| 1 | package movement |
| 2 | |
| 3 | import ( |
| 4 | "crypto/sha256" |
| 5 | "math/bits" |
| 6 | "strings" |
| 7 | ) |
| 8 | |
| 9 | // The proof of work is the price of submitting an address. The client |
| 10 | // searches for a counter whose hash starts with enough zero bits; the server |
| 11 | // runs that hash once and compares. |
| 12 | // |
| 13 | // The address is part of the preimage, and that is the whole point. An |
| 14 | // unbound challenge is solved once and replayed against every address in a |
| 15 | // list, so it costs an attacker one solve in total. Bound, it costs one solve |
| 16 | // per address — which is exactly the axis a bulk signup attack scales along. |
| 17 | // It is also why signing up needs JavaScript at all: the work cannot begin |
| 18 | // until the visitor has typed the address it is bound to. |
| 19 | // |
| 20 | // What this does not do: 2^18 SHA-256 is under a millisecond on a commodity |
| 21 | // GPU. Proof of work prices out casual and scripted abuse and nothing more. |
| 22 | // The defence against a serious bulk attack is the persisted mail budget, |
| 23 | // not this. |
| 24 | |
| 25 | // DefaultDifficulty is an estimate, not a measurement — roughly 262k |
| 26 | // expected hashes. It must be re-set against real mid-range hardware before |
| 27 | // launch, recording p95 and p99 rather than the mean: solve time is |
| 28 | // geometric, so p99 is about 4.6x the average, and calibrating on the |
| 29 | // average ships a form that hangs for one visitor in a hundred. |
| 30 | const DefaultDifficulty = 18 |
| 31 | |
| 32 | // NormalizeEmail lowercases ASCII and nothing else. |
| 33 | // |
| 34 | // Go's strings.ToLower and JavaScript's toLowerCase disagree on parts of |
| 35 | // Unicode, and the preimage has to be byte-identical on both sides. A |
| 36 | // disagreement here is a silent pow_short that no visitor can diagnose and |
| 37 | // no log explains, so the two implementations are kept deliberately dumb and |
| 38 | // deliberately identical. |
| 39 | func NormalizeEmail(s string) string { |
| 40 | b := []byte(strings.TrimSpace(s)) |
| 41 | for i, c := range b { |
| 42 | if c >= 'A' && c <= 'Z' { |
| 43 | b[i] = c + 32 |
| 44 | } |
| 45 | } |
| 46 | return string(b) |
| 47 | } |
| 48 | |
| 49 | // PowPreimage is the exact string both sides hash. pow.js builds the same |
| 50 | // one; the browser test is what proves they agree. |
| 51 | func PowPreimage(nonce, normalizedEmail, counter string) string { |
| 52 | return nonce + ":" + normalizedEmail + ":" + counter |
| 53 | } |
| 54 | |
| 55 | func leadingZeroBits(sum []byte) int { |
| 56 | n := 0 |
| 57 | for _, b := range sum { |
| 58 | if b == 0 { |
| 59 | n += 8 |
| 60 | continue |
| 61 | } |
| 62 | return n + bits.LeadingZeros8(b) |
| 63 | } |
| 64 | return n |
| 65 | } |
| 66 | |
| 67 | // VerifyPow checks one candidate. O(1) here, ~2^difficulty for whoever had |
| 68 | // to find it. |
| 69 | func VerifyPow(nonce, email, counter string, difficulty int) bool { |
| 70 | if counter == "" { |
| 71 | return false |
| 72 | } |
| 73 | sum := sha256.Sum256([]byte(PowPreimage(nonce, NormalizeEmail(email), counter))) |
| 74 | return leadingZeroBits(sum[:]) >= difficulty |
| 75 | } |
| 76 | |
amadan