Let StorableBuilder take data_encryption_key

There is now reason `StorableBuilder` needs to permanently own the
`data_encryption_key`, which just risks that the key will linger around
in memory after `StorableBuilder` has been dropped. Here, we switch the
API to simply take `data_encryption_key` by reference for the
`build`/`deconstruct` operations.

Author: tnull

src/util/storable_builder.rs

@@ -6,16 +6,16 @@ use std::io;
 use std::io::{Error, ErrorKind};
 
 /// [`StorableBuilder`] is a utility to build and deconstruct [`Storable`] objects.
+///
 /// It provides client-side Encrypt-then-MAC using ChaCha20-Poly1305.
 pub struct StorableBuilder<T: EntropySource> {
-	data_encryption_key: [u8; 32],
 	entropy_source: T,
 }
 
 impl<T: EntropySource> StorableBuilder<T> {
 	/// Constructs a new instance.
-	pub fn new(data_encryption_key: [u8; 32], entropy_source: T) -> StorableBuilder<T> {
-		Self { data_encryption_key, entropy_source }
+	pub fn new(entropy_source: T) -> StorableBuilder<T> {
+		Self { entropy_source }
 	}
 }
 
@@ -40,13 +40,15 @@ impl<T: EntropySource> StorableBuilder<T> {
 	/// Refer to docs on [`Storable`] for more information.
 	///
 	/// [`PutObjectRequest`]: crate::types::PutObjectRequest
-	pub fn build(&self, input: Vec<u8>, version: i64, aad: &[u8]) -> Storable {
+	pub fn build(
+		&self, input: Vec<u8>, version: i64, data_encryption_key: &[u8; 32], aad: &[u8],
+	) -> Storable {
 		let mut nonce = vec![0u8; 12];
 		self.entropy_source.fill_bytes(&mut nonce[4..]);
 
 		let mut data_blob = PlaintextBlob { value: input, version }.encode_to_vec();
 
-		let mut cipher = ChaCha20Poly1305::new(&self.data_encryption_key, &nonce, aad);
+		let mut cipher = ChaCha20Poly1305::new(data_encryption_key, &nonce, aad);
 		let mut tag = vec![0u8; 16];
 		cipher.encrypt_inplace(&mut data_blob, &mut tag);
 		Storable {
@@ -63,12 +65,14 @@ impl<T: EntropySource> StorableBuilder<T> {
 	/// corresponding version as stored at the time of [`PutObjectRequest`].
 	///
 	/// [`PutObjectRequest`]: crate::types::PutObjectRequest
-	pub fn deconstruct(&self, mut storable: Storable, aad: &[u8]) -> io::Result<(Vec<u8>, i64)> {
+	pub fn deconstruct(
+		&self, mut storable: Storable, data_encryption_key: &[u8; 32], aad: &[u8],
+	) -> io::Result<(Vec<u8>, i64)> {
 		let encryption_metadata = storable
 			.encryption_metadata
 			.ok_or_else(|| Error::new(ErrorKind::InvalidData, "Invalid Metadata"))?;
 		let mut cipher =
-			ChaCha20Poly1305::new(&self.data_encryption_key, &encryption_metadata.nonce, aad);
+			ChaCha20Poly1305::new(data_encryption_key, &encryption_metadata.nonce, aad);
 
 		cipher
 			.decrypt_inplace(&mut storable.data, encryption_metadata.tag.borrow())
@@ -100,16 +104,15 @@ mod tests {
 		let test_entropy_provider = TestEntropyProvider;
 		let mut data_key = [0u8; 32];
 		test_entropy_provider.fill_bytes(&mut data_key);
-		let storable_builder = StorableBuilder {
-			data_encryption_key: data_key,
-			entropy_source: test_entropy_provider,
-		};
+		let storable_builder = StorableBuilder::new(test_entropy_provider);
 		let expected_data = b"secret".to_vec();
 		let expected_version = 8;
 		let aad = b"A";
-		let storable = storable_builder.build(expected_data.clone(), expected_version, aad);
+		let storable =
+			storable_builder.build(expected_data.clone(), expected_version, &data_key, aad);
 
-		let (actual_data, actual_version) = storable_builder.deconstruct(storable, aad).unwrap();
+		let (actual_data, actual_version) =
+			storable_builder.deconstruct(storable, &data_key, aad).unwrap();
 		assert_eq!(actual_data, expected_data);
 		assert_eq!(actual_version, expected_version);
 	}
@@ -119,25 +122,24 @@ mod tests {
 		let test_entropy_provider = TestEntropyProvider;
 		let mut data_key = [0u8; 32];
 		test_entropy_provider.fill_bytes(&mut data_key);
-		let storable_builder = StorableBuilder {
-			data_encryption_key: data_key,
-			entropy_source: test_entropy_provider,
-		};
+		let storable_builder = StorableBuilder::new(test_entropy_provider);
 
 		let expected_data_a = b"secret_a".to_vec();
 		let expected_version_a = 8;
 		let aad_a = b"A";
-		let storable_a = storable_builder.build(expected_data_a.clone(), expected_version_a, aad_a);
+		let storable_a =
+			storable_builder.build(expected_data_a.clone(), expected_version_a, &data_key, aad_a);
 
 		let expected_data_b = b"secret_b".to_vec();
 		let expected_version_b = 8;
 		let aad_b = b"B";
-		let storable_b = storable_builder.build(expected_data_b.clone(), expected_version_b, aad_b);
+		let storable_b =
+			storable_builder.build(expected_data_b.clone(), expected_version_b, &data_key, aad_b);
 
 		let (actual_data, actual_version) =
-			storable_builder.deconstruct(storable_a, aad_a).unwrap();
+			storable_builder.deconstruct(storable_a, &data_key, aad_a).unwrap();
 		assert_eq!(actual_data, expected_data_a);
 		assert_eq!(actual_version, expected_version_a);
-		assert!(storable_builder.deconstruct(storable_b, aad_a).is_err());
+		assert!(storable_builder.deconstruct(storable_b, &data_key, aad_a).is_err());
 	}
 }