How do I create a SecretVec which length is unknown?

[sandevins]

In this example I want to encrypt a text that is stored on a file (I tried first to have it provided by stdin but I couldn't make it work).

use std::fs::File;
use std::io::Read;

use libsodium_sys as sodium;
use secrets::{SecretBox, SecretVec};

const KEY_LEN   : usize = sodium::crypto_secretbox_KEYBYTES   as _;
const NONCE_LEN : usize = sodium::crypto_secretbox_NONCEBYTES as _;
const MAC_LEN   : usize = sodium::crypto_secretbox_MACBYTES   as _;

pub fn get_ciphertext(key_path: &str, nonce_path: &str, text_path: &str) -> std::io::Result<SecretVec::<u8>>{
    let mut key        = SecretBox::<[u8; KEY_LEN]>::zero();
    let mut nonce      = [0; NONCE_LEN];
    let mut plaintext = SecretVec::zero(14);
    

    File::open(key_path)?
        .read_exact(key.borrow_mut().as_mut())?;

    File::open(nonce_path)?
        .read_exact(&mut nonce)?;

    File::open(text_path)?
        .read_exact(plaintext.borrow_mut().as_mut())?;

    Ok(SecretVec::<u8>::new(plaintext.len() - MAC_LEN, |s| {
        if -1 == unsafe {
            sodium::crypto_secretbox_easy(
                s.as_mut_ptr(),
                plaintext.borrow().as_ptr(),
                plaintext.len() as _,
                nonce.as_ptr(),
                key.borrow().as_ptr(),
            )
        } {
            panic!("failed");
        }
    }))
}

This is like the decrypt example but using the sodium::crypto_secretbox_easy instead.

It gives the error thread 'main' panicked at 'attempt to subtract with overflow' probably because of the size given by plaintext.len() - MAC_LEN.

Have you any idea on how to fix this?

[sandevins]

Nevermind, I found the error. It was a dumb problem due to copying.

The libsodium call should be like this.

Ok(SecretVec::<u8>::new(plaintext.len() + MAC_LEN, |s| {
        if -1 == unsafe {
            sodium::crypto_secretbox_easy(
                s.as_mut_ptr(),
                plaintext.borrow().as_ptr(),
                plaintext.len() as _,
                nonce.as_ptr(),
                key.borrow().as_ptr(),
            )
        } {
            panic!("failed");
        }
    }))

The MAC len has to be added to the length.