| 1 | // Hashcash admission for the fetch service (docs/fetch.md § Abuse posture). |
| 2 | // The service is identity-free by design and, behind the router, can't even |
| 3 | // tell callers apart by IP — so before it becomes load-bearing for the hosted |
| 4 | // fleet, anonymous callers pay in CPU instead of identity: solve one |
| 5 | // challenge, receive a sealed ~1h ticket, and every request rides (and is |
| 6 | // rate-bucketed by) that ticket. One solve per client per hour, not per |
| 7 | // request — an unfurl must stay instant. |
| 8 | // |
| 9 | // The scheme is home/pow.go's (space creation, #561), kept deliberately in |
| 10 | // step: SHA-256(challenge ":" nonce) clearing `bits` leading zero bits, |
| 11 | // sealed expiring single-use challenges, verification costing one hash. Off |
| 12 | // by default (LCHAT_FETCH_POW_BITS unset/0) — self-hosters and dev setups |
| 13 | // never see it. |
| 14 | package main |
| 15 | |
| 16 | import ( |
| 17 | "crypto/aes" |
| 18 | "crypto/cipher" |
| 19 | "crypto/rand" |
| 20 | "crypto/sha256" |
| 21 | "encoding/base64" |
| 22 | "encoding/binary" |
| 23 | "fmt" |
| 24 | "strconv" |
| 25 | "sync" |
| 26 | "time" |
| 27 | ) |
| 28 | |
| 29 | const ( |
| 30 | fetchPowChallengeWindow = 5 * time.Minute |
| 31 | fetchPowTicketWindow = time.Hour |
| 32 | ) |
| 33 | |
| 34 | // fetchPowGate is the admission gate; a nil *fetchPowGate means off. |
| 35 | type fetchPowGate struct { |
| 36 | bits int |
| 37 | aead cipher.AEAD // sealed challenges + tickets; key minted into the service DB |
| 38 | mu sync.Mutex |
| 39 | used map[string]int64 // consumed challenge -> expiry (replay guard) |
| 40 | } |
| 41 | |
| 42 | func newFetchPowGate(bits int, key []byte) (*fetchPowGate, error) { |
| 43 | if bits <= 0 { |
| 44 | return nil, nil |
| 45 | } |
| 46 | block, err := aes.NewCipher(key) |
| 47 | if err != nil { |
| 48 | return nil, err |
| 49 | } |
| 50 | aead, err := cipher.NewGCM(block) |
| 51 | if err != nil { |
| 52 | return nil, err |
| 53 | } |
| 54 | return &fetchPowGate{bits: bits, aead: aead, used: map[string]int64{}}, nil |
| 55 | } |
| 56 | |
| 57 | // seal/open: base64url(iv||ciphertext), the crypto.js convention like the |
| 58 | // preview cache's. |
| 59 | func (g *fetchPowGate) seal(payload []byte) (string, error) { |
| 60 | iv := make([]byte, g.aead.NonceSize()) |
| 61 | if _, err := rand.Read(iv); err != nil { |
| 62 | return "", err |
| 63 | } |
| 64 | return base64.RawURLEncoding.EncodeToString(g.aead.Seal(iv, iv, payload, nil)), nil |
| 65 | } |
| 66 | |
| 67 | func (g *fetchPowGate) open(blob string) ([]byte, bool) { |
| 68 | raw, err := base64.RawURLEncoding.DecodeString(blob) |
| 69 | if err != nil || len(raw) < g.aead.NonceSize() { |
| 70 | return nil, false |
| 71 | } |
| 72 | payload, err := g.aead.Open(nil, raw[:g.aead.NonceSize()], raw[g.aead.NonceSize():], nil) |
| 73 | if err != nil { |
| 74 | return nil, false |
| 75 | } |
| 76 | return payload, true |
| 77 | } |
| 78 | |
| 79 | // mintChallenge issues one solvable challenge: seal(expiry || random). |
| 80 | // Stateless until solved — only CONSUMED challenges take memory. |
| 81 | func (g *fetchPowGate) mintChallenge() (string, error) { |
| 82 | return g.sealExpiring(time.Now().Add(fetchPowChallengeWindow)) |
| 83 | } |
| 84 | |
| 85 | func (g *fetchPowGate) sealExpiring(expiry time.Time) (string, error) { |
| 86 | payload := make([]byte, 8+16) |
| 87 | binary.BigEndian.PutUint64(payload, uint64(expiry.Unix())) |
| 88 | if _, err := rand.Read(payload[8:]); err != nil { |
| 89 | return "", err |
| 90 | } |
| 91 | return g.seal(payload) |
| 92 | } |
| 93 | |
| 94 | // verifyAndConsume checks one solution: the challenge opens, hasn't expired |
| 95 | // or been used, and the nonce clears the difficulty. A passing challenge is |
| 96 | // consumed — one solve, one ticket. |
| 97 | func (g *fetchPowGate) verifyAndConsume(challenge, nonce string) error { |
| 98 | payload, ok := g.open(challenge) |
| 99 | if !ok || len(payload) != 24 { |
| 100 | return fmt.Errorf("bad challenge") |
| 101 | } |
| 102 | expiry := int64(binary.BigEndian.Uint64(payload)) |
| 103 | if time.Now().Unix() > expiry { |
| 104 | return fmt.Errorf("challenge expired") |
| 105 | } |
| 106 | if !fetchPowSolves(challenge, nonce, g.bits) { |
| 107 | return fmt.Errorf("proof of work does not check out") |
| 108 | } |
| 109 | g.mu.Lock() |
| 110 | defer g.mu.Unlock() |
| 111 | now := time.Now().Unix() |
| 112 | for c, exp := range g.used { // expire the replay guard as we go |
| 113 | if exp < now { |
| 114 | delete(g.used, c) |
| 115 | } |
| 116 | } |
| 117 | if _, replayed := g.used[challenge]; replayed { |
| 118 | return fmt.Errorf("challenge already used") |
| 119 | } |
| 120 | g.used[challenge] = expiry |
| 121 | return nil |
| 122 | } |
| 123 | |
| 124 | // mintTicket issues the post-solve pass: same sealed shape, longer window, |
| 125 | // no replay guard (a ticket is MEANT to be reused; the per-ticket rate |
| 126 | // bucket is what bounds it). |
| 127 | func (g *fetchPowGate) mintTicket() (string, error) { |
| 128 | return g.sealExpiring(time.Now().Add(fetchPowTicketWindow)) |
| 129 | } |
| 130 | |
| 131 | func (g *fetchPowGate) ticketOK(t string) bool { |
| 132 | payload, ok := g.open(t) |
| 133 | if !ok || len(payload) != 24 { |
| 134 | return false |
| 135 | } |
| 136 | return time.Now().Unix() <= int64(binary.BigEndian.Uint64(payload)) |
| 137 | } |
| 138 | |
| 139 | // fetchPowSolves / fetchLeadingZeroBits / fetchSolvePow mirror home/pow.go |
| 140 | // (same package-main symbols exist there, unexported in that sibling binary's |
| 141 | // package — keep the two in step). |
| 142 | func fetchPowSolves(challenge, nonce string, bits int) bool { |
| 143 | h := sha256.Sum256([]byte(challenge + ":" + nonce)) |
| 144 | return fetchLeadingZeroBits(h[:]) >= bits |
| 145 | } |
| 146 | |
| 147 | func fetchLeadingZeroBits(h []byte) int { |
| 148 | bits := 0 |
| 149 | for _, b := range h { |
| 150 | if b == 0 { |
| 151 | bits += 8 |
| 152 | continue |
| 153 | } |
| 154 | for mask := byte(0x80); mask != 0; mask >>= 1 { |
| 155 | if b&mask != 0 { |
| 156 | return bits |
| 157 | } |
| 158 | bits++ |
| 159 | } |
| 160 | } |
| 161 | return bits |
| 162 | } |
| 163 | |
| 164 | // fetchSolvePow grinds nonces until one clears the difficulty — tests (at low |
| 165 | // bits); the web and native clients carry their own solver loops. |
| 166 | func fetchSolvePow(challenge string, bits int) string { |
| 167 | for n := 0; ; n++ { |
| 168 | nonce := strconv.Itoa(n) |
| 169 | if fetchPowSolves(challenge, nonce, bits) { |
| 170 | return nonce |
| 171 | } |
| 172 | } |
| 173 | } |
| 174 | |
amadan