fido_buf_write
int fido_buf_write(unsigned char **, size_t *, const void *, size_t);
if (fido_buf_write(buf, count, f, (size_t)(n - 2)) < 0) {
if (fido_buf_write(&buf, &count, sw, sizeof(sw)) < 0) {
if (fido_buf_write(&ptr, &len, &authdata, sizeof(authdata)) < 0 ||
fido_buf_write(&ptr, &len, &attcred_raw, sizeof(attcred_raw)) < 0 ||
fido_buf_write(&ptr, &len, kh, kh_len) < 0 ||
fido_buf_write(&ptr, &len, pk_blob.ptr, pk_blob.len) < 0) {