Set up Linux development environment

AGENTS.md

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+# AGENTS.md
+
+Mixmaster 3.0 — vintage Type II anonymous remailer (C). Two binaries share a
+`Mix/` spool: `mixmaster` (client + legacy remailer) and `mixremailer` (server).
+See `README.md`, `MODERNIZATION.md`, and `Src/remailer/ARCHITECTURE.md` for design.
+
+## Cursor Cloud specific instructions
+
+The cloud VM is **Linux (Ubuntu)**, but the repo's documented build path
+(`scripts/build-macos.sh`, `make build`, CI) targets **macOS/Homebrew**. Use the
+Linux path instead:
+
+- **Build:** `./scripts/build-linux.sh` (added for Linux; mirrors the macOS
+  script). Outputs `bin/mixmaster` and `bin/mixremailer`. `./scripts/build-linux.sh clean` cleans.
+- **System libs** (installed by the startup update script): `libssl-dev`,
+  `libpcre3-dev` (PCRE **1** — the vintage code uses the old `pcre.h` API, not
+  pcre2), `libncurses-dev`, `zlib1g-dev`.
+- **`-fcommon` is mandatory.** Legacy globals are declared (not just `extern`)
+  in a shared header; modern GCC/Clang default to `-fno-common` and would fail
+  linking with "multiple definition" errors. `build-linux.sh` adds `-fcommon`.
+- **bison is NOT needed:** `Src/parsedate.tab.c` is committed for bison-free builds.
+
+### Run / test (from repo root, after build)
+
+Always export the spool path first: `export MIXPATH="$PWD/Mix"`.
+
+- **Spool setup:** `./scripts/setup-mixdir.sh` seeds `Mix/` from `conf/`
+  (config, keyrings, lists). Re-seed config with `SETUP_FORCE_CFG=1`.
+- **Client:** `./bin/mixmaster` (no args) opens the ncurses menu; needs a TTY.
+  CLI flags skip the menu (`--version`, `--help`, `-T` list remailers,
+  `-l <chain> -t <addr>` encrypt, `-S` flush pool, `-G` generate keys).
+- **Server smoke test (matches CI):** `MIXPATH="$PWD/Mix" ./bin/mixremailer -M`.
+  The first `-M`/`-G` auto-generates the remailer's RSA key (OpenSSL 3 path).
+- **Unit test:** `cc -DHAVE_SETENV -include string.h -o /tmp/t Src/tests/test-parse_yearmonthday.c && /tmp/t`
+  (the `-include string.h` works around a missing include the source omits; do
+  not edit the source). The same function is asserted at every `mixmaster` startup.
+
+### Non-obvious runtime gotchas
+
+- **No MTA on the VM.** `mix.cfg` defaults `SENDMAIL` to `/usr/lib/sendmail -t`,
+  which does not exist here. For local delivery tests, point `SENDMAIL` at a
+  capture script in `Mix/mix.cfg` (env `SENDMAIL` does **not** override the
+  config value). Outbound mail is then written to a file instead of sent.
+- **Pool batching holds single messages.** With default `POOLSIZE`/`RATE`, one
+  pooled message is held for anonymity and won't flush. Force it for testing by
+  setting `POOLSIZE 0` / `RATE 100` in `mix.cfg` (or per the `mixmaster(1)`
+  `--POOLSIZE=0 --RATE=100` overrides).
+- **Self-addressed messages loop back:** sending to the remailer's own
+  `REMAILERADDR` stores the packet in the pool (prefix `i*`) for local
+  processing rather than mailing it; feed it to `./bin/mixremailer -R < <file>`
+  to decrypt the next hop.
+- **Bundled sample keys (2007) are expired** — the `Key ... has expired` and
+  `Statistics are older than one day` warnings are benign. For a working hop,
+  publish the locally generated key: `cat Mix/key.txt >> Mix/pubring.mix`.
+- `Mix/`, `bin/`, `Src/Makefile`, secrets/keyrings are gitignored runtime
+  artifacts — safe to delete and regenerate.

scripts/build-linux.sh

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+#!/usr/bin/env bash
+# Author: BrainX
+# Date: 2026-06-17
+# Description: Build Mixmaster 3.0 on Linux (Debian/Ubuntu) using system libraries.
+# Usage:
+#   ./scripts/build-linux.sh
+#   ./scripts/build-linux.sh clean
+#
+# Notes:
+#   Requires dev packages: libssl-dev libpcre3-dev libncurses-dev zlib1g-dev
+#   (PCRE1 / libpcre3-dev is required — the vintage code uses the old pcre.h API.)
+#   bison is NOT needed: Src/parsedate.tab.c is tracked for bison-free builds.
+#   -fcommon is required: legacy code declares globals in a shared header, which
+#   modern GCC/Clang (-fno-common default) would otherwise reject at link time.
+
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+SRC="${ROOT}/Src"
+BIN="${ROOT}/bin"
+
+cleanup() {
+  local code=$?
+  if [[ $code -ne 0 ]]; then
+    echo "build-linux.sh: interrupted or failed (exit $code)" >&2
+  fi
+  exit "$code"
+}
+trap cleanup INT TERM
+
+check_headers() {
+  local missing=()
+  [[ -f /usr/include/openssl/ssl.h ]] || missing+=("libssl-dev")
+  [[ -f /usr/include/pcre.h ]]        || missing+=("libpcre3-dev")
+  [[ -f /usr/include/ncurses.h || -f /usr/include/curses.h ]] || missing+=("libncurses-dev")
+  [[ -f /usr/include/zlib.h ]]        || missing+=("zlib1g-dev")
+  if [[ ${#missing[@]} -gt 0 ]]; then
+    echo "Missing dev libraries. Install with:" >&2
+    echo "  sudo apt-get install -y ${missing[*]}" >&2
+    exit 1
+  fi
+}
+
+if [[ "${1:-}" == "clean" ]]; then
+  make -C "$SRC" clean 2>/dev/null || true
+  rm -f "$SRC/Makefile" "$BIN/mixmaster" "$BIN/mixremailer"
+  echo "Clean complete."
+  exit 0
+fi
+
+check_headers
+
+INC="-I/usr/include"
+DEF="-DUSE_ZLIB -DUSE_PCRE -DUSE_NCURSES -DHAVE_NCURSES_H -DUSE_AES -DHAVE_SETENV -DSPOOL='\"${ROOT}\"' -fcommon"
+LIBS=""
+LDFLAGS="-lcrypto -lpcre -lncurses -lz"
+
+mkdir -p "$BIN"
+cd "$SRC"
+
+HOST="$(hostname -s 2>/dev/null || echo linux)"
+{
+  echo "# Makefile generated on ${HOST} $(date)"
+  sed -e "s#%MIXDIR##" \
+      -e "s#%LIBS#${LIBS}#" \
+      -e "s#%LDFLAGS#${LDFLAGS}#" \
+      -e "s#%INC#${INC}#" \
+      -e "s#%DEF#${DEF}#" < Makefile.in
+  echo "CC = cc"
+} > Makefile
+
+echo "Building mixmaster + mixremailer in ${SRC}..."
+make mixmaster mixremailer
+
+install -m 755 mixmaster "${BIN}/mixmaster"
+install -m 755 mixremailer "${BIN}/mixremailer"
+echo "Success: ${BIN}/mixmaster ($(file -b "${BIN}/mixmaster"))"
+echo "Success: ${BIN}/mixremailer ($(file -b "${BIN}/mixremailer"))"