role of firewalls is:

Intercepting incoming traffic between network and trust levels
Authorizing or reject access
Enabling a more secure connection to open networks
Audit network usage

Netfilter

Netfilter is a Linux kernel module that provides the ability to monitor, modify and filter IP packets, and track connections. It thus provides firewall functions, Internet connection sharing and network traffic authorization.

IpTables

IpTable in a CMD interface to configure Netfilter

Commands

The commands tells iptables what to do with the rest of the rule that is sent to the interpreter.

The chain's name : INPUT

Rules' List

iptable -L --line-numbers

Create a Chain (named allowed)

iptables -N allowed

Erase a chain

iptables -X allowed

Modify the authorization

iptables -P INPUT DROP

Add rule to end of the chain

iptables -A INPUT

Drop rule

iptables -D INPUT --dport 80 -j DROP, iptables -D INPUT 1

Replace a rule

iptables -R INPUT 1 -s 192.168.0.1 -j DROP

Insert a rule anywhere

iptables -I INPUT 1 --dport 80 -j ACCEPT

Clear the chain's rules

iptables -F INPUT

Encryption

Encryption is used to garantee Confidentiality.

Symmetric Encryption

Use one shared encryption key between sender and receiver.

Encrypt the file :

Binary format (default)
```
gpg -c <filename>
```
ASCII format
```
gpg -c --armor(or -a) <filename>
```

Get all the algos

gpg --version

Examples of symmetric algorithms:

AES (Advanced Encryption Standard), DES (Data Encryption Standard), IDEA (International Data Encryption Algorithm),RC4/5/6 (Rivest Cipher 4/5/6)

Encrypt the file and specify the algo (cipher)

gpg -c -a --cipher-algo <filename>

Decrypt

gpg -d <filename>

Advantages ++	Disadvantages --
Fast	non secure key
-	Large number of keys
-	Without signature

Feistel

It's a block cipher

Encryption algorithm:

The plain text is divided into 2 equal halves : G and D
The encryption function is applied on the right half D of the plaintext block. The result of this encryption function is then XORed with the left half G . The result of XOR function becomes the new right half R .

feistel

f : is the the encryption function that takes (data block) as input and returns one output of the same size as the data block.

Decryption Algorithm :

D = L and G = R XOR f(L)

Know that:

We could repeat this algo as many time as we need if we want to increase the security.

The security also increases when the block is larger and the round function is more complex, but it may decrease the speed of encryption and decryption.

Asymmetric Encryption

Use public keys for encryption and private keys for decryption

Generate Keys

gpg --full-generate-key
gpg --gen-key

Get the Key List

gpg --list -keys

Export a public key

gpg -o KEY -a --export <KeyID>

Import Key

gpg --import KEY

Encryption

gpg -o file.enc -a -r file < publicKey > -encrypt

Sign

gpg -o file.signed --sign-file

Verify a signature

gpg --verify file.signed

Decrypt

gpg -o file --decrypt file.enc

Advantages ++	Disadvantages --
Secured	Slow
Signature
Less number of keys

Needham-Schroeder

Authentication Protocole

pka : public key of na
pkb : pyblic key of nb
Na / Nb : random numbers

Algorithm: A wants to prove his identity to B

A sends his name and random number Na,this message is encrypted by an asymmetric encryption algorithmwith the public key of B (pkb ),
B decrypts the message with his private key
returns the random number generated by A (so A could make sure that the message is comming from B) and a new random number Nb : encrypted by the public Key of A
A receives the message and decrypt it with his private key
A sends Nb to B encrypted with pkb (to tell B that he successfully get his message)
Indentity proved !

Problem:

This algo could easly be attacked by the Man in the midlle

Solution:

By adding b (prove the identity of b) , a will make sure if he is talking with b or with not

protocol

RSA

NP-complete problem

Parameters:

choose 2 prime numbers p & q
N= p*q
L = (p-1)*(q-1)
Generate the public Key (e,N): e is between 1 and L and coprime with N and L
private key (d,N) : d*e mod L =1

Encryption : $\Large M^e$ mod N = C

Decryption : M = $\Large C^d$ mod N

ELGamal

DLP : Discrete logarithm problem

Public Infos:

q : prime number

p = 2 q +1

Gq: cyclic group

g: generator of Gq

Choose a private key X from Gq
Public Key: y = g^X mod p

Encryption :

Choose a number k from Gq
u = g^k mod p
v = y^k M mod p ( M : message )
publish : (u,v)

Decryption:

D(u,v) = v/u^X = m

Hybrid Encryption

DH

p (prime number) and g are public infos

Algorithm: A and B want to communicate with each other by sharing the same Key

A chooses a random number "a" between 0 and p-1
B chooses a random number "b" between 0 and p-1
u = $\Large g^a$ mod p (published by A) ; v= $\Large g^b$ mod p (published by B)
Key of B : Kb= $\Large u^b$ = $\Large g^ab$ mod p
Key of A : Ka= $\Large v^a$ = $\Large g^ba$ mod p

=> Ka = Kb

Kerberos

it is responsible for authenticating, authorizing and monitoring users who want to access resources and services on your network. It acts as a watchdog against intruders on your network services.

Based on

“Ticket” Granting
Secret Key Cryptography
Mutual authentication
Time-limited tickets
Anti-replay mechanisms
It is based on 3 tiers architecture :
- Client/user : Who wants to access a resource from the server
- Ressources server : the machine that has resources
- KDC: Key Distribution Center: Grant ticket for users allowed to access a service

Principal : Each user and service on the network is a principal.

Implementation : https://github.com/rihemebh/Kerberos-project

Elliptic Curve

ECC is based on the elliptic curve discrete logarithm problem, or (ECDLP), Instead of numbers, the elliptic curve’s problems operate on points, and multiplication is used instead of exponentiation : Finding the number k given a base point P where the point Q = kP.

ECC Auth

The standard algorithm used for signing with ECC is ECDSA, which stands for elliptic curve digital signature algorithm. This algorithm has replaced RSA signatures and classical DSA signatures in many applications. It is, for example, the only signature algorithm used in Bitcoin and is supported by many TLS and SSH implementations.

Signature generation

Parameters

n is the number of point in the curve

G = (x,y) is a base point

private key = d

public Key P = dG

Algorithm

Hash a message (SHA-256 or BLAKE2) ⇒ generate h between 0 and n-1
Pick a random number k between 1 and n-1
compute kG
set r = x mod n
compute s = (h+rd)/ k mod n
signature = (r,s)

Signature Verification

compute w = 1/s = k / (h+rd) mod n
wh = hk(h+rd) = u
wr = rk(h+rd) = v
Q = uG + vP

The verifier only accepts the signature if the x coordinate of Q is equal to the value r from the signature.

Signature Verification Encryption

We have:

(q ,G, G1, e), P un point de la CE E tq q divise p^k - 1
n longueur du message
H1 : (0, 1)-> G
H2 : G1 -> (0, 1)^n
PKG : calcule P pub = rP

Decryption

RSA VS ECC

Elliptic curve cryptography is often viewed as an alternative to RSA for public-key cryptography, but ECC and RSA don’t have much in common.

RSA is only used for encryption and signatures, whereas ECC is a family of algorithms that can be used to perform encryption, generate signatures, perform key agreement, and offer advanced cryptographic functionalities such as identity-based encryption.

The significant difference: => The same level of security with small numbers

cryptography

ECC has two major advantages over RSA: shorter signatures and signing speed.

Because ECC works with shorter numbers, it produces shorter signatures than RSA (hundreds of bits long, not thousands of bits), which is an obvious benefit if you have to store or transmit numerous signatures.

Signing with ECDSA is also much faster than signing with RSA (though signature verification is about as fast) because ECDSA works with much smaller numbers than RSA does for a similar security level.

Reference : Serious Cryptography A Practical Introduction to Modern Encryption by --Jean-Philippe Aumasson

Hash

Hash is used to garantee Integrity : We use non-bijective functions to hash the message.

Whatever the size of the real message, the size of a hash message is always the same.

Examples of Hash functions : sha, sha256, md5

Hash a file

asc25sum file
md5sum file
sha256sum file

OpenSSL

Definition

SSL (Secure Sockets Layer)

Is an encryption-based protocol
Ensure confidentiality,,Authentication and Integrity
the predecessor of modern TLS(Transport Layer Security) encryption in use today

=> A website that implements SSL / TLS has an “HTTPS” in its URL instead of an “HTTP”.

OpenSSL is a software library for applications that secure communications over computer networks against eavesdropping or need to identify the party at the other end. It contains an open-source implementation of the SSL and TLS protocols. The core library, written in C programming language, implements basic cryptographic functions and provides various utility functions.

CMD

Encrypt the "file" with "aes-128-cbc" algorithm and put the result in "file.enc"

openssl enc -aes-128-cbc -in file -out file.enc

// Apply the algoritm 2 times

openssl enc -aes-128-cbc -iter 2 -in file -out file.enc

Decrypt

openssl enc -d -aes-128-cbc -in file.enc -out filerestored
openssl enc -d -aes-128-cbc -iter 2 -in file.enc -out filerestored

Generate RSA Key:

openssl genrsa -out mykey 2048

Generate RSA params:

openssl rsa -in mykey -text -noout

-noout : print the output in the terminal

Generate a public key from a private key

openssl rsa -in mykey -pubout -out pub

Private Key Encryption :

openssl rsa -in mykey -des -out mykey.enc

Encrypt with public key

openssl rsault -encrypt -pubin -inkey PUB -in file -out file.enc

Decrypt

openssl rsautl -decrypt -inkey mykey.enc -in file -out rsa.enc

Signature

Sign with the private key

openssl rsault -sign -inkey myKey.enc -in file -out fileRSA.sign

Verify signature

openssl rsault -verify -pubin -inkey PUB -in fileRSA.sign

Encrypt + Sign

openssl dgst -sha256 -verify PUB -signature fileHashSign file

Certificate

Digital Certificates are verifiable small data files that contain identity credentials to help websites, people, and devices represent their authentic online identity.
Digital certificates cover three main uses:
- Authentication: used to validate the identity of issuers as part of an authentication process, a crucial element of computer network security.
- Signing: sed to sign a document or a file and to guarantee its integrity
- Encryption: Garantee the security and integrity of information exchanged between a website and a browser, by means of a cryptographic key enabling a secure session to be activated (HTTPS protocol)
Certification Authority issues digital certificates
Get the Certification of the www.google.com website:

openssl s_client www.google.com:443

Get the authority of a certification :

google.cert is the file that contains google certification : output of the above cmd

openssl x509 -in google.cert -subject -issuer -noout

=> google trust

Goal:

Create a certification authority called "INSAT"
Create a certification for INSAT
GL4 will ask a certification from INSAT
INSAT will verify this request and generate a certificate
Now INSAT is the authority of GL4
Create RSA keys for INSAT and put it in INSAT.key

openssl genrsa -des3 -out INSAT.key 3072

Create a certification for INSAT

openssl req -new -x509 -days 730 -key INSAT.key -out INSAT.cert

=> In this certificate INSAT is the subject and issuer : Self Signed Certificate

Create keys for GL4

openssl genrsa -des3 -out gl4.key 3072

Create request for a certiif from INSAT

openssl req -new -key gl4.key -out gl4.req

Generate a Certificate for GL4:

openssl x509 -req -in gl4.req -out gl4.cert -CA INSAT.cert -CAKey INSAT.key -CAcreateserial -CAserial gl4.srl

Export the certificate

openssl pkcs12 -export -out gl4.pfx -in gl4.cert -inkey gl4.key -name "certificat de gl4"

=> pkcs12 : put the certificate and the keys in the same file "gl4.pfx" so you can import it in the browser

Import it

preference -> certificate manager -> my Certificates -> import

Add Authority

preference -> certificate manager -> Authorities -> import -> choose INSAT.cert

Cheat sheet

Some important notions​

Frequently used CMDs​

Attacks​

Firewalls​

Netfilter​

IpTables​

Commands​

Encryption​

Symmetric Encryption​

Feistel​

Asymmetric Encryption​

Needham-Schroeder​

RSA​

ELGamal​

Hybrid Encryption​

DH​

Kerberos​

Elliptic Curve​

ECC Auth​

Signature generation​

Parameters​

Algorithm​

Signature Verification​

Signature Verification Encryption​

Decryption​

RSA VS ECC​

Hash​

OpenSSL​

Definition​

CMD​

Signature​

Certificate​