[PM-22812] Attachments get corrupted when downgrading from cipherkeys (#328)

## 🎟️ Tracking
https://bitwarden.atlassian.net/browse/PM-22812
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point to where this change is coming from. -->

## 📔 Objective
Editing a cipher with attachments causes attachment corruption when
switching between cipher-key encryption and user-key encryption in
either direction. The SDK's AttachmentView was not providing the
decrypted attachment key needed for re-encryption scenarios. When the
cipher gets re-encrypted between different encryption contexts
(cipher-key ↔ user-key) during save, the client needs the decrypted
attachment key to properly re-encrypt it under the new encryption
context. Without this, null gets posted for the attachment key, breaking
decryption.

This is a temporary solution during the migration from TypeScript to the
SDK. The decrypted_key field should be removed once all encryption logic
is handled within the SDK.

Cleanup tracked in: https://bitwarden.atlassian.net/browse/PM-23005
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you're fixing or new feature you're adding. -->

## ⏰ Reminders before review

- Contributor guidelines followed
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- Written new unit and / or integration tests where applicable
- Protected functional changes with optionality (feature flags)
- Used internationalization (i18n) for all UI strings
- CI builds passed
- Communicated to DevOps any deployment requirements
- Updated any necessary documentation (Confluence, contributing docs) or
informed the documentation
  team

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Author: gbubemismith

crates/bitwarden-vault/src/cipher/attachment.rs

@@ -37,6 +37,18 @@ pub struct AttachmentView {
     pub size_name: Option<String>,
     pub file_name: Option<String>,
     pub key: Option<EncString>,
+    /// The decrypted attachmentkey in base64 format.
+    ///
+    /// **TEMPORARY FIELD**: This field is a temporary workaround to provide
+    /// decrypted attachment keys to the TypeScript client during the migration
+    /// process. It will be removed once the encryption/decryption logic is
+    /// fully migrated to the SDK.
+    ///
+    /// **Ticket**: <https://bitwarden.atlassian.net/browse/PM-23005>
+    ///
+    /// Do not rely on this field for long-term use.
+    #[cfg(feature = "wasm")]
+    pub decrypted_key: Option<String>,
 }
 
 #[allow(missing_docs)]
@@ -164,13 +176,27 @@ impl Decryptable<KeyIds, SymmetricKeyId, AttachmentView> for Attachment {
         ctx: &mut KeyStoreContext<KeyIds>,
         key: SymmetricKeyId,
     ) -> Result<AttachmentView, CryptoError> {
+        #[cfg(feature = "wasm")]
+        let decrypted_key = if let Some(attachment_key) = &self.key {
+            let content_key_id = ctx.unwrap_symmetric_key(key, ATTACHMENT_KEY, attachment_key)?;
+
+            #[allow(deprecated)]
+            let actual_key = ctx.dangerous_get_symmetric_key(content_key_id)?;
+
+            Some(actual_key.to_base64())
+        } else {
+            None
+        };
+
         Ok(AttachmentView {
             id: self.id.clone(),
             url: self.url.clone(),
             size: self.size.clone(),
             size_name: self.size_name.clone(),
             file_name: self.file_name.decrypt(ctx, key)?,
             key: self.key.clone(),
+            #[cfg(feature = "wasm")]
+            decrypted_key,
         })
     }
 }
@@ -227,6 +253,7 @@ mod tests {
             size_name: Some("100 Bytes".into()),
             file_name: Some("Test.txt".into()),
             key: None,
+            decrypted_key: None,
         };
 
         let contents = b"This is a test file that we will encrypt. It's 100 bytes long, the encrypted version will be longer!";
@@ -283,6 +310,7 @@ mod tests {
             size_name: Some("161 Bytes".into()),
             file_name: Some("Test.txt".into()),
             key: Some("2.r288/AOSPiaLFkW07EBGBw==|SAmnnCbOLFjX5lnURvoualOetQwuyPc54PAmHDTRrhT0gwO9ailna9U09q9bmBfI5XrjNNEsuXssgzNygRkezoVQvZQggZddOwHB6KQW5EQ=|erIMUJp8j+aTcmhdE50zEX+ipv/eR1sZ7EwULJm/6DY=".parse().unwrap()),
+            decrypted_key: None,
         };
 
         let cipher  = Cipher {
@@ -340,6 +368,7 @@ mod tests {
             size_name: Some("161 Bytes".into()),
             file_name: Some("Test.txt".into()),
             key: None,
+            decrypted_key: None,
         };
 
         let cipher  = Cipher {

crates/bitwarden-vault/src/cipher/cipher.rs

@@ -993,6 +993,7 @@ mod tests {
             size_name: None,
             file_name: Some("Attachment test name".into()),
             key: None,
+            decrypted_key: None,
         };
         cipher.attachments = Some(vec![attachment]);
 
@@ -1062,6 +1063,7 @@ mod tests {
             size_name: None,
             file_name: Some("Attachment test name".into()),
             key: None,
+            decrypted_key: None,
         };
         cipher.attachments = Some(vec![attachment]);
 
@@ -1105,6 +1107,7 @@ mod tests {
             size_name: None,
             file_name: Some("Attachment test name".into()),
             key: Some(attachment_key_enc),
+            decrypted_key: None,
         };
         cipher.attachments = Some(vec![attachment]);
         let cred = generate_fido2(&mut key_store.context(), SymmetricKeyId::User);
@@ -1180,6 +1183,7 @@ mod tests {
             size_name: None,
             file_name: Some("Attachment test name".into()),
             key: Some(attachment_key_enc.clone()),
+            decrypted_key: None,
         };
         cipher.attachments = Some(vec![attachment]);