Question

To encrypt 3 KB of plain-text using a One Time Pad encryption algorithm, how long a truly random key would be required?

Answer 1

One Time Pad Encryption algorithm needs a Random key at least as the Message or the data that is to be encrypted. Here the message is given as 3 Kb of plain text.

But the length of key depends on the characters not on the size. For example if the 3 Kb plain text file is encoded using

(i)ASCII: The message file of 3Kb has (3*1000)=3000 or  characters , so a random key of atleast 3000 characters is needed

(ii)UCS-4 encoding:There can be nearly 3000/4 = 750 characters accommodated in a 3Kb unicode UCS-4 encoded file. So the Random key has to be atleast of 750 characters

(iii) UTF-8 encoding: This is a more complex encoding scheme where we cannot simply draw an approximation. It varies according to the language in which the message is present. As the mandarin may have more than 1 lakh characters and Japanese has more than 4000 characters in alphabet. So it can take 8 bytes to store a character sometimes

Similarly UTF 16 encoding can take 4 bytes to store a character

Therefore a Plain text file of 3 KB can vary from 400 to 3000 characters long depending on encoding