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Is there any way to encrypt the whole Input data into 16 to 24 character alphanumeric string and again able to decrypt it into normal string in c#?
Yes, only if input is no more than 24 characters alphanumeric string.
Beware, with utf-8, some characters are encoded on more than an 8 bits char.
If your question imply changing the size of input to less than 24 chars, it is compression. And nothing can guaranty a given output size.
Basically, encryption does not change the size of input, or makes it bigger with bloc cypher, because input is expanded to multiple of bloc size.