Crypto Lab 1 - Using Symmetric keys

Symmetric encryption, also known as shared secret encryption, uses the same key for both encryption and decryption. If one user encrypts a message with a secret key, the recipient must use the same key to decrypt it.

Let’s walk through an example of symmetric key encryption on the Linux command line involving two users, mikey and bart, using gpg . In this case the shared key is the passphrase used to encrypt and decrypt.

Please refer to pre-requisite for this blog to ensure that you have two users setup on Linux.

Scenario

• mikey wants to send an encrypted file to bart using a shared passphrase.
• Both mikey and bart will use the same symmetric key (passphrase) for encryption and decryption.
• The file to be encrypted is turing_bio stored at /srv/shared

head turing_bio

The file contains a paragraph from the wikipedia page for Alan Turing

Alan Mathison Turing was an English mathematician, computer scientist, logician, cryptanalyst, philosopher and theoretical biologist.

mikey’s Side (Encrypting the File)

mikey encrypts the file using a symmetric key with gpg:

gpg --symmetric --cipher-algo AES256 turing_bio

This command will prompt mikey to enter a passphrase. The result is a file called turing_bio.gpg created in the same folder

ls 

turing_bio  turing_bio.gpg

Checking contents of the file reveals that contains encrypted text

head turing_bio.gpg 

�   ��������z���%�׈�?�

bart’s Side (Decrypting the File)

bart decrypts the file using the same passphrase:

gpg --output turing_bio_decrypted --decrypt turing_bio.gpg

This command will prompt bart to enter the passphrase that mikey used to encrypt the file.

bart can now read the decrypted message and check that it is identical to original

diff turing_bio turing_bio_decrypted