diff --git a/services/tiktak/keygen/keygen.go b/services/tiktak/keygen/keygen.go
index 61aba65..169d17b 100644
--- a/services/tiktak/keygen/keygen.go
+++ b/services/tiktak/keygen/keygen.go
@@ -1,27 +1,42 @@
 package keygen
 
-/*
-#cgo LDFLAGS: ${SRCDIR}/legacy/keygen.a -lm
-#include <keygen.h>
-*/
-import "C"
 import (
-	"strings"
-	"unsafe"
+	"crypto/hmac"
+	"crypto/sha256"
+	"strconv"
+)
+
+// The legacy C implementation (legacy/keygen.a, no source shipped) derived the
+// share token from the video id ALONE -- no key, no salt. Worse, its state
+// collapsed to (seed*17+42)%62 after the first round, so the token depended
+// only on id%62: 62 distinct tokens for the whole service. Anyone could upload
+// 62 private videos, copy the tokens off their own /home and unlock every
+// private video on every team's box. Replaced with a keyed HMAC.
+//
+// Same signatures, same 30-char base62 alphabet, so nothing else changes.
+// keygen.h / legacy/keygen.a are no longer part of the build (cgo is gone).
+const (
+	tokenLen = 30
+	alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
+
+	// Server-side secret. Rotating it invalidates every token already handed
+	// out, so change it only between rounds.
+	secret = "D1gkxNv7_HphLJjNuFag_stjaByXLZ1Y0l12RGppZtw"
 )
 
 func GenerateKey(vid int) string {
-	var key [C.TOK_SIZE]byte
-	keyPtr := (*C.char)(unsafe.Pointer(&key[0]))
-	C.GenerateToken(C.int(vid), keyPtr)
-	res := strings.Builder{}
-	for _, v := range key {
-		res.WriteByte(v)
+	mac := hmac.New(sha256.New, []byte(secret))
+	mac.Write([]byte(strconv.Itoa(vid)))
+	sum := mac.Sum(nil)
+
+	out := make([]byte, tokenLen)
+	for i := 0; i < tokenLen; i++ {
+		out[i] = alphabet[int(sum[i%len(sum)])%len(alphabet)]
 	}
-	return res.String()
+	return string(out)
 }
 
 func ValidateKey(token string, vid int) bool {
-	res := C.ValidateToken(C.int(vid), C.CString(token))
-	return res == 1
+	// Constant-time: never leak how much of the token was correct.
+	return hmac.Equal([]byte(token), []byte(GenerateKey(vid)))
 }
diff --git a/services/tiktak/keygen/keygen_test.go b/services/tiktak/keygen/keygen_test.go
new file mode 100644
index 0000000..6aa3685
--- /dev/null
+++ b/services/tiktak/keygen/keygen_test.go
@@ -0,0 +1,34 @@
+package keygen
+
+import "testing"
+
+func TestTokenProperties(t *testing.T) {
+	seen := map[string]int{}
+	for vid := 1; vid <= 5000; vid++ {
+		tok := GenerateKey(vid)
+		if len(tok) != 30 {
+			t.Fatalf("vid %d: len %d, want 30", vid, len(tok))
+		}
+		for _, ch := range tok {
+			if !((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z') || (ch >= '0' && ch <= '9')) {
+				t.Fatalf("vid %d: bad char %q", vid, ch)
+			}
+		}
+		if prev, dup := seen[tok]; dup {
+			t.Fatalf("COLLISION: vid %d and vid %d share a token", prev, vid)
+		}
+		seen[tok] = vid
+		if !ValidateKey(tok, vid) {
+			t.Fatalf("vid %d: own token rejected", vid)
+		}
+		if ValidateKey(tok, vid+1) || ValidateKey(tok, vid+62) {
+			t.Fatalf("vid %d: token accepted for another video", vid)
+		}
+	}
+	t.Logf("5000 ids -> %d distinct tokens (legacy gave 62)", len(seen))
+	for _, bad := range []string{"", "aaaa", GenerateKey(1)[:29], GenerateKey(1) + "x"} {
+		if ValidateKey(bad, 1) {
+			t.Fatalf("garbage token %q accepted", bad)
+		}
+	}
+}