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Lab 2: Cryptography
Date: 12/05/2021 Author: David Glance
- 1.Using CyberChef to explore encodings and encryption using operations and recipes
- 2.Understand different number representations such as binary and hexadecimal and character representations such as ASCII
- 3.Understand the use of shift ciphers, XOR and modern cryptography systems like AES and PGP
- ASCII, Base64 encoding
- Binary, Hexadecimal
- CyberChef
- Caesar Cipher
- XOR encryption
- AES
- PGP
CyberChef is a website that has been developed by the UK GCHQ a government cyber and security agency. It allows the simple chaining of a large number of different operations on inputs such as text to create an output. If we want to use the Caesar cipher to encode text for example, we can search for ROT13 in the operations, drag it into the Recipe window and simply type in the input window. The output will be transformed using the recipe automatically. In case you were wondering, RO T13 stands for rotate (Shift) by 13. The original Caesar cipher performed a shift of 3.
- 1.
- 2.Search for ROT13 and drag the operation to the Recipe window
- 3.Type "The quick brown fox" into the input window
- 4.Verify that the output is "Gur dhvpx oebja sbk"
We can now try some "cryptanalysis" albeit using the unsophisticated approach of brute force. You have intercepted some encrypted text:
Vogvizvetv zj kyv kvrtyvi fw rcc kyzexj zj r hlfkv wifd Alczlj Trvjri
You suspect it has been encrypted using a Caesar cipher but you need to find out the shift number. How would you find out?
Remember that to decrypt a Caesar cipher, you need to use a shift that is 26 - shift number
Flag: Enter the plaintext
We are going to use CyberChef to explore how text is represented by the ASCII system. ASCII stands for the American Standard Code for Information Interchange. Eac h character is represented by a number between 0 - 254. So the letter 'a' is 97 and the ltter 'z' is 122. You can find the full chart here http://www.asciitable. com/.
To convert text to ASCII, we can use to "To Decimal" recipe in CyberChef and then type the text "The quick brown fox". Your screen should look like this:
The ASCII in decimal is:
84 104 101 32 113 117 105 99 107 32 98 114 111 119 110 32 102 111 120
Remember that the space character is converted as well (to 32)
The binary representation of these numbers can be found using he "To Binary" operation:
Note: Normally ASCII is represented as Hexadecimal when programming
01010100 01101000 01100101 00100000 01110001 01110101 01101001 01100011 01101011 00100000 01100010 01110010 01101111 01110111 01101110 00100000 01100110 01101111 01111000
We can pick a key to encrypt our text using the XOR operation. Remember that XOR works as follows:
Bit 1 | Bit 2 | XOR Result |
0 | 0 | 0 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 0 |
Let us take the binary from the previous example and XOR it with a key which is 1 byte long 00000001
To XOR the key against the binary for "The quick brown fox", we can repeat the key until it is the correct length (19 characters = 19 bytes)
00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001 00000001
Try doing the XOR operation by hand and taking the binary output and converting it back to decimal and then to ASCII
There is an easier way of doing this of course, we can just use the XOR operation in CyberChef and take as the key the value 1. If you do this in CyberChef using "The quick brown fox", you will get
Uid!pthbj!csnvo!gny
The exclamation marks are the result of the space character being XOR'd.
Often the result of an XOR on ASCII characters will be non-printable characters. If we use the key 01111111 in binary or 127 in decimal, the output will be non-printable characters. You can see this by adding a second operation "To Decimal" to the recipe to see the actual ASCII decimal numbers. You will get
43 23 26 95 14 10 22 28 20 95 29 13 16 8 17 95 25 16 7
To handle this situation, we can encode the output into Base64 which is a binary-to-text encoding designed to carry binary data in text form.
If you replace the "To Decimal" operation with the "To Base64" operation in your recipe, you will get
KxcaXw4KFhwUXx0NEAgRXxkQBw==
Whenever you get text which ends in two "==" it is likely to be Base64 but remember that it doesn't always end this way.
To test that we are not making things up and that our encryption worked, we can take the Base64 string as an input and then create the recipe usng the operations:
- 1.From Base64
- 2.XOR with the key 127 in decimal
We should get our original input "The quick brown fox"
You are given the following Base64 encoded text
3PO19LX49Pb9/PvwtfzmtfDt5fD24fDxteH6tffwtfz78/T5+fz3+fC5tfzhtfb0+/v64bX0+eb6tffwtfz74fD5+fzy8Pvhuw==
You don't know the key but you can assume it is one byte long and it uses XOR as the algorithm. You are going to try and brute force it to find the key.
Use the operation "XOR Brute Force" in CyberChef to find the key in Hexadecimal. What was the original message?
Flag: Enter the plaintext
The Advanced Encryption Standard (AES) is the accepted standard block cipher that can use a variety of modes of operation. You can select AES Encrypt in CyberChef and input the text "Please keep this secret". Select CBC Mode and for the IV (Initialisation Vector) and HEX as the format and enter:
0102030405060708090a0b0c0d0e0f10
AES IV length is 16 bytes. The key length is:
- 16 bytes = AES-128
- 24 bytes = AES-192
- 32 bytes = AES-256
Enter the key in UTF8 format:
specialsecretkey
UTF8 is Unicode which expanded the number of bytes used to encode characters to 4 bytes (ASCII uses 1 byte). It is backwards compatible with ASCII but supports up to 1,112,064 characters and so supports Chinese and other languages.
The hex output of he encrypted text is:
0f1ee3bc7b64c9a266bb19aa3eb7e4a780bc1d201a444b3106d8e8ca4c7e7dc5
You can verify that you can decrypt this by adding the operation AES Decrypt to the recipe and use the same key and IV to decrypt.
An interesting thing to note is if you change a single character in the IV, you still get most of the output correct. Changing a single character in the key however will cause the decryption to fail.
You are given the following Base64 encoded text
9GUTUbeUQOm5inHgz+I2DcJ7MoBC3LyEuFzB6GcQSnuoWPGE9FazhYtEbdyomYmfJhjz18bdFm0bJPM6GQgg+clX+xE0/FN6D+wWDSndJII=
The key is snowdenquotation and the IV is 0102030405060708090a0b0c0d0e0f10
What is the decrypted text?
Flag: Enter the plaintext
Pretty Good Privacy (PGP) is an encryption program that uses public key cryptography to encrypt and digitally sign data. The following diagram illustrates how it works:
How PGP works (From Wikipedia: https://en.wikipedia.org/wiki/Pretty_Good_Privacy)
Note that we still use symmetric encryption to encrypt the main data because it is much faster than using the recipient's public key to encrypt.
To start with, you will need to generate public and private keys which you can do in CyberChef using the "Generate PGP Key Pair" operation. It is a good idea to put in your name and email address because it will be incorporated into the public key. This way, when you send someone your public key, this information will automatically be available to them making it easier to add your key to a Keyring and retrieve it later.
Note that for the Key type, there are several options RSA of various key lengths and ECC. ECC is Elliptic Curve Cryptography and uses smaller keys than RSA. Since it is a newer protocol, it may not be as widely supported as RSA is.
Select RSA-1024 as the Key type and enter your name and email address into the fields provided. The output will be a private and public key. Select the text for the private key and copy it to a file called private.key and save it in a folder on your computer. Take all of the text starting and ending with the comment lines:
-----BEGIN PGP PRIVATE KEY BLOCK-----
-----END PGP PRIVATE KEY BLOCK-----
Do the same with the public key, saving it to a file called public.key.
We are now going to send an encrypted and digitally signed email to Edward Snowden. We have been given his public and private key:
-----BEGIN PGP PRIVATE KEY BLOCK-----
Version: Keybase OpenPGP v2.1.15
Comment: https://keybase.io/crypto
xcEYBGCfS8kBBACZxxp9RsQ+pi+kZZ+AhhKVrQboxypxB8mPOnEsTUfobrq3xRdy
oBiOK5dhXSxJ8C7l/77gC8CnBCqLx6+bTgV12byvVLcMv20v+BLMn3bqyDuxnRdM
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AAP8DHhXXxJWhqgOU/wDr7XvmuChn03LVMgnYayBEfEgZCpajN14NcziwOCrEXOv
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CxNxWgm96a2+qk8mLxj7C2dgdm1pVTpdsn8vu/gSdl/jrrie1AaVhR03cAlG+Svv
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Ch4=
=Vslj
-----END PGP PRIVATE KEY BLOCK-----
-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: Keybase OpenPGP v2.1.15
Comment: https://keybase.io/crypto
xo0EYJ9LyQEEAJnHGn1GxD6mL6Rln4CGEpWtBujHKnEHyY86cSxNR+huurfFF3Kg
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Kwp5hrmJ5/y/ALOUCh4=
=BE7f
-----END PGP PUBLIC KEY BLOCK-----
Use the PGP Encrypt and Sign operation to encrypt the following text:
This is a secret email
Remember that you need to use
- 1.Snowden's public key to encrypt the message
- 2.Your private key to sign the message
Take the output which should look something like this:
-----BEGIN PGP MESSAGE-----
Version: Keybase OpenPGP v2.1.15
Comment: https://keybase.io/crypto
wYwDR0od+ZVXKhUBA/0aapS0kDJoo8nppiFll/knSuKzNvZI1IpIiekNbf/4KVoq
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gI3IWf+gdgln7qQmK2KxjTw=
=xO3N
-----END PGP MESSAGE-----
And use the operation PGP Decrypt and Verify to make sure that you can check the signature and decrypt the message. Remember to do this you need:
- 1.Snowden's private key to decrypt the message
- 2.Your public key to verify the signature
And that is it, you can now send encrypted and digitally signed messages to your friends, colleagues and family!
There are public servers where you can list your public key for others to use. You can also simply append it to your emails so that anyone wanting to send you encrypted messages can use that.
There are numerous command line and GUI-based software products that allow you to use PGP more easily. Some integrate with email clients as well.
Let's say you have the following PGP private and public key pair.
-----BEGIN PGP PRIVATE KEY BLOCK-----
Version: Keybase OpenPGP v2.1.15
Comment: https://keybase.io/crypto
xcEYBGDdpbUBBAC8XmTQUTvFOCDJ60yeGmGfyw1cpXUKSVjNJXGcwTerOX7KVZMN
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AAP9GrV1rPX/uBibyZWkAPzzn7EuXVcWj9VTko7/G39oEc6utCjLF3/0MnpPkD8c
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aMwZU4IVkW327+yEhnqMCvG7Nnc7i3VXvjNlqjfGEmiI+3S6C5OYl22QdwPh9ID9
macf5Hk=
=Cjcr
-----END PGP PRIVATE KEY BLOCK-----
-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: Keybase OpenPGP v2.1.15
Comment: https://keybase.io/crypto
xo0EYN2ltQEEALxeZNBRO8U4IMnrTJ4aYZ/LDVyldQpJWM0lcZzBN6s5fspVkw0n
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C5OYl22QdwPh9ID9macf5Hk=
=vGS8
-----END PGP PUBLIC KEY BLOCK-----
And you receive the following message from Edward Snowden.
-----BEGIN PGP MESSAGE-----
Version: Keybase OpenPGP v2.1.15
Comment: https://keybase.io/crypto
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6AyOXYaVYFFS0K9npykbfw==
=Epd7
-----END PGP MESSAGE-----
FLAG: What is the secret message that he sent to you and was it really him?
One of my friends sent me the following text and it looks horrible! They said they made it by combining multiple codes but I wasn't able to decode the message.
Can you retrieve the plaintext?
4qCo4qCo4qCo4qCo4qCk4qCA4qCo4qCo4qCo4qCk4qCkCuKgqOKgqOKgqOKgqOKgpOKggOKgpOKgpOKgpOKgpOKgqArioKjioKjioKjioKjioKjioIDioKjioKjioKjioKjioKQK4qCo4qCo4qCo4qCo4qCo4qCA4qCo4qCo4qCo4qCk4qCkCuKgqOKgqOKgqOKgpOKgpOKggOKgqOKgpOKgpOKgpOKgpArioKjioKjioKjioKTioKTioIDioKTioKTioKTioKTioKQK4qCo4qCo4qCo4qCk4qCk4qCA4qCk4qCk4qCk4qCk4qCkCuKgqOKgqOKgqOKgpOKgpOKggOKgqOKgqOKgqOKgpOKgpArioKTioKTioKjioKjioKjioIDioKTioKjioKjioKgK4qCk4qCo4qCo4qCo4qCo4qCA4qCo4qCo4qCo4qCk4qCkCuKgqOKgqOKgqOKgpOKgpOKggOKgpOKgpOKgpOKgpOKgpArioKjioKjioKjioKTioKTioIDioKTioKTioKTioKTioKQK4qCk4qCo4qCo4qCo4qCo4qCA4qCk4qCo4qCo4qCoCuKgqOKgqOKgqOKgpOKgpOKggOKgqOKgpOKgpOKgpOKgpArioKTioKjioKjioKjioKjioIDioKgK4qCo4qCo4qCo4qCo4qCk4qCA4qCk4qCk4qCo4qCo4qCoCuKgqOKgqOKgqOKgqOKgqOKggOKgqOKgqOKgpOKgqArioKTioKTioKjioKjioKjioIDioKjioKjioKjioKTioKQK4qCo4qCo4qCo4qCk4qCk4qCA4qCk4qCk4qCk4qCk4qCkCuKgpOKgqOKgqOKgqOKgqOKggOKgpOKgqOKgqArioKjioKjioKjioKTioKTioIDioKjioKjioKjioKTioKQK4qCo4qCo4qCo4qCo4qCo4qCA4qCo4qCo4qCk4qCoCuKgpOKgqOKgqOKgqOKgqOKggOKgqOKgqOKgqOKgpOKgpArioKjioKjioKjioKTioKTioIDioKjioKTioKTioKTioKQK4qCk4qCk4qCo4qCo4qCo4qCA4qCk4qCk4qCk4qCk4qCkCuKgqOKgqOKgqOKgqOKgpOKggOKgpOKgpOKgpOKgqOKgqArioKjioKjioKjioKTioKTioIDioKjioKjioKjioKTioKQK4qCk4qCk4qCo4qCo4qCo4qCA4qCo4qCo4qCk4qCk4qCkCuKgpOKgpOKgqOKgqOKgqOKggOKgqOKgqOKgqOKgqOKgpArioKjioKjioKjioKjioKTioIDioKjioKjioKjioKjioKgK4qCk4qCk4qCo4qCo4qCo4qCA4qCk4qCk4qCk4qCo4qCoCuKgqOKgqOKgqOKgqOKgqOKggOKgqOKgqOKgqOKgqOKgpArioKjioKjioKTioKTioKTioIDioKjioKTioKTioKTioKQK4qCo4qCo4qCo4qCo4qCk4qCA4qCk4qCk4qCk4qCk4qCkCuKgqOKgqOKgqOKgpOKgpOKggOKgqOKgpOKgpOKgpOKgpArioKjioKjioKjioKTioKTioIDioKjioKTioKTioKTioKQK4qCo4qCo4qCo4qCk4qCk4qCA4qCo4qCk4qCk4qCk4qCkCuKgpOKgpOKgqOKgqOKgqOKggOKgpOKgqOKgqA==
Flag: Enter the plaintext