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**).

- 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.

{"**salt**": "9757a3a22a9937ca0e0f2b5f2a4a11b4", "**iv**": "2ce8c035d50f7a6ee6509c14fe11725a", "**ciphertext**": "bb435d8ad048c240b50f0e4a191605d9", "**mac**": "02cf870ad1f7453c339dac06edbd648c455f5e8abbf6f2716cbc2d164b644200"}

{"**salt**": "b243f0ac10ef358ff0d37f1e30ef19c2", "**iv**": "fdeff97e89705289d99751f079e2a308", "**ciphertext**": "ea76bc60799c5824627a8c1276b48ab70e24011b6654f8ffb019a4f6876485af", "**mac**": "34085e1a47ae53e154b7466336efee386c2f1ed61a0105183ef016af794da58f"}

Note that the above input will be different in your case, bÐµcause 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).

`Decrypted:`

+ the original `Decryption failed!`

in case or wrong password or other problem.Write your code in programming language of choice.

Decrypted: SecretMsg

Decryption failed!

Last modified 7mo ago