Background
I wrote a web version of Slowly using Vue.js last year, which provided some useful functions unsupported, even now, by the official app. It ran well till yesterday when I failed to sign in. At first, I suspected the server might have detected what I had done was illegal and blocked me from further use, since my app can extract accurate locations of friends, show content of a letter even before it has arrived, and send photos without acceptances from a receiver, which, to some extent, may violate their licences.
After some investigations, it turned out, luckily, that they just upgraded the sercurity level of servers to protect information of users. I spent several hours studing their strategies and modifying my code and eventually made the app sail out again.
This post elaborates on the problems I encountered during the process and how I finally solved them.
Charles didn’t help
I created the first version with help of Charles to intercept requests sent from the Slowly app running in my phone with Android 7 installed. In this way, I was able to see the content sealed with https and record all important APIs and hence build my own version. In order to figure out what went wrong I had to check first the requests sent by the latest version of Slowly. However, installation of certificates is forbidden on Android with platform version above 7, so I turned to VirtualXposed, which promises a way to hack https in Android phones running on an OS of any version without root. It didn’t work either.
Before digging into VirtualXposed I tried doing it on Windows instead. Although there were many problmes, like mismatch of architectures, missing of services from google play required by Slowly and my inability to separate a runnable apk from a bundle, I solved them patiently and finally intercepted requests in Charles.
It turned out that besides using a new API, the login request also brought an encrypted parameter of otp, changing each time, which obviously was generated in client. My next task was to find out how to generate otp.
otp:qxg6AGYQ4y5rDVaSaiRYjZ3WNwbalO
P.S. Charles did help. A lot!
Generate otp
There was little I could do withought reading the code. So I downloaded the latest apk, unpacked it, and found the source file index.android.bundle directly under the folder assets, which turned out to be a large compressed JavaScript file with the size of nearly 3.5 Mb. I copied it to another file named index.js, opening it with VS Code. The messy code soon filled the screen and made my computer roar heavily.
I extracts a piece of code related to the new api /users/me/v2, and it’s easy to find otp is passed from the caller side.
__d(function(g, r, i, a, m, e, d) {
......
e.getMe = function(n) {
var P = n.otp,
y = void 0 === P ? null : P,
S = {
......
otp: y
};
return fetch(t.API_URL + '/users/me/v2?token=' + o, {
method: 'POST',
headers: t.headers,
body: JSON.stringify(S)
}).then(t.handleApiErrors).then(function(n) {
return n.json()
}).then(function(n) {
return n
}).catch(function(n) {
throw n
})
};
}, 962, [1, 924, 598]);
Code blocks referencing getMe are all surrounded with a large piece of messed code, which makes it difficult to figure out what’s going on. Instead, I searched otp and found the core logic to generate otp:
__d(function(g, r, i, a, m, e, d) {
......
var t = r(d[0]);
var o = t(r(d[2]))
e.genOTP = function(t) {
var n = t.timestamp,
c = t.uid;
return new o.default(
'+DP;=SW`DGX&n|]OGoGkj/4XqPw?^Fclc2F-_V~D=rquG+L(kW_xzVR=slp+Yj;B',
30
).encode(parseInt(n), parseInt(c), Math.floor(1e5 + 9e5 * Math.random()))
};
......
}, 923, [1, 924, 925]);
genOTP accepts an object containing two keys, one is timestam, another one is uid.
Before going further, the structure of a module needs to be explainded a little. Each module is defined in a factory function passed along with another two parameters——an integer and an array——as arguments to a function _d. The second parameter, i.e. the integer, indicates the index of the module, and the third parameter is an array of index of module it depends on.
__d(function(g,r,i,a,m,e,d){
......
},923,[1,924,925]);
Each factory function accepts 7 parameters, the most important three ones are:
- r: require,
r(12)means to import a module with index 12 - e: exports,
e.getMe = functionmeans to export a functiongetMefrom this module - d: array of dependencies,
r(d[1])means to import a module whose index isd[1]
In this case, d is [1, 924, 925], then t = r(d[0]) equals t = r(1) and o = t(r(d[2])) equals o = r(1)(r(925)), hence the code above can be transformed into something below:
var otp = new (__r(1)(__r(925)).default)(
'+DP;=SW`DGX&n|]OGoGkj/4XqPw?^Fclc2F-_V~D=rquG+L(kW_xzVR=slp+Yj;B',
30
).encode(
parseInt(Date.now()),
parseInt(0),
Math.floor(1e5 + 9e5 * Math.random())
)
console.log(otp)
Put this piece of code at the bottom of index.js and run node index.js. After supressing some(a lot of) errors related to ReactNative, we will get an output, pretty much looking like the one capured in a previous request:
j4P89Am5g96GPkKwuYiZ5xE3wgbLYO
Now, let’s look at the detail of module 1:
__d(function(g, r, i, a, m, e, d) {
m.exports = function(n) {
return n && n.__esModule ? n : {
default: n
}
}
}, 1, []);
Since module 1 does nothing except returning the input function back, the invocation of module 1 can be eliminated. Next, we will see what module 925 looks like:
__d(function(g, r, i, a, m, e, d) {
!(function(t, s) {
if ("function" == typeof define && define.amd) define(["exports"], s);
else if (void 0 !== e) s(e);
else {
var h = {};
s(h), t.Hashids = h
}
})(this, function(t) {
......
var h = (function() {
......
})();
t.default = h
})
}, 925, [])
Module 925 is an immediate function which at last export function h as default property of e, and it can be simplified to:
__d(function(g, r, i, a, m, e, d) {
var h = (function() {
// a large piece code
......
})();
e.default = h
}, 925, [])
What is still unkown to us is how to get timestamp and uid for genOTP. After investigating the code again I sifted the most important logic related to process of sending request of getMe:
__d(function(g, r, i, a, m, e, d) {
......
var p = r(d[7]),
E = r(d[10]),
......
function X() {
var t, n, s, o, x, _, S, y, h, R, T, b, O, w;
return c.default.wrap(function(c) {
for (;;) switch (c.prev = c.next) {
......
case 11:
return S = c.sent, c.prev = 12, c.next = 15, (0, u.call)(p.getTime);
case 15:
return y = c.sent, c.next = 18, (0, u.select)(E.getMyID);
case 18:
return h = c.sent, c.next = 21, (0, u.call)(p.genOTP, {
timestamp: parseInt(y.now),
uid: parseInt(h)
});
case 21:
return R = c.sent, c.next = 24, (0, u.call)(l.getMe, {
token: n,
location_code: s,
location: o,
device: S.device,
otp: R
});
......
}
}, C, null, [
[12, 45]
])
}
}, 969, [383, 1, 46, 136, 623, 948, 962, 923, 968, 952, 963, 633, 2, 634, 636, 738, 920, 353])
This code block shows that we can get uid through E.getMyID, where E is imported from module 923 and get timestamp by calling p.getTime, where p implies module 963, same one as genOTP.
It’s time to uncover the real face of getTime and getMyID:
__d(function(g, r, i, a, m, e, d) {
......
e.getTime = function() {
return fetch(n.API_URL + '/timestamp', {
timeout: 2e3
}).then(n.handleApiErrors).then(function(t) {
return t.json()
}).then(function(t) {
return t
}).catch(function(t) {
throw t
})
}
}, 962, [1, 924, 598]);
__d(function(g, r, i, a, m, e, d) {
......
var _ = function(t) {
return t.me.id ? t.me.id : 0
};
e.getMyID = _;
}, 963, [1, 18, 46, 964, 356, 919, 965]);
Since we don’t have any information before signing in, thus uid can always be set to 0.
Combining all the code and optimizations metioned above together, we get to a final, feasible piece of code:
const Encryption = (function() {
......
})()
export default function getOtp() {
getTimestamp().then((time) => {
return new Encryption(
"+DP;=SW`DGX&n|]OGoGkj/4XqPw?^Fclc2F-_V~D=rquG+L(kW_xzVR=slp+Yj;B",
30
).encode(
parseInt(time),
parseInt(0),
Math.floor(1e5 + 9e5 * Math.random())
);
})
}