Expanding randomness. Alice wants to implement AES in CTR mode on a small IoT device that doesn’t have a good source of randomness. She has observed that, in practice, she can obtain four random bits for each encryption. To produce the 64 bits of nonce needed for each encryption, she proceeds as follows. She takes the SHA256 of the four random bits. Then she uses the first 64 bits of the SHA256 output as the nonce. Is this method secure? Explain your reasoning. (Hint: consider what happens if the adversary tries to guess the nonce. How many possibilities are there? Would guessing the nonce help them decrypt the message?)
Expanding randomness. Alice wants to implement AES in CTR mode on a small IoT device that doesn’t have a good source of randomness. She has observed that, in practice, she can obtain four random bits for each encryption. To produce the 64 bits of nonce needed for each encryption, she proceeds as follows. She takes the SHA256 of the four random bits. Then she uses the first 64 bits of the SHA256 output as the nonce. Is this method secure? Explain your reasoning. (Hint: consider what happens if the adversary tries to guess the nonce. How many possibilities are there? Would guessing the nonce help them decrypt the message?)
Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
Section: Chapter Questions
Problem 1PE
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Question
Expanding randomness. Alice wants to implement AES in CTR
mode on a small IoT device that doesn’t have a good source of randomness.
She has observed that, in practice, she can obtain four random
bits for each encryption. To produce the 64 bits of nonce needed for
each encryption, she proceeds as follows. She takes the SHA256 of the
four random bits. Then she uses the first 64 bits of the SHA256 output
as the nonce. Is this method secure? Explain your reasoning. (Hint:
consider what happens if the adversary tries to guess the nonce. How
many possibilities are there? Would guessing the nonce help them
decrypt the message?)
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