This is part of modern encryption. To be more specific, we have blocks and a deterministic cryptographically secure rng, we seed the rng with a public key that is broadcasted, we then encrypt blocks and then combine them together with the number produced by the rng and create the next block of encrypted data.
You may be wondering, why do we even do this? The answer is simple, "the image is encrypted, but everyone can see the penguin". In a less cryptic manner, encrypting blocks simply maps them from one space to another, but is a reversible operation and, as any function does, is deterministic and returns the exact same value every time. The consequence of this determinism, is that identical blocks will always produce the same value and thus, the content is still somewhat visible and in some cases it could leak information. By combining blocks together along with a PRNG, we hide the mapping by requiring that the recipient solves the first block before decrypting further data and we don't end up leaking information.
You may be wondering, why do we even do this? The answer is simple, "the image is encrypted, but everyone can see the penguin". In a less cryptic manner, encrypting blocks simply maps them from one space to another, but is a reversible operation and, as any function does, is deterministic and returns the exact same value every time. The consequence of this determinism, is that identical blocks will always produce the same value and thus, the content is still somewhat visible and in some cases it could leak information. By combining blocks together along with a PRNG, we hide the mapping by requiring that the recipient solves the first block before decrypting further data and we don't end up leaking information.