Raise MemoryError when backend.derive_scrypt can't malloc enough (#4592)

* Raise MemoryError when backend.derive_scrypt can't malloc enough

* Expose ERR_R_MALLOC_FAILURE and use the reason_match pattern to catch it

* Add test_scrypt_malloc_failure in test_scrypt

* let's see if this passes

* add comment to filippo's blog post about scrypt's params

2 files changed, 20 insertions, 1 deletions

diff --git a/src/_cffi_src/openssl/err.py b/src/_cffi_src/openssl/err.py
index 59751355..b4d053c6 100644
--- a/src/_cffi_src/openssl/err.py
+++ b/src/_cffi_src/openssl/err.py
@@ -22,6 +22,8 @@ static const int ERR_LIB_PKCS12;
 static const int ERR_LIB_SSL;
 static const int ERR_LIB_X509;
 
+static const int ERR_R_MALLOC_FAILURE;
+
 static const int ASN1_R_BOOLEAN_IS_WRONG_LENGTH;
 static const int ASN1_R_BUFFER_TOO_SMALL;
 static const int ASN1_R_CIPHER_HAS_NO_OBJECT_IDENTIFIER;
diff --git a/src/cryptography/hazmat/backends/openssl/backend.py b/src/cryptography/hazmat/backends/openssl/backend.py
index ae966cd0..fda6293c 100644
--- a/src/cryptography/hazmat/backends/openssl/backend.py
+++ b/src/cryptography/hazmat/backends/openssl/backend.py
@@ -2120,7 +2120,24 @@ class Backend(object):
             key_material, len(key_material), salt, len(salt), n, r, p,
             scrypt._MEM_LIMIT, buf, length
         )
-        self.openssl_assert(res == 1)
+        if res != 1:
+            errors = self._consume_errors()
+            if not self._lib.CRYPTOGRAPHY_OPENSSL_LESS_THAN_111:
+                # This error is only added to the stack in 1.1.1+
+                self.openssl_assert(
+                    errors[0]._lib_reason_match(
+                        self._lib.ERR_LIB_EVP,
+                        self._lib.ERR_R_MALLOC_FAILURE
+                    )
+                )
+
+            # memory required formula explained here:
+            # https://blog.filippo.io/the-scrypt-parameters/
+            min_memory = 128 * n * r // (1024**2)
+            raise MemoryError(
+                "Not enough memory to derive key. These parameters require"
+                " {} MB of memory.".format(min_memory)
+            )
         return self._ffi.buffer(buf)[:]
 
     def aead_cipher_supported(self, cipher):