fix(app): harden the setup key imports

- PGP key import reads at most 1 MiB (bounded read, buffer zeroed) and
  refuses larger files with a new message.
- SshKeyViewModel catches any Exception from import and generation
  (CancellationException rethrown) instead of a short list, so an
  unexpected provider or sshj error no longer crashes the app.
- Pending PGP key bytes are zeroed in a finally block, also on
  cancellation during addKey after a replace, and an earlier pending
  pick is zeroed before a new one takes its place.
- PGPKeyManager never replaces a stored secret key with a public-only
  key of the same ID, with or without replace (KeyDowngradeException);
  the setup ViewModel shows a localized message for it. Unit test added.
- storeRecipients strips '#' comments and a trailing '!' as pass reads
  .gpg-id; tests added.
- GitServerViewModel.clearStore returns the failure when the store
  folder cannot be emptied, so the clone does not start; the plan's
  Task 3 call site reports it.
- readGpgId opens the file with NOFOLLOW_LINKS and reads at most
  64 KiB + 1 bytes, refusing more.
- SshKey.import writes the new private key to a temp file and moves it
  over atomically before dropping the old key's Keystore entry, public
  key and passphrase, so an I/O error keeps the old key.
- KeyUtils.isSecretKey notes (ponytail) that GnuPG stub and smartcard
  keyrings count as secret.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

4 files changed, 21 insertions, 0 deletions

diff --git a/crypto/common/src/main/kotlin/app/passwordstore/crypto/errors/CryptoException.kt b/crypto/common/src/main/kotlin/app/passwordstore/crypto/errors/CryptoException.kt
index af6145d0..9589ceec 100644
--- a/crypto/common/src/main/kotlin/app/passwordstore/crypto/errors/CryptoException.kt
+++ b/crypto/common/src/main/kotlin/app/passwordstore/crypto/errors/CryptoException.kt
@@ -39,6 +39,10 @@ public class KeyNotFoundException(keyId: String) :
 public class KeyAlreadyExistsException(keyId: String) :
   KeyManagerException("Pre-existing key was found for $keyId")
 
+/** Attempting to replace a stored secret key with a public-only key of the same `keyId`. */
+public class KeyDowngradeException(keyId: String) :
+  KeyManagerException("A secret key is stored for $keyId; a public key cannot replace it")
+
 /** Sealed exception types for [app.passwordstore.crypto.CryptoHandler]. */
 public sealed class CryptoHandlerException(message: String? = null, cause: Throwable? = null) :
   CryptoException(message, cause)
diff --git a/crypto/pgpainless/src/main/kotlin/app/passwordstore/crypto/KeyUtils.kt b/crypto/pgpainless/src/main/kotlin/app/passwordstore/crypto/KeyUtils.kt
index 311e9049..9acd0489 100644
--- a/crypto/pgpainless/src/main/kotlin/app/passwordstore/crypto/KeyUtils.kt
+++ b/crypto/pgpainless/src/main/kotlin/app/passwordstore/crypto/KeyUtils.kt
@@ -41,6 +41,8 @@ public object KeyUtils {
   }
 
   /** True when [key] parses as a secret key ring, the only kind that can decrypt. */
+  // ponytail: GnuPG stub and smartcard keyrings (no usable secret material) count as secret too.
+  // Upgrade path: check for a secret subkey usable for decryption.
   public fun isSecretKey(key: PGPKey): Boolean = tryParseKeyring(key) is PGPSecretKeyRing
 
   /**
diff --git a/crypto/pgpainless/src/main/kotlin/app/passwordstore/crypto/PGPKeyManager.kt b/crypto/pgpainless/src/main/kotlin/app/passwordstore/crypto/PGPKeyManager.kt
index 6f852af4..ef58ad19 100644
--- a/crypto/pgpainless/src/main/kotlin/app/passwordstore/crypto/PGPKeyManager.kt
+++ b/crypto/pgpainless/src/main/kotlin/app/passwordstore/crypto/PGPKeyManager.kt
@@ -14,6 +14,7 @@ import app.passwordstore.crypto.errors.InvalidKeyException
 import app.passwordstore.crypto.errors.KeyAlreadyExistsException
 import app.passwordstore.crypto.errors.KeyDeletionFailedException
 import app.passwordstore.crypto.errors.KeyDirectoryUnavailableException
+import app.passwordstore.crypto.errors.KeyDowngradeException
 import app.passwordstore.crypto.errors.KeyNotFoundException
 import app.passwordstore.crypto.errors.NoKeysAvailableException
 import app.passwordstore.crypto.errors.UnusableKeyException
@@ -59,6 +60,9 @@ constructor(filesDir: String, private val dispatcher: CoroutineDispatcher) :
               keyFile.writeBytes(keyBytes)
               return@runSuspendCatching key
             }
+            // Never with replace either: it would delete the only copy of the private key.
+            existingKeyRing is PGPSecretKeyRing && incomingKeyRing is PGPPublicKeyRing ->
+              throw KeyDowngradeException(tryGetId(key)?.toString() ?: "Failed to retrieve key ID")
           }
           // Check for replace flag first and if it is false, throw an error
           if (!replace)
diff --git a/crypto/pgpainless/src/test/kotlin/app/passwordstore/crypto/PGPKeyManagerTest.kt b/crypto/pgpainless/src/test/kotlin/app/passwordstore/crypto/PGPKeyManagerTest.kt
index 0525c28a..81a666f4 100644
--- a/crypto/pgpainless/src/test/kotlin/app/passwordstore/crypto/PGPKeyManagerTest.kt
+++ b/crypto/pgpainless/src/test/kotlin/app/passwordstore/crypto/PGPKeyManagerTest.kt
@@ -9,6 +9,7 @@ import app.passwordstore.crypto.KeyUtils.tryGetId
 import app.passwordstore.crypto.PGPIdentifier.KeyId
 import app.passwordstore.crypto.PGPIdentifier.UserId
 import app.passwordstore.crypto.errors.KeyAlreadyExistsException
+import app.passwordstore.crypto.errors.KeyDowngradeException
 import app.passwordstore.crypto.errors.KeyNotFoundException
 import app.passwordstore.crypto.errors.NoKeysAvailableException
 import app.passwordstore.crypto.errors.UnusableKeyException
@@ -177,6 +178,16 @@ class PGPKeyManagerTest {
     }
 
   @Test
+  fun replaceSecretKeyWithPublicKeyIsRefusedEvenWithReplace() =
+    runTest(dispatcher) {
+      keyManager.addKey(secretKey).unwrap()
+      assertIs<KeyDowngradeException>(keyManager.addKey(publicKey, replace = true).unwrapError())
+      assertIs<KeyDowngradeException>(keyManager.addKey(publicKey, replace = false).unwrapError())
+      // The private key is still the stored one.
+      assertContentEquals(secretKey.contents, keyManager.getAllKeys().unwrap().single().contents)
+    }
+
+  @Test
   fun replacePublicKeyWithSecretKey() =
     runTest(dispatcher) {
       assertTrue(keyManager.addKey(publicKey).isOk)