# Aero CTF 2020 : Old Crypto Server
**category** : crypto
**points** : 100
## write-up
AES ECB encrypt `msg + flag`, `msg` is controllable.
Send `msg = 'a' * 15`, and the first block will be `'a' * 15 + flag[0]`  Then send `msg = 'a' * 15 + '0'`, `msg = 'a' * 15 + '1'`, ..., until you find a match with the previous result and you got `flag[0]`.
Send `msg = 'a' * 14`, and the first block will be `'a' * 14 + flag[0] + flag[1]`  Then send `msg = 'a' * 14 + flag[0] + '0'`, `msg = 'a' * 14 + flag[0] + '1'`, ..., until you find a match with the previous result and got `flag[1]`
Keep going and you will find the whole flag.
See `solve.py` for more detail.
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