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Ruthie avatar Crypto

Is it possible to decrypt my message if encrypt it twice using different keys each time?

ke flag
Ruthie

Suppose I want to encrypt $mssg$ using One-Time-Pad, and I want the $mssg$ to be encrypted twice.

Once with $k_1$ and second with $k_2$

Is it still possible to detect my $mssg$?

lets say:

  • $c_1 = mssg \oplus k_1$
  • $c_2 = mssg \oplus k_2$

$c_1 \oplus c_2 = k_1 \oplus k_2$

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meshcollider avatar Crypto
gb flag
meshcollider

That is perfectly safe, there is no way you could recover the message from that as long as k1 and k2 truely are uniformly random and independent.

This is because both ciphertexts are perfectly secure, and xoring them would give you the xor of two entirely random things (k1 and k2) which would be completely meaningless.

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kelalaka avatar
in flag
kelalaka
Yes, I did. There is another issue that if the two messages are sent to two parties, then the observer can conclude that the same message is sent to two parties. Generally, I do not upvote OTP answers anymore...
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meshcollider avatar
gb flag
meshcollider
How can they do that? Just assume due to length?
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kelalaka avatar
in flag
kelalaka
Of course, that is a general issue that is not considered theoretically, however, exists in practice. For example, CryptDB solved this issue to extend the string to the same size, otherwise, there is a distinction on the database, etc... However, if a new long string is added, it is a hell of a mess...
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meshcollider avatar
gb flag
meshcollider
Yeah, agreed. But if you are sending only two messages then it would be fine to pad both to an arbitrary length. There is no other way to tell the messages are the same.
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kelalaka avatar
in flag
kelalaka
That is a solution in this case, in general, this is not as easy as this.
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