[PM-24377] Adds decrypt_user_key_with_master_key into PureCrypto (#465)

## 🎟️ Tracking

https://bitwarden.atlassian.net/browse/PM-24377

## 📔 Objective

Create a new SDK method in
`PureCrypto.decrypt_user_key_with_master_key`.

## ⏰ Reminders before review

- Contributor guidelines followed
- All formatters and local linters executed and passed
- 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

## 🦮 Reviewer guidelines

<!-- Suggested interactions but feel free to use (or not) as you desire!
-->

- 👍 (`:+1:`) or similar for great changes
- 📝 (`:memo:`) or ℹ️ (`:information_source:`) for notes or general info
- ❓ (`:question:`) for questions
- 🤔 (`:thinking:`) or 💭 (`:thought_balloon:`) for more open inquiry
that's not quite a confirmed
  issue and could potentially benefit from discussion
- 🎨 (`:art:`) for suggestions / improvements
- ❌ (`:x:`) or ⚠️ (`:warning:`) for more significant problems or
concerns needing attention
- 🌱 (`:seedling:`) or ♻️ (`:recycle:`) for future improvements or
indications of technical debt
- ⛏ (`:pick:`) for minor or nitpick changes

Author: mzieniukbw

crates/bitwarden-wasm-internal/src/pure_crypto.rs

@@ -323,6 +323,20 @@ impl PureCrypto {
         #[allow(deprecated)]
         dangerous_derive_kdf_material(password, salt, &kdf)
     }
+
+    pub fn decrypt_user_key_with_master_key(
+        encrypted_user_key: String,
+        master_key: Vec<u8>,
+    ) -> Result<Vec<u8>, CryptoError> {
+        let master_key = &BitwardenLegacyKeyBytes::from(master_key);
+        let master_key = &SymmetricCryptoKey::try_from(master_key)?;
+        let master_key = MasterKey::try_from(master_key)?;
+        let encrypted_user_key = EncString::from_str(&encrypted_user_key)?;
+        let result = master_key
+            .decrypt_user_key(encrypted_user_key)
+            .map_err(|_| CryptoError::InvalidKey)?;
+        Ok(result.to_encoded().to_vec())
+    }
 }
 
 #[cfg(test)]
@@ -665,4 +679,25 @@ DnqOsltgPomWZ7xVfMkm9niL2OA=
         let derived_key = PureCrypto::derive_kdf_material(password, email, kdf).unwrap();
         assert_eq!(derived_key, DERIVED_KDF_MATERIAL_ARGON2ID);
     }
+
+    #[test]
+    fn test_decrypt_user_key_with_master_key() {
+        let password = "test_password";
+        let email = "test_email@example.com";
+        let kdf = &Kdf::Argon2id {
+            iterations: NonZero::try_from(3).unwrap(),
+            memory: NonZero::try_from(64).unwrap(),
+            parallelism: NonZero::try_from(4).unwrap(),
+        };
+        let master_key = MasterKey::derive(password, email, kdf).unwrap();
+        let (user_key, encrypted_user_key) = master_key.make_user_key().unwrap();
+        let master_key_bytes = master_key.to_base64().into_bytes();
+
+        let decrypted_user_key = PureCrypto::decrypt_user_key_with_master_key(
+            encrypted_user_key.to_string(),
+            master_key_bytes,
+        )
+        .unwrap();
+        assert_eq!(user_key.0.to_encoded().to_vec(), decrypted_user_key);
+    }
 }