c
// Example using a hypothetical checked arithmetic library 
#include <safemath.h> // Include a safe math library

// ... (rest of your code)

ret = key->type->instantiate(key, prep);
if (ret == 0) {
    uint64_t safe_expiry;
    if (safe_add_u64(&safe_expiry, prep->expiry, 0) != SAFE_MATH_SUCCESS) { 
        // Handle error - Overflow detected!
    } else {
        key_set_expiry(key, safe_expiry); 
    }
}
