Refactor piv-p256-specific stanza unwrapping onto RecipientLine

src/format.rs

@@ -1,7 +1,7 @@
 use age_core::{
-    format::{FileKey, Stanza},
-    primitives::aead_encrypt,
-    secrecy::ExposeSecret,
+    format::{FileKey, Stanza, FILE_KEY_BYTES},
+    primitives::{aead_decrypt, aead_encrypt, hkdf},
+    secrecy::{zeroize::Zeroize, ExposeSecret},
 };
 use base64::{prelude::BASE64_STANDARD_NO_PAD, Engine};
 use p256::{
@@ -11,8 +11,9 @@ use p256::{
 use rand::rngs::OsRng;
 use sha2::Sha256;
 
-use crate::{p256::Recipient, STANZA_TAG};
+use crate::{key::Connection, p256::Recipient};
 
+const STANZA_TAG: &str = "piv-p256";
 pub(crate) const STANZA_KEY_LABEL: &[u8] = b"piv-p256";
 
 const TAG_BYTES: usize = 4;
@@ -117,9 +118,7 @@ impl RecipientLine {
 
         let shared_secret = esk.diffie_hellman(pk.public_key());
 
-        let mut salt = vec![];
-        salt.extend_from_slice(epk_bytes.as_bytes());
-        salt.extend_from_slice(pk.to_encoded().as_bytes());
+        let salt = salt(&epk_bytes, pk);
 
         let enc_key = {
             let mut okm = [0; 32];
@@ -142,4 +141,34 @@ impl RecipientLine {
             encrypted_file_key,
         }
     }
+
+    pub(crate) fn unwrap_file_key(&self, conn: &mut Connection) -> Result<FileKey, ()> {
+        assert_eq!(self.tag, conn.recipient().tag());
+
+        // The YubiKey API for performing scalar multiplication takes the point in its
+        // uncompressed SEC-1 encoding.
+        let shared_secret = conn.p256_ecdh(self.epk_bytes.decompress().as_bytes())?;
+
+        let salt = salt(&self.epk_bytes, conn.recipient());
+
+        let enc_key = hkdf(&salt, STANZA_KEY_LABEL, shared_secret.as_ref());
+
+        // A failure to decrypt is fatal, because we assume that we won't
+        // encounter 32-bit collisions on the key tag embedded in the header.
+        aead_decrypt(&enc_key, FILE_KEY_BYTES, &self.encrypted_file_key)
+            .map_err(|_| ())
+            .map(|mut pt| {
+                FileKey::init_with_mut(|file_key| {
+                    file_key.copy_from_slice(&pt);
+                    pt.zeroize();
+                })
+            })
+    }
+}
+
+fn salt(epk_bytes: &EphemeralKeyBytes, pk: &Recipient) -> Vec<u8> {
+    let mut salt = vec![];
+    salt.extend_from_slice(epk_bytes.as_bytes());
+    salt.extend_from_slice(pk.to_encoded().as_bytes());
+    salt
 }

src/key.rs

@@ -1,10 +1,6 @@
 //! Structs for handling YubiKeys.
 
-use age_core::{
-    format::{FileKey, FILE_KEY_BYTES},
-    primitives::{aead_decrypt, hkdf},
-    secrecy::{zeroize::Zeroize, ExposeSecret, SecretString},
-};
+use age_core::secrecy::{ExposeSecret, SecretString};
 use age_plugin::{identity, Callbacks};
 use bech32::{ToBase32, Variant};
 use dialoguer::Password;
@@ -25,7 +21,7 @@ use yubikey::{
 use crate::{
     error::Error,
     fl,
-    format::{RecipientLine, STANZA_KEY_LABEL},
+    format::RecipientLine,
     p256::{Recipient, TAG_BYTES},
     util::{otp_serial_prefix, Metadata},
     IDENTITY_PREFIX,
@@ -623,7 +619,6 @@ impl Stub {
             cert,
             pk,
             slot: self.slot,
-            tag: self.tag,
             identity_index: self.identity_index,
             cached_metadata: None,
             last_touch: None,
@@ -636,7 +631,6 @@ pub(crate) struct Connection {
     cert: Certificate,
     pk: Recipient,
     slot: RetiredSlotId,
-    tag: [u8; 4],
     identity_index: usize,
     cached_metadata: Option<Metadata>,
     last_touch: Option<Instant>,
@@ -705,8 +699,10 @@ impl Connection {
         Ok(Ok(()))
     }
 
-    pub(crate) fn unwrap_file_key(&mut self, line: &RecipientLine) -> Result<FileKey, ()> {
-        assert_eq!(self.tag, line.tag);
+    pub(crate) fn p256_ecdh(&mut self, epk_bytes: &[u8]) -> Result<yubikey::Buffer, ()> {
+        // The YubiKey API for performing scalar multiplication takes the point in its
+        // uncompressed SEC-1 encoding.
+        assert_eq!(epk_bytes.len(), 65);
 
         // Check if the touch policy requires a touch.
         let needs_touch = match (
@@ -718,11 +714,9 @@ impl Connection {
             _ => false,
         };
 
-        // The YubiKey API for performing scalar multiplication takes the point in its
-        // uncompressed SEC-1 encoding.
         let shared_secret = match decrypt_data(
             &mut self.yubikey,
-            line.epk_bytes.decompress().as_bytes(),
+            epk_bytes,
             AlgorithmId::EccP256,
             SlotId::Retired(self.slot),
         ) {
@@ -739,22 +733,7 @@ impl Connection {
             }
         }
 
-        let mut salt = vec![];
-        salt.extend_from_slice(line.epk_bytes.as_bytes());
-        salt.extend_from_slice(self.pk.to_encoded().as_bytes());
-
-        let enc_key = hkdf(&salt, STANZA_KEY_LABEL, shared_secret.as_ref());
-
-        // A failure to decrypt is fatal, because we assume that we won't
-        // encounter 32-bit collisions on the key tag embedded in the header.
-        aead_decrypt(&enc_key, FILE_KEY_BYTES, &line.encrypted_file_key)
-            .map_err(|_| ())
-            .map(|mut pt| {
-                FileKey::init_with_mut(|file_key| {
-                    file_key.copy_from_slice(&pt);
-                    pt.zeroize();
-                })
-            })
+        Ok(shared_secret)
     }
 
     /// Close this connection without resetting the YubiKey.

src/main.rs

@@ -28,7 +28,6 @@ const PLUGIN_NAME: &str = "yubikey";
 const BINARY_NAME: &str = "age-plugin-yubikey";
 const RECIPIENT_PREFIX: &str = "age1yubikey";
 const IDENTITY_PREFIX: &str = "age-plugin-yubikey-";
-const STANZA_TAG: &str = "piv-p256";
 
 const USABLE_SLOTS: [RetiredSlotId; 20] = [
     RetiredSlotId::R1,

src/plugin.rs

@@ -252,7 +252,7 @@ impl IdentityPluginV1 for IdentityPlugin {
                 }
 
                 for (stanza_index, line) in stanzas.iter().enumerate() {
-                    match conn.unwrap_file_key(line) {
+                    match line.unwrap_file_key(&mut conn) {
                         Ok(file_key) => {
                             // We've managed to decrypt this file!
                             file_keys.entry(file_index).or_insert(Ok(file_key));