Number Theory operations in Sage - Some common number theory operations in SageMath with explained examples.
Crypto 101 - 254 page book on basics of cryptography including block ciphers, stream ciphers, key exchange, public-key encryption, hash functions, message authentication codes, signature algorithms, key derivation functions, random number generators, SSL & TLS, OpenPGP & GPG
An Overview of Cryptography - Gary C. Kessler - Online book on fundamentals of cryptography and a variety of cryptographic algorithms
Cryptography: An Introduction - Nigel Smart - ~430 page book on both symmetric and asymmetric encryption with chapters on security issues and more advanced protocols
Twenty Years of Attacks on the RSA Cryptosystem - Dan Boneh - A survey that goes through multiple vulnerabilities with RSA (with mathematical proofs)
Elliptic Curve Cryptography: a gentle introduction - A good introduction to ECC and the basics of the mathematics behind it.
cryptopals - Self guided challenge sets
Crypton - A collection of both explanations and challenges for various cryptography systems
factordb - A tool used to store known factorisations for numbers
Alpertron Integer factorisation calculator - Web assembly implementation of the Elliptic Curve Method and Self-Initializing Quadratic Sieve for factorising integers
SageMath - Python-based mathematics software system built on top of various scientific python packages.
OWASP XSS Filter Evasion Cheatsheet
The Basics of Web Application Security - Blog post about common security issues in web applications (2017)
OWASP Top 10 - 2017 - The ten most critical web application security risks
Exotic HTTP Headers - Blog post about non-typical HTTP headers
Sonic Visualiser - View and analyse the contents of audio files
Audacity - Multi-track audio editor with various filters and effects
Online Image Stego Tool - Online tool that can occasionaly extract hidden data in image files
Steghide - Extracts data from image and audio files (jpeg, bmp, wav au)
Stegsolve - GUI tool that automatically applies colour filters on images
Exiftool - View metadata of files (mostly works with image files)
Binwalk - Searches binary files for embedded files and data
Hausec's Pentesting Cheatsheet
HighOn.Coffee Pentesting Tools Cheatsheet
IppSec (Youtube) - HTB video writeups
0xRick (Blog) - HTB writeups
Hack The Box Pen-Testing Labs - boot2root style challenges