Improve security of encrypted notes

Closes #1.

The password is hashed using Argon2, with the salt being `SHA256(password)`. The output hash is then encoded using hex.

The password-checking function checks if the "note database" is encrypted using the plain password (and returns it) or if it is encrypted using the hashed+salted password (and returns the value). This allows older databases to work properly.

A migration path may be added in the future.

Cargo.toml

@@ -16,3 +16,6 @@ rand = "0.8"
 pwbox = "0.5"
 directories = "4.0"
 native-dialog = "0.6"
+hex = "0.4"
+sha2 = "0.10"
+argon2 = "0.5"

src/main.rs

@@ -16,10 +16,12 @@ use log::info;
 use native_dialog::MessageDialog;
 use simple_logger::SimpleLogger;
 use crate::notes::{Note, NoteMetadata};
+use crate::password::{check_password, generate_new_password};
 use crate::saving::Saving;
 
 mod notes;
 mod saving;
+mod password;
 
 #[derive(PartialEq)]
 enum CurrentMode {
@@ -101,7 +103,9 @@ impl eframe::App for App {
 
         if !self.tested_password && self.password.is_some() {
             if let Some(note) = self.notes.get(0) {
-                if note.decrypt(self.password.as_ref().unwrap()).is_err() {
+                if let Ok(password) = check_password(self.password.as_ref().unwrap(), note) {
+                    self.password = Some(password);
+                } else {
                     let _ = MessageDialog::new()
                         .set_title("invalid password")
                         .set_text("failed to verify the password against an existing note. please try again")
@@ -111,9 +115,11 @@ impl eframe::App for App {
                     self.tested_password = false;
                     return;
                 }
-
-                self.tested_password = true;
+            } else {
+                self.password = Some(generate_new_password(self.password.as_ref().unwrap()))
             }
+
+            self.tested_password = true;
         }
 
         if self.update_notes_next {

src/password/mod.rs

@@ -0,0 +1,35 @@
+use anyhow::bail;
+use argon2::Argon2;
+use sha2::{Sha256, Digest};
+use crate::notes::Note;
+
+pub fn generate_new_password(password: &str) -> String {
+    let mut output = [0_u8; 32];
+    let mut output_key = [0u8; 256];
+
+    hash_password(password.as_bytes(), &mut output);
+    Argon2::default().hash_password_into(&password.as_bytes(), &output, &mut output_key).unwrap();
+
+    hex::encode(output_key)
+}
+
+pub fn check_password(password: &str, note: &Note) -> anyhow::Result<String> {
+    if note.decrypt(password).is_err() {
+        let new_password = generate_new_password(password);
+        if note.decrypt(&new_password).is_err() {
+            bail!("invalid password");
+        } else {
+            Ok(new_password)
+        }
+    } else {
+        Ok(password.into())
+    }
+}
+
+fn hash_password(password: &[u8], output: &mut [u8]) {
+    let mut hasher = Sha256::new();
+    hasher.update(password);
+    let result = hasher.finalize();
+
+    output.clone_from_slice(&result[..])
+}