# lfsr

Linear Feedback Shift Registers for OCaml.

Configurable LFSRs (Fibonacci form) with up to 62-bit state. Includes a preset for CCSDS OID frame randomization (32-cell, polynomial x^32 + x^22 + x^2 + x + 1, per CCSDS 132.0-B-3 Annex D). Used for TRANSEC traffic flow confidentiality in satellite links.

## Installation

Install with opam:

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```sh
$ opam install lfsr
```

If opam cannot find the package, it may not yet be released in the public
`opam-repository`. Add the overlay repository, then install it:

<!-- $MDX skip -->
```sh
$ opam repo add samoht https://tangled.org/gazagnaire.org/opam-overlay.git
$ opam update
$ opam install lfsr
```

## Usage

```ocaml
(* CCSDS OID frame randomization: noise XORs with frame data *)
let ccsds = Lfsr.ccsds_oid ()
let noise = Lfsr.generate ccsds 256
let () = Printf.printf "generated %d bytes of noise\n" (Bytes.length noise)

(* Custom LFSR *)
let custom = Lfsr.v ~taps:0xC0000401 ~seed:0xFFFFFFFF ~width:32
let () = Printf.printf "0x%02X\n" (Lfsr.next_byte custom)
```

## API Overview

- **`type t`** -- Mutable LFSR state
- **`v`** -- Create LFSR with custom taps, seed, and width (1-62 bits)
- **`ccsds_oid`** -- Preset for CCSDS 132.0-B-3 OID frame randomization
- **`step`** -- Advance one bit, return output bit (0 or 1)
- **`next_byte`** -- Advance 8 bits, return byte (MSB-first)
- **`fill`** -- Fill a buffer with pseudo-noise
- **`generate`** -- Generate n bytes of pseudo-noise
- **`state`**, **`width`** -- Query register state

## License

MIT