Salmon: generate private key with native crypto if available.

lib/pleroma/web/salmon/salmon.ex

@@ -73,6 +73,18 @@ def encode_key({:RSAPublicKey, modulus, exponent}) do
     "RSA.#{modulus_enc}.#{exponent_enc}"
   end
 
+  # Native generation of RSA keys is only available since OTP 20+ and in default build conditions
+  # We try at compile time to generate natively an RSA key otherwise we fallback on the old way.
+  try do
+    _ = :public_key.generate_key({:rsa, 2048, 65537})
+    def generate_rsa_pem do
+      key = :public_key.generate_key({:rsa, 2048, 65537})
+      entry = :public_key.pem_entry_encode(:RSAPrivateKey, key)
+      pem = :public_key.pem_encode([entry]) |> String.trim_trailing
+      {:ok, pem}
+    end
+  rescue
+    _ ->
       def generate_rsa_pem do
         port = Port.open({:spawn, "openssl genrsa"}, [:binary])
         {:ok, pem} = receive do
@@ -85,6 +97,7 @@ def generate_rsa_pem do
           :error
         end
       end
+  end
 
   def keys_from_pem(pem) do
     [private_key_code] = :public_key.pem_decode(pem)