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diff --git a/src/include/ipxe/x25519.h b/src/include/ipxe/x25519.h
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+++ b/src/include/ipxe/x25519.h
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+#ifndef _IPXE_X25519_H
+#define _IPXE_X25519_H
+
+/** @file
+ *
+ * X25519 key exchange
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <stdint.h>
+#include <ipxe/bigint.h>
+
+/** X25519 unsigned big integer size
+ *
+ * X25519 uses the finite field of integers modulo the prime
+ * p=2^255-19.  The canonical representations of integers in this
+ * field therefore require only 255 bits.
+ *
+ * For internal calculations we use big integers containing up to 267
+ * bits, since this ends up allowing us to avoid some unnecessary (and
+ * expensive) intermediate reductions modulo p.
+ */
+#define X25519_SIZE bigint_required_size ( ( 267 /* bits */ + 7 ) / 8 )
+
+/** An X25519 unsigned big integer used in internal calculations */
+typedef bigint_t ( X25519_SIZE ) x25519_t;
+
+/** An X25519 unsigned 258-bit integer
+ *
+ * This is an unsigned integer N in the finite field of integers
+ * modulo the prime p=2^255-19.
+ *
+ * In this representation, N is encoded as any big integer that is in
+ * the same congruence class as N (i.e that has the same value as N
+ * modulo p) and that lies within the 258-bit range [0,8p-1].
+ *
+ * This type can be used as an input for multiplication (but not for
+ * addition or subtraction).
+ *
+ * Addition or subtraction will produce an output of this type.
+ */
+union x25519_oct258 {
+	/** Big integer value */
+	x25519_t value;
+};
+
+/** An X25519 unsigned 257-bit integer
+ *
+ * This is an unsigned integer N in the finite field of integers
+ * modulo the prime p=2^255-19.
+ *
+ * In this representation, N is encoded as any big integer that is in
+ * the same congruence class as N (i.e that has the same value as N
+ * modulo p) and that lies within the 257-bit range [0,4p-1].
+ *
+ * This type can be used as an input for addition, subtraction, or
+ * multiplication.
+ *
+ * Multiplication will produce an output of this type.
+ */
+union x25519_quad257 {
+	/** Big integer value */
+	x25519_t value;
+	/** X25519 unsigned 258-bit integer
+	 *
+	 * Any value in the range [0,4p-1] is automatically also
+	 * within the range [0,8p-1] and so may be consumed as an
+	 * unsigned 258-bit integer.
+	 */
+	const union x25519_oct258 oct258;
+};
+
+/** An X25519 32-byte value */
+struct x25519_value {
+	/** Raw value */
+	uint8_t raw[32];
+};
+
+extern void x25519_multiply ( const union x25519_oct258 *multiplicand,
+			      const union x25519_oct258 *multiplier,
+			      union x25519_quad257 *result );
+extern void x25519_invert ( const union x25519_oct258 *invertend,
+			    union x25519_quad257 *result );
+extern void x25519_reduce ( union x25519_quad257 *value );
+extern void x25519_key ( const struct x25519_value *base,
+			 const struct x25519_value *scalar,
+			 struct x25519_value *result );
+
+#endif /* _IPXE_X25519_H */