One Time Pad - Authentication problem

• The oponent can change the CipherText, without the recipient's knowledge, keeping the message “logical”.

• Example:

  • Assuming message structure from a bank to ATM machine is:
  • If we toggle bit no. 120 in the cipher text, the decrypted balance value will change. A 16.7 milion NIS change...

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Another example:

- Software liscence expiry date which is kept encrypted like that in a specific place on the HD.

- All we have to do is toggle the bit of the year in this date.

Solution:

Digital Signatures.