First, let's check if "mtrjm" can be decoded. If I shift each letter forward by one: m→n, t→u, r→s, j→k, m→n. That gives "nuskn", which doesn't make sense. Try shifting backwards by one: m→l, t→s, r→q, j→i, m→l. "lsqil" – nope. Maybe shifting by 2? m→k, t→r, r→p, j→h, m→k: "krphk" – not helpful. What if it's a different shift? Let's try shifting each letter by its position. Alternatively, maybe it's using numbers: m(13), t(20), r(18), j(10), m(13). If adding a number... Not sure. Alternatively, maybe replacing letters with numbers like a=1, b=2, etc. So m=13, t=20, r=18, j=10, m=13. Not sure. Maybe reversing the letters? mtrjm reversed is mjrtm? No.