Full writeup

A detailled writeup can be found here.

Summarized Solution

• Decompile the apk using jadx
• Extract a base64 encoded string from MainActivity.java, the string is AzE9Omd0eG8XHhEcHTx1Nz0dN2MjfzF2MDYdICE6fyMa.
• Decode the string and then xor it with the key SPONGEBOBSPONGEBOBSPONGEBOBSPONGEBOBSPONGEBOB, then you'll obtain Part 1: UMASS{0ur_d3l1v3ry_squ1d_
• In the decompiled apk, look for a shared library named libfreedelivery.so, decompile it and extract some data that was xored with the byte 0x55, the xored bytes are \x30\x36\x3d\x3a\x75\x77\x05\x34\x27\x21\x75\x01\x22\x3a\x6f\x75\x22\x64\x39\x39\x0a\x37\x27\x64\x3b\x32\x0a\x64\x21\x0a\x27\x64\x32\x3d\x21\x0a\x65\x23\x66\x27\x0a\x74\x28\x77\x55, xor them with 0x55 and you'll obtain echo "Part Two: w1ll_br1ng_1t_r1ght_0v3r_!}"\x00'
• Put together with the first part, we get the full flag: UMASS{0ur_d3l1v3ry_squ1d_w1ll_br1ng_1t_r1ght_0v3r_!}