feat(captcha): FNV-1a + xorshift32 PRNG matching Cap.js widget

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

captcha/prng.go

@@ -0,0 +1,36 @@
+package captcha
+
+import (
+	"fmt"
+	"strings"
+)
+
+const prngMask = 0xFFFFFFFF
+
+// prng generates a deterministic lowercase-hex string of length chars from
+// seed, matching the Cap.js widget PRNG exactly: FNV-1a seeds a 32-bit state,
+// then xorshift32 produces the hex output. Must stay byte-for-byte compatible
+// with the widget or solutions will never verify.
+func prng(seed string, length int) string {
+	state := fnv1a(seed)
+	var b strings.Builder
+	for b.Len() < length {
+		state ^= (state << 13) & prngMask
+		state ^= state >> 17
+		state ^= (state << 5) & prngMask
+		state &= prngMask
+		fmt.Fprintf(&b, "%08x", state)
+	}
+	return b.String()[:length]
+}
+
+// fnv1a returns the 32-bit FNV-1a hash of s. Iterates over runes (code points)
+// to match the JS/Python reference (which use charCodeAt / ord).
+func fnv1a(s string) uint32 {
+	var h uint32 = 2166136261
+	for _, ch := range s {
+		h ^= uint32(ch)
+		h = (h + (h << 1) + (h << 4) + (h << 7) + (h << 8) + (h << 24)) & prngMask
+	}
+	return h
+}

captcha/prng_test.go

@@ -0,0 +1,37 @@
+package captcha
+
+import "testing"
+
+func TestPrngKnownVectors(t *testing.T) {
+	cases := []struct {
+		seed   string
+		length int
+		want   string
+	}{
+		{"hello", 8, "eb492c6e"},
+		{"hello", 16, "eb492c6e1655ea8c"},
+		{"test", 32, "9c7ca3730a4a283aa6e4bc1c1d83b14f"},
+		{"a", 8, "441aaeb8"},
+		{"z", 8, "da40eb31"},
+	}
+	for _, c := range cases {
+		if got := prng(c.seed, c.length); got != c.want {
+			t.Errorf("prng(%q,%d)=%q want %q", c.seed, c.length, got, c.want)
+		}
+	}
+}
+
+func TestPrngLengthAndDeterminism(t *testing.T) {
+	for _, n := range []int{1, 8, 16, 32, 64} {
+		if got := prng("x", n); len(got) != n {
+			t.Errorf("len(prng(\"x\",%d))=%d want %d", n, len(got), n)
+		}
+	}
+	if prng("seed1", 16) == prng("seed2", 16) {
+		t.Error("different seeds produced identical output")
+	}
+	a, b := prng("test", 16), prng("test", 16)
+	if a != b {
+		t.Error("prng not deterministic")
+	}
+}