Encrypted text question

Hello! I’m new here and I dont even know if I’m at the right place. I have a question about encrypted text. The thing I want to know is… let’s say someone sends me encrypted text
Example:

2CUInbU28fz5viYV0FZlctOgo5IPTAHK4sjhu59s0xlvG0gGE7SGpyNGsmlbMULFdVMyPLPP93cE3javfT4vhH3AUeCy1d541HpGi8lh1oMRzMdEx81EES6oYxN

(Encrypted with thedecryptor website)

And that someone still has the key to the encrypted text. Now let’s say that the text says “Hello123”. Now I only have the encrypted text saved that reads “Hello123”. Is there a way for that someone when he sends me the key for the encrypted text to change the content somehow something like… when i decrypt the text with the key is says something other that “Hello123”? We’re talking about 32 character long key.
Or is there a way to decrypt it without the key using a website or something?
Please answer and thank you!

Unspecific to Whonix: → Potential Solutions Beyond Whonix!

Needs to be resolved as per Self Support First Policy for Whonix.