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README.md

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+# repo-delivery-middleware
+
+HTTP middleware that serves encrypted GitLab repo bundles to customers.
+
+## Building
+
+```sh
+nix build                # Go binary -> ./result/bin/repo-delivery-middleware
+nix build .#docker-image # streaming Docker image script -> ./result
+```
+
+## Running (binary)
+
+Directly from the flake:
+
+```sh
+nix run . -- --config config.toml
+nix run . -- --config config.toml --listen :9090
+```
+
+Or after `nix build`:
+
+```sh
+./result/bin/repo-delivery-middleware --config config.toml
+```
+
+| Flag       | Default | Description        |
+|------------|---------|--------------------|
+| `--config` |         | Path to TOML config file |
+| `--listen` | `:8080` | Address to listen on     |
+
+The `CONFIG_PATH` environment variable can be used instead of `--config`.
+The flag takes precedence if both are set.
+
+## Running (Docker)
+
+The flake produces a `streamLayeredImage` script rather than a tarball, so
+building and loading is a two-step pipe:
+
+```sh
+nix build .#docker-image
+./result | docker load
+```
+
+This streams and loads the image as `repo-delivery-middleware:latest`.
+
+```sh
+docker run --rm -p 8080:8080 \
+  -e GITLAB_API_KEY=glpat-xxxx \
+  -v /path/to/config.toml:/config.toml \
+  repo-delivery-middleware:latest --config /config.toml
+```
+
+The image has no shell. The entrypoint is the binary; extra arguments are
+passed directly as flags.
+
+## Configuration
+
+```toml
+schema = "v1"
+
+[config]
+gitlab_url = "https://gitlab.example.com"
+# Supports environment variable expansion ($VAR or ${VAR})
+gitlab_api_key = "${GITLAB_API_KEY}"
+
+[customers.acme]
+# AES-256 key: exactly 64 hex characters (32 bytes)
+key = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"
+repos = [
+    "group/project-a",
+    "group/subgroup/project-b",
+]
+```
+
+- `schema` must be `"v1"`.
+- At least one customer is required.
+- Each customer key must be exactly 64 hex characters.
+- Each customer must list at least one repo.
+
+## API
+
+```
+GET /getPayload
+X-API-Key: <customer_name>
+```
+
+Success response:
+
+```
+200 OK
+Content-Type: application/octet-stream
+Content-Disposition: attachment; filename="payload.bin"
+```
+
+Error codes:
+
+| Code | Meaning |
+|------|---------|
+| 401  | Missing or unknown `X-API-Key` header |
+| 405  | HTTP method other than GET |
+| 500  | Failed to build or encrypt payload |
+| 502  | Failed to fetch repositories from GitLab |
+
+## Payload format
+
+The response body is a single binary blob:
+
+```
+nonce (12 bytes) || ciphertext || GCM tag (16 bytes)
+```
+
+- Encrypted with AES-256-GCM using the customer's key.
+- Plaintext is a tar.gz archive containing one directory per repo, named
+  after the repo basename (GitLab's internal prefix is stripped).
+- Repos are sorted alphabetically for deterministic output.
+
+## Development
+
+```sh
+nix develop   # shell with go and gopls
+```