How to only encrypt a subset of the plaintext I was wondering if there is a smart way for a user to only encrypt a subset of a plaintext.

I'll try to be more specific. Let's suppose the user U wants to use a special cipher such that given a plaintext P and some additional information, it only encrypts the subset requested by the user, the rest of the plaintext is not modified.

Does it exist such a cipher? If so, where can I find it and how does it work? If not, how would you do design such a cipher?

Thanks

EDIT:
I've read about selective encryption but it looks like it's mainly meant for images and video streams
You can get what you want programmatically. No special ciphers or modes needed.

You say there is a single, continuous subset that needs to be encrypted. Thus, you could have a function where the programmer specifies the start of the portion of the plaintext that needs to be encrypted and the number of bytes to encrypt. The function could pull that part out, encrypt it (using a good block cipher and mode) into a buffer. Then write out the part preceding it, the ciphtertext, then the part following the portion that needs to be encrypted.

If you require integrity checks too, a simple HMAC of the plain-part-one + ciphertext + plain-part-two using a different key would do.