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client- enters company name

server- responds with random number

client- receives random number and uses it to create a string from a text file on their local computer.
client then encrypts and sends back.

server has same text file for that client and has encrypted and loaded to database ready to compare with client response.

I want to know how to open local file from client machine to read. I want to know for example how to start at character x and read x number of characters to return as string to encrypt before sending back.

I know very little of javascript

Thanks