Malleability is a physical property of matter, signifying its capability of deformation, especially by hammering or rolling. Malleability is an important issue in for example stamping and form pressing materials such as metals and plastics.
Malleable is a term used in the analyses of cryptographic algorithms:
A malleable encryption algorithm allows transformations on the ciphertext to produce meaningful changes in the plaintext. That is, given a plaintext P and the corresponding ciphertext C = E(P), it is possible to generate C1 = f(C) so that the decryption of C1 is a function P1 = f'(P) of the original plaintext P, with arbitrary but known functions f and f'. Stream ciphers are examples of malleable encryption algorithms.