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exercises-aes-encrypt-decrypt.md

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Exercises: AES Encrypt / Decrypt

In this exercise we shall encrypt and decrypt a text message using a symmetric cipher AES-CBC-256, combined with Scrypt password-to-key derivation and HMAC message authentication code. In fact we shall implement a password-based symmetric authenticated encryption scheme.

Symmetric Encryption (AES + Scrypt + HMAC)

Write a program to encrypt a text message using given password. Use the following steps:

  • Derive a 512-bit key from the password using Scrypt (n=16384, r=16, p=1) with random salt (128 bits).
    • Split the derived key into two 256-bit sub-keys: encryption key and HMAC key.
  • Pad the input message using the PKCS7 algorithm to length, which is multiple of 16 bytes (128 bits).
  • Encrypt the padded message using AES-256-CBC using the encryption key. The obtained result is the ciphertext. Its length should be a multiple of 16 bytes (128 bits), which is the block size in the AES cipher.
    • Use a randomly generated 128-bit initial vector (IV).
  • Calculate message authentication code (MAC) using HMAC-SHA256(hmac_key, ciphertext).

Input: message + password (space separated).

Output: JSON document (see the example below), holding the following assets:

  • The Scrypt randomly-generated salt (in hex format).
  • The randomly-generated iv (in hex format), used for the AES cipher.
  • The encrypted message ciphertext (in hex format) from the AES cipher.
  • The message authentication code - mac (in hex format).

Write your code in programming language of choice.
Input Output
p@sSw0rd~123 SecretMsg {"salt": "9757a3a22a9937ca0e0f2b5f2a4a11b4", "iv": "2ce8c035d50f7a6ee6509c14fe11725a", "ciphertext": "bb435d8ad048c240b50f0e4a191605d9", "mac": "02cf870ad1f7453c339dac06edbd648c455f5e8abbf6f2716cbc2d164b644200"}
stupid!pass longer-message-for-encryption {"salt": "b243f0ac10ef358ff0d37f1e30ef19c2", "iv": "fdeff97e89705289d99751f079e2a308", "ciphertext": "ea76bc60799c5824627a8c1276b48ab70e24011b6654f8ffb019a4f6876485af", "mac": "34085e1a47ae53e154b7466336efee386c2f1ed61a0105183ef016af794da58f"}

Note that the above input will be different in your case, bеcause of the randomly generated salt and iv.

Symmetric Decryption (AES + Scrypt + HMAC)

Write a program to decrypt an encrypted message (coming as input) using given password.

  • Derive a 512-bit key from the password using Scrypt (n=16384, r=16, p=1) with the salt (from the JSON).
    • Split the derived key into two 256-bit sub-keys: encryption key and HMAC key.
  • Calculate message authentication code (MAC) using HMAC-SHA256(hmac_key, ciphertext).
    • Compare the MAC with the MAC in the JSON document.
    • Same MAC means "correct password / successful decryption".
    • Different MAC means "wrong password / incorrect input data".
  • Decrypt the ciphertext from the input using AES-256-CBC using the encryption key and the IV from the JSON.
  • Unpad the decrypted message using the PKCS7 algorithm from length, which is multiple of 16 bytes (128 bits) to its original length (usually smaller).

Input: password + JSON (space separated). The JSON is in exactly the same format, like in the output from the previous exercise (it holds salt, iv, ciphertext and mac, all as hex numbers)

Output: Decrypted: + the original decrypted message or the text Decryption failed! in case or wrong password or other problem.

Write your code in programming language of choice.
Input Output
p@sSw0rd~123 {"salt": "9757a3a22a9937ca0e0f2b5f2a4a11b4", "iv": "2ce8c035d50f7a6ee6509c14fe11725a", "ciphertext": "bb435d8ad048c240b50f0e4a191605d9", "mac": "02cf870ad1f7453c339dac06edbd648c455f5e8abbf6f2716cbc2d164b644200"} Decrypted: SecretMsg
wrong!pass {"salt": "9757a3a22a9937ca0e0f2b5f2a4a11b4", "iv": "2ce8c035d50f7a6ee6509c14fe11725a", "ciphertext": "bb435d8ad048c240b50f0e4a191605d9", "mac": "02cf870ad1f7453c339dac06edbd648c455f5e8abbf6f2716cbc2d164b644200"} Decryption failed!