Introduction to Cryptography

This guide contains the answer and steps necessary to get to them for the Introduction to Cryptography room.

Table of contents

• Introduction
• Symmetric Encryption
• Asymmetric Encryption
• Diffie-Hellman Key Exchange
• Hashing
• PKI and SSL/TLS
• Authenticating with Passwords
• Cryptography and Data - Example

Introduction

1. You have received the following encrypted message:

“Xjnvw lc sluxjmw jsqm wjpmcqbg jg wqcxqmnvw; xjzjmmjd lc wjpm sluxjmw jsqm bqccqm zqy.” Zlwvzjxj Zpcvcol

You can guess that it is a quote. Who said it?

We can use the Quipquip site to try and decode the message.

Click for answer

Miyamoto Musashi

Symmetric Encryption

1. Decrypt the file quote01 encrypted (using AES256) with the key s!kR3T55 using gpg. What is the third word in the file?

gpg --output task02/quote01.txt --decrypt task02/quote01.txt.gpg

Click for answer

waste

1. Decrypt the file quote02 encrypted (using AES256-CBC) with the key s!kR3T55 using openssl. What is the third word in the file?

openssl enc -aes-256-cbc -d -in task02/quote02 -out task02/quote02.txt

Click for answer

science

1. Decrypt the file quote03 encrypted (using CAMELLIA256) with the key s!kR3T55 using gpg. What is the third word in the file?

gpg --output task02/quote03.txt --decrypt task02/quote03.txt.gpg

Click for answer

understand

Asymmetric Encryption

1. Bob has received the file ciphertext_message sent to him from Alice. You can find the key you need in the same folder. What is the first word of the original plaintext?

For this, we need the cipher text and Bob's private key.

openssl pkeyutl -decrypt -in ciphertext_message -inkey private-key-bob.pem -out plaintext.txt

This will put the plaintext into a file for us to read.

Click for answer

Perception

1. Take a look at Bob’s private RSA key. What is the last byte of p?

To view the real RSA variable we can use the following command:

openssl rsa -in private-key-bob.pem -text -noout

The p variable will be prime1.

Click for answer

e7

1. Take a look at Bob’s private RSA key. What is the last byte of q?

This can be found with the same command. q will be prime2.

Click for answer

27

Diffie-Hellman Key Exchange

1. A set of Diffie-Hellman parameters can be found in the file dhparam.pem. What is the size of the prime number in bits?

To view the real varibales of the Diffie-Hellman key, we can use the same command as in the previous task.

openssl dhparam -in dhparams.pem -text -noout

Click for answer

4096

1. What is the prime number’s last byte (least significant byte)?

This can be found together with the previous question.

Click for answer

4f

Hashing

1. What is the SHA256 checksum of the file order.json?

Using sha256sum we can calculate the SHA-256 hash of the file.

sha256sum order.json

Click for answer

2c34b68669427d15f76a1c06ab941e3e6038dacdfb9209455c87519a3ef2c660

1. Open the file order.json and change the amount from 1000 to 9000. What is the new SHA256 checksum?

After changing the content of the file, we can use the same command to re-calculate the hash.

sha256sum order.json

Click for answer

11faeec5edc2a2bad82ab116bbe4df0f4bc6edd96adac7150bb4e6364a238466

1. Using SHA256 and the key 3RfDFz82, what is the HMAC of order.txt?

Using the following command will give us the hash we are looking for.

hmac256 3RfDFz82 order.txt

Click for answer

c7e4de386a09ef970300243a70a444ee2a4ca62413aeaeb7097d43d2c5fac89f

PKI and SSL/TLS

1. What is the size of the public key in bits?

Using the following command we can view the contents of the certificate.

openssl x509 -in cert.pem -text -noout

Click for answer

4096

1. Till which year is this certificate valid?

This can be found in the same image as the previous question.

Click for answer

2039

Authenticating with Passwords

1. You were auditing a system when you discovered that the MD5 hash of the admin password is 3fc0a7acf087f549ac2b266baf94b8b1. What is the original password?

We can use hashcat to crack the hash. Knowing it is an MD5 hash we use the following command:

hashcat -m 0 3fc0a7acf087f549ac2b266baf94b8b1 /usr/share/wordlists/rockyou.txt

Click for answer

qwerty123