Config: split configuration into persistent and internal, avoiding "k…

…ey" outside MLA

mla/src/config.rs

@@ -1,12 +1,25 @@
 use crate::errors::ConfigError;
 use crate::layers::compress::CompressionConfig;
 use crate::layers::encrypt::{
-    EncryptionConfig, EncryptionPersistentConfig, EncryptionReaderConfig,
+    EncryptionConfig, EncryptionPersistentConfig, EncryptionReaderConfig, InternalEncryptionConfig,
 };
 use crate::Layers;
 use serde::{Deserialize, Serialize};
 
 /// This module implements the configuration capabilities of MLA Archive
+///
+/// Workflow:
+/// ```asciiart
+///                      `ArchiveWriterConfig`  <--> User, add recipients, set the compression level, etc.
+///                              |
+///                              v
+///                      -------------- MLA ---
+///                              |
+///                       .to_persistent()
+///                           /       \
+/// MLA specifics <= InternalConfig  PersistentConfig => to be stored in the header,
+///                                                      for futur reloaded by the Reader
+/// ```
 
 /// User's configuration used to prepare an archive
 pub struct ArchiveWriterConfig {
@@ -17,7 +30,7 @@ pub struct ArchiveWriterConfig {
     pub(crate) encrypt: EncryptionConfig,
 }
 
-/// Internal configuration stored in the header, to be reloaded
+/// Configuration stored in the header, to be reloaded
 #[derive(Serialize, Deserialize)]
 pub struct ArchivePersistentConfig {
     pub layers_enabled: Layers,
@@ -26,6 +39,13 @@ pub struct ArchivePersistentConfig {
     pub encrypt: Option<EncryptionPersistentConfig>,
 }
 
+/// Internal config, to be used only during MLA processing, by MLA
+#[derive(Default)]
+pub(crate) struct InternalConfig {
+    // Layers specifics
+    pub(crate) encrypt: Option<InternalEncryptionConfig>,
+}
+
 pub type ConfigResult<'a> = Result<&'a mut ArchiveWriterConfig, ConfigError>;
 
 impl ArchiveWriterConfig {
@@ -57,17 +77,24 @@ impl ArchiveWriterConfig {
     }
 
     /// Get the persistent version, to be stored in the header
-    pub fn to_persistent(&self) -> Result<ArchivePersistentConfig, ConfigError> {
-        Ok(ArchivePersistentConfig {
-            layers_enabled: self.layers_enabled,
-            encrypt: {
-                if self.is_layers_enabled(Layers::ENCRYPT) {
-                    Some(self.encrypt.to_persistent()?)
-                } else {
-                    None
-                }
+    pub(crate) fn to_persistent(
+        &self,
+    ) -> Result<(ArchivePersistentConfig, InternalConfig), ConfigError> {
+        let (encrypt_persistent, encrypt_internal) = if self.is_layers_enabled(Layers::ENCRYPT) {
+            let (persistent, internal) = self.encrypt.to_persistent()?;
+            (Some(persistent), Some(internal))
+        } else {
+            (None, None)
+        };
+        Ok((
+            ArchivePersistentConfig {
+                layers_enabled: self.layers_enabled,
+                encrypt: encrypt_persistent,
+            },
+            InternalConfig {
+                encrypt: encrypt_internal,
             },
-        })
+        ))
     }
 
     /// Check if layers are enabled

mla/src/errors.rs

@@ -186,6 +186,9 @@ pub enum ConfigError {
     // Compression specifics
     CompressionLevelOutOfRange,
     // Encryption specifics
+    /// No recipients provided, encryption can't continue
+    NoRecipients,
+    /// Internal state has not yet been created. A call to `to_persistent` might be missing
     EncryptionKeyIsMissing,
     PrivateKeyNotSet,
     PrivateKeyNotFound,

mla/src/layers/encrypt.rs

@@ -17,6 +17,7 @@ use rand_chacha::{rand_core::CryptoRngCore, ChaChaRng};
 use x25519_dalek::{PublicKey, StaticSecret};
 
 use serde::{Deserialize, Deserializer, Serialize};
+use zeroize::Zeroize;
 
 use super::traits::InnerReaderTrait;
 
@@ -135,6 +136,46 @@ impl<'de> Deserialize<'de> for KeyCommitmentAndTag {
     }
 }
 
+/// Return a Cryptographic random number generator
+fn get_crypto_rng() -> impl CryptoRngCore {
+    // Use OsRng from crate rand, that uses getrandom() from crate getrandom.
+    // getrandom provides implementations for many systems, listed on
+    // https://docs.rs/getrandom/0.1.14/getrandom/
+    // On Linux it uses `getrandom()` syscall and falls back on `/dev/urandom`.
+    // On Windows it uses `RtlGenRandom` API (available since Windows XP/Windows Server 2003).
+    //
+    // So this seems to be secure, but unfortunately there is no strong
+    // warranty that this would stay this way forever.
+    // In order to be "better safe than sorry", seed `ChaChaRng` from the
+    // bytes generated by `OsRng` in order to build a CSPRNG
+    // (Cryptographically Secure PseudoRandom Number Generator).
+    // This is actually what `ChaChaRng::from_entropy()` does:
+    // https://github.com/rust-random/rand/blob/rand_core-0.5.1/rand_core/src/lib.rs#L378
+    // and this function is documented as "secure" in
+    // https://docs.rs/rand/0.7.3/rand/trait.SeedableRng.html#method.from_entropy
+    //
+    // For the same reasons, force at compile time that the Rng implements CryptoRngCore
+    ChaChaRng::from_entropy()
+}
+
+#[derive(Zeroize)]
+/// Cryptographic material used for encryption in the Encrypt layer
+/// Part of this data must be kept secret and drop as soon as possible
+pub(crate) struct InternalEncryptionConfig {
+    pub(crate) key: Key,
+    pub(crate) nonce: [u8; NONCE_SIZE],
+}
+
+impl Default for InternalEncryptionConfig {
+    fn default() -> Self {
+        let mut csprng = get_crypto_rng();
+        let key = csprng.gen::<Key>();
+        let nonce = csprng.gen::<[u8; NONCE_SIZE]>();
+
+        Self { key, nonce }
+    }
+}
+
 /// Configuration stored in the header, to be reloaded
 #[derive(Serialize, Deserialize)]
 pub struct EncryptionPersistentConfig {
@@ -146,78 +187,55 @@ pub struct EncryptionPersistentConfig {
     key_commitment: KeyCommitmentAndTag,
 }
 
+#[derive(Default)]
+/// ArchiveWriterConfig specific configuration for the Encryption, to let API users specify encryption options
 pub struct EncryptionConfig {
-    /// Public keys with which to encrypt the symmetric encryption key below
+    /// Public keys of recipients
     ecc_keys: Vec<PublicKey>,
-    /// Symmetric encryption Key
-    key: Key,
-    /// Symmetric encryption nonce
-    nonce: [u8; NONCE_SIZE],
-}
-
-/// Return a Cryptographic random number generator
-fn get_crypto_rng() -> impl CryptoRngCore {
-        // Use OsRng from crate rand, that uses getrandom() from crate getrandom.
-        // getrandom provides implementations for many systems, listed on
-        // https://docs.rs/getrandom/0.1.14/getrandom/
-        // On Linux it uses `getrandom()` syscall and falls back on `/dev/urandom`.
-        // On Windows it uses `RtlGenRandom` API (available since Windows XP/Windows Server 2003).
-        //
-        // So this seems to be secure, but unfortunately there is no strong
-        // warranty that this would stay this way forever.
-        // In order to be "better safe than sorry", seed `ChaChaRng` from the
-        // bytes generated by `OsRng` in order to build a CSPRNG
-        // (Cryptographically Secure PseudoRandom Number Generator).
-        // This is actually what `ChaChaRng::from_entropy()` does:
-        // https://github.com/rust-random/rand/blob/rand_core-0.5.1/rand_core/src/lib.rs#L378
-        // and this function is documented as "secure" in
-        // https://docs.rs/rand/0.7.3/rand/trait.SeedableRng.html#method.from_entropy
-    //
-    // For the same reasons, force at compile time that the Rng implements CryptoRngCore
-    ChaChaRng::from_entropy()
-}
-
-impl std::default::Default for EncryptionConfig {
-    fn default() -> Self {
-        let mut csprng = get_crypto_rng();
-        let key = csprng.gen::<Key>();
-        let nonce = csprng.gen::<[u8; NONCE_SIZE]>();
-        EncryptionConfig {
-            ecc_keys: Vec::new(),
-            key,
-            nonce,
-        }
-    }
 }
 
 impl EncryptionConfig {
     /// Consistency check
     pub fn check(&self) -> Result<(), ConfigError> {
         if self.ecc_keys.is_empty() {
-            Err(ConfigError::EncryptionKeyIsMissing)
+            Err(ConfigError::NoRecipients)
         } else {
             Ok(())
         }
     }
 
-    pub fn to_persistent(&self) -> Result<EncryptionPersistentConfig, ConfigError> {
-        let mut rng = ChaChaRng::from_entropy();
-
-        if let Ok(multi_recipient) =
-            store_key_for_multi_recipients(&self.ecc_keys, &self.key, &mut rng)
-        {
-            if let Ok(key_commitment) = build_key_commitment_chain(&self.key, &self.nonce) {
-                Ok(EncryptionPersistentConfig {
-                    multi_recipient,
-                    nonce: self.nonce,
-                    key_commitment,
-                })
-            } else {
-                Err(ConfigError::KeyCommitmentComputationError)
-            }
-        } else {
-            Err(ConfigError::ECIESComputationError)
-        }
+    /// Create a persistent version, to be reloaded, of the configuration
+    /// This method also create the cryptographic material, so it can only be called once
+    pub(crate) fn to_persistent(
+        &self,
+    ) -> Result<(EncryptionPersistentConfig, InternalEncryptionConfig), ConfigError> {
+        // This will generate the main encrypt layer key & nonce
+        let cryptographic_material = InternalEncryptionConfig::default();
+
+        // Store a wrapped version of the key for each recipient
+        // As this function call be call only once, recipient list can't be changed after
+        let multi_recipient = store_key_for_multi_recipients(
+            &self.ecc_keys,
+            &cryptographic_material.key,
+            &mut get_crypto_rng(),
+        )
+        .or(Err(ConfigError::ECIESComputationError))?;
+
+        // Add a key commitment
+        let key_commitment =
+            build_key_commitment_chain(&cryptographic_material.key, &cryptographic_material.nonce)
+                .or(Err(ConfigError::KeyCommitmentComputationError))?;
+
+        // Create the persistent version, to be exported
+        let nonce = cryptographic_material.nonce;
+        Ok((
+            EncryptionPersistentConfig {
+                multi_recipient,
+                nonce,
+                key_commitment,
+            },
+            cryptographic_material,
+        ))
     }
 }
 
@@ -227,16 +245,6 @@ impl ArchiveWriterConfig {
         self.encrypt.ecc_keys.extend_from_slice(keys);
         self
     }
-
-    /// Return the key used for encryption
-    pub fn encryption_key(&self) -> &Key {
-        &self.encrypt.key
-    }
-
-    /// Return the nonce used for encryption
-    pub fn encryption_nonce(&self) -> &[u8; NONCE_SIZE] {
-        &self.encrypt.nonce
-    }
 }
 
 #[derive(Default)]
@@ -295,7 +303,7 @@ impl ArchiveReaderConfig {
 
 // ---------- Writer ----------
 
-pub struct EncryptionLayerWriter<'a, W: 'a + InnerWriterTrait> {
+pub(crate) struct EncryptionLayerWriter<'a, W: 'a + InnerWriterTrait> {
     inner: InnerWriterType<'a, W>,
     cipher: AesGcm256,
     /// Symmetric encryption Key
@@ -307,14 +315,17 @@ pub struct EncryptionLayerWriter<'a, W: 'a + InnerWriterTrait> {
 }
 
 impl<'a, W: 'a + InnerWriterTrait> EncryptionLayerWriter<'a, W> {
-    pub fn new(inner: InnerWriterType<'a, W>, config: &EncryptionConfig) -> Result<Self, Error> {
+    pub fn new(
+        inner: InnerWriterType<'a, W>,
+        internal_config: &InternalEncryptionConfig,
+    ) -> Result<Self, Error> {
         Ok(Self {
             inner,
-            key: config.key,
-            nonce_prefix: config.nonce,
+            key: internal_config.key,
+            nonce_prefix: internal_config.nonce,
             cipher: AesGcm256::new(
-                &config.key,
-                &build_nonce(config.nonce, FIRST_DATA_CHUNK_NUMBER),
+                &internal_config.key,
+                &build_nonce(internal_config.nonce, FIRST_DATA_CHUNK_NUMBER),
                 ASSOCIATED_DATA,
             )?,
             current_chunk_offset: 0,
@@ -682,8 +693,7 @@ mod tests {
         let mut encrypt_w = Box::new(
             EncryptionLayerWriter::new(
                 Box::new(RawLayerWriter::new(file)),
-                &EncryptionConfig {
-                    ecc_keys: Vec::new(),
+                &InternalEncryptionConfig {
                     key: KEY,
                     nonce: NONCE,
                 },
@@ -802,8 +812,7 @@ mod tests {
         let mut encrypt_w = Box::new(
             EncryptionLayerWriter::new(
                 Box::new(RawLayerWriter::new(file)),
-                &EncryptionConfig {
-                    ecc_keys: Vec::new(),
+                &InternalEncryptionConfig {
                     key: KEY,
                     nonce: NONCE,
                 },