Using a SHA value as a key for encryption

Using a file's own SHA value is great for a encryption key because:

• It is a very good password
• You can check if the password is incorrect
• The data/checking is two sided

LINPUT "Password:"; PASSWORD$

'Insert decryption stuff here

IF SHA(PRG$) != PASSWORD$ THEN
 PRINT "Incorrect password!"
ENDIF

Here's a relatively simple image about the idea: