Describe how an attacker can obtain the one-time pad that is used to encrypt a message
CSCI 455: Computer Security
Spring 2019
Homework Assignment 2
Problem 1
a) Describe how an attacker can obtain the one-time pad that is used to encrypt a message, given both the message and the ciphertext, and explain why your method works.
b) Suppose that two equal-sized messages M1 and M2 are encrypted with the same one-time pad and let C1 and C2 be the resulting ciphertexts. Suppose further that an attacker captures both ciphertexts C1 and C2, and knows one of the two messages, say M1. Based on Part a), describe how the attacker can obtain the other message M2, and explain why your method works.
c) Let’s think about a more realistic situation than part b) that the attacker captures both ciphertexts C1 and C2, but does not know message M1. Based on part b), can you provide an example showing that the attacker can still obtain the message M2 ? You can do research online and find approaches. By answering this question, you can have an idea on how an attacker can do in practice and why the one-time pad cannot be used more than once.
Hint
Management "b) Since two equal sized messages (M1 amd M2) are encrypted with same OTP and same stream ciphertect (C1 and C2),Here, C1 xor C2 results in M1 xor M2.The messages can be recovered using a technique known as crib dragging. One (attacker) can take a common word or phrase that may appear in the message (such as "" the "") and xor that against the result of M1 xor M2, and if one of th...
"b) Since two equal sized messages (M1 amd M2) are encrypted with same OTP and same stream ciphertect (C1 and C2),
Here, C1 xor C2 results in M1 xor M2.
The messages can be recovered using a technique known as crib dragging. One (attacker) can take a common word or phrase that may appear in the message (such as "" the "") and xor that against the result of M1 xor M2, and if one of the messages had the text of the crib ("" the "" in our example), then the result of the xor is what the other message had in that position. If neither message contains the text of the crib, it is very likely that the result of the xor is just gibberish."
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