grpc-web reverse engineer

decompiler.py

import jsbeautifier
import requests
import pprint

enumerateMessagesSearchString = 'exportSymbol("proto.'

r = requests.get('http://localhost:8081/dist/main.js')
jsInput = r.text

pretty_js = jsbeautifier.beautify(jsInput).split('\n')
messages = {}
namespaces = set()

for s in pretty_js:
    if enumerateMessagesSearchString in s:
        symbols = s.split("exportSymbol")
        for sym in symbols:
            if 'proto' in sym:
                messages[sym.split('"')[1]] = {}
                namespaces.add(sym.split('"')[1].split(".")[1])

print '==================== URL Endpoints ===================='
for namespace in list(namespaces):
    for s in pretty_js:
        if "/{}.".format(namespace) in s:
            print "Discovered Endpoint: {}".format(s.split('"')[1])

for message in messages:
    for idx, s in enumerate(pretty_js):
        if '{}.toObject ='.format(message) in s:
            searchIndex = idx + 1
            endString = ""
            while (endString.strip() != "};"):
                search_js = pretty_js[searchIndex] 
                # edge case when empty message
                if "{};" in search_js:
                    endString = "};"
                    break

                fieldParts = search_js.strip().split(":")
                if len(fieldParts) > 1:
                    fieldName = fieldParts[0]
                    fieldMeta = fieldParts[1].strip()
                    if 'getFieldWithDefault' in fieldMeta:
                        fieldDefault = fieldMeta.strip().split(")")[0].split(",")[-1]
                        fieldType = "Primitive with default value {}".format(fieldDefault)
                    elif 'getRepeatedField' in fieldMeta:
                        fieldType = "Repeated Field"
                    elif 'toObjectList' in fieldMeta:
                        nestedObject = fieldMeta.split(",")[1]
                        fieldType = "Nested Object: {}".format(nestedObject)
                    else:
                        fieldType = "Unknown"

                    messages[message][fieldName]=fieldType

                searchIndex+=1
                endString = search_js

            break

pp = pprint.PrettyPrinter(indent=4)
print '==================== Messages ===================='
pp.pprint(messages)

helloworld.proto

syntax = "proto3";

package helloworld;

service Greeter {
    rpc SayHello (HelloRequest) returns (HelloResponse);
    rpc SearchResults (Empty) returns (SearchResponse);
}

message HelloRequest {
    string name = 1;
}

message HelloResponse {
    string message = 1;
    string test = 2;
}

message SearchResponse {
    repeated Result results = 1;
}
  
message Result {
    string url = 1;
    string title = 2;
    repeated string snippets = 3;
}

message Empty {

}

results.txt

==================== URL Endpoints ====================

Discovered Endpoint: /helloworld.Greeter/SayHello
Discovered Endpoint: /helloworld.Greeter/SearchResults

==================== Messages ====================
{
    u'proto.helloworld.Empty': {   },
    u'proto.helloworld.HelloRequest': {   u'name': 'Primitive with default value  ""'},
    u'proto.helloworld.HelloResponse': {   u'message': 'Primitive with default value  ""',
                                           u'test': 'Primitive with default value  ""'},
    u'proto.helloworld.Result': {   u'snippetsList': 'Repeated Field',
                                    u'title': 'Primitive with default value  ""',
                                    u'url': 'Primitive with default value  ""'},
    u'proto.helloworld.SearchResponse': {   u'resultsList': 'Nested Object:  proto.helloworld.Result.toObject'}
}