have you heard of pgp, a well known encryption software? As their
manual says ..

"With PGP..., you can protect the privacy of your e-mail messages and
files by encrypting them so that only the intended recipients can read
them. You can also digitally sign messages and files, which ensures
their authenticity. A signed message verifies that the information
within it has not been tampered with in any way."

raqueeb hassan
bangladesh