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)  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>
2026-03-13 00:38:41 +01:00
parent f2e7a6ec85
commit 75cecd9803
14 changed files with 1795 additions and 1008 deletions
--- a/internal/hashcash/hashcash.go
+++ b/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)
+		}
+	}
+}