feat: verify sha256sum of downloaded attachments

presage/Cargo.toml

@@ -19,6 +19,7 @@ thiserror = "1.0"
 url = "2.2"
 parking_lot = "0.11"
 tokio = { version = "1.0", default-features = false, features = ["sync", "time"] }
+sha2 = "0.10.8"
 
 [dev-dependencies]
 quickcheck = "1.0.3"

presage/src/errors.rs

@@ -8,6 +8,7 @@ use crate::store::StoreError;
 
 /// The error type of Signal manager
 #[derive(thiserror::Error, Debug)]
+#[non_exhaustive]
 pub enum Error<S: std::error::Error> {
     #[error("captcha from https://signalcaptchas.org/registration/generate.html required")]
     CaptchaRequired,
@@ -65,6 +66,8 @@ pub enum Error<S: std::error::Error> {
     PushChallengeRequired,
     #[error("Not allowed to request verification code, reason unknown: {0:?}")]
     RequestingCodeForbidden(libsignal_service::push_service::RegistrationSessionMetadataResponse),
+    #[error("attachment sha256 checksum did not match")]
+    UnexpectedAttachmentChecksum,
     #[error("Unverified registration session (i.e. wrong verification code)")]
     UnverifiedRegistrationSession,
 }

presage/src/manager/registered.rs

@@ -41,6 +41,7 @@ use log::{debug, error, info, trace, warn};
 use rand::rngs::StdRng;
 use rand::SeedableRng;
 use serde::{Deserialize, Serialize};
+use sha2::Digest;
 use tokio::sync::Mutex;
 
 use crate::cache::CacheCell;
@@ -845,15 +846,24 @@ impl<S: Store> Manager<S, Registered> {
         &self,
         attachment_pointer: &AttachmentPointer,
     ) -> Result<Vec<u8>, Error<S::Error>> {
+        let expected_digest = attachment_pointer
+            .digest
+            .as_ref()
+            .ok_or_else(|| Error::UnexpectedAttachmentChecksum)?;
+
         let mut service = self.identified_push_service();
         let mut attachment_stream = service.get_attachment(attachment_pointer).await?;
 
         // We need the whole file for the crypto to check out
         let mut ciphertext = Vec::new();
         let len = attachment_stream.read_to_end(&mut ciphertext).await?;
-
         trace!("downloaded encrypted attachment of {} bytes", len);
 
+        let digest = sha2::Sha256::digest(&ciphertext);
+        if &digest[..] != expected_digest {
+            return Err(Error::UnexpectedAttachmentChecksum);
+        }
+
         let key: [u8; 64] = attachment_pointer.key().try_into()?;
         decrypt_in_place(key, &mut ciphertext)?;