Parsing Protocol-Buffers without knowing the .prot

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.