Bmap file integrity check

Before using a Bmap file checks if its checksum is correct for the
current bmap file.
Bmap checksum is the application of Sha256 to the file data. When the
bmap file is created, the value of the checksum has to be zero (all ASCII
"0" symbols). Once calculated, zeros are replaced by the checksum, notice
this modifies the file itself.
In order to calculate the checksum before using it and compare it with
the original, we need to set the field as all "0" before applying Sha256.

Closes: #50

Signed-off-by: Rafael Garcia Ruiz <[email protected]>

bmap-rs/Cargo.toml

@@ -12,4 +12,6 @@ anyhow = "1.0.66"
 nix = "0.26.1"
 flate2 = "1.0.24"
 clap = { version = "4.0.18", features = ["derive"] }
-indicatif = "0.17.1"
+indicatif = "0.17.1"
+sha2 = { version = "0.10.6", features = [ "asm" ] }
+hex = "0.4.3"

bmap-rs/src/main.rs

@@ -1,9 +1,10 @@
-use anyhow::{anyhow, bail, Context, Result};
+use anyhow::{anyhow, bail, ensure, Context, Result};
 use bmap::{Bmap, Discarder, SeekForward};
 use clap::Parser;
 use flate2::read::GzDecoder;
 use indicatif::{ProgressBar, ProgressState, ProgressStyle};
 use nix::unistd::ftruncate;
+use sha2::{Digest, Sha256};
 use std::ffi::OsStr;
 use std::fmt::Write;
 use std::fs::File;
@@ -89,6 +90,24 @@ fn setup_input(path: &Path) -> Result<Decoder> {
     }
 }
 
+fn bmap_integrity(checksum: String, xml: String) -> Result<()> {
+    //Unset the checksum
+    let mut bmap_hash = Sha256::new();
+    let default = "0".repeat(64);
+    let before_checksum = xml.replace(&checksum, &default);
+
+    //Compare given and created checksum
+    bmap_hash.update(before_checksum);
+    let digest = bmap_hash.finalize_reset();
+    let new_checksum = hex::encode(digest.as_slice());
+    ensure!(
+        checksum == new_checksum,
+        "Bmap file doesn't match its checksum. It could be corrupted or compromised."
+    );
+    println!("Bmap integrity checked!");
+    Ok(())
+}
+
 fn copy(c: Copy) -> Result<()> {
     if !c.image.exists() {
         bail!("Image file doesn't exist")
@@ -102,6 +121,7 @@ fn copy(c: Copy) -> Result<()> {
     b.read_to_string(&mut xml)?;
 
     let bmap = Bmap::from_xml(&xml)?;
+    bmap_integrity(bmap.bmap_file_checksum(), xml)?;
     let output = std::fs::OpenOptions::new()
         .write(true)
         .create(true)