ACE414 Security of Systems and Services

Assignment 2

Theodoraki Emmanouela

Implementation of AES cryptographuc algorithm in ECB mode with both 128, 256 bit modes, using OpenSSL toolkit in C.

Tool Usage:

command: ./assign_2 -i in_file -o out_file -p password -b bits [-d | -e | -s | -v]

Options: -i path Path to input file -o path Path to output file -p psswd Password for key generation -b bits Bit mode (128 or 256 only) -d Decrypt input and store results to output -e Encrypt input and store results to output -s Encrypt+sign input and store results to output -v Decrypt+verify input and store results to output -h This help message

A. Key Derivation Function

First, the main function provides the password and the bits mode obtained by the user on build and declares the key and key_length. The function keygen(password, key, key_len, iv, bit_mode) generates a key based on the given password with the help of EVP_BytesToKey() function after setting a cipher for both cases of aes-128-ecb and aes-256-ecb and the digest method is set to SHA1. The derived key is printed at the end in hex.

B. Data Encryption

Input file is read and stored as “plaintext” buffer through the readFromFile function. The function encrypt() takes the plaintext, encrypts it with the key derived at Task A for aes-128-ecb and aes-256-ecb mode and stores it to the ciphertext buffer using the EVP API functions:

EVP_CIPHER_CTX_new() EVP_EncryptInit_ex() EVP_EncryptUpdate() EVP_EncryptFinal_ex() EVP_CIPHER_CTX_free()

The encrypted message (ciphertext) is stored to the output file through the function writeToFile().

C. Data Decryption

Here the implementaion of Task B is reversed. We give an encrypted message (ciphertext) as input file in order to decrypt it, giving the initial message (plaintext) and store it to an output file. Functions of EVP API used:

EVP_CIPHER_CTX_new() EVP_DecryptInit_ex() EVP_DecryptUpdate() EVP_DecryptFinal_ex() EVP_CIPHER_CTX_free()

D. Data Signing (CMAC)

At the option of signing, input file is read for encryption and saved as plaintext, while the encrypted data are saved as ciphertext. Then a CMAC is generated through the function gen_cmac() using the functions:

CMAC_CTX_new() CMAC_Init(ctx, key, key_len, cipher_mode, NULL) CMAC_Update(ctx, data, data_len) CMAC_Final(ctx, cmac, cmac_len) CMAC_CTX_free(ctx). Continuing, the generated CMAC and the ciphertext concatenate through the function concat_CMAC_cipher and the result is stored to the output file.

E. Data Verification (CMAC)

In the verification stage, the input file has the concatenated CMAC and cipher provided by the signing. So, the input splits to obtain the ciphertext and CMAC separately. Since we have the ciphertext, we can now decrypt it using the process of Task C and obtain the plaintext. Afterwards, a new CMAC is generated by the process of Task D and used for comparison with the CMAC found on separation of the concatenated input. If the two CMACs match, the verification has succeeded and the plaintext can be stored to the output file. The match is checked through the function strcmp().

F. Using the tool

1. Encryption of file “encryptme_256.txt” using the password: TUC2014030238 and output file name “decryptme_256.txt”.

   Key:
   79 78 F2 F9 31 4C 06 E0 B7 93 C3 5E 70 AC 08 70 
   AB 27 1B 9A 31 BC 4C 2B C4 C4 46 77 1C AA D9 54 
   Message to encrypt (plaintext): 
   Encrypt this file using  a 256-bit key.
   The key should be generated using TUC<AM> as password.
   For example, if your AM is 2020123456 the password should be TUC2020123456.
   
   Encrypted message (ciphertext): 
   40 19 F1 CF C7 3F E4 33 47 22 83 D8 B4 97 C5 C7 
   FC DF 52 16 63 74 0A 40 3F 6F 2D 7E E5 19 89 D6 
   19 77 01 19 ED 18 12 33 B7 30 23 E0 24 20 86 4A 
   FC A3 B9 30 A5 68 B9 29 F3 23 4F 24 DC 6A DF 50 
   0A 7A 9D 9D F6 0B BE E0 26 7F 37 40 4A CA FC 12 
   B6 F2 75 53 5F 84 7F 1A 1D 73 F4 3A AA 1D E1 4A 
   F4 F3 AB E9 86 DE E3 5D 17 DE E6 D4 02 A2 52 0E 
   F2 9C 71 8B 26 FB B3 24 B7 09 C2 ED F6 06 C7 3C 
   67 79 E5 D0 AE EE 61 CC FE 2C FE F5 07 E7 7D A8 
   89 5E 0A D6 19 E1 3C 76 69 D3 58 31 76 59 07 C6 
   08 09 B1 DA 6B 1C A1 EE 1D BC EF 22 A1 78 64 C4 
   

2. Decryption of file “hpy414_decryptme_128.txt” using a 128-bit key derived by the password “hpy414” and output filename “hpy414_encryptme_128.txt”.

   Key:
   8D 3B 78 B5 73 3F 18 9D 72 1A A1 52 48 6A B7 EE 
   Message to decrypt (ciphertext): 
   A7 22 4B B8 66 17 E2 A2 06 EA 33 E3 34 7E 94 1C 
   FC 61 9E 38 71 F8 66 8C 1B 88 24 15 0A 88 CC 29 
   A9 14 7E 3E FF 10 3B C4 D2 B9 9B 91 93 04 CC 15 
   7F 4E 49 7C 54 1F 16 80 2D E1 FE C0 09 6E 8E 7F 
   Decrypted message (plaintext): 
   Hello HPY414
   
   This file was encrypted with a 128-bit key
   
   BB!
   

3. Sgning the file “signme_128.txt” using a 128-bit key derived from the password TUC2014030238 and output filename to store the ciphertext concatenated with its CMAC in a file named “verifyme_128.txt”.

   Key:
   79 78 F2 F9 31 4C 06 E0 B7 93 C3 5E 70 AC 08 70 
   CMAC: 
   E5 51 EA F3 06 7E 19 2B 06 B7 2A B5 5B 15 8F 3A 
   

4. Verification of the files “hpy414_verifyme_256.txt” and “hpy414_verifyme_128.txt” using the appropriate key size, as the filename specifies. The keys derived by the password “hpy414”.

   for “hpy414_verifyme_128.txt”
   Key:
   8D 3B 78 B5 73 3F 18 9D 72 1A A1 52 48 6A B7 EE 
   CMAC1: 
   A6 18 94 13 55 F2 6B D4 77 C0 CF AF C3 B7 CD A1 
   CMAC2: 
   66 4C 57 6F 54 4B DC 7B 8E 10 FC F2 E2 4C BD B5 
   [FALSE] Verification failed. CMACs do not match.