Encryption - Crypto 101
Ciphertext - The result of encrypting a plain text, encrypted data
Cipher - A method of encrypting or decrypting data. Modern ciphers are cryptographic, but there are many non cryptographic ciphers like Caesar.
Plaintext - Data before encryption, often text but not always. Could be a photograph or other file
Encryption - Transforming data into ciphertext, using a cipher.
Encoding - NOT a form of encryption, just a form of data representation like base64. Immediately reversible.
Key - Some information that is needed to correctly decrypt the ciphertext and obtain the plaintext.
Passphrase - Separate to the key, a passphrase is similar to a password and used to protect a key.
Asymmetric encryption - Uses different keys to encrypt and decrypt.
Symmetric encryption - Uses the same key to encrypt and decrypt
Brute force - Attacking cryptography by trying every different password or every different key
Cryptanalysis - Attacking cryptography by finding a weakness in the underlying maths
Alice and Bob - Used to represent 2 people who generally want to communicate. They’re named Alice and Bob because this gives them the initials A and B.
Why is encryption important?
Cryptography is used to protect confidentiality, ensure integrity, ensure authenticity. You use cryptography every day most likely, and you’re almost certainly reading this now over an encrypted connection.
When logging into TryHackMe, your credentials were sent to the server. These were encrypted, otherwise someone would be able to capture them by snooping on your connection.
When you connect to SSH, your client and the server establish an encrypted tunnel so that no one can snoop on your session.
When you connect to your bank, there’s a certificate that uses cryptography to prove that it is actually your bank rather than a hacker.
When you download a file, how do you check if it downloaded right? You can use cryptography here to verify a checksum of the data.
You rarely have to interact directly with cryptography, but it silently protects almost everything you do digitally.
Whenever sensitive user data needs to be stored, it should be encrypted. Standards like PCI-DSS state that the data should be encrypted both at rest (in storage) AND while being transmitted. If you’re handling payment card details, you need to comply with these PCI regulations. Medical data has similar standards. With legislation like GDPR and California’s data protection, data breaches are extremely costly and dangerous to you as either a consumer or a business.
DO NOT encrypt passwords unless you’re doing something like a password manager. Passwords should not be stored in plain text, and you should use hashing to manage them safely.
RSA
The math(s) side
RSA is based on the mathematically difficult problem of working out the factors of a large number. It’s very quick to multiply two prime numbers together, say 17*23 = 391, but it’s quite difficult to work out what two prime numbers multiply together to make 14351 (113x127 for reference).
The attacking side
The maths behind RSA seems to come up relatively often in CTFs, normally requiring you to calculate variables or break some encryption based on them. The wikipedia page for RSA seems complicated at first, but will give you almost all of the information you need in order to complete challenges.
There are some excellent tools for defeating RSA challenges in CTFs, and my personal favorite is https://github.com/Ganapati/RsaCtfTool which has worked very well for me. I’ve also had some success with https://github.com/ius/rsatool.
The key variables that you need to know about for RSA in CTFs are p, q, m, n, e, d, and c.
“p” and “q” are large prime numbers, “n” is the product of p and q.
The public key is n and e, the private key is n and d.
“m” is used to represent the message (in plaintext) and “c” represents the ciphertext (encrypted text).
CTFs involving RSA
Crypto CTF challenges often present you with a set of these values, and you need to break the encryption and decrypt a message to retrieve the flag.
There’s a lot more maths to RSA, and it gets quite complicated fairly quickly. If you want to learn the maths behind it, I recommend reading MuirlandOracle’s blog post here: https://muirlandoracle.co.uk/2020/01/29/rsa-encryption/.
Establishing Keys Using Asymmetric Cryptography
Metaphor time
Imagine you have a secret code, and instructions for how to use the secret code. If you want to send your friend the instructions without anyone else being able to read it, what you could do is ask your friend for a lock.
Only they have the key for this lock, and we’ll assume you have an indestructible box that you can lock with it.
If you send the instructions in a locked box to your friend, they can unlock it once it reaches them and read the instructions.
After that, you can communicate in the secret code without risk of people snooping.
In this metaphor, the secret code represents a symmetric encryption key, the lock represents the server’s public key, and the key represents the server’s private key.
You’ve only used asymmetric cryptography once, so it’s fast, and you can now communicate privately with symmetric encryption.
The Real World
In reality, you need a little more cryptography to verify the person you’re talking to is who they say they are, which is done using digital signatures and certificates. You can find a lot more detail on how HTTPS (one example where you need to exchange keys) really works from this excellent blog post. https://robertheaton.com/2014/03/27/how-does-https-actually-work/
Digital Signatures and Certificates
What’s a Digital Signature?
Digital signatures are a way to prove the authenticity of files, to prove who created or modified them. Using asymmetric cryptography, you produce a signature with your private key and it can be verified using your public key. As only you should have access to your private key, this proves you signed the file. Digital signatures and physical signatures have the same value in the UK, legally.
The simplest form of digital signature would be encrypting the document with your private key, and then if someone wanted to verify this signature they would decrypt it with your public key and check if the files match.
Certificates - Prove who you are!
Certificates are also a key use of public key cryptography, linked to digital signatures. A common place where they’re used is for HTTPS. How does your web browser know that the server you’re talking to is the real tryhackme.com?
The answer is certificates. The web server has a certificate that says it is the real tryhackme.com. The certificates have a chain of trust, starting with a root CA (certificate authority). Root CAs are automatically trusted by your device, OS, or browser from install. Certs below that are trusted because the Root CAs say they trust that organisation. Certificates below that are trusted because the organisation is trusted by the Root CA and so on. There are long chains of trust. Again, this blog post explains this much better than I can. https://robertheaton.com/2014/03/27/how-does-https-actually-work/
You can get your own HTTPS certificates for domains you own using Let’s Encrypt for free. If you run a website, it’s worth setting it up.
Cracking SSH ID_RSA
https://vk9-sec.com/ssh2john-how-to/
PGP, GPG and AES
What is PGP?
PGP stands for Pretty Good Privacy. It’s a software that implements encryption for encrypting files, performing digital signing and more.
What is GPG?
GnuPG or GPG is an Open Source implementation of PGP from the GNU project. You may need to use GPG to decrypt files in CTFs. With PGP/GPG, private keys can be protected with passphrases in a similar way to SSH private keys. If the key is passphrase protected, you can attempt to crack this passphrase using John The Ripper and gpg2john. The key provided in this task is not protected with a passphrase.
The man page for GPG can be found online here.
What about AES?
AES, sometimes called Rijndael after its creators, stands for Advanced Encryption Standard. It was a replacement for DES which had short keys and other cryptographic flaws.
AES and DES both operate on blocks of data (a block is a fixed size series of bits).
AES is complicated to explain, and doesn’t seem to come up as often. If you’d like to learn how it works, here’s an excellent video from Computerphile https://www.youtube.com/watch?v=O4xNJsjtN6E