Encryption
Key Terms
Ciphertext
The result of encrypting a plaintext, encrypted data
Cipher
A method of encrypting or decrypting data. Modern ciphers are cryptographic, but there are many non cryptographic ciphers like Caesar.
Encryption
Transforming data into ciphertext, using a cipher.
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. Examples are RSA and Elliptic Curve Cryptography. Normally these keys are referred to as a public key and a private key. Data encrypted with the private key can be decrypted with the public key, and vice versa. Your private key needs to be kept private, hence the name. Asymmetric encryption tends to be slower and uses larger keys, for example RSA typically uses 2048 to 4096 bit keys.
Symmetric encryption
Uses the same key to encrypt and decrypt. Examples of Symmetric encryption are DES (Broken) and AES.These algorithms tend to be faster than asymmetric cryptography, and use smaller keys (128 or 256 bit keys are common for AES, DES keys are 56 bits long).
Brute force
Attacking cryptography by trying every different password or every different key
Rivest Shamir Adleman (RSA)
Maths 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).
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 RsaCtfTool which has worked very well for me. I’ve also had some success with 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).
See MuirlandOracle’s blog post for Learning Maths behind it.
Establishing Keys Using Asymmetric Cryptography
A very common use of asymmetric cryptography is exchanging keys for symmetric encryption.
Asymmetric encryption tends to be slower, so for things like HTTPS symmetric encryption is better.
Metaphorically
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.
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.
Digital Signatures and Certificates
Digital Signatures
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.
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.
Your web browser know that the server you’re talking to is the real website using certificates.
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. Blog Post in Detail
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.
SSH authentication
By default, SSH is authenticated using usernames and passwords in the same way that you would log in to the physical machine.
SSH can also be configured with key authentication instead. This uses public and private keys to prove that the client is a valid and authorised user on the server. By default, SSH keys are RSA keys. You can choose which algorithm to generate, and/or add a passphrase to encrypt the SSH key. ssh-keygen is the program used to generate pairs of keys most of the time.
SSH Private Keys
You should treat your private SSH keys like passwords. Don’t share them, they’re called private keys for a reason. If someone has your private key, they can use it to log in to servers that will accept it unless the key is encrypted.
It’s very important to mention that the passphrase to decrypt the key isn’t used to identify you to the server at all, all it does is decrypt the SSH key. The passphrase is never transmitted, and never leaves your system.
Using tools like John the Ripper, you can attack an encrypted SSH key to attempt to find the passphrase, which highlights the importance of using a secure passphrase and keeping your private key private.
When generating an SSH key to log in to a remote machine, you should generate the keys on your machine and then copy the public key over as this means the private key never exists on the target machine.
How to use these keys
The ~/.ssh folder is the default place to store these keys for OpenSSH. The authorized_keys file in this directory holds public keys that are allowed to access the server if key authentication is enabled. By default on many distros, key authentication is enabled as it is more secure than using a password to authenticate. Normally for the root user, only key authentication is enabled.
In order to use a private SSH key, the permissions must be set up correctly otherwise your SSH client will ignore the file with a warning. Only the owner should be able to read or write to the private key (600 or stricter). ssh -i keyNameGoesHere user@host is how you specify a key for the standard Linux OpenSSH client.
Using SSH keys to get a better shell
SSH keys are an excellent way to “upgrade” a reverse shell, assuming the user has login enabled (www-data normally does not, but regular users and root will). Leaving an SSH key in authorized_keys on a box can be a useful backdoor, and you don't need to deal with any of the issues of unstabilised reverse shells like Control-C or lack of tab completion.
Practical
Generate a ssh key-pair on your machine using
ssh-keygenCopy your public key and paste it into target
authorized_keysfile manually or by usingssh-copy-idNext time when logging using ssh, use your private key while connecting to target with
-iYour backdoor is created.
Diffie Hellman Key Exchange
Key exchange allows 2 people/parties to establish a set of common cryptographic keys without an observer being able to get these keys. Generally, to establish common symmetric keys.
Working
Alice and Bob want to talk securely. They want to establish a common key, so they can use symmetric cryptography, but they don’t want to use key exchange with asymmetric cryptography. This is where DH Key Exchange comes in.
Alice and Bob both have secrets that they generate, let’s call these A and B. They also have some common material that’s public, let’s call this C.
We need to make some assumptions. Firstly, whenever we combine secrets/material it’s impossible or very very difficult to separate. Secondly, the order that they're combined in doesn’t matter.
Alice and Bob will combine their secrets with the common material, and form AC and BC. They will then send these to each other, and combine that with their secrets to form two identical keys, both ABC. Now they can use this key to communicate.
DH Key Exchange is often used alongside RSA public key cryptography, to prove the identity of the person you’re talking to with digital signing. This prevents someone from attacking the connection with a man-in-the-middle attack by pretending to be Bob.
PGP
PGP stands for Pretty Good Privacy. It’s a software that implements encryption for encrypting files, performing digital signing and more.
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.
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).
Excellent Video from Computerphile
Quantum Computers and Encryption
Asymmetric and Quantum
While it’s unlikely we’ll have sufficiently powerful quantum computers until around 2030, once these exist encryption that uses RSA or Elliptical Curve Cryptography will be very fast to break. This is because quantum computers can very efficiently solve the mathematical problems that these algorithms rely on for their strength.
AES/DES and Quantum
AES with 128 bit keys is also likely to be broken by quantum computers in the near future, but 256 bit AES can’t be broken as easily. Triple DES is also vulnerable to attacks from quantum computers
Current Recommendations
The NSA recommends using RSA-3072 or better for asymmetric encryption and AES-256 or better for symmetric encryption. There are several competitions currently running for quantum safe cryptographic algorithms, and it’s likely that we will have a new encryption standard before quantum computers become a threat to RSA and AES.
Other Resources
NIST has resources that detail what the issues with current encryption is and the currently proposed solutions for these. Link
Book "Cryptography Apocalypse" By Roger A. Grimes
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