046

• Description: Flag inside the file.

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🔎 Solution

The provided file contains the following information:

x=W0oyRmlZMlJsWm1kb2FXcHJiRzF1YjNCeGNuTjBkWFozZUhsNlFVSkRSRVZHUjBoSlNrdE1UVTVQVUZGU1UxUlZWbGRZV1ZvbklDZDZlWGgzZG5WMGMzSnhjRzl1Yld4cm;
y=FtbG9aMlpsWkdOaVlWcFpXRmRXVlZSVFVsRlFUMDVOVEV0S1NVaEhSa1ZFUTBKQkp3bz0sIEhWMUR2M1FvQjNFYngyb2V5bTBQXQo=;
x+y

Combining the two strings x and y yields:

W0oyRmlZMlJsWm1kb2FXcHJiRzF1YjNCeGNuTjBkWFozZUhsNlFVSkRSRVZHUjBoSlNrdE1UVTVQVUZGU1UxUlZWbGRZV1ZvbklDZDZlWGgzZG5WMGMzSnhjRzl1Yld4cmFtbG9aMlpsWkdOaVlWcFpXRmRXVlZSVFVsRlFUMDVOVEV0S1NVaEhSa1ZFUTBKQkp3bz0sIEhWMUR2M1FvQjNFYngyb2V5bTBQXQo=

Decoding this Base64 string results in:

[J2FiY2RlZmdoaWprbG1ub3BxcnN0dXZ3eHl6QUJDREVGR0hJSktMTU5PUFFSU1RVVldYWVonICd6eXh3dnV0c3JxcG9ubWxramloZ2ZlZGNiYVpZWFdWVVRTUlFQT05NTEtKSUhHRkVEQ0JBJwo=, HV1Dv3QoB3Ebx2oeym0P]

The result is divided into two parts. Decoding the first part reveals:

'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ' 'zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA'

This shows that if the first string is the plaintext alphabet, the second string is its reverse. This defines an Atbash cipher, where each letter is replaced by its symmetrical counterpart in the alphabet.

Therefore, the remaining string HV1Dv3QoB3Ebx2oeym0P is the ciphertext that needs to be decrypted.

Using CyberChef to perform the Atbash decryption yields the flag.

🚩Flag

HMV{recursion}