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diff --git a/include/curve25519.h b/include/curve25519.h
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+/* Copyright 2017 The Chromium OS Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef __CROS_EC_CURVE25519_H
+#define __CROS_EC_CURVE25519_H
+
+#include <stdint.h>
+
+/* 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.
+ */
+
+#define X25519_PRIVATE_KEY_LEN 32
+#define X25519_PUBLIC_VALUE_LEN 32
+
+/**
+ * Generate a public/private key pair.
+ * @param out_public_value generated public key.
+ * @param out_private_value generated private key.
+ */
+void X25519_keypair(uint8_t out_public_value[32], uint8_t out_private_key[32]);
+
+/**
+ * Diffie-Hellman function.
+ * @param out_shared_key
+ * @param private_key
+ * @param out_public_value
+ * @return one on success and zero on error.
+ *
+ * X25519() writes a shared key to @out_shared_key that is calculated from the
+ * given private key and the peer's public value.
+ *
+ * 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]);
+
+/**
+ * Compute the matching public key.
+ * @param out_public_value computed public key.
+ * @param private_key private key to use.
+ *
+ * X25519_public_from_private() calculates a Diffie-Hellman public value from
+ * the given private key and writes it to @out_public_value.
+ */
+void X25519_public_from_private(uint8_t out_public_value[32],
+				const uint8_t private_key[32]);
+
+#endif /* __CROS_EC_CURVE25519_H */