Added C crypto.c for digestSha256
diff --git a/ndn-cpp/c/util/crypto.c b/ndn-cpp/c/util/crypto.c
new file mode 100644
index 0000000..68af3e4
--- /dev/null
+++ b/ndn-cpp/c/util/crypto.c
@@ -0,0 +1,14 @@
+/**
+ * @author: Jeff Thompson
+ * See COPYING for copyright and distribution information.
+ */
+
+#include "crypto.h"
+
+void ndn_digestSha256(const unsigned char *data, unsigned int dataLength, unsigned char *digest)
+{
+ SHA256_CTX sha256;
+ SHA256_Init(&sha256);
+ SHA256_Update(&sha256, data, dataLength);
+ SHA256_Final(digest, &sha256);
+}