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I have short (8-byte) messages which are effectively true random numbers, and I want to encrypt these messages with a (pre-shared) key....

I'm using AES-CTR for this purpose -- but with the SAME IV each time (which is seemingly contrary to best practice).... but since EVERY message that I'll encrypt with this key is unique, do I really have a security issue?

Maarten Bodewes
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biosbob
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1 Answers1

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Yes, this is an issue. Essentially you are encrypting each 8-bytes by XORing it with the same secret key. If we write $P_i$ for the true random 8-byte values, then the cipher texts are $C_i=P_i\oplus K$ for some fixed 8-byte value $K$.

Compromise of any one of the $(P_i,C_i)$ pairs will now compromise all of the pairs as $K$ can be recovered and the same $K$ is used for all pairs.

Moreover if there is any bias in your "effective" randomness, this will lead to bias in the cipher texts which in turn will reveal information about the corresponding plaintext.

Daniel S
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