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diff --git a/src/lib/libcrypto/curve25519/curve25519.h b/src/lib/libcrypto/curve25519/curve25519.h
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+++ b/src/lib/libcrypto/curve25519/curve25519.h
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+/*
+ * Copyright (c) 2015, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#ifndef HEADER_CURVE25519_H
+#define HEADER_CURVE25519_H
+
+#include <stdint.h>
+
+#include <openssl/opensslconf.h>
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+/*
+ * Curve25519.
+ *
+ * Curve25519 is an elliptic curve. See https://tools.ietf.org/html/rfc7748.
+ */
+
+/*
+ * X25519.
+ *
+ * X25519 is the Diffie-Hellman primitive built from curve25519. It is
+ * sometimes referred to as curve25519, but X25519 is a more precise name.
+ * See http://cr.yp.to/ecdh.html and https://tools.ietf.org/html/rfc7748.
+ */
+
+/*
+ * X25519_keypair sets |out_public_value| and |out_private_key| to a freshly
+ * generated, public/private key pair.
+ */
+void X25519_keypair(uint8_t out_public_value[32], uint8_t out_private_key[32]);
+
+/*
+ * X25519 writes a shared key to |out_shared_key| that is calculated from the
+ * given private key and the peer's public value. It returns one on success and
+ * zero on error.
+ *
+ * Don't use the shared key directly, rather use a KDF and also include the two
+ * public values as inputs.
+ */
+int X25519(uint8_t out_shared_key[32], const uint8_t private_key[32],
+    const uint8_t peers_public_value[32]);
+
+#if defined(__cplusplus)
+}  /* extern C */
+#endif
+
+#endif  /* HEADER_CURVE25519_H */