Malleability

Updated 13 Aug 2026

•
Definition
- •
  the property (present in the one-time pad and stream ciphers generally) that an attacker can modify a ciphertext and produce a predictable, specific change in the resulting plaintext, without knowing the key or the plaintext at all
•
The attack
- •
  given $c = m \oplus k$ , an attacker computes $c' := c \oplus p$ for a chosen "patch" $p$ ; decrypting $c'$ yields $m \oplus p$ — the attacker fully controls the effect on the plaintext
•
Worked example
- •
  an intercepted (but unread) ciphertext is known to decrypt to an email starting "From: Bob"; the attacker XORs the ciphertext with "Bob" XOR "Eve" at the right offset, and the victim's decrypted email now reads "From: Eve" — the attacker never learned the key or needed to construct a valid ciphertext from scratch
•
Significance
- •
  stream ciphers and the one-time pad provide confidentiality only — zero integrity; a message can be silently, undetectably, and precisely tampered with in transit
- •
  this motivates adding a separate integrity mechanism (message authentication codes) on top of encryption, covered later in the course
•
Related
- •
  One-Time Pad
- •
  Stream Cipher

Linked references 4