coffee_break

crypto - Points: 60

The program “encrypt.py” gets one string argument and outputs ciphertext.

Example:

$ python encrypt.py "test_text"

gYYpbhlXwuM59PtV1qctnQ==

The following text is ciphertext with “encrypt.py”.

FyRyZNBO2MG6ncd3hEkC/yeYKUseI/CxYoZiIeV2fe/Jmtwx+WbWmU1gtMX9m905

Please download “encrypt.py” from the following url.

• encrypt.py

encrypt.py

In this challenge we have to decrypt the given ciphertext that was encrypted with the given script.

The encryption process takes each character of the plaintext and adds the corresponding character of the key at the same index, whereby the key is repeated and the output shifted in printable ASCII range.

Afterwards the output is encrypted with AES and another key and then base64 encoded.

So first of all we decode the base64 encoded ciphertext and decrypt the resulting AES encrypted ciphertext with the key. Afterwards we have now to subtract from each character of the next ciphertext the corresponding character of the key to get the plaintext and remove the padding to get the flag.

flag: SECCON{Success_Decryption_Yeah_Yeah_SECCON}