If you ignore the prepend part, it is in essence taking known input and calculate a collision.
> This is an identical-prefix collision attack, where a given prefix P is extended with two distinct near-collision block pairs such that they collide for any suffix S.
And
> Our example colliding files only differ in two successive random-looking message
blocks generated by our attack. We exploit these limited differences to craft two colliding
PDF documents containing arbitrary distinct images.
> This is an identical-prefix collision attack, where a given prefix P is extended with two distinct near-collision block pairs such that they collide for any suffix S.
And
> Our example colliding files only differ in two successive random-looking message blocks generated by our attack. We exploit these limited differences to craft two colliding PDF documents containing arbitrary distinct images.