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https://github.com/cybershadow/rabcdasm

Robust ABC (ActionScript Bytecode) [Dis-]Assembler
https://github.com/cybershadow/rabcdasm

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Robust ABC (ActionScript Bytecode) [Dis-]Assembler

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Robust ABC (ActionScript Bytecode) [Dis-]Assembler

==================================================



[RABCDAsm][] is a collection of utilities including an ActionScript 3 

assembler/disassembler, and a few tools to manipulate SWF files. These are:



 * `rabcdasm` - ABC disassembler

 * `rabcasm` - ABC assembler

 * `abcexport` - extracts ABC from SWF files

 * `abcreplace` - replaces ABC in SWF files

 * `swfdecompress` - decompresses zlib-compressed SWF files

 * `swf7zcompress` - (re-)compress the contents of a SWF using 7-Zip

 * `swflzmacompress` - compress the contents of a SWF using LZMA

 * `swfbinexport` / `swfbinreplace` - extract/replace contents of binary data 

   tags from SWF files



`abcexport` and `abcreplace` are reimplementations of similar utilities from 

my [swfutilsex][] Java package, however these work faster as they do not parse 

the SWF files as deeply.  

`swfdecompress` is ancillary and is only useful for debugging and studying of 

the SWF file format, and not required for ABC manipulation. It is functionally 

equivalent to [flasm][]'s `-x` option. If you frequently work on compressed 

SWF files, you may want to decompress them to speed processing up.  

`swf7zcompress` is an utility to further reduce the size of SWF files. It uses 

[7-Zip][] to compress the data better than the standard zlib library would. It 

requires that the `7z` command-line program be installed and in `PATH`.  

`swflzmacompress` compresses SWF files using the [LZMA][] algorithm, support 

for which was introduced in Flash 11. It will only work with SWF files with 

version 13 or higher.  

`swfbinexport` and `swfbinreplace` aid in the manipulation of 

`DefineBinaryData` tags in SWF files (some files may contain nested SWF files 

stored in these tags).



  [RABCDAsm]: http://github.com/CyberShadow/RABCDAsm

  [swfutilsex]: http://github.com/CyberShadow/swfutilsex

  [flasm]: http://flasm.sourceforge.net/

  [7-Zip]: http://www.7-zip.org/

  [LZMA]: http://en.wikipedia.org/wiki/Lempel-Ziv-Markov_chain_algorithm



Motivation and goals

--------------------



This package was created due to lack of similar software out there. 

Particularly, I needed an utility which would allow me to edit ActionScript 3 

bytecode with the following properties:



 1. Speed. Less waiting means more productivity. `rabcasm` can assemble large 

    projects (>200000 LOC) in under a second on modern machines.

 2. Comfortably-editable output. Each class is decompiled to its own file, 

    with files arranged in subdirectories representing the package hierarchy. 

    Class files are `#include`d from the main file.

 3. Most importantly - robustness! If the Adobe AVM can load and run the file, 

    then it must be editable - no matter if the file is obfuscated or 

    otherwise mutilated to prevent reverse-engineering. RABCDAsm achieves this 

    by using a textual representation closer to the ABC file format, rather 

    than to what an ActionScript compiler would generate.



Compiling from source

---------------------



RABCDAsm is written in the [D programming language, version 2][d2].



Assuming you have [git][] and a D2 compiler, such as [dmd][] or [gdc][] 

installed, compiling should be as straight-forward as:



    git clone git://github.com/CyberShadow/RABCDAsm.git

    cd RABCDAsm

    dmd -run build_rabcdasm.d



Substitute `dmd` with `gdmd` if you're using gdc. You can use the `DC` and 

`DCFLAGS` environment variables to override the detected compiler and default 

compilation flags (`-O -inline`).



To be able to manipulate SWF files packed with LZMA compression, you'll need 

to have the [liblzma library][] and development files installed on your system.



Note: DMD 2.066 is required for long path support on Windows since RABCDAsm 

1.16.



  [d2]: http://dlang.org/

  [dmd]: http://www.digitalmars.com/d/download.html

  [gdc]: http://bitbucket.org/goshawk/gdc/

  [git]: http://git-scm.com/

  [liblzma library]: http://tukaani.org/xz/



Pre-compiled binaries

---------------------



You can find pre-compiled Windows binaries on [my website][downloads]. 

However, please don't expect them to be up-to-date with the latest source 

versions.



  [downloads]: http://files.thecybershadow.net/RABCDAsm/



Usage

-----



To begin hacking on a SWF file:



    abcexport file.swf



This will create `file-0.abc` ... `file-N.abc` (often just `file-0.abc`). Each 

file corresponds to an ABC block inside the SWF file.



To disassemble one of the `.abc` files:



    rabcdasm file-0.abc



This will create a `file-0` directory, which will contain `file-0.main.asasm` 

(the main program file) and files for ActionScript scripts, classes, and 

orphan and script-level methods.



To assemble the `.asasm` files back, and update the SWF file:



    rabcasm file-0/file-0.main.asasm

    abcreplace file.swf 0 file-0/file-0.main.abc



The second `abcreplace` argument represents the index of the ABC block in the 

SWF file, and corresponds to the number in the filename created by `abcexport`.



`swfbinexport` and `swfbinreplace` are used in the same manner as `abcexport` 

and `abcreplace`.



Syntax

======



The syntax of the disassembly was designed to be very simple and allow fast 

and easy parsing. It is a close representation of the `.abc` file format, and 

thus it is somewhat verbose. All constant pool elements (signed/unsigned 

integers, doubles, strings, namespaces, namespace sets, multinames) are always 

*expanded inline*, for ease of editing. Similarly, classes, instances, methods 

and method bodies are also defined inline, in the context of their "parent" 

object. By-index references of classes and methods (used in the `newclass`, 

`newfunction` and `callstatic` instructions) are represented via 

automatically-generated unique "reference strings", declared as `refid` fields.



If you haven't yet, I strongly recommend that you look through Adobe's 

[ActionScript Virtual Machine 2 (AVM2) Overview][avm2]. You will most likely 

need to consult it for the instruction reference anyway (although you can also 

use [this handy list][avm2i] as well). You will find it difficult to 

understand the disassembly without good understanding of concepts such as 

namespaces and multinames.



  [avm2]: https://www.adobe.com/content/dam/acom/en/devnet/pdf/avm2overview.pdf

  [avm2i]: https://web.archive.org/web/20160215185222/www.anotherbigidea.com/javaswf/avm2/AVM2Instructions.html



Overview

--------



In order to guarantee unambiguity and data preservation, all strings read from 

the input file - including identifiers (variable/function/class names) - are 

represented as string literals. Thus, the syntax does not have any "reserved 

words" or such - an unrecognized word is treated as an error, not as an 

identifier.



Whitespace (outside string literals, of course) is completely ignored, except 

where required to separate words. Comments are Intel-assembler-style: a single 

`;` demarks a comment until the next end-of-line. Control directives (such as 

`#include`) are allowed anywhere where whitespace is allowed.



The syntax is comprised of hierarchical blocks. Each block contains a number 

of fields - starting with a keyword specifying the field type. A block is 

terminated with the `end` keyword. Some fields contain a limited number of 

parameters, and others are, or contain blocks.



Hierarchy

---------



The topmost block in the hierarchy is the `program` block. This must be the 

first block in the file (thus, `program` must be the first word in the file as 

well). The `program` block contains `script` fields, and `class` / `method` 

fields for "orphan" classes and methods (not owned by other objects in the 

hierarchy). Orphan methods are usually anonymous functions. The file version 

is also specified in the `program` block, using the `minorversion` and 

`majorversion` fields (both unsigned integers).



`script` blocks have one mandatory `sinit` field (the script initialization 

method) and `trait` fields.



A "trait" can be one of several kinds. The kind is specified right after the 

`trait` keyword, followed by the trait name (a multiname). Following the name 

are the trait fields, varying by trait kind:



 * `slot` / `const` : `slotid` (unsigned integer), `type` (multiname), `value`

 * `class` : `slotid`, `class` (the actual class block)

 * `function` : `slotid`, `method` (the actual method block)

 * `method` / `getter` / `setter` : `dispid` (unsigned integer), `method`



Additionally, all traits may have `flag` fields, describing the trait's 

attributes (`FINAL` / `OVERRIDE` / `METADATA`), and `metadata` blocks.



`metadata` blocks (which are ignored by the AVM) consist of a name string, and 

a series of `item` fields - each item having a key and value string.



`class` blocks have mandatory `instance` and `cinit` fields, defining the 

class instance and the class initializer method respectively. They may also 

have `trait` fields and a `refid` field (the `refid` field is not part of the 

file format - it's an unique string to allow referencing the class, see above).



`instance` blocks - always declared inline of their `class` block - must 

contain one `iinit` field (the instance initializer method), and may contain 

one `extends` field (multiname), `implements` fields (multinames), `flag` 

fields (`SEALED` / `FINAL` / `INTERFACE` / `PROTECTEDNS`), one `protectedns` 

field (namespace), and `trait` fields.



`method` blocks may contain one `name` field (multiname), a `refid` field, 

`param` fields (multinames - this represents the parameter types), one 

`returns` field (multiname), `flag` fields (`NEED_ARGUMENTS` / 

`NEED_ACTIVATION` / `NEED_REST` / `HAS_OPTIONAL` / `SET_DXNS` / 

`HAS_PARAM_NAMES`), `optional` fields (values), `paramname` fields (strings), 

and a `body` field (method body).



`body` blocks - always declared inline of their `method` block - must contain 

the `maxstack`, `localcount`, `initscopedepth` and `maxscopedepth` fields 

(unsigned integers), and a `code` field. It may also contain `try` and `trait` 

fields.



`code` blocks - always declared inline of their `body` block - are somewhat 

different in syntax from other blocks - mostly in that they may contain 

labels. Labels follow the most common syntax - a word followed by a `:` 

character, optionally followed by a relative byte offset (in case of pointers 

inside instructions). Multiple instruction arguments are comma-separated. 

Instruction arguments' types depend on the instruction - see the `OpcodeInfo` 

array in `abcfile.d` for a reference.



`try` blocks - always declared inline of their `body` block - represent an 

"exception" (try/catch) block. They contain five mandatory fields: `from`, 

`to` and `target` (names of labels representing start and end of the "try" 

block, and start of the "catch" block respectively), and `type` and `name` 

(multinames), representing the type and name of the exception variable.



Values have the syntax *type* `(` *value* `)` . *type* can be one of 

`Integer`, `UInteger`, `Double`, `Utf8`, `Namespace`, `PackageNamespace`, 

`PackageInternalNs`, `ProtectedNamespace`, `ExplicitNamespace`, 

`StaticProtectedNs`, `PrivateNamespace`, `True`, `False`, `Null` or 

`Undefined`. The type of the value depends on *type*. Types `True`, `False`, 

`Null` and `Undefined` have no value.



Constants

---------



Multinames have the syntax *type* `(` *parameters* `)` . *type* can be one of 

`QName` / `QNameA`, `RTQName` / `RTQNameA`, `RTQNameL` / `RTQNameLA`, 

`Multiname` / `MultinameA`, `MultinameL` / `MultinameLA`, or `TypeName`. 

*parameters* depends on *type*:



 * `QName` / `QNameA` `(` *namespace* `,` *string* `)`

 * `RTQName` / `RTQNameA` `(` *string* `)`

 * `RTQNameL` / `RTQNameLA` `(` `)`

 * `Multiname` / `MultinameA` `(` *string* `,` *namespace-set* `)`

 * `MultinameL` / `MultinameLA` `(` *namespace-set* `)`

 * `TypeName` `(` *multiname* `<` *multiname [* `,` *multiname ... ]* `>` `)`



Namespace sets have the syntax `[` *[ namespace [* `,` *namespace ... ] ]* `]` 

(that is, a comma-separated list of namespaces in square brackets). Empty 

namespace sets can be specified using `[]`.



Namespaces have the syntax *type* `(` *string [* `,` *string ]* `)` . The 

first string indicates the namespace name. In the case that there are multiple 

distinct namespaces with the same type and name (as `PrivateNamespace` 

namespaces usually are), a second parameter may be present to uniquely 

distinguish them. Internally (the ABC file format), namespaces are 

distinguished by their numerical index. When disassembling, `rabcdasm` will 

attempt to assign descriptive labels to homonym namespaces based on their 

context.



Strings have a syntax similar to C string literals. Strings start and end with 

a `"`. Supported escape sequences (a backslash followed by a letter) are `\n` 

(generates ASCII 0x0A), `\r` (ASCII 0x0D), and `\x` followed by two 

hexadecimal digits, which inserts the ASCII character with that code. Any 

other characters following a backslash generate that character - thus, you can 

escape backslashes using `\\` and double quotes using `\"`. When decompiling, 

high-ASCII characters (usually UTF-8) are not escaped - if you see gibberish 

instead of international text, configure your editor to open the files in 

UTF-8 encoding.



Additionally, constant pool types (signed/unsigned integers, doubles, strings, 

namespaces, namespace sets and multinames) may also have the value `null` 

(which represents the index 0 in the ABC file). Note that `null` is 

conceptually different from zero, an empty string or empty namespace set.



Macros

------



RABCDAsm has some basic macro-like capabilities, controlled by directives and 

variables. These bear some similarity to the C preprocessor, however these are 

processed in-loop rather than as a separate pre-processing step.



### Directives



Directives start with a `#`, followed by a word identifying the directive:



  * `#include` *string* - inserts the contents of the file by the specified 

    filename inline. Functionally equivalent to `#mixin #get` *string* , but 

    faster.

  * `#mixin` *string* - inserts the contents of the specified string inline. 

    Not very useful on its own.

  * `#call` *string* `(` *[ string [* `,` *string ... ] ]* `)` - same as 

    `#mixin`, however it additionally sets the special variables `$1`, `$2` 

    etc. to the contents of the specified arguments. When the end of the 

    inserted string is reached, the old values of `$1`, `$2` etc. are restored.

  * `#get` *string* - inserts **a string containing** the contents of the file 

    by the specified filename inline. Similar to #include, but it inserts a 

    string (surrounded by `"` etc.) instead.

  * `#set` *word* *string* - assigns the contents of the string to the 

    variable *word*.

  * `#unset` *word* - deletes the variable *word*.

  * `#privatens` *number* *string* - deprecated, currently ignored.

  * `#version` specifies the syntax version of the disassembly. Newer RABCDAsm 

    versions may emit disassembly output that is not backwards-compatible, but 

    should still understand older disassemblies. The versions are:

     1. The first version.

     2. Introduced in v1.11 to work around error in ABC format specification.

     3. Introduced in v1.12 to support multiple non-private namespaces with 

        the same name. This is the current version.



### Variables



Variables are manipulated with the `#set` and `#unset` directives, and can be 

instantiated in two ways:



  1. `$`*name* - this inserts the contents of the variable inline. Note that 

     although variables are defined using a string syntax, they are not 

     inserted as a string using this syntax. Thus, the code:



        #set str "Hello, world!"

        ...

        pushstring $str



     will expand to `pushstring Hello, world!`, which will result in an error.

     To correct the problem, add escaped quotes around the variable contents

     ( `#set str "\"Hello, world!\""` ), or use the second syntax:

  

  2. `$"`*name*`"` - this inserts a string containing the contents of the 

     variable inline. This syntax also works for `#call` arguments (e.g. 

     `$"1"`).



### Example



Here's an example of how to use the above features to create a macro which 

logs a string literal and the contents of a register:



    #set log "

        findpropstrict      QName(PackageNamespace(\"\"), \"log\")

        pushstring          $\"1\"

        getlocal            $2

        callpropvoid        QName(PackageNamespace(\"\"), \"log\"), 2

    "

    

    ; ...

    

    pushbyte 2

    pushbyte 2

    add_i

    setlocal1

    #call $"log"("two plus two equals", "1")

     

Highlighting

------------



Included with the project is the file `asasm.hrc`, a simple syntax definition 

for the [Colorer take5][] syntax highlighting library. It should be 

straight-forward to adapt it to other syntax highlighting systems.



  [Colorer take5]: http://colorer.sourceforge.net/



Hacking

=======



ABC is internally represented in two forms. The `ABCFile` class stores the raw 

data structures, as they appear in the binary file. `ASProgram` uses pointers 

instead of indexes, allowing easy manipulation without having to worry about 

record order or constant pools. Conversion between various states is done as 

follows:



                                 file.abc

                                   |  ^

                  ------ ABCReader |  | ABCWriter ----

                 /                 v  |               \

                /                ABCFile               \

               /                   |  ^                 \

        rabcdasm---------- ABCtoAS |  | AStoABC --------rabcasm

               \                   v  |                 /

                \               ASProgram              /

                 \                 |  ^               /

                  --- Disassembler |  | Assembler ----

                                   v  |

                                file.asasm



`AStoABC` will rebuild the constant pools, in a manner similar to Adobe's 

compilers (reverse-sorted by reference count). The exact order will almost 

surely be different, however.



Should you need to write an utility to manipulate ABC, you can use the 

existing code to load the file to either an `ABCFile` or `ASProgram` instance, 

and perform the necessary manipulations using those classes.



Tips

====



The following tips come from the author's experience and may be useful for 

RABCDAsm users.



1. Once you have disassembled a SWF file you intend to modify, you should 

   immediately add the directory to a distributed source control system, such 

   as [Git][] or [Mercurial][]. This will allow you to easily track and undo 

   your changes, and easily merge your changes with new versions of SWF files.



  [Git]: http://git-scm.com/

  [Mercurial]: http://mercurial.selenic.com/



2. If you plan on making non-trivial changes to SWF files, you should install 

   the [debug Flash Player][]. This will allow you to see validation and 

   run-time error messages, instead of simply getting an empty window.



  [debug Flash Player]: http://www.adobe.com/support/flashplayer/downloads.html



3. The [Fiddler][] Web Debugging Proxy can be very useful for analyzing 

   websites with SWF content. The following script fragment (which is to be 

   placed in the `OnBeforeResponse` function) will automatically save all SWF 

   files while preserving the directory structure.



        if (oSession.oResponse.headers.ExistsAndContains("Content-Type",

                "application/x-shockwave-flash")) {

            // Set desired path here

            var path:String = "C:\\Temp\\FiddlerCapture\\" +

                oSession.host + oSession.PathAndQuery;

            if (path.Contains('?'))

                path = path.Substring(0, path.IndexOf('?'));

            var dir:String = Path.GetDirectoryName(path);

            if (!Directory.Exists(dir))

                Directory.CreateDirectory(dir);

            oSession.utilDecodeResponse();

            oSession.SaveResponseBody(path);

        }



   A more robust version of the above snippet is available as a Fiddler plugin 

   [here][FiddlerAutoCapture].



   Once you have edited a SWF file, you can use Fiddler's [AutoResponder][] to 

   replace the original file with your modified version.



  [Fiddler]: http://www.fiddler2.com/fiddler2/

  [AutoResponder]: http://www.fiddler2.com/fiddler2/help/AutoResponder.asp

  [FiddlerAutoCapture]: https://github.com/CyberShadow/FiddlerAutoCapture



Limitations

===========



* None known.



License

=======



RABCDAsm is distributed under the terms of the GPL v3 or later, with the 

exception of `murmurhash2a.d`, `zlibx.d` and LZMA components, which are in the 

public domain, and `asasm.hrc`, which is tri-licensed under the MPL 1.1/GPL 

2.0/LGPL 2.1. The full text of the GNU General Public License can be found in 

the file `COPYING`.