Ravindra BagaleCourses & study guides

33. Cryptography Basics

33.3 Symmetric Encryption

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Symmetric encryption uses one shared key to encrypt and decrypt. It is fast, so it protects bulk data.

  • AES (Advanced Encryption Standard) with 128 or 256-bit keys is the standard today. Modes matter: AES-GCM gives encryption plus integrity; avoid ECB mode, which leaks patterns.
  • ChaCha20-Poly1305 is a fast modern alternative, common on mobiles.
  • Old and unsafe: DES, 3DES, RC4.
echo "Kolhapur shop sales: 4,20,000" > secret.txt
openssl enc -aes-256-cbc -pbkdf2 -salt -in secret.txt -out secret.enc     # asks for a passphrase
openssl enc -d -aes-256-cbc -pbkdf2 -in secret.enc -out back.txt
gpg -c secret.txt                                                         # symmetric with GPG, creates secret.txt.gpg

The big problem with symmetric crypto is key exchange: how do two people who have never met share the key safely? Asymmetric crypto solves that.

Ravindra Bagale's Tip

Encryption strong asla tari key file sobat ch thevli tar kahi upyog nahi – jasa kulup lavun chavi darwajavar latkavli. Keys vegli theva: AWS KMS, Secrets Manager, password manager. Aani "maza swatacha encryption algorithm" kadhich banvu naka – tested standards (AES) vapra.

Lab

Encrypt a file with openssl enc -aes-256-cbc -pbkdf2, send the .enc file to a partner in class, and share the passphrase by a different channel (spoken, not in the same message). Ask them to decrypt it. Then try decrypting with a wrong passphrase and note the error.