Adding unsigned 256 bit integers

I was wondering if there would be any way to do the following in PostgreSQL:

    UPDATE cryptotable SET work = work + 'some big hexadecimal number'

where work is an unsigned 256 bit integer. Right now my column is a character varying(64) column (hexadecimal representation of the number) but I would be happy to switch to another data type if it lets me do the operation above.

If it's not possible with vanilla PostgreSQL, are there extensions that could help me?

- Oli

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