commit f27626ad70300494a6f90254b8c57206d112afe9
author Alice Wang <aliceywang@google.com> Tue Sep 27 12:36:22 2022 +0000
committer Alice Wang <aliceywang@google.com> Tue Sep 27 17:56:28 2022 +0000
tree 086799fcdc69e16e27d6c9f6317d8e5fa684bfba
parent 39915e965d81d4ba91c2e3bf6384d11b43df21a5

[apk_digest] Move the logic of extracting v4 digest to v4 module

Bug: 248999133
Test: libapkverify.integration_test
Change-Id: Ib0424bef92708de8b2a9f9d9bfa53bce69f971c8

libs/apkverify/src/v3.rs

diff --git a/libs/apkverify/src/v3.rs b/libs/apkverify/src/v3.rs
index fac0a7f..db7d8cc 100644
--- a/libs/apkverify/src/v3.rs
+++ b/libs/apkverify/src/v3.rs
@@ -76,9 +76,9 @@
 }
 
 #[derive(Debug)]
-struct Signature {
+pub(crate) struct Signature {
     /// Option is used here to allow us to ignore unsupported algorithm.
-    signature_algorithm_id: Option<SignatureAlgorithmID>,
+    pub(crate) signature_algorithm_id: Option<SignatureAlgorithmID>,
     signature: LengthPrefixed<Bytes>,
 }
 
@@ -127,9 +127,9 @@
 }
 
 impl Signer {
-    /// Select the signature that uses the strongest algorithm according to the preferences of the
-    /// v4 signing scheme.
-    fn strongest_signature(&self) -> Result<&Signature> {
+    /// Selects the signature that has the strongest supported `SignatureAlgorithmID`.
+    /// The strongest signature is used in both v3 verification and v4 apk digest computation.
+    pub(crate) fn strongest_signature(&self) -> Result<&Signature> {
         Ok(self
             .signatures
             .iter()
@@ -138,14 +138,13 @@
             .context("No supported signatures found")?)
     }
 
-    pub(crate) fn pick_v4_apk_digest(&self) -> Result<(SignatureAlgorithmID, Box<[u8]>)> {
-        let strongest_algorithm_id = self
-            .strongest_signature()?
-            .signature_algorithm_id
-            .context("Strongest signature should contain a valid signature algorithm.")?;
+    pub(crate) fn find_digest_by_algorithm(
+        &self,
+        algorithm_id: SignatureAlgorithmID,
+    ) -> Result<Box<[u8]>> {
         let signed_data: SignedData = self.signed_data.slice(..).read()?;
-        let digest = signed_data.find_digest_by_algorithm(strongest_algorithm_id)?;
-        Ok((strongest_algorithm_id, digest.digest.as_ref().to_vec().into_boxed_slice()))
+        let digest = signed_data.find_digest_by_algorithm(algorithm_id)?;
+        Ok(digest.digest.as_ref().to_vec().into_boxed_slice())
     }
 
     /// Verifies the strongest signature from signatures against signed data using public key.

libs/apkverify/src/v4.rs

diff --git a/libs/apkverify/src/v4.rs b/libs/apkverify/src/v4.rs
index d0522a7..9012479 100644
--- a/libs/apkverify/src/v4.rs
+++ b/libs/apkverify/src/v4.rs
@@ -18,7 +18,7 @@
 //!
 //! [v4]: https://source.android.com/security/apksigning/v4
 
-use anyhow::{ensure, Result};
+use anyhow::{ensure, Context, Result};
 use std::io::{Read, Seek};
 
 use crate::algorithms::SignatureAlgorithmID;
@@ -34,13 +34,17 @@
     verify: bool,
 ) -> Result<(SignatureAlgorithmID, Box<[u8]>)> {
     let (signer, mut sections) = extract_signer_and_apk_sections(apk)?;
-    let (signature_algorithm_id, extracted_digest) = signer.pick_v4_apk_digest()?;
+    let strongest_algorithm_id = signer
+        .strongest_signature()?
+        .signature_algorithm_id
+        .context("Strongest signature should contain a valid signature algorithm.")?;
+    let extracted_digest = signer.find_digest_by_algorithm(strongest_algorithm_id)?;
     if verify {
-        let computed_digest = sections.compute_digest(signature_algorithm_id)?;
+        let computed_digest = sections.compute_digest(strongest_algorithm_id)?;
         ensure!(
             computed_digest == extracted_digest.as_ref(),
             "Computed digest does not match the extracted digest."
         );
     }
-    Ok((signature_algorithm_id, extracted_digest))
+    Ok((strongest_algorithm_id, extracted_digest))
 }