If you have any kind of interest in crypto you should probably check this out. Recently, I started doing the Cryptopals, which are a series of cryptography challenges created by the formerly Matasano Security team. They are a collection of 48 exercises divided in 8 sets. I have just finished the first set but I can guarantee you that it was already a lot of fun and learning.
SPOILER ALERT: This is post will give the solutions for the first set of exercises, if you are planning to do it, stop reading this now and try them by yourself. However, if you are still unsure if this is for you or not, maybe try skim reading through this post to see if you get interested.
While resolving the first set I had mixed feelings of love and hatred. Sometimes I was able to get the idea but ended up making a programming mistake that would keep me stuck for while in one challenge or another. They stated in their website that this is a “relatively easy” set of challenges with the except of one. I can’t tell you about the further sets but one challenge in this set took me 2 days to get it right.
I decided to write my solutions in Node.js and this was a good way to learn about some quirks of the language that I was not used to face in a common daily programming session. You might also want to choose a new programming language to kill two birds with one stone.
Challenge 1: Convert hex to base64
Challenge 2: Fixed XOR
This challenge will introduce you to XOR operation for cryptographic purposes. The simple XOR cipher is a type of additive cipher that was used in the past to encrypt text, but it is clearly not safe since the ciphertext could be decrypted using character frequency analysis. On the other hand, the XOR operation is still frequently used in more complex ciphers, due to its simplicity and performance.
The challenge asks for a function that takes two equal-length buffers and produces their XOR combination. A solution for this exercise would be to loop through one of the buffers doing a XOR with each correspondent byte from the other buffer. The code that can do this will be shown further in this post.
Challenge 3: Single-byte XOR cipher
Now that we already know how to cipher and decipher messages using XOR, things are going to become interesting. The challenge presents a hex encoded string that was XOR’d against a single character and require us to discover this key and decrypt the message. This can be achieved by brute-forcing the key and analysing the character frequency of the output messages. The output with the best score is probably the message decrypted.
To calculate the score of a string according to the frequency of english letters you can find the percentual values on the Internet and sum them. Don’t forget to add the space character to your list, it took me some time to notice that.
If everything works fine, we will get the decrypted message:
Cooking MC\'s like a pound of bacon
Challenge 4: Detect single-character XOR
This time we were given a file containing a lot of encrypted strings. The goal was to find the string that was encrypted with single-character XOR, but we know from the previous exercise that decrypting it would be really easy too. We can use the function from the previous challenge to brute-force each string as a single-byte XOR cipher and find out the one with the best score.
If everything works fine, we will get the decrypted message:
Now that the party is jumping\n
Challenge 5: Implement repeating-key XOR
This challenge is similar to the Challenge #2, but this time the message length is bigger than the key length, so we will use the Vigenère cipher. In this case, the key needs to be repeated to cipher the whole message. Maybe this challenge could be positioned after the second one.
In order to reuse code, some of the functions were created in a separated file. Since XOR is a commonly used operation, I decided to create a function that will XOR two buffers repeating the smallest one which would be the key. This function could be used with fixed or repeating-key XOR.
Ciphering the message with the above function and converting the result to a hex string will give the expected output.
Challenge 6: Break repeating-key XOR
“It is officially on, now.” This one took me a while to make it work right. They give you a file containing a message that is encrypted with repeating-xor and encoded in base64. This challenge is when you put together everything you learned until now. Fortunately, they give you the steps to do it.
In order to get this one right, you will need to use something called the Hamming distance, which is the number of different position between two symbols. In this case, we need to calculate the number of differing bits between the two strings.
The first thing you need to do is to figure out the size of the key. Unless you have a crystal ball at home, you can start guessing. They suggest values from 2 to 40.
If you are wondering what kind of test is that, no worries, they explain you how to do it. It basically consists of getting the first two chunks of KEYSIZE bytes from the ciphertext, computing the Hamming distance between them, repeating this process a couple of times and normalizing the result. The KEYSIZE which gives the least distance between the chunks of bytes in the ciphertext is probably the right one.
Once you have the KEYSIZE, you can use the following strategy to solve the challenge:
- Divide the ciphertext into blocks of KEYSIZE length;
- Transpose the blocks: make a block that is the first byte of every block, and a block that is the second byte of every block, and so on;
- Solve each block as it was single-character XOR;
- For each block, the single-byte XOR key that produces the best output is the XOR key byte for that block;
- Put them together and you have the key;
- Decipher the message with the key you obtained.
Using this key against the file will give you the lyrics of this song.
Challenge 7: AES in ECB mode
After solving the previous challenge the last two seemed like a walk in the park. This is an introduction to the block cipher AES. A block cipher is a deterministic algorithm which operates on fixed-length groups of bits. Examples of block ciphers are DES, RC5, Blowfish, AES. The following code will do the job of decrypting a base64 encoded ciphertext in Node.js:
If everything works fine, we will get the same lyrics of the previous challenge.
Challenge 8: Detect AES in ECB mode
This challenges requires you to find which one of the ciphertexts in a file was encrytpted with AES in ECB mode. The ECB mode is simplest mode of encryption in which the message is divided into blocks and they are independently encrypted. The problem with this mode is that a block is always encrypted into the same ciphertext block. This is exactly the weakness you need to abuse in order to detect which ciphertext was encrypted in ECB mode, you will count the repetitions.
The complete source code of the solutions you can find on my Github. If you solved this first set of exercises you have the knowledge of performing the following tasks:
- Convert between data base64, hex, binary
- Detect and decrypt single-character XOR ciphers
- Break repeating-key XOR ciphers
- Detect and decipher AES in ECB mode
I am planning on doing the other sets soon. I will probably release the solutions on Github and make my comments about them here. Stay tuned!