1 files changed, 67 insertions, 0 deletions
diff --git a/src/lib/x509/x509_obj.cpp b/src/lib/x509/x509_obj.cpp
index 01a92a001..78413c121 100644
--- a/src/lib/x509/x509_obj.cpp
+++ b/src/lib/x509/x509_obj.cpp
@@ -12,6 +12,7 @@
 #include <botan/ber_dec.h>
 #include <botan/parsing.h>
 #include <botan/pem.h>
+#include <botan/emsa.h>
 #include <algorithm>
 
 namespace Botan {
@@ -284,4 +285,70 @@ std::vector<uint8_t> X509_Object::make_signed(PK_Signer* signer,
    .get_contents_unlocked();
    }
 
+/*
+* Choose a signing format for the key
+*/
+std::unique_ptr<PK_Signer> X509_Object::choose_sig_format(AlgorithmIdentifier& sig_algo,
+                                                          const Private_Key& key,
+                                                          RandomNumberGenerator& rng,
+                                                          const std::string& hash_fn,
+                                                          const std::string& padding_algo)
+   {
+   const std::string algo_name = key.algo_name();
+
+   std::string padding;
+
+   // check algo_name and set default
+   if(algo_name == "RSA")
+      {
+      // set to EMSA3 for compatibility reasons, originally it was the only option
+      padding = "EMSA3(" + hash_fn + ")";
+      }
+   else if(algo_name == "DSA" ||
+           algo_name == "ECDSA" ||
+           algo_name == "ECGDSA" ||
+           algo_name == "ECKCDSA" ||
+           algo_name == "GOST-34.10")
+      {
+      padding = "EMSA1(" + hash_fn + ")";
+      }
+   else
+      {
+      throw Invalid_Argument("Unknown X.509 signing key type: " + algo_name);
+      }
+
+   if(padding_algo.empty() == false)
+      {
+      padding = padding_algo;
+      }
+
+   // try to construct an EMSA object from the padding options or default
+   std::unique_ptr<EMSA> emsa = nullptr;
+   try
+      {
+      emsa.reset(get_emsa(padding));
+      }
+   /*
+    * get_emsa will throw if opts contains {"padding",<valid_padding>} but
+    * <valid_padding> does not specify a hash function.
+    * Omitting it is valid since it needs to be identical to hash_fn.
+    * If it still throws, something happened that we cannot repair here,
+    * e.g. the algorithm/padding combination is not supported.
+    */
+   catch(...)
+      {
+      emsa.reset(get_emsa(padding + "(" + hash_fn + ")"));
+      }
+   if(emsa == nullptr)
+      {
+      throw Invalid_Argument("Could not parse padding scheme " + padding);
+      }
+
+   const Signature_Format format = (key.message_parts() > 1) ? DER_SEQUENCE : IEEE_1363;
+
+   sig_algo = emsa->config_for_x509(key, hash_fn);
+
+   return std::unique_ptr<PK_Signer>(new PK_Signer(key, rng, emsa->name(), format));
+   }
+
 }