Parsing Protocol-Buffers without knowing the .proto

Question

I know that protocol-buffers are a serialized format that requires a message format in the .proto in order to read back properly. But I have a file that I do not know the proper message format for because it isn't published. What I am trying to do is to reverse engineer the data myself so i can reconstruct the messages. To do this I need to read the raw file out where I can pick up the field numbers, types and values.

Is there a program that will do this (preferrably in python but C/C++ is cool too)?

Answer 1

After doing some digging, I wrote my own tool to do this. There were other ways to do this, I'm sure, but this tool looks at the description in the source binary. It reads in the description stream and spits out a pseudo-.proto file. From that .proto file you can compile your own pb file and decode your stream.

import sys import struct  # Helper functions ------------------------------------------------------------ # this comes largely straight out of the google protocol-buffers code for DecodeVarint(internal\decoder.py) # with a few tweaks to make it work for me def readVarInt(buffer, pos):   mask = (1 << 64) - 1   result = 0   shift = 0   startPos = pos   while 1:     b = ord(buffer[pos])     result |= ((b & 0x7f) << shift)     pos += 1     if not (b & 0x80):       if result > 0x7fffffffffffffff:         result -= (1 << 64)         result |= ~mask       else:         result &= mask       return (result, pos, pos-startPos)     shift += 7     if shift >= 64:       raise Error('Too many bytes when decoding varint.')  def readQWORD(d, pos):     try:         v = struct.unpack("<Q", d[pos:pos+8])[0]     except:         print "Exception in readQWORD"         print sys.exc_info()         return (None, pos)     pos += 8     return (v, pos);  def readDWORD(d, pos):     try:         v = struct.unpack("<L", d[pos:pos+4])[0]     except:         print "Exception in readDWORD"         print sys.exc_info()         return (None, pos)     pos += 4     return (v, pos);  def readBYTE(d, pos):     try:         v = struct.unpack("<B", d[pos:pos+1])[0]     except:         print "Exception in readBYTE"         print sys.exc_info()         return (None, pos)     pos += 1     return (v, pos);  # returns (value, new position, data type, field ID, and value's length) def readField(d, pos):     # read field and type info     (v, p) = readBYTE(d, pos);     datatype = v & 7;     fieldnum = v >> 3;      if datatype == 0:       # varint         (v, p, l) = readVarInt(d, p)         return (v, p, datatype, fieldnum, l)         elif datatype == 1: # 64-bit         (v,p) = readQWORD(d, p)         return (v, p, datatype, fieldnum, 8)         elif datatype == 2: # varlen string/blob         (v, p, l) = readVarInt(d, p)    # get string length         return (d[p:p+v], p+v, datatype, fieldnum, v)            elif datatype == 5: # 32-bit value         (v,p) = readDWORD(d, p)         return (v, p, datatype, fieldnum, 4)     else:         print "Unknown type: %d [%x]\n" % (datatype, pos)         return (None, p, datatype, fieldnum, 1);  # PARSERS ---------------------------------------------------------------------  #  Parse DescriptorProto field def PrintDescriptorProto(data, size, prefix):     pos = 0      while pos < size:         (d, p, t, fid, l)  = readField(data, pos);         pos = p          if fid == 1: print "%smessage %s {" % (prefix,d)         elif fid == 2: PrintFieldDescriptorProto(d, l, prefix+"\t") # FieldDescriptorProto         elif fid == 3: PrintDescriptorProto(d, l, prefix+"\t") # DescriptorProto         elif fid == 4: PrintEnumDescriptorProto(d, l, prefix+"\t") # EnumDescriptorProto         elif fid == 5:              print "%sextension_range:" % (prefix)             PrintDescriptorProto(d, l, prefix+"\t") # ExtensionRange         elif fid == 6: print "%sextension: %s" % (prefix,d) # FieldDescriptorProto         elif fid == 7: print "%soptions: %s" % (prefix,d) # MessageOptions         else: print "***UNKNOWN fid in PrintDescriptorProto %d" % fid      print "%s}" % prefix  # Parse EnumDescriptorProto def PrintEnumDescriptorProto(data, size, prefix):     pos = 0     while pos < size:         (d, p, t, fid, l)  = readField(data, pos);         pos = p          if fid == 1: print "%senum %s {" % (prefix,d)         elif fid == 2: PrintEnumValueDescriptorProto(d, l, prefix+"\t") # EnumValueDescriptorProto         elif fid == 3: # EnumOptions             print "%soptions" % prefix         else: print "***UNKNOWN fid in PrintDescriptorProto %d" % fid     print "%s};" % prefix   # Parse EnumValueDescriptorProto def PrintEnumValueDescriptorProto(data, size, prefix):     pos = 0     enum = {"name": None, "number": None}     while pos < size:         (d, p, t, fid, l)  = readField(data, pos);         pos = p         if fid == 1: enum['name'] = d         elif fid == 2: enum['number'] = d         elif fid == 3: # EnumValueOptions             print "%soptions: %s" % (prefix,d)         else: print "***UNKNOWN fid in PrintDescriptorProto %d" % fid      print "%s%s = %s;" % (prefix, enum['name'], enum['number'])  # Parse FieldDescriptorProto def PrintFieldDescriptorProto(data, size, prefix):     pos = 0     field = {"name": None, "extendee": None, "number": None, "label": None, "type": None, "type_name": None, "default_value": None, "options": None}     while pos < size:         (d, p, t, fid, l)  = readField(data, pos);         pos = p          if fid == 1: field['name'] = d         elif fid == 2: field['extendee'] = d         elif fid == 3: field['number'] = d         elif fid == 4:              if d == 1: field['label'] = "optional"             elif d == 2: field['label'] = "required"             elif d == 3: field['label'] = "repeated"             else: print "{{Label: UNKNOWN (%d)}}" % (prefix,d)          elif fid == 5:              types = {1: "double",                               2: "float",                               3: "int64",                               4: "uint64",                               5: "int32",                               6: "fixed64",                              7: "fixed32",                              8: "bool",                              9: "string",                              10: "group",                               11: "message",                              12: "bytes",                              13: "uint32",                              14: "enum",                              15: "sfixed32",                              16: "sfixed64",                              17: "sint32",                              18: "sint64" }             if d not in types:                 print "%sType: UNKNOWN(%d)" % (prefix,d)             else:                 field['type'] = types[d]           elif fid == 6: field["type_name"] = d         elif fid == 7: field["default_value"] = d         elif fid == 8: field["options"] = d         else: print "***UNKNOWN fid in PrintFieldDescriptorProto %d" % fid      output = prefix      if field['label'] is not None: output += " %s" % field['label']     output += " %s" % field['type']     output += " %s" % field['name']     output += " = %d" % field['number']     if field['default_value']: output += " [DEFAULT = %s]" % field['default_value']     output += ";"     print output   #  Parse ExtensionRange field def PrintExtensionRange(data, size, prefix):     pos = 0     while pos < size:         (d, p, t, fid, l)  = readField(data, pos);         pos = p         print "%stype %d, field %d, length %d" % (prefix, t, fid, l)          if fid == 1: print "%sstart: %d" % (prefix,d)         elif fid == 2: print "%send: %d" % (prefix,d)         else: print "***UNKNOWN fid in PrintExtensionRange %d" % fid   def PrintFileOptions(data, size, prefix):     pos = 0     while pos < size:         (d, p, t, fid, l)  = readField(data, pos);         pos = p          if fid == 1: print "%soption java_package = \"%s\";" % (prefix,d)         elif fid == 8: print "%soption java_outer_classname = \"%s\"" % (prefix,d)         elif fid == 10: print "%soption java_multiple_files = %d" % (prefix,d)         elif fid == 20: print "%soption java_generate_equals_and_hash = %d" % (prefix,d)         elif fid == 9: print "%soption optimize_for = %d" % (prefix,d)         elif fid == 16: print "%soption cc_generic_services = %d" % (prefix,d)         elif fid == 17: print "%soption java_generic_services = %d" % (prefix,d)         elif fid == 18: print "%soption py_generic_services = %d" % (prefix,d)         elif fid == 999: print "%soption uninterpreted_option = \"%s\"" % (prefix,d)        # UninterpretedOption         else: print "***UNKNOWN fid in PrintFileOptions %d" % fid  # ----------------------------------------------------------------------------- # Main function.  def ParseProto(filename, offset, size):     f = open(filename, "rb").read()      data = f[offset:offset+size]      pos = 0     while pos < size:         (d, p, t, fid, l)  = readField(data, pos);         pos = p         #print "type %d, field %d, length %d" % (t, fid, l)          if fid == 1: print "// source filename: %s" % d         elif fid == 2: print "package %s;" % d         elif fid == 3: print "import \"%s\"" % d         elif fid == 4: PrintDescriptorProto(d, l, "")         elif fid == 5: print "EnumDescriptorProto: %s" % d         elif fid == 6: print "ServiceDescriptorProto: %s" % d         elif fid == 7: print "FieldDescriptorProto: %s" % d         elif fid == 8: PrintFileOptions(d, l, "")         else: print "***UNKNOWN fid in ParseProto %d" % fid     return {}    # main if __name__ == "__main__":     if len(sys.argv) != 4:         print "Usage: %s binaryfile offset size" % sys.argv[0]         sys.exit(0)      ParseProto(sys.argv[1], int(sys.argv[2]), int(sys.argv[3])) 

Answer 2

I found it useful to convert the raw message to text using protoc --decode_raw < file. If the file actually contains multiple (length-prefixed) messages, save them into separate files first.