diff --git a/firmware/inc/u2f.h b/firmware/inc/u2f.h
index 7d54e29..417cee2 100644
--- a/firmware/inc/u2f.h
+++ b/firmware/inc/u2f.h
@@ -83,7 +83,7 @@ uint16_t u2f_attestation_cert_size();
 // callback for u2f to send back response data
 //  @buf data to write back
 //  @len length of buf in bytes
-void u2f_response_writeback(uint8_t * buf, uint8_t len);
+void u2f_response_writeback(uint8_t * buf, uint16_t len);
 
 // callback when u2f finishes and will
 // indicate when all buffer data, if any, should be written
diff --git a/firmware/inc/u2f_hid.h b/firmware/inc/u2f_hid.h
index c85e0c5..3d27b6f 100644
--- a/firmware/inc/u2f_hid.h
+++ b/firmware/inc/u2f_hid.h
@@ -101,7 +101,7 @@ void u2f_hid_set_len(uint16_t len);
 // @payload the buffer to write
 // @len length of buffer
 // @@prereq is that hid_layer.current_cid, hid_layer.res_len each set to correct values
-void u2f_hid_writeback(uint8_t * payload, uint8_t len);
+void u2f_hid_writeback(uint8_t * payload, uint16_t len);
 
 // u2f_hid_flush flush any remaining data that may be buffered.
 void u2f_hid_flush();
diff --git a/firmware/src/u2f-atecc.c b/firmware/src/u2f-atecc.c
index d008daf..1416df6 100644
--- a/firmware/src/u2f-atecc.c
+++ b/firmware/src/u2f-atecc.c
@@ -73,7 +73,7 @@ void u2f_init()
 	}
 }
 
-void u2f_response_writeback(uint8_t * buf, uint8_t len)
+void u2f_response_writeback(uint8_t * buf, uint16_t len)
 {
 	u2f_hid_writeback(buf, len);
 }
diff --git a/firmware/src/u2f.c b/firmware/src/u2f.c
index 90c9145..3e80715 100644
--- a/firmware/src/u2f.c
+++ b/firmware/src/u2f.c
@@ -52,6 +52,7 @@ static int16_t u2f_register(struct u2f_register_request * req)
 
     uint8_t key_handle[U2F_KEY_HANDLE_SIZE];
     uint8_t pubkey[64];
+    const uint16_t attest_size = u2f_attestation_cert_size();
 
     if (u2f_get_user_feedback() != 0)
     {
@@ -83,8 +84,8 @@ static int16_t u2f_register(struct u2f_register_request * req)
     i[0] = U2F_KEY_HANDLE_SIZE;
     u2f_response_writeback(i,1);
     u2f_response_writeback(key_handle,U2F_KEY_HANDLE_SIZE);
-    u2f_response_writeback(u2f_get_attestation_cert(),u2f_attestation_cert_size()-254/2);
-    u2f_response_writeback(u2f_get_attestation_cert()+u2f_attestation_cert_size(),254/2);
+
+    u2f_response_writeback(u2f_get_attestation_cert(),u2f_attestation_cert_size());
 
     u2f_response_writeback((uint8_t*)req, 64);
 
diff --git a/firmware/src/u2f_hid.c b/firmware/src/u2f_hid.c
index 5ff9a13..1876f4c 100644
--- a/firmware/src/u2f_hid.c
+++ b/firmware/src/u2f_hid.c
@@ -101,7 +101,7 @@ void u2f_hid_flush()
 }
 
 
-void u2f_hid_writeback(uint8_t * payload, uint8_t len)
+void u2f_hid_writeback(uint8_t * payload, uint16_t len)
 {
 	struct u2f_hid_msg * r = (struct u2f_hid_response *) _hid_pkt;
 	_hid_in_session = 1;