## Caesar Cipher

Caesar Cipher-encryption just involves shifting each alphabetic letter by some number.

Automatically guess shift number: https://www.xarg.org/tools/caesar-cipher/

Print all shifts: caesar_all.py

## Substitution Cipher

Substitution ciphers substitute each letter with another letter. They can be cracked by frequency analysis.

Guess cipher: http://quipqiup.com/index.php

View n-grams to help guess letters: subst.py and https://ehsandev.com/pico2014/cryptography/substitution.html

## Block Ciphers

Simple XOR CBC key finding script: bricks_of_gold_csaw_2015.py

## RSA

### Setup

Use this script to install a few useful utilities for working with RSA problems. You will need git, python, pip, and virtualenv already installed on your system.

install_rsa.sh

### General RSA Attack Notes

- Small public modulus n - use https://factordb.com/index.php to find p and q.
- Given multiple keys - see if any of the keys have common factors using the Euclidean Algorithm.

1 | import fractions |

- p and q are close to each other - use YAFU or https://www.alpertron.com.ar/ECM.HTM
- Two ciphertexts use the same modulus n but different exponents e - use: same_modulus.py
- Small p or q - use YAFU or https://www.alpertron.com.ar/ECM.HTM
- Large e or d - Wiener’s attack. Use attackrsa tool.
- Same m and e for multiple messages - Hastad’s Broadcast Attack. Use attackrsa tool.
- If num_ciphertexts >= e then you can use Chinese Remainder Theorem to calculate the message (but gcd of all n’s must be 1 - if the gcd between any two n’s is not 1, then you can just find a common factor between them).

### Basic RSA Decryption

If you can easily find p and q or d, then you can use this script to decrypt a ciphertext to ASCII.

decrypt_rsa.py

### Commands

Here are some commands to transform and work with keys.

#### Given n and d, print e, p, q.

1 | python rsatool.py -n 13826123222358393307 -d 9793706120266356337 |

#### Given n and d, print PEM format.

1 | # cd rsatool |

#### Given p and q, print DER format.

1 | # cd rsatool |

#### Factorize with YAFU.

1 | ./yafu "factor(0xD8E24C12B7B99EFE0A9BC04A6A3DF58A2A944269B492B7376DF129023F2061B9)" -threads 5 |