feat: implement hashcash proof-of-work for session creation

Add SHA-256-based hashcash proof-of-work requirement to POST /session
to prevent abuse via rapid session creation. The server advertises the
required difficulty via GET /server (hashcash_bits field), and clients
must include a valid stamp in the X-Hashcash request header.

Server-side:
- New internal/hashcash package with stamp validation (format, bits,
  date, resource, replay prevention via in-memory spent set)
- Config: NEOIRC_HASHCASH_BITS env var (default 20, set 0 to disable)
- GET /server includes hashcash_bits when > 0
- POST /session validates X-Hashcash header when enabled
- Returns HTTP 402 for missing/invalid stamps

Client-side:
- SPA: fetches hashcash_bits from /server, computes stamp using Web
  Crypto API with batched SHA-256, shows 'Computing proof-of-work...'
  feedback during computation
- CLI: api package gains MintHashcash() function, CreateSession()
  auto-fetches server info and computes stamp when required

Stamp format: 1:bits:YYMMDD:resource::counter (standard hashcash)

closes #11

README.md

@@ -987,7 +987,18 @@ the format:
 
 Create a new user session. This is the entry point for all clients.
 
-**Request:**
+If the server requires hashcash proof-of-work (see
+[Hashcash Proof-of-Work](#hashcash-proof-of-work)), the client must include a
+valid stamp in the `X-Hashcash` request header. The required difficulty is
+advertised via `GET /api/v1/server` in the `hashcash_bits` field.
+
+**Request Headers:**
+
+| Header       | Required | Description |
+|--------------|----------|-------------|
+| `X-Hashcash` | Conditional | Hashcash stamp (required when server has `hashcash_bits` > 0) |
+
+**Request Body:**
 ```json
 {"nick": "alice"}
 ```
@@ -1016,12 +1027,15 @@ Create a new user session. This is the entry point for all clients.
 | Status | Error | When |
 |--------|-------|------|
 | 400    | `nick must be 1-32 characters` | Empty or too-long nick |
+| 402    | `hashcash proof-of-work required` | Missing `X-Hashcash` header when hashcash is enabled |
+| 402    | `invalid hashcash stamp: ...` | Stamp fails validation (wrong bits, expired, reused, etc.) |
 | 409    | `nick already taken` | Another active session holds this nick |
 
 **curl example:**
 ```bash
 TOKEN=$(curl -s -X POST http://localhost:8080/api/v1/session \
   -H 'Content-Type: application/json' \
+  -H 'X-Hashcash: 1:20:260310:neoirc::3a2f1' \
   -d '{"nick":"alice"}' | jq -r .token)
 echo $TOKEN
 ```
@@ -1363,15 +1377,17 @@ Return server metadata. No authentication required.
 {
   "name": "My NeoIRC Server",
   "motd": "Welcome! Be nice.",
-  "users": 42
+  "users": 42,
+  "hashcash_bits": 20
 }
 ```
 
 | Field           | Type    | Description |
-|---------|---------|-------------|
+|-----------------|---------|-------------|
 | `name`          | string  | Server display name |
 | `motd`          | string  | Message of the day |
 | `users`         | integer | Number of currently active user sessions |
+| `hashcash_bits` | integer | Required proof-of-work difficulty (leading zero bits). Only present when > 0. See [Hashcash Proof-of-Work](#hashcash-proof-of-work). |
 
 ### GET /.well-known/healthcheck.json — Health Check
 
@@ -1805,6 +1821,7 @@ directory is also loaded automatically via
 | `SENTRY_DSN`       | string  | `""`                                 | Sentry error tracking DSN (optional) |
 | `METRICS_USERNAME` | string  | `""`                                 | Basic auth username for `/metrics` endpoint. If empty, metrics endpoint is disabled. |
 | `METRICS_PASSWORD` | string  | `""`                                 | Basic auth password for `/metrics` endpoint |
+| `NEOIRC_HASHCASH_BITS` | int | `20`                               | Required hashcash proof-of-work difficulty (leading zero bits in SHA-256) for session creation. Set to `0` to disable. |
 | `MAINTENANCE_MODE` | bool    | `false`                              | Maintenance mode flag (reserved) |
 
 ### Example `.env` file
@@ -1816,6 +1833,7 @@ MOTD=Welcome! Be excellent to each other.
 DEBUG=false
 DBURL=file:///var/lib/neoirc/state.db?_journal_mode=WAL
 SESSION_IDLE_TIMEOUT=24h
+NEOIRC_HASHCASH_BITS=20
 ```
 
 ---
@@ -2098,62 +2116,102 @@ Clients should handle these message commands from the queue:
 
 ## Rate Limiting & Abuse Prevention
 
-Session creation (`POST /api/v1/session`) will require a
+### Hashcash Proof-of-Work
+
+Session creation (`POST /api/v1/session`) requires a
 [hashcash](https://en.wikipedia.org/wiki/Hashcash)-style proof-of-work token.
 This is the primary defense against resource exhaustion — no CAPTCHAs, no
 account registration, no IP-based rate limits that punish shared networks.
 
 ### How It Works
 
-1. Client requests a challenge: `GET /api/v1/challenge`
+1. Client fetches server info: `GET /api/v1/server` returns a `hashcash_bits`
-   ```json
+   field (e.g., `20`) indicating the required difficulty.
-   → {"nonce": "random-hex-string", "difficulty": 20, "expires": "2026-02-10T20:01:00Z"}
+2. Client computes a hashcash stamp: find a counter value such that the
-   ```
+   SHA-256 hash of the stamp string has the required number of leading zero
-2. Server returns a nonce and a required difficulty (number of leading zero
+   bits.
-   bits in the SHA-256 hash)
+3. Client includes the stamp in the `X-Hashcash` request header when creating
-3. Client finds a counter value such that `SHA-256(nonce || ":" || counter)`
+   a session: `POST /api/v1/session`.
-   has the required number of leading zero bits:
+4. Server validates the stamp:
-   ```
+   - Version is `1`
-   SHA-256("a1b2c3:0")     = 0xf3a1...  (0 leading zeros — no good)
+   - Claimed bits ≥ required bits
-   SHA-256("a1b2c3:1")     = 0x8c72...  (0 leading zeros — no good)
+   - Resource matches the server name
-   ...
+   - Date is within 48 hours (not expired, not too far in the future)
-   SHA-256("a1b2c3:94217") = 0x00003a... (20 leading zero bits — success!)
+   - SHA-256 hash has the required leading zero bits
-   ```
+   - Stamp has not been used before (replay prevention)
-4. Client submits the proof with the session request:
-   ```json
-   POST /api/v1/session
-   {"nick": "alice", "proof": {"nonce": "a1b2c3", "counter": 94217}}
-   ```
-5. Server verifies:
-   - Nonce was issued by this server and hasn't expired
-   - Nonce hasn't been used before (prevent replay)
-   - `SHA-256(nonce || ":" || counter)` has the required leading zeros
-   - If valid, create the session normally
 
-### Adaptive Difficulty
+### Stamp Format
 
-The required difficulty scales with server load. Under normal conditions, the
+Standard hashcash format:
-cost is negligible (a few milliseconds of CPU). As concurrent sessions or
-session creation rate increases, difficulty rises — making bulk session creation
-exponentially more expensive for attackers while remaining cheap for legitimate
-single-user connections.
 
-| Server Load        | Difficulty (bits) | Approx. Client CPU |
+```
-|--------------------|-------------------|--------------------|
+1:bits:date:resource::counter
-| Normal (< 100/min) | 16                | ~1ms               |
+```
-| Elevated           | 20                | ~15ms              |
-| High               | 24                | ~250ms             |
-| Under attack       | 28+               | ~4s+               |
 
-Each additional bit of difficulty doubles the expected work. An attacker
+| Field      | Description |
-creating 1000 sessions at difficulty 28 needs ~4000 CPU-seconds; a legitimate
+|------------|-------------|
-user creating one session needs ~4 seconds once and never again for the
+| `1`        | Version (always `1`) |
-duration of their session.
+| `bits`     | Claimed difficulty (must be ≥ server's `hashcash_bits`) |
+| `date`     | Date stamp in `YYMMDD` or `YYMMDDHHMMSS` format (UTC) |
+| `resource` | The server name (from `GET /api/v1/server`; defaults to `neoirc`) |
+| (empty)    | Extension field (unused) |
+| `counter`  | Hex counter value found by the client to satisfy the PoW |
+
+**Example stamp:** `1:20:260310:neoirc::3a2f1b`
+
+The SHA-256 hash of this entire string must have at least 20 leading zero bits.
+
+### Computing a Stamp
+
+```bash
+# Pseudocode
+bits = 20
+resource = "neoirc"
+date = "260310"         # YYMMDD in UTC
+counter = 0
+
+loop:
+    stamp = "1:{bits}:{date}:{resource}::{hex(counter)}"
+    hash = SHA-256(stamp)
+    if leading_zero_bits(hash) >= bits:
+        return stamp
+    counter++
+```
+
+At difficulty 20, this requires approximately 2^20 (~1M) hash attempts on
+average, taking roughly 0.5–2 seconds on modern hardware.
+
+### Client Integration
+
+Both the embedded web SPA and the CLI client automatically handle hashcash:
+
+1. Fetch `GET /api/v1/server` to read `hashcash_bits`
+2. If `hashcash_bits > 0`, compute a valid stamp
+3. Include the stamp in the `X-Hashcash` header on `POST /api/v1/session`
+
+The web SPA uses the Web Crypto API (`crypto.subtle.digest`) for SHA-256
+computation with batched parallelism. The CLI client uses Go's `crypto/sha256`.
+
+### Configuration
+
+Set `NEOIRC_HASHCASH_BITS` to control difficulty:
+
+| Value | Effect | Approx. Client CPU |
+|-------|--------|---------------------|
+| `0`   | Disabled (no proof-of-work required) | — |
+| `16`  | Light protection | ~1ms |
+| `20`  | Default — good balance | ~0.5–2s |
+| `24`  | Strong protection | ~10–30s |
+| `28`  | Very strong (may frustrate users) | ~2–10min |
+
+Each additional bit doubles the expected work. An attacker creating 1000
+sessions at difficulty 20 needs ~1000–2000 CPU-seconds; a legitimate user
+creating one session pays once and keeps their session.
 
 ### Why Hashcash and Not Rate Limits?
 
 - **No state to track**: No IP tables, no token buckets, no sliding windows.
-  The server only needs to verify a hash.
+  The server only needs to verify a single hash.
 - **Works through NATs and proxies**: Doesn't punish shared IPs (university
   campuses, corporate networks, Tor exits). Every client computes their own
   proof independently.
@@ -2161,36 +2219,9 @@ duration of their session.
   (one SHA-256 hash) regardless of difficulty. Only the client does more work.
 - **Fits the "no accounts" philosophy**: Proof-of-work is the cost of entry.
   No registration, no email, no phone number, no CAPTCHA. Just compute.
-- **Trivial for legitimate clients**: A single-user client pays ~1ms of CPU
-  once. A botnet trying to create thousands of sessions pays exponentially more.
 - **Language-agnostic**: SHA-256 is available in every programming language.
   The proof computation is trivially implementable in any client.
 
-### Challenge Endpoint (Planned)
-
-```
-GET /api/v1/challenge
-```
-
-**Response:** `200 OK`
-```json
-{
-  "nonce": "a1b2c3d4e5f6...",
-  "difficulty": 20,
-  "algorithm": "sha256",
-  "expires": "2026-02-10T20:01:00Z"
-}
-```
-
-| Field        | Type    | Description |
-|--------------|---------|-------------|
-| `nonce`      | string  | Server-generated random hex string (32+ chars) |
-| `difficulty` | integer | Required number of leading zero bits in the hash |
-| `algorithm`  | string  | Hash algorithm (always `sha256` for now) |
-| `expires`    | string  | ISO 8601 expiry time for this challenge |
-
-**Status:** Not yet implemented. Tracked for post-MVP.
-
 ---
 
 ## Roadmap
@@ -2219,7 +2250,7 @@ GET /api/v1/challenge
 
 ### Post-MVP (Planned)
 
-- [ ] **Hashcash proof-of-work** for session creation (abuse prevention)
+- [x] **Hashcash proof-of-work** for session creation (abuse prevention)
 - [ ] **Queue pruning** — delete old queue entries per `QUEUE_MAX_AGE`
 - [ ] **Message rotation** — enforce `MAX_HISTORY` per channel
 - [ ] **Channel modes** — enforce `+i`, `+m`, `+s`, `+t`, `+n`

cmd/neoirc-cli/api/client.go

@@ -43,13 +43,34 @@ func NewClient(baseURL string) *Client {
 }
 
 // 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) {
-	data, err := client.do(
+	// Fetch server info to check for hashcash requirement.
+	info, err := client.GetServerInfo()
+
+	var headers map[string]string
+
+	if err == nil && info.HashcashBits > 0 {
+		resource := info.Name
+		if resource == "" {
+			resource = "neoirc"
+		}
+
+		stamp := MintHashcash(info.HashcashBits, resource)
+		headers = map[string]string{
+			"X-Hashcash": stamp,
+		}
+	}
+
+	data, err := client.doWithHeaders(
 		http.MethodPost,
 		"/api/v1/session",
 		&SessionRequest{Nick: nick},
+		headers,
 	)
 	if err != nil {
 		return nil, err
@@ -261,6 +282,16 @@ func (client *Client) GetServerInfo() (
 func (client *Client) do(
 	method, path string,
 	body any,
+) ([]byte, error) {
+	return client.doWithHeaders(
+		method, path, body, nil,
+	)
+}
+
+func (client *Client) doWithHeaders(
+	method, path string,
+	body any,
+	extraHeaders map[string]string,
 ) ([]byte, error) {
 	var bodyReader io.Reader
 
@@ -293,6 +324,10 @@ func (client *Client) do(
 		)
 	}
 
+	for key, val := range extraHeaders {
+		request.Header.Set(key, val)
+	}
+
 	resp, err := client.HTTPClient.Do(request)
 	if err != nil {
 		return nil, fmt.Errorf("http: %w", err)

cmd/neoirc-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())
+}

cmd/neoirc-cli/api/types.go

@@ -66,6 +66,7 @@ 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.

internal/config/config.go

@@ -44,6 +44,7 @@ type Config struct {
 	ServerName         string
 	FederationKey      string
 	SessionIdleTimeout string
+	HashcashBits       int
 	params             *Params
 	log                *slog.Logger
 }
@@ -74,6 +75,7 @@ func New(
 	viper.SetDefault("SERVER_NAME", "")
 	viper.SetDefault("FEDERATION_KEY", "")
 	viper.SetDefault("SESSION_IDLE_TIMEOUT", "24h")
+	viper.SetDefault("NEOIRC_HASHCASH_BITS", "20")
 
 	err := viper.ReadInConfig()
 	if err != nil {
@@ -98,6 +100,7 @@ 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

@@ -144,6 +144,33 @@ func (hdlr *Handlers) handleCreateSession(
 	writer http.ResponseWriter,
 	request *http.Request,
 ) {
+	// Validate hashcash proof-of-work if configured.
+	if hdlr.params.Config.HashcashBits > 0 {
+		stamp := request.Header.Get("X-Hashcash")
+		if stamp == "" {
+			hdlr.respondError(
+				writer, request,
+				"hashcash proof-of-work required",
+				http.StatusPaymentRequired,
+			)
+
+			return
+		}
+
+		err := hdlr.hashcashVal.Validate(
+			stamp, hdlr.params.Config.HashcashBits,
+		)
+		if err != nil {
+			hdlr.respondError(
+				writer, request,
+				"invalid hashcash stamp: "+err.Error(),
+				http.StatusPaymentRequired,
+			)
+
+			return
+		}
+	}
+
 	type createRequest struct {
 		Nick string `json:"nick"`
 	}
@@ -2335,10 +2362,18 @@ func (hdlr *Handlers) HandleServerInfo() http.HandlerFunc {
 			return
 		}
 
-		hdlr.respondJSON(writer, request, map[string]any{
+		resp := map[string]any{
 			"name":  hdlr.params.Config.ServerName,
 			"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(),
+		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)
+		}
+	}
+}