In symmetric encryption, what does the key relationship look like and what security property does it provide?

In symmetric encryption $K_E = K_D$ — the same secret key is used for both encryption and decryption, providing only confidentiality (not integrity).

How it works:

• Alice and Bob share a secret key $K$
• Alice encrypts: $c = E(K, m)$
• Bob decrypts: $m = D(K, c)$

Provides: Confidentiality (eavesdropping protection only)

Does NOT provide:

• Data integrity — ciphertext can be modified without detection
• Authentication — anyone who knows $K$ can encrypt
• Non-repudiation — both parties know $K$

Critical misconception: Many people assume encryption = security. But encryption without integrity checking is dangerous. An attacker can flip bits in the ciphertext, which causes predictable changes in the plaintext (especially with stream ciphers or CTR mode). This is why modern protocols always combine encryption with a MAC (authenticated encryption).

Go deeper:

• Symmetric-key algorithm — the shared-key model and its confidentiality-only guarantee.