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 — an attacker can alter the ciphertext in transit and Bob decrypts the altered version without noticing
• Authentication — decryption has no verification step: any bit-string decrypts to something, so Bob can't tell a ciphertext Alice made with $K$ from one an attacker injected or tampered with. Proving origin needs a keyed check value (a MAC), and encryption isn't one
• Non-repudiation — even with a MAC, Alice and Bob both hold $K$, so Bob can't prove to a third party that Alice (and not Bob himself) produced a message

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.