Reused Keystream
Two messages, encrypted under what should have been two different one-time keys.
An operator accidentally reused the same keystream to encrypt two different messages — the exact mistake a one-time pad (and any stream cipher) can never survive.
You know the first message in full. You don't know the second — except that it's the flag.
the meeting is at midnight in the usual place bring the
a44dbe2a28e1e9fdb58fe600a9b94f8a4a3bac949c84a3442125e67755270766541826b14886fc7e691c0a06a53b013603937dfb8c41a356
b346af7131f3bcd6a8d0ec459fba5b8f6169a4c99486b37c7822996a53660751540c20bb6293a0017b025851ab66
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Background on this technique
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Forge the Cookie →A session cookie is encrypted with AES-CBC. You don't have the key. You don't need it.