Why does encryption NOT protect against modification of data?

Encryption (Verschlüsselung) only prevents unauthorized reading — it does NOT prevent an attacker from modifying the ciphertext, which will decrypt to corrupted or attacker-controlled plaintext.

A core principle of applied cryptography, memorably stated in German as "Verschlüsseln schützt nicht vor Veränderung!" — encryption does not protect against modification.

Why this matters:

• An attacker can flip bits in ciphertext without knowing the plaintext
• The recipient will decrypt the modified ciphertext to different, corrupted plaintext
• In some cipher modes (like CTR or stream ciphers), bit-flipping attacks allow targeted modifications — e.g., changing "transfer $100" to "transfer $900"

What you need instead:

• For integrity (detecting changes): use a hash function or MAC (Message Authentication Code)
• For authenticity (proving origin + integrity): use a digital signature or HMAC
• Best practice: Encrypt-then-MAC — encrypt first, then compute a MAC over the ciphertext

Tip: This is one of the most dangerous misconceptions in applied cryptography. Many real-world vulnerabilities (like padding oracle attacks) exploit the fact that developers assumed encryption provided integrity.