Symmetric Encryption

Overview 
 Symmetric encryption, or secret-key encryption, is a fundamental cryptographic method where the same key (the secret key) is used for both encryption (converting plaintext to ciphertext) and decryption (recovering the plaintext from the ciphertext). The communication parties must agree upon and securely exchange this secret key beforehand. 
 
 Key Information 
 Terminology : 
 
 Cryptographic Algorithm or Cipher This algorithm defines the encryption and decryption processes. 
 Key The cryptographic algorithm needs a key to convert the plaintext into ciphertext and vice versa. 
 plaintext is the original message that we want to encrypt 
 ciphertext is the message in its encrypted form 
 
 A symmetric encryption algorithm uses the same key for encryption and decryption. 
 
 
 
 Encryption Algorithm 
 Notes 
 
 
 
 
 AES, AES192, and AES256 
 AES with a key size of 128, 192, and 256 bits 
 
 
 IDEA 
 International Data Encryption Algorithm (IDEA) 
 
 
 3DES 
 Triple DES (Data Encryption Standard) and is based on DES. We should note that 3DES will be deprecated in 2023 and disallowed in 2024. 
 
 
 CAST5 
 Also known as CAST-128. Some sources state that CAST stands for the names of its authors: Carlisle Adams and Stafford Tavares. 
 
 
 BLOWFISH 
 Designed by Bruce Schneier 
 
 
 TWOFISH 
 Designed by Bruce Schneier and derived from Blowfish 
 
 
 CAMELLIA128, CAMELLIA192, and CAMELLIA256 
 Designed by Mitsubishi Electric and NTT in Japan. Its name is derived from the flower camellia japonica. 
 
 
 
 
 Notes 
 Popular tools for symmetric encryption: 
 
 GNU Priacy Guard : The GNU Privacy Guard, also known as GnuPG or GPG, implements the OpenPGP standard. 
 OpenSSL Project : The OpenSSL Project maintains the OpenSSL software. 
 
 GNU Privacy Guard 
 
 Command to ecnrypt gpg --symmetric --cipher-algo CIPHER message.txt 
 Ascii armorded output gpg --armor --symmetric --cipher-algo CIPHER message.tx 
 command to decryp gpg --output original_message.txt --decrypt message.gpg 
 
 OpenSSL Project 
 
 command to encrypt openssl aes-256-cbc -e -in message.txt -out encrypted_message 
 command to decrypt openssl aes-256-cbc -d -in encrypted_message -out original_message.txt 
 Password-Based Key Derivation Function 2 openssl aes-256-cbc -pbkdf2 -iter 10000 -e -in message.txt -out encrypted_message 
 
 
 Task 
 
 Decrypt the file quote01 encrypted (using AES256) with the key s!kR3T55 using gpg . What is the third word in the file?
 
 gpg --output quote1.txt --decrypt quote01.txt.gpg 
 Third Word waste 
 
 
 Decrypt the file quote02 encrypted (using AES256-CBC) with the key s!kR3T55 using openssl . What is the third word in the file?
 
 openssl aes-256-cbc -d -in quote02 -out quote2 
 Third Word science 
 
 
 Decrypt the file quote03 encrypted (using CAMELLIA256) with the key s!kR3T55 using gpg . What is the third word in the file?
 
 gpg --output quote3.txt --decrypt quote03.txt.gpg 
 Third Word understand 
 
 
 
 
 Conclusion 
 Symmetric encryption is a cryptographic method where a single secret key is used to encrypt plaintext into ciphertext and decrypt it back. While historical algorithms like DES (56-bit key) were broken, modern standards like AES (128/192/256-bit keys) remain secure and provide confidentiality, integrity, and authenticity. Popular implementations include GnuPG (GPG) and OpenSSL. Despite its security benefits, symmetric encryption suffers from a scalability problem because the number of required keys grows quadratically with the number of users, making it impractical for large-scale key distribution. 
 
 Resources 
 
 TryHackMe : Intro to Cryptography 
 OpenSSL Project : OpenSSL 
 GNU Privacy Guard : GPG