Import code from previous AssetBuilder version

thirdparty/libtomcrypt/stream/chacha/chacha_keystream.c

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+/* LibTomCrypt, modular cryptographic library -- Tom St Denis
+ *
+ * LibTomCrypt is a library that provides various cryptographic
+ * algorithms in a highly modular and flexible manner.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ */
+
+/* The implementation is based on:
+ * chacha-ref.c version 20080118
+ * Public domain from D. J. Bernstein
+ */
+
+#include "tomcrypt.h"
+
+#ifdef LTC_CHACHA
+
+/**
+  Generate a stream of random bytes via ChaCha
+  @param st      The ChaCha20 state
+  @param out     [out] The output buffer
+  @param outlen  The output length
+  @return CRYPT_OK on success
+ */
+int chacha_keystream(chacha_state *st, unsigned char *out, unsigned long outlen)
+{
+   if (outlen == 0) return CRYPT_OK; /* nothing to do */
+   LTC_ARGCHK(out != NULL);
+   XMEMSET(out, 0, outlen);
+   return chacha_crypt(st, out, outlen, out);
+}
+
+#endif
+
+/* ref:         HEAD -> master, tag: v1.18.2 */
+/* git commit:  7e7eb695d581782f04b24dc444cbfde86af59853 */
+/* commit time: 2018-07-01 22:49:01 +0200 */