> If a ciphertext is not authenticated, it can be trivially tampered with. Shouldn't it be impossible to forge a plaintext without the key for a good encryptio

> If a ciphertext is not authenticated, it can be trivially tampered with. Shouldn't it be impossible to forge a plaintext without the key for a good encryption algorithm? I imagine a good algorithm not to be just key -> pseudorandom stuff that is XORed with data, but something that has cascading. Change a bit anywhere, and a whole block changes unpredictably. Include the physical position of the block in the key, so that it impossible to copy blocks around to duplicate data.