diff --git a/sys/crypto/twofish/twofish.c b/sys/crypto/twofish/twofish.c
index 0000000..1111111 100644
--- a/sys/crypto/twofish/twofish.c
+++ b/sys/crypto/twofish/twofish.c
@@ -430,7 +430,21 @@
     l_key = ctx->l_key;
     s_key = ctx->s_key;
 
-    ctx->k_len = key_len_bits / 64;   /* 2, 3 or 4 */
+    /* Defense-in-depth: only 128/192/256-bit keys are valid for Twofish.
+     * The kernel cryptoapi wrappers (twofish_cbc_setkey, twofish_xts_setkey)
+     * already validate this, but clamp here as well so a future caller
+     * cannot trigger the OOB stack/struct writes that the k_len-driven
+     * loops below would otherwise perform. */
+    switch (key_len_bits) {
+    case 128:
+    case 192:
+    case 256:
+        ctx->k_len = key_len_bits / 64;   /* 2, 3 or 4 */
+        break;
+    default:
+        ctx->k_len = 0;
+        return;
+    }
 
     for(i = 0; i < ctx->k_len; ++i)
     {