[flareon2018] chal5 WebAssembly writeup: side channel attack

fetch("test.wasm").then(response =>
  response.arrayBuffer()
).then(bytes =>
  WebAssembly.instantiate(bytes, {
    env: {
    /// comment
     },
      __syscall6: function __syscall6(n, a, b, c, d, e, f) { return syscall(instance, n, [a, b, c, d, e, f]); },

      putc_js: function (c) {
        c = String.fromCharCode(c);
        if (c == "\n") {
          console.log(wasm_stdout);
          wasm_stdout  = "";
        } else {
          wasm_stdout += c;
        }
      }
    }
  })
).then(results => {
    instance = results.instance;

    let a = new Uint8Array([
        0xE4, 0x47, 0x30, 0x10, 0x61, 0x24, 0x52, 0x21, 0x86, 0x40, 0xAD, 0xC1, 0xA0, 0xB4, 0x50, 0x22, 0xD0, 0x75, 0x32, 0x48, 0x24, 0x86, 0xE3, 0x48, 0xA1, 0x85, 0x36, 0x6D, 0xCC, 0x33, 0x7B, 0x6E, 0x93, 0x7F, 0x73, 0x61, 0xA0, 0xF6, 0x86, 0xEA, 0x55, 0x48, 0x2A, 0xB3, 0xFF, 0x6F, 0x91, 0x90, 0xA1, 0x93, 0x70, 0x7A, 0x06, 0x2A, 0x6A, 0x66, 0x64, 0xCA, 0x94, 0x20, 0x4C, 0x10, 0x61, 0x53, 0x77, 0x72, 0x42, 0xE9, 0x8C, 0x30, 0x2D, 0xF3, 0x6F, 0x6F, 0xB1, 0x91, 0x65, 0x24, 0x0A, 0x14, 0x21, 0x42, 0xA3, 0xEF, 0x6F, 0x55, 0x97, 0xD6
    ]);

    let b = new Uint8Array(new TextEncoder().encode(getParameterByName("q")));

    let pa = wasm_alloc(instance, 0x200);
    wasm_write(instance, pa, a);

    let pb = wasm_alloc(instance, 0x200);
    wasm_write(instance, pb, b);

    if (instance.exports.Match(pa, a.byteLength, pb, b.byteLength) == 1) {
        // PARTY POPPER
        document.getElementById("container").innerText = "🎉";
    } else {
        // PILE OF POO
        document.getElementById("container").innerText = "💩";
    }
});

/*
 * User-facing API for dynamic analyses.
 */

sa = {

};

function resetSA() {
    sa = {
        start: 0,
        nop: 0,
        unreachable: 0,
        if_: 0,
        br: 0,
        br_if: 0,
        br_table: 0,
        begin: 0,
        end: 0,
        drop: 0,
        select: 0,
        call_pre: 0,
        call_post: 0,
        return_: 0,
        load: 0,
        store: 0
    };
}

function printSA(prefix) {
    console.log(prefix, "sa:",
        "start=" , sa.start,
        "nop=" , sa.nop,
        "unreachable=" , sa.unreachable,
        "if_=" , sa.if_,
        "br=" , sa.br,
        "br_if=" , sa.br_if,
        "br_table=" , sa.br_table,
        "begin=" , sa.begin,
        "end=" , sa.end,
        "drop=" , sa.drop,
        "select=" , sa.select,
        "call_pre=" , sa.call_pre,
        "call_post=" , sa.call_post,
        "load=" , sa.load,
        "store=" , sa.store
        );
};

function getSAStr(prefix) {
    return JSON.stringify(sa);
};

Wasabi.analysis = {

    start(location) {
        sa.start++;
    },
    if_(location, condition) {
        sa.if_++;
    },
    br(location, target) {
        sa.br++;
    },
    br_if(location, conditionalTarget, condition) {
        sa.if_++;
    },
    br_table(location, table, defaultTarget, tableIdx) {
        sa.br_table++;
    },
    begin(location, type) {
        sa.begin++;
    },
    end(location, type, beginLocation, ifLocation) {
        sa.end++;
    },
    drop(location, value) {
        sa.drop++;
    },
    select(location, cond, first, second) {
        sa.select++;
    },
    call_pre(location, targetFunc, args, indirectTableIdx) {
        sa.call_pre++;
    },
    call_post(location, values) {
        sa.call_post++;
    },
    return_(location, values) {
        sa.return_++;
    },
    load(location, op, memarg, value) {
        sa.load++;
    },
    store(location, op, memarg, value) {
        sa.store++;
    },

};

.then(results => {
    instance = results.instance;
  /// comment

  function run(key) {
      var q = key;
      resetSA();
      // let b = new Uint8Array(new TextEncoder().encode(getParameterByName("q")));
      let b = new Uint8Array(new TextEncoder().encode(key));
      let pa = wasm_alloc(instance, 0x200);
      wasm_write(instance, pa, a);

      let pb = wasm_alloc(instance, 0x200);
      wasm_write(instance, pb, b);


      if (instance.exports.Match(pa, a.byteLength, pb, b.byteLength) == 1) {
          // PARTY POPPER
          document.getElementById("container").innerText = "🎉";
          window.stop();
      } else {
          // PILE OF POO
          document.getElementById("container").innerText = "💩";
          return getSAStr();
      }
  }

  var choices = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789-~!@#$%^&*()_+[]\{}|;':<>?/,."

  for (i = 0; i < choices.length; i++) {
    var ch = choices.charAt(i);
    resetSA()
    var trySA = run(ch);
    printSA(ch);
  }
});

.then(results => {
    instance = results.instance;
    /// comment

  function run(key) {
      var q = key;
      resetSA();
      // let b = new Uint8Array(new TextEncoder().encode(getParameterByName("q")));
      let b = new Uint8Array(new TextEncoder().encode(key));
      let pa = wasm_alloc(instance, 0x200);
      wasm_write(instance, pa, a);

      let pb = wasm_alloc(instance, 0x200);
      wasm_write(instance, pb, b);


      if (instance.exports.Match(pa, a.byteLength, pb, b.byteLength) == 1) {
          // PARTY POPPER
          document.getElementById("container").innerText = "🎉";
          window.stop();
      } else {
          // PILE OF POO
          document.getElementById("container").innerText = "💩";
          return getSAStr();
      }
  }

  var choices = " abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789-~!@#$%^&*()_+[]\{}|;':<>?/,."
  var cur = "";
  for (var pos = 0; pos < 50; pos++) {
    // console.log("key_len:", pos);
    var i;
    var countSA = null;
    for (i = 0; i < choices.length; i++) {
        var ch = choices.charAt(i);
        var q = cur + ch;
        var trySA = run(q);
        if (countSA == null) {
          // set on first sight
          countSA = trySA;
        }
        if (countSA != trySA) {
          cur += ch;
          console.log("-> " + cur);
          break;
        }
      }

      if (i == choices.length) {
        console.log("flag on len: " + pos);
        break;
      }
  }
  console.log("flag: ", cur);
});