I messed up the task description. The original description is the following.
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it's all math anyways. I heard predicting the future has been in vogue recently, so I hid the flag in a black-box oracle. Connect to the service and submit one OpenQASM-like circuit ending with END. nc challs.umdctf.io 30309 The circuit has 16 input qubits q[0] through q[15], one ancilla qubit q[16], and a 16-bit classical register c[0] through c[15]. Supported instructions:

h q[i];
x q[i];
z q[i];
mcx q[i],...,q[j];
oracle q[0],q[1],q[2],q[3],q[4],q[5],q[6],q[7],q[8],q[9],q[10],q[11],q[12],q[13],q[14],q[15],q[16];
diffuse q[0],q[1],q[2],q[3],q[4],q[5],q[6],q[7],q[8],q[9],q[10],q[11],q[12],q[13],q[14],q[15];
measure q[i] -> c[i];

You may include the usual OPENQASM 2.0;, include "qelib1.inc";, qreg q[17];, and creg c[16]; lines. Measure every input qubit as q[i] -> c[i]. Limits: at most 250 oracle calls, 200000 bytes of input, and 512 shots. If your circuit places enough probability mass on the hidden marked state, the service prints the flag.