feat(notsecrets): Decryptor — identity-driven age decryption with MAC verification

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/notsecrets/src/decrypt.rs

@@ -0,0 +1,164 @@
+use crate::error::AgeError;
+use crate::format::{header_bytes_up_to_footer, parse_header};
+use crate::identities::{FileKey, Identity};
+use chacha20poly1305::{
+    aead::Aead,
+    ChaCha20Poly1305, Key, KeyInit, Nonce,
+};
+use hkdf::Hkdf;
+use hmac::{Hmac, Mac};
+use sha2::Sha256;
+
+type HmacSha256 = Hmac<Sha256>;
+
+pub struct Decryptor {
+    identities: Vec<Box<dyn Identity>>,
+}
+
+impl Decryptor {
+    pub fn with_identities(identities: Vec<Box<dyn Identity>>) -> Self {
+        Self { identities }
+    }
+
+    pub fn decrypt(&self, ciphertext: &[u8]) -> Result<Vec<u8>, AgeError> {
+        let (header, payload_offset) = parse_header(ciphertext)?;
+
+        // Try to recover file key: try each identity against each stanza.
+        // `None` means the stanza type doesn't match; `Some(Err)` means the
+        // type matched but key material didn't — treat both as "keep trying".
+        let mut file_key: Option<FileKey> = None;
+        'outer: for identity in &self.identities {
+            for stanza in &header.recipients {
+                if let Some(Ok(fk)) = identity.unwrap_file_key(stanza) {
+                    file_key = Some(fk);
+                    break 'outer;
+                }
+            }
+        }
+        let file_key = file_key.ok_or(AgeError::NoMatch)?;
+
+        // Verify header MAC
+        let mac_key = derive_mac_key(&file_key)?;
+        let header_input = header_bytes_up_to_footer(ciphertext)?;
+        let mut mac = <HmacSha256 as Mac>::new_from_slice(&mac_key)
+            .map_err(|e| AgeError::CryptoError(format!("HMAC init: {e}")))?;
+        mac.update(&header_input);
+        mac.verify_slice(&header.mac)
+            .map_err(|_| AgeError::MacMismatch)?;
+
+        // Decrypt payload
+        let payload = &ciphertext[payload_offset..];
+        if payload.len() < 16 {
+            return Err(AgeError::ParseError(
+                "payload too short to contain nonce".to_string(),
+            ));
+        }
+        let nonce_bytes: [u8; 16] = payload[..16].try_into().expect("slice is exactly 16 bytes");
+        let payload_ciphertext = &payload[16..];
+
+        let payload_key = derive_payload_key(&file_key, &nonce_bytes)?;
+        let cipher = ChaCha20Poly1305::new(Key::from_slice(&payload_key));
+        let counter_nonce = Nonce::default();
+        let plaintext = cipher
+            .decrypt(&counter_nonce, payload_ciphertext)
+            .map_err(|_| AgeError::CryptoError("payload decryption failed".to_string()))?;
+
+        Ok(plaintext)
+    }
+}
+
+fn derive_mac_key(file_key: &FileKey) -> Result<[u8; 32], AgeError> {
+    let hk = Hkdf::<Sha256>::new(Some(b""), file_key.as_bytes());
+    let mut mac_key = [0u8; 32];
+    hk.expand(b"header", &mut mac_key)
+        .map_err(|e| AgeError::CryptoError(format!("HKDF mac key: {e}")))?;
+    Ok(mac_key)
+}
+
+fn derive_payload_key(file_key: &FileKey, nonce: &[u8; 16]) -> Result<[u8; 32], AgeError> {
+    let hk = Hkdf::<Sha256>::new(Some(nonce.as_slice()), file_key.as_bytes());
+    let mut payload_key = [0u8; 32];
+    hk.expand(b"payload", &mut payload_key)
+        .map_err(|e| AgeError::CryptoError(format!("HKDF payload key: {e}")))?;
+    Ok(payload_key)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::identities::x25519::X25519Identity;
+    use crate::recipients::x25519::X25519Recipient;
+    use crate::Encryptor;
+    use rand::rngs::OsRng;
+    use x25519_dalek::{PublicKey, StaticSecret};
+
+    fn make_x25519_pair() -> (X25519Identity, X25519Recipient) {
+        let secret = StaticSecret::random_from_rng(OsRng);
+        let public = PublicKey::from(&secret);
+        (
+            X25519Identity::from_static_secret(secret),
+            X25519Recipient::from_public_key(public),
+        )
+    }
+
+    #[test]
+    fn decryptor_roundtrip_single_recipient() {
+        let (identity, recipient) = make_x25519_pair();
+        let encryptor = Encryptor::with_recipients(vec![Box::new(recipient)]).unwrap();
+        let plaintext = b"the quick brown fox";
+        let ciphertext = encryptor.encrypt(plaintext).unwrap();
+        let decryptor = Decryptor::with_identities(vec![Box::new(identity)]);
+        let decrypted = decryptor.decrypt(&ciphertext).unwrap();
+        assert_eq!(decrypted, plaintext);
+    }
+
+    #[test]
+    fn decryptor_roundtrip_multiple_recipients() {
+        let (identity1, recipient1) = make_x25519_pair();
+        let (identity2, recipient2) = make_x25519_pair();
+        let encryptor = Encryptor::with_recipients(vec![
+            Box::new(recipient1),
+            Box::new(recipient2),
+        ])
+        .unwrap();
+        let plaintext = b"multi-recipient test";
+        let ciphertext = encryptor.encrypt(plaintext).unwrap();
+        let decryptor1 = Decryptor::with_identities(vec![Box::new(identity1)]);
+        assert_eq!(decryptor1.decrypt(&ciphertext).unwrap(), plaintext);
+        let decryptor2 = Decryptor::with_identities(vec![Box::new(identity2)]);
+        assert_eq!(decryptor2.decrypt(&ciphertext).unwrap(), plaintext);
+    }
+
+    #[test]
+    fn decryptor_no_matching_identity_returns_no_match() {
+        let (_, recipient) = make_x25519_pair();
+        let (wrong_identity, _) = make_x25519_pair();
+        let encryptor = Encryptor::with_recipients(vec![Box::new(recipient)]).unwrap();
+        let ciphertext = encryptor.encrypt(b"data").unwrap();
+        let decryptor = Decryptor::with_identities(vec![Box::new(wrong_identity)]);
+        let err = decryptor.decrypt(&ciphertext).unwrap_err();
+        assert!(matches!(err, crate::error::AgeError::NoMatch));
+    }
+
+    #[test]
+    fn decryptor_corrupted_mac_returns_mac_mismatch() {
+        let (identity, recipient) = make_x25519_pair();
+        let encryptor = Encryptor::with_recipients(vec![Box::new(recipient)]).unwrap();
+        let mut ciphertext = encryptor.encrypt(b"data").unwrap();
+        // Flip a byte in the MAC region (after "--- ")
+        let footer_pos = ciphertext
+            .windows(4)
+            .position(|w| w == b"--- ")
+            .expect("footer present");
+        ciphertext[footer_pos + 4] ^= 0xff;
+        let decryptor = Decryptor::with_identities(vec![Box::new(identity)]);
+        let err = decryptor.decrypt(&ciphertext).unwrap_err();
+        assert!(
+            matches!(
+                err,
+                crate::error::AgeError::MacMismatch | crate::error::AgeError::ParseError(_)
+            ),
+            "expected MacMismatch or ParseError, got: {err:?}"
+        );
+    }
+}

crates/notsecrets/src/identities/encrypted.rs

@@ -3,36 +3,53 @@ use crate::identities::{FileKey, Identity, Stanza};
 
 /// An identity whose key material is stored in an age-encrypted file.
 ///
-/// The outer file is decrypted on each call to `unwrap_file_key` using
-/// `passphrase_identity`. The decrypted content must be an `AGE-SECRET-KEY-1...`
-/// bech32 string, which is parsed as an `X25519Identity`.
+/// The outer file is decrypted on each call to `unwrap_file_key` using a
+/// scrypt passphrase identity. The decrypted content must be an
+/// `AGE-SECRET-KEY-1...` bech32 string, which is parsed as an `X25519Identity`.
 pub struct EncryptedIdentity {
     /// Raw bytes of the age-encrypted identity file.
     encrypted_data: Vec<u8>,
-    /// Identity used to decrypt the outer age file (typically `ScryptIdentity`).
-    passphrase_identity: Box<dyn Identity>,
+    /// Passphrase used to decrypt the outer age file via `ScryptIdentity`.
+    passphrase: String,
 }
 
 impl EncryptedIdentity {
-    pub fn new(encrypted_data: Vec<u8>, passphrase_identity: Box<dyn Identity>) -> Self {
+    pub fn new(encrypted_data: Vec<u8>, passphrase: String) -> Self {
         Self {
             encrypted_data,
-            passphrase_identity,
+            passphrase,
         }
     }
 }
 
 impl Identity for EncryptedIdentity {
-    fn unwrap_file_key(&self, _stanza: &Stanza) -> Option<Result<FileKey, AgeError>> {
-        // Full implementation in Task 9 after Decryptor is available.
-        // The integration test below is marked #[ignore] until then.
-        let _ = &self.encrypted_data;
-        let _ = self.passphrase_identity.as_ref();
-        Some(Err(AgeError::CryptoError(
-            "EncryptedIdentity requires Decryptor — implemented in Task 9".to_string(),
-        )))
+    fn unwrap_file_key(&self, stanza: &Stanza) -> Option<Result<FileKey, AgeError>> {
+        let decryptor = crate::Decryptor::with_identities(vec![Box::new(
+            crate::identities::scrypt::ScryptIdentity::new(self.passphrase.clone()),
+        )]);
+        let plaintext = match decryptor.decrypt(&self.encrypted_data) {
+            Ok(p) => p,
+            Err(e) => return Some(Err(e)),
+        };
+        let key_str = match std::str::from_utf8(&plaintext) {
+            Ok(s) => s.trim().to_string(),
+            Err(_) => {
+                return Some(Err(AgeError::ParseError(
+                    "decrypted identity file is not valid UTF-8".to_string(),
+                )))
+            }
+        };
+        // Parse inner identity — only X25519 supported for now
+        let inner: Box<dyn Identity> = if key_str.starts_with("AGE-SECRET-KEY-1") {
+            match crate::identities::x25519::X25519Identity::from_bech32(&key_str) {
+                Ok(id) => Box::new(id),
+                Err(e) => return Some(Err(e)),
+            }
+        } else {
+            return Some(Err(AgeError::UnsupportedKeyType(
+                "encrypted identity file must contain AGE-SECRET-KEY-1".to_string(),
+            )));
+        };
+        inner.unwrap_file_key(stanza)
     }
 }
-
-// Integration test for EncryptedIdentity lives in tests/integration.rs and is
-// activated in Task 9 once Encryptor and Decryptor are available.

crates/notsecrets/src/lib.rs

@@ -1,3 +1,4 @@
+pub mod decrypt;
 pub mod encrypt;
 pub mod error;
 pub mod format;
@@ -8,6 +9,7 @@ pub mod _legacy;
 
 pub use error::AgeError;
 pub use identities::{FileKey, Header, Identity, Stanza, X25519Identity, ScryptIdentity, SshEd25519Identity, SshRsaIdentity, EncryptedIdentity};
+pub use decrypt::Decryptor;
 pub use encrypt::Encryptor;
 pub use recipients::{Recipient, X25519Recipient, ScryptRecipient, SshEd25519Recipient, SshRsaRecipient};
 pub use sources::{BitwardenSource, FileSource, PromptSource};