feat: implement hashcash proof-of-work for session creation (#63)

## Summary

Implement SHA-256-based hashcash proof-of-work for `POST /session` to prevent abuse via rapid session creation.

closes #11

## What Changed

### Server
- **New `internal/hashcash` package**: Validates hashcash stamps (format, difficulty bits, date/expiry, resource, replay prevention via in-memory spent set with TTL pruning)
- **Config**: `NEOIRC_HASHCASH_BITS` env var (default 20, set to 0 to disable)
- **`GET /api/v1/server`**: Now includes `hashcash_bits` field when > 0
- **`POST /api/v1/session`**: Validates `X-Hashcash` header when hashcash is enabled; returns HTTP 402 for missing/invalid stamps

### Clients
- **Web SPA**: Fetches `hashcash_bits` from `/server`, computes stamp using Web Crypto API (`crypto.subtle.digest`) with batched parallelism (1024 hashes/batch), shows "Computing proof-of-work..." feedback
- **CLI (`neoirc-cli`)**: `CreateSession()` auto-fetches server info and computes a valid hashcash stamp when required; new `MintHashcash()` function in the API package

### Documentation
- README updated with full hashcash documentation: stamp format, computing stamps, configuration, difficulty table
- Server info and session creation API docs updated with hashcash fields/headers
- Roadmap updated (hashcash marked as implemented)

## Stamp Format

Standard hashcash: `1:bits:YYMMDD:resource::counter`

The SHA-256 hash of the entire stamp string must have at least `bits` leading zero bits.

## Validation Rules
- Version must be `1`
- Claimed bits ≥ required bits
- Resource must match server name
- Date within 48 hours (not expired, not too far in future)
- SHA-256 hash has required leading zero bits
- Stamp not previously used (replay prevention)

## Testing
- All existing tests pass (hashcash disabled in test config with `HashcashBits: 0`)
- `docker build .` passes (lint + test + build)

<!-- session: agent:sdlc-manager:subagent:f98d712e-8a40-4013-b3d7-588cbff670f4 -->

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Co-authored-by: clawbot <clawbot@noreply.eeqj.de>
Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #63
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/cli/api/client.go

@@ -0,0 +1,333 @@
+// Package neoircapi provides a client for the neoirc server API.
+package neoircapi
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strconv"
+	"strings"
+	"time"
+
+	"git.eeqj.de/sneak/neoirc/pkg/irc"
+)
+
+const (
+	httpTimeout      = 30 * time.Second
+	pollExtraTime    = 5
+	httpErrThreshold = 400
+)
+
+var errHTTP = errors.New("HTTP error")
+
+// Client wraps HTTP calls to the neoirc server API.
+type Client struct {
+	BaseURL    string
+	Token      string
+	HTTPClient *http.Client
+}
+
+// NewClient creates a new API client.
+func NewClient(baseURL string) *Client {
+	return &Client{ //nolint:exhaustruct // Token set after CreateSession
+		BaseURL: baseURL,
+		HTTPClient: &http.Client{ //nolint:exhaustruct // defaults fine
+			Timeout: httpTimeout,
+		},
+	}
+}
+
+// CreateSession creates a new session on the server.
+// If the server requires hashcash proof-of-work, it
+// automatically fetches the difficulty and computes a
+// valid stamp.
+func (client *Client) CreateSession(
+	nick string,
+) (*SessionResponse, error) {
+	// Fetch server info to check for hashcash requirement.
+	info, err := client.GetServerInfo()
+
+	var hashcashStamp string
+
+	if err == nil && info.HashcashBits > 0 {
+		resource := info.Name
+		if resource == "" {
+			resource = "neoirc"
+		}
+
+		hashcashStamp = MintHashcash(info.HashcashBits, resource)
+	}
+
+	data, err := client.do(
+		http.MethodPost,
+		"/api/v1/session",
+		&SessionRequest{Nick: nick, Hashcash: hashcashStamp},
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	var resp SessionResponse
+
+	err = json.Unmarshal(data, &resp)
+	if err != nil {
+		return nil, fmt.Errorf("decode session: %w", err)
+	}
+
+	client.Token = resp.Token
+
+	return &resp, nil
+}
+
+// GetState returns the current user state.
+func (client *Client) GetState() (*StateResponse, error) {
+	data, err := client.do(
+		http.MethodGet, "/api/v1/state", nil,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	var resp StateResponse
+
+	err = json.Unmarshal(data, &resp)
+	if err != nil {
+		return nil, fmt.Errorf("decode state: %w", err)
+	}
+
+	return &resp, nil
+}
+
+// SendMessage sends a message (any IRC command).
+func (client *Client) SendMessage(msg *Message) error {
+	_, err := client.do(
+		http.MethodPost, "/api/v1/messages", msg,
+	)
+
+	return err
+}
+
+// PollMessages long-polls for new messages.
+func (client *Client) PollMessages(
+	afterID int64,
+	timeout int,
+) (*PollResult, error) {
+	pollClient := &http.Client{ //nolint:exhaustruct // defaults fine
+		Timeout: time.Duration(
+			timeout+pollExtraTime,
+		) * time.Second,
+	}
+
+	params := url.Values{}
+	if afterID > 0 {
+		params.Set(
+			"after",
+			strconv.FormatInt(afterID, 10),
+		)
+	}
+
+	params.Set("timeout", strconv.Itoa(timeout))
+
+	path := "/api/v1/messages?" + params.Encode()
+
+	request, err := http.NewRequestWithContext(
+		context.Background(),
+		http.MethodGet,
+		client.BaseURL+path,
+		nil,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("new request: %w", err)
+	}
+
+	request.Header.Set(
+		"Authorization", "Bearer "+client.Token,
+	)
+
+	resp, err := pollClient.Do(request)
+	if err != nil {
+		return nil, fmt.Errorf("poll request: %w", err)
+	}
+
+	defer func() { _ = resp.Body.Close() }()
+
+	data, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("read poll body: %w", err)
+	}
+
+	if resp.StatusCode >= httpErrThreshold {
+		return nil, fmt.Errorf(
+			"%w %d: %s",
+			errHTTP, resp.StatusCode, string(data),
+		)
+	}
+
+	var wrapped MessagesResponse
+
+	err = json.Unmarshal(data, &wrapped)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"decode messages: %w", err,
+		)
+	}
+
+	return &PollResult{
+		Messages: wrapped.Messages,
+		LastID:   wrapped.LastID,
+	}, nil
+}
+
+// JoinChannel joins a channel.
+func (client *Client) JoinChannel(channel string) error {
+	return client.SendMessage(
+		&Message{ //nolint:exhaustruct // only command+to needed
+			Command: irc.CmdJoin, To: channel,
+		},
+	)
+}
+
+// PartChannel leaves a channel.
+func (client *Client) PartChannel(channel string) error {
+	return client.SendMessage(
+		&Message{ //nolint:exhaustruct // only command+to needed
+			Command: irc.CmdPart, To: channel,
+		},
+	)
+}
+
+// ListChannels returns all channels on the server.
+func (client *Client) ListChannels() (
+	[]Channel, error,
+) {
+	data, err := client.do(
+		http.MethodGet, "/api/v1/channels", nil,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	var channels []Channel
+
+	err = json.Unmarshal(data, &channels)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"decode channels: %w", err,
+		)
+	}
+
+	return channels, nil
+}
+
+// GetMembers returns members of a channel.
+func (client *Client) GetMembers(
+	channel string,
+) ([]string, error) {
+	name := strings.TrimPrefix(channel, "#")
+
+	data, err := client.do(
+		http.MethodGet,
+		"/api/v1/channels/"+url.PathEscape(name)+
+			"/members",
+		nil,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	var members []string
+
+	err = json.Unmarshal(data, &members)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"unexpected members format: %w", err,
+		)
+	}
+
+	return members, nil
+}
+
+// GetServerInfo returns server info.
+func (client *Client) GetServerInfo() (
+	*ServerInfo, error,
+) {
+	data, err := client.do(
+		http.MethodGet, "/api/v1/server", nil,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	var info ServerInfo
+
+	err = json.Unmarshal(data, &info)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"decode server info: %w", err,
+		)
+	}
+
+	return &info, nil
+}
+
+func (client *Client) do(
+	method, path string,
+	body any,
+) ([]byte, error) {
+	var bodyReader io.Reader
+
+	if body != nil {
+		data, err := json.Marshal(body)
+		if err != nil {
+			return nil, fmt.Errorf("marshal: %w", err)
+		}
+
+		bodyReader = bytes.NewReader(data)
+	}
+
+	request, err := http.NewRequestWithContext(
+		context.Background(),
+		method,
+		client.BaseURL+path,
+		bodyReader,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("request: %w", err)
+	}
+
+	request.Header.Set(
+		"Content-Type", "application/json",
+	)
+
+	if client.Token != "" {
+		request.Header.Set(
+			"Authorization", "Bearer "+client.Token,
+		)
+	}
+
+	resp, err := client.HTTPClient.Do(request)
+	if err != nil {
+		return nil, fmt.Errorf("http: %w", err)
+	}
+
+	defer func() { _ = resp.Body.Close() }()
+
+	data, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("read body: %w", err)
+	}
+
+	if resp.StatusCode >= httpErrThreshold {
+		return data, fmt.Errorf(
+			"%w %d: %s",
+			errHTTP, resp.StatusCode, string(data),
+		)
+	}
+
+	return data, nil
+}

internal/cli/api/hashcash.go

@@ -0,0 +1,79 @@
+package neoircapi
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"math/big"
+	"time"
+)
+
+const (
+	// bitsPerByte is the number of bits in a byte.
+	bitsPerByte = 8
+	// fullByteMask is 0xFF, a mask for all bits in a byte.
+	fullByteMask = 0xFF
+	// counterSpace is the range for random counter seeds.
+	counterSpace = 1 << 48
+)
+
+// MintHashcash computes a hashcash stamp with the given
+// difficulty (leading zero bits) and resource string.
+func MintHashcash(bits int, resource string) string {
+	date := time.Now().UTC().Format("060102")
+	prefix := fmt.Sprintf(
+		"1:%d:%s:%s::", bits, date, resource,
+	)
+
+	for {
+		counter := randomCounter()
+		stamp := prefix + counter
+		hash := sha256.Sum256([]byte(stamp))
+
+		if hasLeadingZeroBits(hash[:], bits) {
+			return stamp
+		}
+	}
+}
+
+// hasLeadingZeroBits checks if hash has at least numBits
+// leading zero bits.
+func hasLeadingZeroBits(
+	hash []byte,
+	numBits int,
+) bool {
+	fullBytes := numBits / bitsPerByte
+	remainBits := numBits % bitsPerByte
+
+	for idx := range fullBytes {
+		if hash[idx] != 0 {
+			return false
+		}
+	}
+
+	if remainBits > 0 && fullBytes < len(hash) {
+		mask := byte(
+			fullByteMask << (bitsPerByte - remainBits),
+		)
+
+		if hash[fullBytes]&mask != 0 {
+			return false
+		}
+	}
+
+	return true
+}
+
+// randomCounter generates a random hex counter string.
+func randomCounter() string {
+	counterVal, err := rand.Int(
+		rand.Reader, big.NewInt(counterSpace),
+	)
+	if err != nil {
+		// Fallback to timestamp-based counter on error.
+		return fmt.Sprintf("%x", time.Now().UnixNano())
+	}
+
+	return hex.EncodeToString(counterVal.Bytes())
+}

internal/cli/api/types.go

@@ -0,0 +1,93 @@
+package neoircapi
+
+import "time"
+
+// SessionRequest is the body for POST /api/v1/session.
+type SessionRequest struct {
+	Nick     string `json:"nick"`
+	Hashcash string `json:"pow_token,omitempty"` //nolint:tagliatelle
+}
+
+// SessionResponse is the response from session creation.
+type SessionResponse struct {
+	ID    int64  `json:"id"`
+	Nick  string `json:"nick"`
+	Token string `json:"token"`
+}
+
+// StateResponse is the response from GET /api/v1/state.
+type StateResponse struct {
+	ID       int64    `json:"id"`
+	Nick     string   `json:"nick"`
+	Channels []string `json:"channels"`
+}
+
+// Message represents a neoirc message envelope.
+type Message struct {
+	Command string   `json:"command"`
+	From    string   `json:"from,omitempty"`
+	To      string   `json:"to,omitempty"`
+	Params  []string `json:"params,omitempty"`
+	Body    any      `json:"body,omitempty"`
+	ID      string   `json:"id,omitempty"`
+	TS      string   `json:"ts,omitempty"`
+	Meta    any      `json:"meta,omitempty"`
+}
+
+// BodyLines returns the body as a string slice.
+func (m *Message) BodyLines() []string {
+	switch bodyVal := m.Body.(type) {
+	case []any:
+		lines := make([]string, 0, len(bodyVal))
+
+		for _, item := range bodyVal {
+			if str, ok := item.(string); ok {
+				lines = append(lines, str)
+			}
+		}
+
+		return lines
+	case []string:
+		return bodyVal
+	default:
+		return nil
+	}
+}
+
+// Channel represents a channel in the list response.
+type Channel struct {
+	Name      string `json:"name"`
+	Topic     string `json:"topic"`
+	Members   int    `json:"members"`
+	CreatedAt string `json:"createdAt"`
+}
+
+// ServerInfo is the response from GET /api/v1/server.
+type ServerInfo struct {
+	Name         string `json:"name"`
+	MOTD         string `json:"motd"`
+	Version      string `json:"version"`
+	HashcashBits int    `json:"hashcash_bits"` //nolint:tagliatelle
+}
+
+// MessagesResponse wraps polling results.
+type MessagesResponse struct {
+	Messages []Message `json:"messages"`
+	LastID   int64     `json:"lastId"`
+}
+
+// PollResult wraps the poll response including the cursor.
+type PollResult struct {
+	Messages []Message
+	LastID   int64
+}
+
+// ParseTS parses the message timestamp.
+func (m *Message) ParseTS() time.Time {
+	t, err := time.Parse(time.RFC3339Nano, m.TS)
+	if err != nil {
+		return time.Now()
+	}
+
+	return t
+}

internal/cli/app.go

@@ -0,0 +1,912 @@
+// Package cli implements the neoirc-cli terminal client.
+package cli
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"sync"
+	"time"
+
+	api "git.eeqj.de/sneak/neoirc/internal/cli/api"
+	"git.eeqj.de/sneak/neoirc/pkg/irc"
+)
+
+const (
+	splitParts  = 2
+	pollTimeout = 15
+	pollRetry   = 2 * time.Second
+	timeFormat  = "15:04"
+)
+
+// App holds the application state.
+type App struct {
+	ui     *UI
+	client *api.Client
+
+	mu        sync.Mutex
+	nick      string
+	target    string
+	connected bool
+	lastQID   int64
+	stopPoll  chan struct{}
+}
+
+// Run creates and runs the CLI application.
+func Run() {
+	app := &App{ //nolint:exhaustruct
+		ui:   NewUI(),
+		nick: "guest",
+	}
+
+	app.ui.OnInput(app.handleInput)
+	app.ui.SetStatus(app.nick, "", "disconnected")
+
+	app.ui.AddStatus(
+		"Welcome to neoirc-cli — an IRC-style client",
+	)
+	app.ui.AddStatus(
+		"Type [yellow]/connect <server-url>" +
+			"[white] to begin, " +
+			"or [yellow]/help[white] for commands",
+	)
+
+	err := app.ui.Run()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+		os.Exit(1)
+	}
+}
+
+func (a *App) handleInput(text string) {
+	if strings.HasPrefix(text, "/") {
+		a.handleCommand(text)
+
+		return
+	}
+
+	a.mu.Lock()
+	target := a.target
+	connected := a.connected
+	a.mu.Unlock()
+
+	if !connected {
+		a.ui.AddStatus(
+			"[red]Not connected. Use /connect <url>",
+		)
+
+		return
+	}
+
+	if target == "" {
+		a.ui.AddStatus(
+			"[red]No target. " +
+				"Use /join #channel or /query nick",
+		)
+
+		return
+	}
+
+	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
+		Command: irc.CmdPrivmsg,
+		To:      target,
+		Body:    []string{text},
+	})
+	if err != nil {
+		a.ui.AddStatus(
+			"[red]Send error: " + err.Error(),
+		)
+
+		return
+	}
+
+	timestamp := time.Now().Format(timeFormat)
+
+	a.mu.Lock()
+	nick := a.nick
+	a.mu.Unlock()
+
+	a.ui.AddLine(target, fmt.Sprintf(
+		"[gray]%s [green]<%s>[white] %s",
+		timestamp, nick, text,
+	))
+}
+
+func (a *App) handleCommand(text string) {
+	parts := strings.SplitN(text, " ", splitParts)
+	cmd := strings.ToLower(parts[0])
+
+	args := ""
+	if len(parts) > 1 {
+		args = parts[1]
+	}
+
+	a.dispatchCommand(cmd, args)
+}
+
+func (a *App) dispatchCommand(cmd, args string) {
+	switch cmd {
+	case "/connect":
+		a.cmdConnect(args)
+	case "/nick":
+		a.cmdNick(args)
+	case "/join":
+		a.cmdJoin(args)
+	case "/part":
+		a.cmdPart(args)
+	case "/msg":
+		a.cmdMsg(args)
+	case "/query":
+		a.cmdQuery(args)
+	case "/topic":
+		a.cmdTopic(args)
+	case "/window", "/w":
+		a.cmdWindow(args)
+	case "/quit":
+		a.cmdQuit()
+	case "/help":
+		a.cmdHelp()
+	default:
+		a.dispatchInfoCommand(cmd, args)
+	}
+}
+
+func (a *App) dispatchInfoCommand(cmd, args string) {
+	switch cmd {
+	case "/names":
+		a.cmdNames()
+	case "/list":
+		a.cmdList()
+	case "/motd":
+		a.cmdMotd()
+	case "/who":
+		a.cmdWho(args)
+	case "/whois":
+		a.cmdWhois(args)
+	default:
+		a.ui.AddStatus(
+			"[red]Unknown command: " + cmd,
+		)
+	}
+}
+
+func (a *App) cmdConnect(serverURL string) {
+	if serverURL == "" {
+		a.ui.AddStatus(
+			"[red]Usage: /connect <server-url>",
+		)
+
+		return
+	}
+
+	serverURL = strings.TrimRight(serverURL, "/")
+
+	a.ui.AddStatus("Connecting to " + serverURL + "...")
+
+	a.mu.Lock()
+	nick := a.nick
+	a.mu.Unlock()
+
+	client := api.NewClient(serverURL)
+
+	resp, err := client.CreateSession(nick)
+	if err != nil {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[red]Connection failed: %v", err,
+		))
+
+		return
+	}
+
+	a.mu.Lock()
+	a.client = client
+	a.nick = resp.Nick
+	a.connected = true
+	a.lastQID = 0
+	a.mu.Unlock()
+
+	a.ui.AddStatus(fmt.Sprintf(
+		"[green]Connected! Nick: %s, Session: %d",
+		resp.Nick, resp.ID,
+	))
+	a.ui.SetStatus(resp.Nick, "", "connected")
+
+	a.stopPoll = make(chan struct{})
+
+	go a.pollLoop()
+}
+
+func (a *App) cmdNick(nick string) {
+	if nick == "" {
+		a.ui.AddStatus(
+			"[red]Usage: /nick <name>",
+		)
+
+		return
+	}
+
+	a.mu.Lock()
+	connected := a.connected
+	a.mu.Unlock()
+
+	if !connected {
+		a.mu.Lock()
+		a.nick = nick
+		a.mu.Unlock()
+
+		a.ui.AddStatus(
+			"Nick set to " + nick +
+				" (will be used on connect)",
+		)
+
+		return
+	}
+
+	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
+		Command: irc.CmdNick,
+		Body:    []string{nick},
+	})
+	if err != nil {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[red]Nick change failed: %v", err,
+		))
+
+		return
+	}
+
+	a.mu.Lock()
+	a.nick = nick
+	target := a.target
+	a.mu.Unlock()
+
+	a.ui.SetStatus(nick, target, "connected")
+	a.ui.AddStatus("Nick changed to " + nick)
+}
+
+func (a *App) cmdJoin(channel string) {
+	if channel == "" {
+		a.ui.AddStatus(
+			"[red]Usage: /join #channel",
+		)
+
+		return
+	}
+
+	if !strings.HasPrefix(channel, "#") {
+		channel = "#" + channel
+	}
+
+	a.mu.Lock()
+	connected := a.connected
+	a.mu.Unlock()
+
+	if !connected {
+		a.ui.AddStatus("[red]Not connected")
+
+		return
+	}
+
+	err := a.client.JoinChannel(channel)
+	if err != nil {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[red]Join failed: %v", err,
+		))
+
+		return
+	}
+
+	a.mu.Lock()
+	a.target = channel
+	nick := a.nick
+	a.mu.Unlock()
+
+	a.ui.SwitchToBuffer(channel)
+	a.ui.AddLine(channel,
+		"[yellow]*** Joined "+channel,
+	)
+	a.ui.SetStatus(nick, channel, "connected")
+}
+
+func (a *App) cmdPart(channel string) {
+	a.mu.Lock()
+	if channel == "" {
+		channel = a.target
+	}
+
+	connected := a.connected
+	a.mu.Unlock()
+
+	if channel == "" ||
+		!strings.HasPrefix(channel, "#") {
+		a.ui.AddStatus("[red]No channel to part")
+
+		return
+	}
+
+	if !connected {
+		a.ui.AddStatus("[red]Not connected")
+
+		return
+	}
+
+	err := a.client.PartChannel(channel)
+	if err != nil {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[red]Part failed: %v", err,
+		))
+
+		return
+	}
+
+	a.ui.AddLine(channel,
+		"[yellow]*** Left "+channel,
+	)
+
+	a.mu.Lock()
+	if a.target == channel {
+		a.target = ""
+	}
+
+	nick := a.nick
+	a.mu.Unlock()
+
+	a.ui.SwitchBuffer(0)
+	a.ui.SetStatus(nick, "", "connected")
+}
+
+func (a *App) cmdMsg(args string) {
+	parts := strings.SplitN(args, " ", splitParts)
+	if len(parts) < splitParts {
+		a.ui.AddStatus(
+			"[red]Usage: /msg <nick> <text>",
+		)
+
+		return
+	}
+
+	target, text := parts[0], parts[1]
+
+	a.mu.Lock()
+	connected := a.connected
+	nick := a.nick
+	a.mu.Unlock()
+
+	if !connected {
+		a.ui.AddStatus("[red]Not connected")
+
+		return
+	}
+
+	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
+		Command: irc.CmdPrivmsg,
+		To:      target,
+		Body:    []string{text},
+	})
+	if err != nil {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[red]Send failed: %v", err,
+		))
+
+		return
+	}
+
+	timestamp := time.Now().Format(timeFormat)
+
+	a.ui.AddLine(target, fmt.Sprintf(
+		"[gray]%s [green]<%s>[white] %s",
+		timestamp, nick, text,
+	))
+}
+
+func (a *App) cmdQuery(nick string) {
+	if nick == "" {
+		a.ui.AddStatus(
+			"[red]Usage: /query <nick>",
+		)
+
+		return
+	}
+
+	a.mu.Lock()
+	a.target = nick
+	myNick := a.nick
+	a.mu.Unlock()
+
+	a.ui.SwitchToBuffer(nick)
+	a.ui.SetStatus(myNick, nick, "connected")
+}
+
+func (a *App) cmdTopic(args string) {
+	a.mu.Lock()
+	target := a.target
+	connected := a.connected
+	a.mu.Unlock()
+
+	if !connected {
+		a.ui.AddStatus("[red]Not connected")
+
+		return
+	}
+
+	if !strings.HasPrefix(target, "#") {
+		a.ui.AddStatus("[red]Not in a channel")
+
+		return
+	}
+
+	if args == "" {
+		err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
+			Command: irc.CmdTopic,
+			To:      target,
+		})
+		if err != nil {
+			a.ui.AddStatus(fmt.Sprintf(
+				"[red]Topic query failed: %v", err,
+			))
+		}
+
+		return
+	}
+
+	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
+		Command: irc.CmdTopic,
+		To:      target,
+		Body:    []string{args},
+	})
+	if err != nil {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[red]Topic set failed: %v", err,
+		))
+	}
+}
+
+func (a *App) cmdNames() {
+	a.mu.Lock()
+	target := a.target
+	connected := a.connected
+	a.mu.Unlock()
+
+	if !connected {
+		a.ui.AddStatus("[red]Not connected")
+
+		return
+	}
+
+	if !strings.HasPrefix(target, "#") {
+		a.ui.AddStatus("[red]Not in a channel")
+
+		return
+	}
+
+	members, err := a.client.GetMembers(target)
+	if err != nil {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[red]Names failed: %v", err,
+		))
+
+		return
+	}
+
+	a.ui.AddLine(target, fmt.Sprintf(
+		"[cyan]*** Members of %s: %s",
+		target, strings.Join(members, " "),
+	))
+}
+
+func (a *App) cmdList() {
+	a.mu.Lock()
+	connected := a.connected
+	a.mu.Unlock()
+
+	if !connected {
+		a.ui.AddStatus("[red]Not connected")
+
+		return
+	}
+
+	channels, err := a.client.ListChannels()
+	if err != nil {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[red]List failed: %v", err,
+		))
+
+		return
+	}
+
+	a.ui.AddStatus("[cyan]*** Channel list:")
+
+	for _, ch := range channels {
+		a.ui.AddStatus(fmt.Sprintf(
+			"  %s (%d members) %s",
+			ch.Name, ch.Members, ch.Topic,
+		))
+	}
+
+	a.ui.AddStatus("[cyan]*** End of channel list")
+}
+
+func (a *App) cmdMotd() {
+	a.mu.Lock()
+	connected := a.connected
+	a.mu.Unlock()
+
+	if !connected {
+		a.ui.AddStatus("[red]Not connected")
+
+		return
+	}
+
+	err := a.client.SendMessage(
+		&api.Message{Command: irc.CmdMotd}, //nolint:exhaustruct
+	)
+	if err != nil {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[red]MOTD failed: %v", err,
+		))
+	}
+}
+
+func (a *App) cmdWho(args string) {
+	a.mu.Lock()
+	connected := a.connected
+	target := a.target
+	a.mu.Unlock()
+
+	if !connected {
+		a.ui.AddStatus("[red]Not connected")
+
+		return
+	}
+
+	channel := args
+	if channel == "" {
+		channel = target
+	}
+
+	if channel == "" ||
+		!strings.HasPrefix(channel, "#") {
+		a.ui.AddStatus(
+			"[red]Usage: /who #channel",
+		)
+
+		return
+	}
+
+	err := a.client.SendMessage(
+		&api.Message{ //nolint:exhaustruct
+			Command: irc.CmdWho, To: channel,
+		},
+	)
+	if err != nil {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[red]WHO failed: %v", err,
+		))
+	}
+}
+
+func (a *App) cmdWhois(args string) {
+	a.mu.Lock()
+	connected := a.connected
+	a.mu.Unlock()
+
+	if !connected {
+		a.ui.AddStatus("[red]Not connected")
+
+		return
+	}
+
+	if args == "" {
+		a.ui.AddStatus(
+			"[red]Usage: /whois <nick>",
+		)
+
+		return
+	}
+
+	err := a.client.SendMessage(
+		&api.Message{ //nolint:exhaustruct
+			Command: irc.CmdWhois, To: args,
+		},
+	)
+	if err != nil {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[red]WHOIS failed: %v", err,
+		))
+	}
+}
+
+func (a *App) cmdWindow(args string) {
+	if args == "" {
+		a.ui.AddStatus(
+			"[red]Usage: /window <number>",
+		)
+
+		return
+	}
+
+	var bufIndex int
+
+	_, _ = fmt.Sscanf(args, "%d", &bufIndex)
+
+	a.ui.SwitchBuffer(bufIndex)
+
+	a.mu.Lock()
+	nick := a.nick
+	a.mu.Unlock()
+
+	if bufIndex >= 0 && bufIndex < a.ui.BufferCount() {
+		buf := a.ui.buffers[bufIndex]
+		if buf.Name != "(status)" {
+			a.mu.Lock()
+			a.target = buf.Name
+			a.mu.Unlock()
+
+			a.ui.SetStatus(
+				nick, buf.Name, "connected",
+			)
+		} else {
+			a.ui.SetStatus(nick, "", "connected")
+		}
+	}
+}
+
+func (a *App) cmdQuit() {
+	a.mu.Lock()
+
+	if a.connected && a.client != nil {
+		_ = a.client.SendMessage(
+			&api.Message{Command: irc.CmdQuit}, //nolint:exhaustruct
+		)
+	}
+
+	if a.stopPoll != nil {
+		close(a.stopPoll)
+	}
+
+	a.mu.Unlock()
+	a.ui.Stop()
+}
+
+func (a *App) cmdHelp() {
+	help := []string{
+		"[cyan]*** neoirc-cli commands:",
+		"  /connect <url>  — Connect to server",
+		"  /nick <name>    — Change nickname",
+		"  /join #channel  — Join channel",
+		"  /part [#chan]    — Leave channel",
+		"  /msg <nick> <text> — Send DM",
+		"  /query <nick>   — Open DM window",
+		"  /topic [text]   — View/set topic",
+		"  /names          — List channel members",
+		"  /list           — List channels",
+		"  /who [#channel] — List users in channel",
+		"  /whois <nick>   — Show user info",
+		"  /motd           — Show message of the day",
+		"  /window <n>     — Switch buffer",
+		"  /quit           — Disconnect and exit",
+		"  /help           — This help",
+		"  Plain text sends to current target.",
+	}
+
+	for _, line := range help {
+		a.ui.AddStatus(line)
+	}
+}
+
+// pollLoop long-polls for messages in the background.
+func (a *App) pollLoop() {
+	for {
+		select {
+		case <-a.stopPoll:
+			return
+		default:
+		}
+
+		a.mu.Lock()
+		client := a.client
+		lastQID := a.lastQID
+		a.mu.Unlock()
+
+		if client == nil {
+			return
+		}
+
+		result, err := client.PollMessages(
+			lastQID, pollTimeout,
+		)
+		if err != nil {
+			time.Sleep(pollRetry)
+
+			continue
+		}
+
+		if result.LastID > 0 {
+			a.mu.Lock()
+			a.lastQID = result.LastID
+			a.mu.Unlock()
+		}
+
+		for i := range result.Messages {
+			a.handleServerMessage(&result.Messages[i])
+		}
+	}
+}
+
+func (a *App) handleServerMessage(msg *api.Message) {
+	timestamp := a.formatTS(msg)
+
+	a.mu.Lock()
+	myNick := a.nick
+	a.mu.Unlock()
+
+	switch msg.Command {
+	case irc.CmdPrivmsg:
+		a.handlePrivmsgEvent(msg, timestamp, myNick)
+	case irc.CmdJoin:
+		a.handleJoinEvent(msg, timestamp)
+	case irc.CmdPart:
+		a.handlePartEvent(msg, timestamp)
+	case irc.CmdQuit:
+		a.handleQuitEvent(msg, timestamp)
+	case irc.CmdNick:
+		a.handleNickEvent(msg, timestamp, myNick)
+	case irc.CmdNotice:
+		a.handleNoticeEvent(msg, timestamp)
+	case irc.CmdTopic:
+		a.handleTopicEvent(msg, timestamp)
+	default:
+		a.handleDefaultEvent(msg, timestamp)
+	}
+}
+
+func (a *App) formatTS(msg *api.Message) string {
+	if msg.TS != "" {
+		return msg.ParseTS().UTC().Format(timeFormat)
+	}
+
+	return time.Now().Format(timeFormat)
+}
+
+func (a *App) handlePrivmsgEvent(
+	msg *api.Message, timestamp, myNick string,
+) {
+	lines := msg.BodyLines()
+	text := strings.Join(lines, " ")
+
+	if msg.From == myNick {
+		return
+	}
+
+	target := msg.To
+	if !strings.HasPrefix(target, "#") {
+		target = msg.From
+	}
+
+	a.ui.AddLine(target, fmt.Sprintf(
+		"[gray]%s [green]<%s>[white] %s",
+		timestamp, msg.From, text,
+	))
+}
+
+func (a *App) handleJoinEvent(
+	msg *api.Message, timestamp string,
+) {
+	if msg.To == "" {
+		return
+	}
+
+	a.ui.AddLine(msg.To, fmt.Sprintf(
+		"[gray]%s [yellow]*** %s has joined %s",
+		timestamp, msg.From, msg.To,
+	))
+}
+
+func (a *App) handlePartEvent(
+	msg *api.Message, timestamp string,
+) {
+	if msg.To == "" {
+		return
+	}
+
+	lines := msg.BodyLines()
+	reason := strings.Join(lines, " ")
+
+	if reason != "" {
+		a.ui.AddLine(msg.To, fmt.Sprintf(
+			"[gray]%s [yellow]*** %s has left %s (%s)",
+			timestamp, msg.From, msg.To, reason,
+		))
+	} else {
+		a.ui.AddLine(msg.To, fmt.Sprintf(
+			"[gray]%s [yellow]*** %s has left %s",
+			timestamp, msg.From, msg.To,
+		))
+	}
+}
+
+func (a *App) handleQuitEvent(
+	msg *api.Message, timestamp string,
+) {
+	lines := msg.BodyLines()
+	reason := strings.Join(lines, " ")
+
+	if reason != "" {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[gray]%s [yellow]*** %s has quit (%s)",
+			timestamp, msg.From, reason,
+		))
+	} else {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[gray]%s [yellow]*** %s has quit",
+			timestamp, msg.From,
+		))
+	}
+}
+
+func (a *App) handleNickEvent(
+	msg *api.Message, timestamp, myNick string,
+) {
+	lines := msg.BodyLines()
+
+	newNick := ""
+	if len(lines) > 0 {
+		newNick = lines[0]
+	}
+
+	if msg.From == myNick && newNick != "" {
+		a.mu.Lock()
+		a.nick = newNick
+
+		target := a.target
+		a.mu.Unlock()
+
+		a.ui.SetStatus(newNick, target, "connected")
+	}
+
+	a.ui.AddStatus(fmt.Sprintf(
+		"[gray]%s [yellow]*** %s is now known as %s",
+		timestamp, msg.From, newNick,
+	))
+}
+
+func (a *App) handleNoticeEvent(
+	msg *api.Message, timestamp string,
+) {
+	lines := msg.BodyLines()
+	text := strings.Join(lines, " ")
+
+	a.ui.AddStatus(fmt.Sprintf(
+		"[gray]%s [magenta]--%s-- %s",
+		timestamp, msg.From, text,
+	))
+}
+
+func (a *App) handleTopicEvent(
+	msg *api.Message, timestamp string,
+) {
+	if msg.To == "" {
+		return
+	}
+
+	lines := msg.BodyLines()
+	text := strings.Join(lines, " ")
+
+	a.ui.AddLine(msg.To, fmt.Sprintf(
+		"[gray]%s [cyan]*** %s set topic: %s",
+		timestamp, msg.From, text,
+	))
+}
+
+func (a *App) handleDefaultEvent(
+	msg *api.Message, timestamp string,
+) {
+	lines := msg.BodyLines()
+	text := strings.Join(lines, " ")
+
+	if text != "" {
+		a.ui.AddStatus(fmt.Sprintf(
+			"[gray]%s [white][%s] %s",
+			timestamp, msg.Command, text,
+		))
+	}
+}

internal/cli/ui.go

@@ -0,0 +1,294 @@
+package cli
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/gdamore/tcell/v2"
+	"github.com/rivo/tview"
+)
+
+// Buffer holds messages for a channel/DM/status window.
+type Buffer struct {
+	Name   string
+	Lines  []string
+	Unread int
+}
+
+// UI manages the terminal interface.
+type UI struct {
+	app       *tview.Application
+	messages  *tview.TextView
+	statusBar *tview.TextView
+	input     *tview.InputField
+	layout    *tview.Flex
+
+	buffers       []*Buffer
+	currentBuffer int
+
+	onInput func(string)
+}
+
+// NewUI creates the tview-based IRC-like UI.
+func NewUI() *UI {
+	ui := &UI{ //nolint:exhaustruct,varnamelen // fields set below; ui is idiomatic
+		app: tview.NewApplication(),
+		buffers: []*Buffer{
+			{Name: "(status)", Lines: nil, Unread: 0},
+		},
+	}
+
+	ui.initMessages()
+	ui.initStatusBar()
+	ui.initInput()
+	ui.initKeyCapture()
+
+	ui.layout = tview.NewFlex().
+		SetDirection(tview.FlexRow).
+		AddItem(ui.messages, 0, 1, false).
+		AddItem(ui.statusBar, 1, 0, false).
+		AddItem(ui.input, 1, 0, true)
+
+	ui.app.SetRoot(ui.layout, true)
+	ui.app.SetFocus(ui.input)
+
+	return ui
+}
+
+// Run starts the UI event loop (blocks).
+func (ui *UI) Run() error {
+	err := ui.app.Run()
+	if err != nil {
+		return fmt.Errorf("run ui: %w", err)
+	}
+
+	return nil
+}
+
+// Stop stops the UI.
+func (ui *UI) Stop() {
+	ui.app.Stop()
+}
+
+// OnInput sets the callback for user input.
+func (ui *UI) OnInput(fn func(string)) {
+	ui.onInput = fn
+}
+
+// AddLine adds a line to the specified buffer.
+func (ui *UI) AddLine(bufferName, line string) {
+	ui.app.QueueUpdateDraw(func() {
+		buf := ui.getOrCreateBuffer(bufferName)
+		buf.Lines = append(buf.Lines, line)
+
+		cur := ui.buffers[ui.currentBuffer]
+		if cur != buf {
+			buf.Unread++
+
+			ui.refreshStatusBar()
+		}
+
+		if cur == buf {
+			_, _ = fmt.Fprintln(ui.messages, line)
+		}
+	})
+}
+
+// AddStatus adds a line to the status buffer.
+func (ui *UI) AddStatus(line string) {
+	ts := time.Now().Format("15:04")
+	ui.AddLine(
+		"(status)",
+		"[gray]"+ts+"[white] "+line,
+	)
+}
+
+// SwitchBuffer switches to the buffer at index n.
+func (ui *UI) SwitchBuffer(bufIndex int) {
+	ui.app.QueueUpdateDraw(func() {
+		if bufIndex < 0 || bufIndex >= len(ui.buffers) {
+			return
+		}
+
+		ui.currentBuffer = bufIndex
+
+		buf := ui.buffers[bufIndex]
+		buf.Unread = 0
+
+		ui.messages.Clear()
+
+		for _, line := range buf.Lines {
+			_, _ = fmt.Fprintln(ui.messages, line)
+		}
+
+		ui.messages.ScrollToEnd()
+		ui.refreshStatusBar()
+	})
+}
+
+// SwitchToBuffer switches to named buffer, creating if
+// needed.
+func (ui *UI) SwitchToBuffer(name string) {
+	ui.app.QueueUpdateDraw(func() {
+		buf := ui.getOrCreateBuffer(name)
+
+		for i, b := range ui.buffers {
+			if b == buf {
+				ui.currentBuffer = i
+
+				break
+			}
+		}
+
+		buf.Unread = 0
+
+		ui.messages.Clear()
+
+		for _, line := range buf.Lines {
+			_, _ = fmt.Fprintln(ui.messages, line)
+		}
+
+		ui.messages.ScrollToEnd()
+		ui.refreshStatusBar()
+	})
+}
+
+// SetStatus updates the status bar text.
+func (ui *UI) SetStatus(
+	nick, target, connStatus string,
+) {
+	ui.app.QueueUpdateDraw(func() {
+		ui.renderStatusBar(nick, target, connStatus)
+	})
+}
+
+// BufferCount returns the number of buffers.
+func (ui *UI) BufferCount() int {
+	return len(ui.buffers)
+}
+
+// BufferIndex returns the index of a named buffer.
+func (ui *UI) BufferIndex(name string) int {
+	for i, buf := range ui.buffers {
+		if buf.Name == name {
+			return i
+		}
+	}
+
+	return -1
+}
+
+func (ui *UI) initMessages() {
+	ui.messages = tview.NewTextView().
+		SetDynamicColors(true).
+		SetScrollable(true).
+		SetWordWrap(true).
+		SetChangedFunc(func() {
+			ui.app.Draw()
+		})
+	ui.messages.SetBorder(false)
+}
+
+func (ui *UI) initStatusBar() {
+	ui.statusBar = tview.NewTextView().
+		SetDynamicColors(true)
+	ui.statusBar.SetBackgroundColor(tcell.ColorNavy)
+	ui.statusBar.SetTextColor(tcell.ColorWhite)
+}
+
+func (ui *UI) initInput() {
+	ui.input = tview.NewInputField().
+		SetFieldBackgroundColor(tcell.ColorBlack).
+		SetFieldTextColor(tcell.ColorWhite)
+
+	ui.input.SetDoneFunc(func(key tcell.Key) {
+		if key != tcell.KeyEnter {
+			return
+		}
+
+		text := ui.input.GetText()
+		if text == "" {
+			return
+		}
+
+		ui.input.SetText("")
+
+		if ui.onInput != nil {
+			ui.onInput(text)
+		}
+	})
+}
+
+func (ui *UI) initKeyCapture() {
+	ui.app.SetInputCapture(
+		func(event *tcell.EventKey) *tcell.EventKey {
+			if event.Modifiers()&tcell.ModAlt == 0 {
+				return event
+			}
+
+			r := event.Rune()
+			if r >= '0' && r <= '9' {
+				idx := int(r - '0')
+				ui.SwitchBuffer(idx)
+
+				return nil
+			}
+
+			return event
+		},
+	)
+}
+
+func (ui *UI) refreshStatusBar() {
+	// Placeholder; full refresh needs nick/target context.
+}
+
+func (ui *UI) renderStatusBar(
+	nick, target, connStatus string,
+) {
+	var unreadParts []string
+
+	for i, buf := range ui.buffers {
+		if buf.Unread > 0 {
+			unreadParts = append(unreadParts,
+				fmt.Sprintf(
+					"%d:%s(%d)",
+					i, buf.Name, buf.Unread,
+				),
+			)
+		}
+	}
+
+	unread := ""
+	if len(unreadParts) > 0 {
+		unread = " [Act: " +
+			strings.Join(unreadParts, ",") + "]"
+	}
+
+	bufInfo := fmt.Sprintf(
+		"[%d:%s]",
+		ui.currentBuffer,
+		ui.buffers[ui.currentBuffer].Name,
+	)
+
+	ui.statusBar.Clear()
+
+	_, _ = fmt.Fprintf(ui.statusBar,
+		" [%s] %s %s %s%s",
+		connStatus, nick, bufInfo, target, unread,
+	)
+}
+
+func (ui *UI) getOrCreateBuffer(name string) *Buffer {
+	for _, buf := range ui.buffers {
+		if buf.Name == name {
+			return buf
+		}
+	}
+
+	buf := &Buffer{Name: name, Lines: nil, Unread: 0}
+	ui.buffers = append(ui.buffers, buf)
+
+	return buf
+}

internal/config/config.go

@@ -45,6 +45,7 @@ type Config struct {
 	ServerName         string
 	FederationKey      string
 	SessionIdleTimeout string
+	HashcashBits       int
 	params             *Params
 	log                *slog.Logger
 }
@@ -76,6 +77,7 @@ func New(
 	viper.SetDefault("SERVER_NAME", "")
 	viper.SetDefault("FEDERATION_KEY", "")
 	viper.SetDefault("SESSION_IDLE_TIMEOUT", "720h")
+	viper.SetDefault("NEOIRC_HASHCASH_BITS", "20")
 
 	err := viper.ReadInConfig()
 	if err != nil {
@@ -101,6 +103,7 @@ func New(
 		ServerName:         viper.GetString("SERVER_NAME"),
 		FederationKey:      viper.GetString("FEDERATION_KEY"),
 		SessionIdleTimeout: viper.GetString("SESSION_IDLE_TIMEOUT"),
+		HashcashBits:       viper.GetInt("NEOIRC_HASHCASH_BITS"),
 		log:                log,
 		params:             &params,
 	}

internal/handlers/api.go

@@ -146,7 +146,8 @@ func (hdlr *Handlers) handleCreateSession(
 	request *http.Request,
 ) {
 	type createRequest struct {
-		Nick string `json:"nick"`
+		Nick     string `json:"nick"`
+		Hashcash string `json:"pow_token,omitempty"` //nolint:tagliatelle
 	}
 
 	var payload createRequest
@@ -162,6 +163,32 @@ func (hdlr *Handlers) handleCreateSession(
 		return
 	}
 
+	// Validate hashcash proof-of-work if configured.
+	if hdlr.params.Config.HashcashBits > 0 {
+		if payload.Hashcash == "" {
+			hdlr.respondError(
+				writer, request,
+				"hashcash proof-of-work required",
+				http.StatusPaymentRequired,
+			)
+
+			return
+		}
+
+		err = hdlr.hashcashVal.Validate(
+			payload.Hashcash, hdlr.params.Config.HashcashBits,
+		)
+		if err != nil {
+			hdlr.respondError(
+				writer, request,
+				"invalid hashcash stamp: "+err.Error(),
+				http.StatusPaymentRequired,
+			)
+
+			return
+		}
+	}
+
 	payload.Nick = strings.TrimSpace(payload.Nick)
 
 	if !validNickRe.MatchString(payload.Nick) {
@@ -2392,11 +2419,19 @@ func (hdlr *Handlers) HandleServerInfo() http.HandlerFunc {
 			return
 		}
 
-		hdlr.respondJSON(writer, request, map[string]any{
+		resp := map[string]any{
 			"name":    hdlr.params.Config.ServerName,
 			"version": hdlr.params.Globals.Version,
 			"motd":    hdlr.params.Config.MOTD,
 			"users":   users,
-		}, http.StatusOK)
+		}
+
+		if hdlr.params.Config.HashcashBits > 0 {
+			resp["hashcash_bits"] = hdlr.params.Config.HashcashBits
+		}
+
+		hdlr.respondJSON(
+			writer, request, resp, http.StatusOK,
+		)
 	}
 }

internal/handlers/api_test.go

@@ -85,6 +85,7 @@ func newTestServer(
 
 				cfg.DBURL = dbURL
 				cfg.Port = 0
+				cfg.HashcashBits = 0
 
 				return cfg, nil
 			},

internal/handlers/handlers.go

@@ -13,6 +13,7 @@ import (
 	"git.eeqj.de/sneak/neoirc/internal/config"
 	"git.eeqj.de/sneak/neoirc/internal/db"
 	"git.eeqj.de/sneak/neoirc/internal/globals"
+	"git.eeqj.de/sneak/neoirc/internal/hashcash"
 	"git.eeqj.de/sneak/neoirc/internal/healthcheck"
 	"git.eeqj.de/sneak/neoirc/internal/logger"
 	"go.uber.org/fx"
@@ -39,6 +40,7 @@ type Handlers struct {
 	log           *slog.Logger
 	hc            *healthcheck.Healthcheck
 	broker        *broker.Broker
+	hashcashVal   *hashcash.Validator
 	cancelCleanup context.CancelFunc
 }
 
@@ -47,11 +49,17 @@ func New(
 	lifecycle fx.Lifecycle,
 	params Params,
 ) (*Handlers, error) {
+	resource := params.Config.ServerName
+	if resource == "" {
+		resource = "neoirc"
+	}
+
 	hdlr := &Handlers{ //nolint:exhaustruct // cancelCleanup set in startCleanup
-		params: &params,
-		log:    params.Logger.Get(),
-		hc:     params.Healthcheck,
-		broker: broker.New(),
+		params:      &params,
+		log:         params.Logger.Get(),
+		hc:          params.Healthcheck,
+		broker:      broker.New(),
+		hashcashVal: hashcash.NewValidator(resource),
 	}
 
 	lifecycle.Append(fx.Hook{

internal/hashcash/hashcash.go

@@ -0,0 +1,277 @@
+// Package hashcash implements SHA-256-based hashcash
+// proof-of-work validation for abuse prevention.
+//
+// Stamp format: 1:bits:YYMMDD:resource::counter.
+//
+// The SHA-256 hash of the entire stamp string must have
+// at least `bits` leading zero bits.
+package hashcash
+
+import (
+	"crypto/sha256"
+	"errors"
+	"fmt"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+)
+
+const (
+	// stampVersion is the only supported hashcash version.
+	stampVersion = "1"
+	// stampFields is the number of fields in a stamp.
+	stampFields = 6
+	// maxStampAge is how old a stamp can be before
+	// rejection.
+	maxStampAge = 48 * time.Hour
+	// maxFutureSkew allows stamps slightly in the future.
+	maxFutureSkew = 1 * time.Hour
+	// pruneInterval controls how often expired stamps are
+	// removed from the spent set.
+	pruneInterval = 10 * time.Minute
+	// dateFormatShort is the YYMMDD date layout.
+	dateFormatShort = "060102"
+	// dateFormatLong is the YYMMDDHHMMSS date layout.
+	dateFormatLong = "060102150405"
+	// dateShortLen is the length of YYMMDD.
+	dateShortLen = 6
+	// dateLongLen is the length of YYMMDDHHMMSS.
+	dateLongLen = 12
+	// bitsPerByte is the number of bits in a byte.
+	bitsPerByte = 8
+	// fullByteMask is 0xFF, a mask for all bits in a byte.
+	fullByteMask = 0xFF
+)
+
+var (
+	errInvalidFields    = errors.New("invalid stamp field count")
+	errBadVersion       = errors.New("unsupported stamp version")
+	errInsufficientBits = errors.New("insufficient difficulty")
+	errWrongResource    = errors.New("wrong resource")
+	errStampExpired     = errors.New("stamp expired")
+	errStampFuture      = errors.New("stamp date in future")
+	errProofFailed      = errors.New("proof-of-work failed")
+	errStampReused      = errors.New("stamp already used")
+	errBadDateFormat    = errors.New("unrecognized date format")
+)
+
+// Validator checks hashcash stamps for validity and
+// prevents replay attacks via an in-memory spent set.
+type Validator struct {
+	resource string
+	mu       sync.Mutex
+	spent    map[string]time.Time
+}
+
+// NewValidator creates a Validator for the given resource.
+func NewValidator(resource string) *Validator {
+	validator := &Validator{
+		resource: resource,
+		mu:       sync.Mutex{},
+		spent:    make(map[string]time.Time),
+	}
+
+	go validator.pruneLoop()
+
+	return validator
+}
+
+// Validate checks a hashcash stamp. It returns nil if the
+// stamp is valid and has not been seen before.
+func (v *Validator) Validate(
+	stamp string,
+	requiredBits int,
+) error {
+	if requiredBits <= 0 {
+		return nil
+	}
+
+	parts := strings.Split(stamp, ":")
+	if len(parts) != stampFields {
+		return fmt.Errorf(
+			"%w: expected %d, got %d",
+			errInvalidFields, stampFields, len(parts),
+		)
+	}
+
+	version := parts[0]
+	bitsStr := parts[1]
+	dateStr := parts[2]
+	resource := parts[3]
+
+	if err := v.validateHeader(
+		version, bitsStr, resource, requiredBits,
+	); err != nil {
+		return err
+	}
+
+	stampTime, err := parseStampDate(dateStr)
+	if err != nil {
+		return err
+	}
+
+	if err := validateTime(stampTime); err != nil {
+		return err
+	}
+
+	if err := validateProof(
+		stamp, requiredBits,
+	); err != nil {
+		return err
+	}
+
+	return v.checkAndRecordStamp(stamp, stampTime)
+}
+
+func (v *Validator) validateHeader(
+	version, bitsStr, resource string,
+	requiredBits int,
+) error {
+	if version != stampVersion {
+		return fmt.Errorf(
+			"%w: %s", errBadVersion, version,
+		)
+	}
+
+	claimedBits, err := strconv.Atoi(bitsStr)
+	if err != nil || claimedBits < requiredBits {
+		return fmt.Errorf(
+			"%w: need %d bits",
+			errInsufficientBits, requiredBits,
+		)
+	}
+
+	if resource != v.resource {
+		return fmt.Errorf(
+			"%w: got %q, want %q",
+			errWrongResource, resource, v.resource,
+		)
+	}
+
+	return nil
+}
+
+func validateTime(stampTime time.Time) error {
+	now := time.Now()
+
+	if now.Sub(stampTime) > maxStampAge {
+		return errStampExpired
+	}
+
+	if stampTime.Sub(now) > maxFutureSkew {
+		return errStampFuture
+	}
+
+	return nil
+}
+
+func validateProof(stamp string, requiredBits int) error {
+	hash := sha256.Sum256([]byte(stamp))
+	if !hasLeadingZeroBits(hash[:], requiredBits) {
+		return fmt.Errorf(
+			"%w: need %d leading zero bits",
+			errProofFailed, requiredBits,
+		)
+	}
+
+	return nil
+}
+
+func (v *Validator) checkAndRecordStamp(
+	stamp string,
+	stampTime time.Time,
+) error {
+	v.mu.Lock()
+	defer v.mu.Unlock()
+
+	if _, ok := v.spent[stamp]; ok {
+		return errStampReused
+	}
+
+	v.spent[stamp] = stampTime
+
+	return nil
+}
+
+// hasLeadingZeroBits checks if the hash has at least n
+// leading zero bits.
+func hasLeadingZeroBits(hash []byte, numBits int) bool {
+	fullBytes := numBits / bitsPerByte
+	remainBits := numBits % bitsPerByte
+
+	for idx := range fullBytes {
+		if hash[idx] != 0 {
+			return false
+		}
+	}
+
+	if remainBits > 0 && fullBytes < len(hash) {
+		mask := byte(
+			fullByteMask << (bitsPerByte - remainBits),
+		)
+
+		if hash[fullBytes]&mask != 0 {
+			return false
+		}
+	}
+
+	return true
+}
+
+// parseStampDate parses a hashcash date stamp.
+// Supports YYMMDD and YYMMDDHHMMSS formats.
+func parseStampDate(dateStr string) (time.Time, error) {
+	switch len(dateStr) {
+	case dateShortLen:
+		parsed, err := time.Parse(
+			dateFormatShort, dateStr,
+		)
+		if err != nil {
+			return time.Time{}, fmt.Errorf(
+				"parse date: %w", err,
+			)
+		}
+
+		return parsed, nil
+	case dateLongLen:
+		parsed, err := time.Parse(
+			dateFormatLong, dateStr,
+		)
+		if err != nil {
+			return time.Time{}, fmt.Errorf(
+				"parse date: %w", err,
+			)
+		}
+
+		return parsed, nil
+	default:
+		return time.Time{}, fmt.Errorf(
+			"%w: %q", errBadDateFormat, dateStr,
+		)
+	}
+}
+
+// pruneLoop periodically removes expired stamps from the
+// spent set.
+func (v *Validator) pruneLoop() {
+	ticker := time.NewTicker(pruneInterval)
+	defer ticker.Stop()
+
+	for range ticker.C {
+		v.prune()
+	}
+}
+
+func (v *Validator) prune() {
+	cutoff := time.Now().Add(-maxStampAge)
+
+	v.mu.Lock()
+	defer v.mu.Unlock()
+
+	for stamp, stampTime := range v.spent {
+		if stampTime.Before(cutoff) {
+			delete(v.spent, stamp)
+		}
+	}
+}

internal/hashcash/hashcash_test.go

@@ -0,0 +1,261 @@
+package hashcash_test
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"math/big"
+	"testing"
+	"time"
+
+	"git.eeqj.de/sneak/neoirc/internal/hashcash"
+)
+
+const testBits = 2
+
+// mintStampWithDate creates a valid hashcash stamp using
+// the given date string.
+func mintStampWithDate(
+	tb testing.TB,
+	bits int,
+	resource string,
+	date string,
+) string {
+	tb.Helper()
+
+	prefix := fmt.Sprintf(
+		"1:%d:%s:%s::", bits, date, resource,
+	)
+
+	for {
+		counterVal, err := rand.Int(
+			rand.Reader, big.NewInt(1<<48),
+		)
+		if err != nil {
+			tb.Fatalf("random counter: %v", err)
+		}
+
+		stamp := prefix + hex.EncodeToString(
+			counterVal.Bytes(),
+		)
+		hash := sha256.Sum256([]byte(stamp))
+
+		if hasLeadingZeroBits(hash[:], bits) {
+			return stamp
+		}
+	}
+}
+
+// hasLeadingZeroBits checks if hash has at least numBits
+// leading zero bits. Duplicated here for test minting.
+func hasLeadingZeroBits(
+	hash []byte,
+	numBits int,
+) bool {
+	fullBytes := numBits / 8
+	remainBits := numBits % 8
+
+	for idx := range fullBytes {
+		if hash[idx] != 0 {
+			return false
+		}
+	}
+
+	if remainBits > 0 && fullBytes < len(hash) {
+		mask := byte(0xFF << (8 - remainBits))
+
+		if hash[fullBytes]&mask != 0 {
+			return false
+		}
+	}
+
+	return true
+}
+
+func todayDate() string {
+	return time.Now().UTC().Format("060102")
+}
+
+func TestMintAndValidate(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewValidator("test-resource")
+	stamp := mintStampWithDate(
+		t, testBits, "test-resource", todayDate(),
+	)
+
+	err := validator.Validate(stamp, testBits)
+	if err != nil {
+		t.Fatalf("valid stamp rejected: %v", err)
+	}
+}
+
+func TestReplayDetection(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewValidator("test-resource")
+	stamp := mintStampWithDate(
+		t, testBits, "test-resource", todayDate(),
+	)
+
+	err := validator.Validate(stamp, testBits)
+	if err != nil {
+		t.Fatalf("first use failed: %v", err)
+	}
+
+	err = validator.Validate(stamp, testBits)
+	if err == nil {
+		t.Fatal("replay not detected")
+	}
+}
+
+func TestResourceMismatch(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewValidator("correct-resource")
+	stamp := mintStampWithDate(
+		t, testBits, "wrong-resource", todayDate(),
+	)
+
+	err := validator.Validate(stamp, testBits)
+	if err == nil {
+		t.Fatal("expected resource mismatch error")
+	}
+}
+
+func TestInvalidStampFormat(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewValidator("test-resource")
+
+	err := validator.Validate(
+		"not:a:valid:stamp", testBits,
+	)
+	if err == nil {
+		t.Fatal("expected error for bad format")
+	}
+}
+
+func TestBadVersion(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewValidator("test-resource")
+	stamp := fmt.Sprintf(
+		"2:%d:%s:%s::abc123",
+		testBits, todayDate(), "test-resource",
+	)
+
+	err := validator.Validate(stamp, testBits)
+	if err == nil {
+		t.Fatal("expected bad version error")
+	}
+}
+
+func TestInsufficientDifficulty(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewValidator("test-resource")
+	// Claimed bits=1, but we require testBits=2.
+	stamp := fmt.Sprintf(
+		"1:1:%s:%s::counter",
+		todayDate(), "test-resource",
+	)
+
+	err := validator.Validate(stamp, testBits)
+	if err == nil {
+		t.Fatal("expected insufficient bits error")
+	}
+}
+
+func TestExpiredStamp(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewValidator("test-resource")
+	oldDate := time.Now().Add(-72 * time.Hour).
+		UTC().Format("060102")
+	stamp := mintStampWithDate(
+		t, testBits, "test-resource", oldDate,
+	)
+
+	err := validator.Validate(stamp, testBits)
+	if err == nil {
+		t.Fatal("expected expired stamp error")
+	}
+}
+
+func TestZeroBitsSkipsValidation(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewValidator("test-resource")
+
+	err := validator.Validate("garbage", 0)
+	if err != nil {
+		t.Fatalf("zero bits should skip: %v", err)
+	}
+}
+
+func TestLongDateFormat(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewValidator("test-resource")
+	longDate := time.Now().UTC().Format("060102150405")
+	stamp := mintStampWithDate(
+		t, testBits, "test-resource", longDate,
+	)
+
+	err := validator.Validate(stamp, testBits)
+	if err != nil {
+		t.Fatalf("long date stamp rejected: %v", err)
+	}
+}
+
+func TestBadDateFormat(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewValidator("test-resource")
+	stamp := fmt.Sprintf(
+		"1:%d:BADDATE:%s::counter",
+		testBits, "test-resource",
+	)
+
+	err := validator.Validate(stamp, testBits)
+	if err == nil {
+		t.Fatal("expected bad date error")
+	}
+}
+
+func TestMultipleUniqueStamps(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewValidator("test-resource")
+
+	for range 5 {
+		stamp := mintStampWithDate(
+			t, testBits, "test-resource", todayDate(),
+		)
+
+		err := validator.Validate(stamp, testBits)
+		if err != nil {
+			t.Fatalf("unique stamp rejected: %v", err)
+		}
+	}
+}
+
+func TestHigherBitsStillValid(t *testing.T) {
+	t.Parallel()
+
+	// Mint with bits=4 but validate requiring only 2.
+	validator := hashcash.NewValidator("test-resource")
+	stamp := mintStampWithDate(
+		t, 4, "test-resource", todayDate(),
+	)
+
+	err := validator.Validate(stamp, testBits)
+	if err != nil {
+		t.Fatalf(
+			"higher-difficulty stamp rejected: %v",
+			err,
+		)
+	}
+}