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AES Mess
Challenge
We encrypted a flag with AES-ECB encryption using a secret key, and got the hash:
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e220eb994c8fc16388dbd60a969d4953f042fc0bce25dbef573cf522636a1ba3fafa1a7c21ff824a5824c5dc4a376e75
However, we lost our plaintext flag and also lost our key and we can’t seem to decrypt the hash back :(. Luckily we encrypted a bunch of other flags with the same key. Can you recover the lost flag using this?
Solution
AES in ECB mode has the unfortunate characteristic that identical plaintext, when encrypted with the same key always outputs the same ciphertext, since we are given a bunch of encryptions of different plaintext using the same key, we can use this to find our decryption.
Each of the blocks in our given plaintext appears exactly in one of the other encryption, so we can reconstruct the plaintext
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e220eb994c8fc16388dbd60a969d4953f042fc0bce25dbef573cf522636a1ba3fafa1a7c21ff824a5824c5dc4a376e75
block1: e220eb994c8fc16388dbd60a969d4953
block2: f042fc0bce25dbef573cf522636a1ba3
block3: fafa1a7c21ff824a5824c5dc4a376e75
abctf{looks_like_gospel_feebly}:
e220eb994c8fc16388dbd60a969d4953 <-- // abctf{looks_like
6d896bd7d6da9c4ce3eac5e4832c2f64 //_gospel_feebly}
abctf{verism_evg_you_can_break_ajugas}:
528c30c67c57968fa131684d07c1fa9c // abctf{verism_evg
f042fc0bce25dbef573cf522636a1ba3 <-- // _you_can_break_a
c0bd6ceeec8e817f1be7b09a9a8b0fb8 // jugas}
abctf{amidin_ogees}:
5f0ec66748ad4e9c512616572dd9197b // abctf{amidin_oge
fafa1a7c21ff824a5824c5dc4a376e75 <-- // es}
so now we just put the three plaintext blocks together to obtain the key:
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block1: e220eb994c8fc16388dbd60a969d4953 - abctf{looks_like
block2: f042fc0bce25dbef573cf522636a1ba3 - _you_can_break_a
block3: fafa1a7c21ff824a5824c5dc4a376e75 - es}
Flag
abctf{looks_like_you_can_break_aes}