diff --git a/src/borg/crypto/key.py b/src/borg/crypto/key.py
index 1ccb679cd..21437d22b 100644
--- a/src/borg/crypto/key.py
+++ b/src/borg/crypto/key.py
@@ -621,7 +621,7 @@ class KeyfileKeyBase(AESKeyBase):
             data = msgpack.unpackb(data)
             key = Key(internal_dict=data)
             if key.version != 1:
-                raise IntegrityError('Invalid key file header')
+                raise Error("key version %d is not supported by this borg version.")
             self.repository_id = key.repository_id
             self.enc_key = key.enc_key
             self.enc_hmac_key = key.enc_hmac_key
@@ -636,8 +636,10 @@ class KeyfileKeyBase(AESKeyBase):
         unpacker.feed(data)
         data = unpacker.unpack()
         enc_key = EncryptedKey(internal_dict=data)
-        assert enc_key.version == 1
-        assert enc_key.algorithm == 'sha256'
+        if enc_key.version != 1:
+            raise Error("encrypted key version %d is not supported by this borg version." % enc_key.version)
+        if enc_key.algorithm != 'sha256':
+            raise Error("encrypted key algorithm '%s' is not supported by this borg version." % enc_key.algorithm)
         key = passphrase.kdf(enc_key.salt, enc_key.iterations, 32)
         data = AES(is_encrypt=False, key=key).decrypt(enc_key.data)
         if compare_digest(hmac_sha256(key, data), enc_key.hash):