[littlestymaar]

> Isn't this a big assumption?

The standard resilience criteria for modern multi-purpose encryption suppose that your scheme should be resistant to adaptive chosen-cipher attack. Chosen plaintext is a way weaker attack (the hierarchy being: known plaintext < chosen plaintext < chosen cipher < adaptative chosen cipher).

It may be OK for some situations, but it requires to be much more cautious than with regular crypto (which is already error-prone…).