### File Permissions

A little break in the exams, so we have some time to discover another topic - which is file permissions on Linux. Let's fully understand how they work.

### Frequency Analysis

**Frequency analysis** is the study of the frequency of letters or groups of letters occuring next to each other. The most ancient description for what we know was made by Al-Kindi, dating back to the IXth century. This attack is used to break **monoalphabetic ciphers**, which work by simple and fixed substitution of letters. We now, that if the **a** in plaintext is encrypted to **x** letter, everytime in the ciphertext that **x** will convert to **a**.

### PicoCTF - SmallRSA

You intercepted a single message. However, it appears to be encrypted. Can you decrypt it? Message

### PicoCTF - ComputeRSA

RSA encryption/decryption is based on a formula that anyone can find and use, as long as they know the values to plug in. Given the encrypted number 150815, d = 1941, and N = 435979, what is the decrypted number?