Skip to content
This repository has been archived by the owner on Feb 15, 2019. It is now read-only.

Latest commit

 

History

History
81 lines (49 loc) · 5.11 KB

index.md

File metadata and controls

81 lines (49 loc) · 5.11 KB
Raw

AES and Encryption Modes

AES is a widely used cipher with no known vulnerabilities. But, like any block cipher, it needs to be used with a block encryption mode. As Wikipedia states:

A block cipher by itself is only suitable for the secure cryptographic transformation (encryption or decryption) of one fixed-length group of bits called a block. A mode of operation describes how to repeatedly apply a cipher’s single-block operation to securely transform amounts of data larger than a block.

You will need pic_original.bmp from CourseSpaces.

Learning Objectives

  • Apply secret-key cryptography using openssl and different ciphers
  • Describe differences between operating modes w.r.t. confidentiality
  • Describe differences between operating modes w.r.t. integrity
  • Describe the PKCS5 padding scheme and the operating modes that need padding

Part 1: Encryption using different ciphers and modes

In this task, we will experiment with various encryption algorithms and modes. You will be using the openssl tool for this purpose. Use the openssl enc command to encrypt/decrypt a file. To see the manuals, you can type man openssl and man enc. Below is an example command:

openssl enc ciphertype -e -in plain.txt -out cipher.bin -K 00112233445566778899aabbccddeeff -iv 0102030405060708

Please replace the ciphertype with a specific AES cipher type, such as -aes-128-cbc, -aes-128-cfb, and -aes-128-ofb. In this task, you should try at least 3 different ciphers and three different modes. you can find the meaning of the command-line options and all the supported cipher types by typing man enc. Below are some common options for the openssl enc command:

Option Description
-in input file
-out output file
-e encrypt
-d decrypt
-K/-iv key/iv in hex is the next argument
-[pP] print the iv/key (then exit if -P)

Part 2: Encryption Mode - ECB vs. CBC

The file pic_original.bmp contains a simple picture. We would like to encrypt this picture so that people without the encryption keys cannot know what is in the picture. Please encrypt the file using the ECB (Electronic Code Book) and CBC (Cipher Block Chaining) modes.

Let us treat the encrypted pictures as a picture, and use a picture viewing software such as eog <filename> to display it. However, for the .bmp file format, the first 54 bytes contain the header information about the picture. We have to set the header correctly so that the encrypted file can still be treated as a legitimate .bmp file.

We will replace the header of the encrypted picture with that of the original picture. You can use the hexedit <filename> tool to directly modify binary files. Use the F2 button to save when you are done, or Ctrl+X to save and exit. The original picture's 54 byte picture header should look something like this:

42 4D E4 54  02 00 00 00  00 00 3E 00  00 00 28 00
00 00 44 04  00 00 43 04  00 00 01 00  01 00 00 00
00 00 00 00  00 00 80 3D  00 00 80 3D  00 00 00 00
00 00 00 00  00 00

  • Note: Instead of entering it in manually, you may be able to copy-paste the following into hexedit if you remove all spaces and newlines in the hex stream and right-click paste at the first byte:

    424DE4540200000000003E000000280000004404000043040000010001000000000000000000803D0000803D00000000000000000000

Question 1: Display the encrypted pictures using any picture viewing software. Can you derive any useful information about the original picture from the encrypted picture? Please explain your observations.

Part 3: Encryption Mode - Corrupted Cipher Text

Question 2: Before doing Part 3, how much information do you expect to recover by decrypting the corrupted file, if the encryption mode is ECB, CBC, CFB, or OFB, respectively? Please explain why.

To understand the properties of various encryption modes, do the following:

  1. Create a text file that is at least 64 bytes long.
  2. Encrypt the file using the AES-128 cipher.
  3. Unfortunately, a single bit of the 30th byte of the encrypted file got corrupted. You can achieve this corruption with hexedit.
  4. Decrypt the corrupted file (encrypted) using the correct key and IV.

Question 3: After doing Part 3, did your expectations from question 2 align with the results? What are the implications of these encryption mode differences?

Part 4: Padding

For block ciphers, when the size of the plaintext is not the multiple of the block size, padding may be required. In this task, we will study padding schemes.

Task 1: The openssl manual says that it uses the PKCS5 standard for its padding. Please design an experiment to verify the existence of padding. In particular, use your experiment to figure out the paddings in the AES encryption when the length of the plaintext is 20 bytes and 32 bytes.

Task 2: Please use ECB, CBC, CFB, and OFB modes to encrypt a file (you can pick any cipher).

Question 4: Please report the findings of your experiment.

Question 5: Which operating modes require padding? Why do the others not require padding?

Submission

You will be submitting one file:

  • report.txt Your answers to the 5 questions