> Welcome to reversing! Prove your worth and get the flag from this
> neat little program!

The program is a bunch of byte-sized integers.  I wrote a throw-away
program in Ruby to turn it into a binary.

    bytes = File.open('program.txt').read.split(', ').map(&:to_i)
    File.open('program', 'wb') { |f| bytes.each { |b| f.write(b.chr) } }

Going by the file size my first hunch was that it's raw shell code.
`radare2` agrees, showing reasonable x64 code doing multiplications
(as suggested by the challenge name):

    $ r2 -q -cpd program
                0x00000000      55             push rbp
                0x00000001      4889e5         mov rbp, rsp
                0x00000004      4883ec18       sub rsp, 0x18
                0x00000008      48c745f84f00.  mov qword [rbp - 8], 0x4f   ; 'O' ; 79
                0x00000010      48b8154fe74b.  movabs rax, 0x14be74f15
                0x0000001a      488945f0       mov qword [rbp - 0x10], rax
                0x0000001e      48c745e80400.  mov qword [rbp - 0x18], 4
                0x00000026      48c745e00300.  mov qword [rbp - 0x20], 3
                0x0000002e      48c745d81300.  mov qword [rbp - 0x28], 0x13
                0x00000036      48c745d01501.  mov qword [rbp - 0x30], 0x115 ; 277
                0x0000003e      48b8615b644b.  movabs rax, 0x77cf4b645b61
                0x00000048      488945c8       mov qword [rbp - 0x38], rax
                0x0000004c      48c745c00200.  mov qword [rbp - 0x40], 2
                0x00000054      48c745b81100.  mov qword [rbp - 0x48], 0x11
                0x0000005c      48c745b0c121.  mov qword [rbp - 0x50], 0x21c1
                0x00000064      48c745a8e965.  mov qword [rbp - 0x58], 0x182265e9
                0x0000006c      48c745a03308.  mov qword [rbp - 0x60], 0x833 ; 2099
                0x00000074      48c74598ab0a.  mov qword [rbp - 0x68], 0xaab ; 2731
                0x0000007c      48c74590adaa.  mov qword [rbp - 0x70], 0x8daaad
                0x00000084      488b45f8       mov rax, qword [rbp - 8]
                0x00000088      480faf45f0     imul rax, qword [rbp - 0x10]
                0x0000008d      48894588       mov qword [rbp - 0x78], rax
                0x00000091      488b45e8       mov rax, qword [rbp - 0x18]
                0x00000095      480faf45e0     imul rax, qword [rbp - 0x20]
                0x0000009a      480faf45d8     imul rax, qword [rbp - 0x28]
                0x0000009f      480faf45d0     imul rax, qword [rbp - 0x30]
                0x000000a4      480faf45c8     imul rax, qword [rbp - 0x38]
                0x000000a9      48894580       mov qword [rbp - 0x80], rax
                0x000000ad      488b45c0       mov rax, qword [rbp - 0x40]
                0x000000b1      480faf45b8     imul rax, qword [rbp - 0x48]
                0x000000b6      480faf45b0     imul rax, qword [rbp - 0x50]
                0x000000bb      480faf45a8     imul rax, qword [rbp - 0x58]
                0x000000c0      48898578ffff.  mov qword [rbp - 0x88], rax
                0x000000c7      488b45a0       mov rax, qword [rbp - 0x60]
                0x000000cb      480faf4598     imul rax, qword [rbp - 0x68]
                0x000000d0      480faf4590     imul rax, qword [rbp - 0x70]
                0x000000d5      48898570ffff.  mov qword [rbp - 0x90], rax
                0x000000dc      b800000000     mov eax, 0
                0x000000e1      c9             leave

There's no way I'm going to reverse-engineer that though.  I'd rather
execute it in GDB and inspect the stack afterwards.  My feeble
attempts at writing a C program that executes shell code failed with
segfaults, but no worries, I can instead just transplant the shell
code into a dummy program doing literally nothing.  For that the
amount of NOP instructions must naturally match the size of the shell
code:

    int main(){
        asm("nop"); // repeat 225 more times
    }

The actual patching is done with `r2 -w program.elf`:

    s main
    wx <shellcode hexstring>
    q

Finally step through it with the debugger.  I like GEF for that
purpose:

    gef➤  start
    gef➤  ni # repeat with return until leave instruction
    gef➤  hexdump $rbp-0x90
    0x00007fffffffea20     7d 6d 34 72 67 30 00 00 72 70 5f 64 31 6c 00 00    }m4rg0..rp_d1l..
    0x00007fffffffea30     34 76 5f 72 33 70 75 73 7b 67 61 6c 66 00 00 00    4v_r3pus{galf...
    0x00007fffffffea40     ad aa 8d 00 00 00 00 00 ab 0a 00 00 00 00 00 00    ................
    0x00007fffffffea50     33 08 00 00 00 00 00 00 e9 65 22 18 00 00 00 00    3........e".....

The flag is encoded in reverse on the stack:

    $ ruby -e "puts '}m4rg0rp_d1l4v_r3pus{galf'.reverse"
    flag{sup3r_v4l1d_pr0gr4m}