dst-codec

Reads and writes AutoCAD / ARES Commander Sheet Set files (*.dst) - plain UTF-8 XML (<AcSmDatabase>…) run through a fixed one-byte substitution cipher when written to disk. No compression, checksum, or header: substitute the bytes back and it's just XML.

100% language coverage. Latin, Greek, Cyrillic, Armenian, Hebrew, Arabic, Devanagari, CJK, emojis, and other scripts all decode and re-encode correctly.

The formula

.dst files contain UTF-8 XML encoded with a fixed byte-substitution cipher.

The encoding is byte-for-byte: each .dst byte is split into a 4-bit block and a 4-bit index, then mapped to an XML byte:

xml = BLOCK_BASE[dst >> 4] + PERMUTE[dst & 0xF]

In other words:

• dst >> 4 selects the block from the byte's high nibble.
• dst & 0xF selects an entry from PERMUTE using the low nibble.
• BLOCK_BASE[block] supplies the starting XML byte for that block.
• The two values are added to produce the final XML byte.

PERMUTE is the same fixed 16-entry permutation for every block, and is self-inverse:

PERMUTE = [13, 12, 15, 14, 9, 8, 11, 10, 5, 4, 7, 6, 1, 0, 3, 2]

BLOCK_BASE provides each block's offset. Only these blocks are confirmed; dstToXml / xmlToDst return undefined for all others.

block	dst range	xml range	base	notes
0x0	0–15	127–142	127	Full
0x1	16–31	111–126	111	o–z, { | } ~
0x2	32–47	159–174	159	Latin-1 Supplement
0x3	48–63	143–158	143	Full
0x4	64–79	191–206	191	UTF-8 lead bytes
0x5	80–95	175–190	175	Full
0x6	96–111	223–238	223	3-byte UTF-8 lead bytes
0x7	112–127	207–222	207	Greek/Cyrillic/etc. lead bytes
0x9	144–159	239–254	239	4-byte UTF-8 lead bytes
0xa	160–175	31–46	31	Punctuation & XML syntax
0xc	192–207	63–78	63	Full
0xd	208–223	47–62	47	Includes <
0xe	224–239	95–110	95	Full
0xf	240–255	79–94	79	Full

The confirmed blocks tile one gapless XML range: 31–254. Every valid UTF-8 lead byte (0xC2–0xF4) falls inside that range, so normal UTF-8 text across all languages and scripts is covered.

Block 0xb (dst 176–191) is the only unconfirmed cipher block; its possible XML range would be 0–30, which contains only C0 control characters and therefore isn't needed for normal text.

Block 0x8 is reserved for the separate TAB/LF substitutions, rather than being an unknown cipher block.

LF encoding: encodeDst writes 131 (AutoCAD's confirmed-safe LF byte) instead of ARES's 135.

Every byte is substituted

The cipher applies to every byte; there is no literal pass-through for XML syntax such as <, >, &, or ". These go through the same block/PERMUTE mapping as everything else.

The only separate handling is for TAB and LF.

Usage

import { decodeDst, encodeDst, dstSheets } from "./dstCodec.js";

const xml = decodeDst(await Deno.readFile("sheetset.dst")); // -> XML text

await Deno.writeFile("sheetset.dst", encodeDst(xml));       // -> .dst bytes

for (const sheet of dstSheets(xml)) {
    console.log(sheet.number, sheet.title, sheet.file, sheet.handle);
}

decodeDst returns valid XML text - entities such as & are left intact, not unescaped. Reading the actual content of a specific field (such as a title) is a separate step handled by dstSheets; unescaping during decoding would make literal & content indistinguishable from XML syntax.

CLI

deno run -A dstCodec.js --to-xml sheetset.dst [--out sheetset.xml]  decode (stdout without --out)
deno run -A dstCodec.js --to-dst sheetset.xml --out sheetset.dst    encode
deno run -A dstCodec.js --sheets sheetset.dst                       one line per sheet

Same flags under Node:

node dstCodec.js --to-xml sheetset.dst ...

Portability

dstCodec.js is a single dependency-free JavaScript file (JSDoc types, // @ts-check, no build step) that runs unmodified in Deno, Node, and browsers.

• Deno: deno run -A dstCodec.js ... for the CLI, or import the functions directly.
• Node (v18+): node dstCodec.js ... with the same CLI flags, or import the functions from a Node ESM script.
• Browser: use <script type="module"> and import the pure functions.

The core functions (dstToXml, xmlToDst, decodeDst, encodeDst, dstSheets) only use standard JavaScript APIs: TextEncoder, TextDecoder, Uint8Array, RegExp, and String. They have no runtime-specific dependencies.

The CLI is isolated at the bottom of the file and setup so it only runs when the file is actually run as a Deno or Node script; importing the library in a browser skips it.

Notable references

• abrman/dst-edit - browser-based .dst sheet set editor, and the starting point for this project. Its bit-flip.ts byte table covers the ASCII range, working out the pattern behind that table gave the block/PERMUTE formula above, which extends it to the full 31–254 range.
• mxwell's dst_format_ctl.py gist - Python script (2020) that recovers the substitution table by diffing a matching .xml/.dst pair, one UTF-8 character at a time. Its Cyrillic dictionary was a useful cross-check for the non-ASCII blocks. The cipher turns out to be per-byte rather than per-character, which is why a fixed formula works without paired files.

License

MIT - see LICENSE.