| 1 | package main |
| 2 | |
| 3 | // Hashcash at space creation (#561): the client burns a few seconds of CPU |
| 4 | // per space, making the Sybil loop (mint invites → fresh accounts → another |
| 5 | // free space each) cost compute instead of nothing. The burn runs during the |
| 6 | // fader's "Hold on…" build phase — the cost IS the animation. |
| 7 | // |
| 8 | // Classic hashcash: the server hands out a sealed, expiring, single-use |
| 9 | // challenge bound to the requesting account; the client finds a nonce where |
| 10 | // SHA-256(challenge ":" nonce) has at least `bits` leading zero bits; |
| 11 | // verification is one hash. Asymmetric by construction — the server never |
| 12 | // works. Off by default (LCHAT_POW_BITS unset/0): self-hosters and dev setups |
| 13 | // never see it. |
| 14 | |
| 15 | import ( |
| 16 | "crypto/rand" |
| 17 | "crypto/sha256" |
| 18 | "encoding/binary" |
| 19 | "fmt" |
| 20 | "strconv" |
| 21 | "sync" |
| 22 | "time" |
| 23 | ) |
| 24 | |
| 25 | // powChallengeWindow bounds how long a minted challenge stays solvable. Long |
| 26 | // enough for slow hardware at production difficulty, short enough that a |
| 27 | // stockpile of pre-mined challenges goes stale. |
| 28 | const powChallengeWindow = 5 * time.Minute |
| 29 | |
| 30 | // powGate is the creation gate. nil = disabled. |
| 31 | type powGate struct { |
| 32 | bits int |
| 33 | keys *stateKeys |
| 34 | mu sync.Mutex |
| 35 | used map[string]int64 // consumed challenge -> expiry (replay guard) |
| 36 | } |
| 37 | |
| 38 | func newPowGate(bits int, keys *stateKeys) *powGate { |
| 39 | if bits <= 0 { |
| 40 | return nil |
| 41 | } |
| 42 | return &powGate{bits: bits, keys: keys, used: map[string]int64{}} |
| 43 | } |
| 44 | |
| 45 | // mint issues a challenge for one account: seal(expiry || random || uid). |
| 46 | // Stateless until solved — only CONSUMED challenges take memory, and those |
| 47 | // expire out of the map. |
| 48 | func (g *powGate) mint(uid string) (string, error) { |
| 49 | return g.mintAt(uid, time.Now().Add(powChallengeWindow)) |
| 50 | } |
| 51 | |
| 52 | // mintAt is the expiry-injectable body of mint, so tests can mint stale |
| 53 | // challenges without a clock. |
| 54 | func (g *powGate) mintAt(uid string, expiry time.Time) (string, error) { |
| 55 | payload := make([]byte, 8+16+len(uid)) |
| 56 | binary.BigEndian.PutUint64(payload, uint64(expiry.Unix())) |
| 57 | if _, err := rand.Read(payload[8:24]); err != nil { |
| 58 | return "", err |
| 59 | } |
| 60 | copy(payload[24:], uid) |
| 61 | return g.keys.seal(payload) |
| 62 | } |
| 63 | |
| 64 | // verify checks one solution: the challenge opens, is bound to this account, |
| 65 | // hasn't expired or been used, and the nonce clears the difficulty. A passing |
| 66 | // challenge is consumed — one challenge, one space. |
| 67 | func (g *powGate) verify(uid, challenge, nonce string) error { |
| 68 | payload, ok := g.keys.open(challenge) |
| 69 | if !ok || len(payload) < 24 { |
| 70 | return fmt.Errorf("bad challenge") |
| 71 | } |
| 72 | expiry := int64(binary.BigEndian.Uint64(payload)) |
| 73 | if time.Now().Unix() > expiry { |
| 74 | return fmt.Errorf("challenge expired") |
| 75 | } |
| 76 | if string(payload[24:]) != uid { |
| 77 | return fmt.Errorf("challenge is not yours") |
| 78 | } |
| 79 | if !powSolves(challenge, nonce, g.bits) { |
| 80 | return fmt.Errorf("proof of work does not check out") |
| 81 | } |
| 82 | g.mu.Lock() |
| 83 | defer g.mu.Unlock() |
| 84 | now := time.Now().Unix() |
| 85 | for c, exp := range g.used { // expire the replay guard as we go |
| 86 | if exp < now { |
| 87 | delete(g.used, c) |
| 88 | } |
| 89 | } |
| 90 | if _, replayed := g.used[challenge]; replayed { |
| 91 | return fmt.Errorf("challenge already used") |
| 92 | } |
| 93 | g.used[challenge] = expiry |
| 94 | return nil |
| 95 | } |
| 96 | |
| 97 | // powSolves reports whether SHA-256(challenge ":" nonce) clears `bits` |
| 98 | // leading zero bits — the one-hash verification (and the test suite's solver |
| 99 | // building block). |
| 100 | func powSolves(challenge, nonce string, bits int) bool { |
| 101 | h := sha256.Sum256([]byte(challenge + ":" + nonce)) |
| 102 | return leadingZeroBits(h[:]) >= bits |
| 103 | } |
| 104 | |
| 105 | func leadingZeroBits(h []byte) int { |
| 106 | bits := 0 |
| 107 | for _, b := range h { |
| 108 | if b == 0 { |
| 109 | bits += 8 |
| 110 | continue |
| 111 | } |
| 112 | for mask := byte(0x80); mask != 0; mask >>= 1 { |
| 113 | if b&mask != 0 { |
| 114 | return bits |
| 115 | } |
| 116 | bits++ |
| 117 | } |
| 118 | } |
| 119 | return bits |
| 120 | } |
| 121 | |
| 122 | // solvePow grinds nonces until one clears the difficulty — used by tests (at |
| 123 | // low bits) and by any Go client; the web client has its own solver loop. |
| 124 | func solvePow(challenge string, bits int) string { |
| 125 | for n := 0; ; n++ { |
| 126 | nonce := strconv.Itoa(n) |
| 127 | if powSolves(challenge, nonce, bits) { |
| 128 | return nonce |
| 129 | } |
| 130 | } |
| 131 | } |
| 132 | |
amadan