.. index:: single: x25519
.. _x25519/0:

.. rst-class:: right

**category**

``x25519``
==========

X25519 Diffie-Hellman key agreement implementation (RFC 7748). Requires exact, unbounded integer arithmetic for arithmetic modulo the 255-bit field prime.

| **Availability:** 
|    ``logtalk_load(crypto(loader))``

| **Author:** Paulo Moura
| **Version:** 1:0:0
| **Date:** 2026-08-03

| **Compilation flags:**
|    ``static``


| **Complements:**
|    :ref:`crypto <crypto/0>`
| **Uses:**
|    :ref:`list <list/0>`
|    :ref:`type <type/0>`

| **Remarks:**
|    (none)

| **Inherited public predicates:**
|    (none)

.. contents::
   :local:
   :backlinks: top

Public predicates
-----------------

.. index:: x25519_keypair/2
.. _x25519/0::x25519_keypair/2:

``x25519_keypair/2``
^^^^^^^^^^^^^^^^^^^^

Generates a fresh random 32-byte PrivateKey and derives the corresponding 32-byte PublicKey. Available only on backends with unbounded integer arithmetic.

| **Compilation flags:**
|    ``static``

| **Template:**
|    ``x25519_keypair(PrivateKey,PublicKey)``
| **Mode and number of proofs:**
|    ``x25519_keypair(-list(byte),-list(byte))`` - ``one``


------------

.. index:: x25519_public_key/2
.. _x25519/0::x25519_public_key/2:

``x25519_public_key/2``
^^^^^^^^^^^^^^^^^^^^^^^

Derives the 32-byte X25519 PublicKey corresponding to a 32-byte PrivateKey. Available only on backends with unbounded integer arithmetic.

| **Compilation flags:**
|    ``static``

| **Template:**
|    ``x25519_public_key(PrivateKey,PublicKey)``
| **Mode and number of proofs:**
|    ``x25519_public_key(+list(byte),-list(byte))`` - ``one_or_error``

| **Exceptions:**
|    ``PrivateKey`` is a partial list or a list with an element which is a variable:
|        ``instantiation_error``
|    ``PrivateKey`` is neither a variable nor a list of 32 bytes:
|        ``type_error(list(byte,32),PrivateKey)``
|    ``PrivateKey`` contains a non-integer byte:
|        ``type_error(integer,Byte)``
|    ``PrivateKey`` contains an integer outside the byte range:
|        ``domain_error(byte,Byte)``


------------

.. index:: x25519_shared_secret/3
.. _x25519/0::x25519_shared_secret/3:

``x25519_shared_secret/3``
^^^^^^^^^^^^^^^^^^^^^^^^^^

Computes the 32-byte X25519 SharedSecret using PrivateKey and PeerPublicKey. Fails when the result is all zeroes. Available only on backends with unbounded integer arithmetic.

| **Compilation flags:**
|    ``static``

| **Template:**
|    ``x25519_shared_secret(PrivateKey,PeerPublicKey,SharedSecret)``
| **Mode and number of proofs:**
|    ``x25519_shared_secret(+list(byte),+list(byte),-list(byte))`` - ``zero_or_one_or_error``

| **Exceptions:**
|    ``PrivateKey`` is a partial list or a list with an element which is a variable:
|        ``instantiation_error``
|    ``PrivateKey`` is neither a variable nor a list of 32 bytes:
|        ``type_error(list(byte,32),PrivateKey)``
|    ``PrivateKey`` contains a non-integer byte:
|        ``type_error(integer,Byte)``
|    ``PrivateKey`` contains an integer outside the byte range:
|        ``domain_error(byte,Byte)``
|    ``PeerPublicKey`` is a partial list or a list with an element which is a variable:
|        ``instantiation_error``
|    ``PeerPublicKey`` is neither a variable nor a list of 32 bytes:
|        ``type_error(list(byte,32),PeerPublicKey)``
|    ``PeerPublicKey`` contains a non-integer byte:
|        ``type_error(integer,Byte)``
|    ``PeerPublicKey`` contains an integer outside the byte range:
|        ``domain_error(byte,Byte)``


------------

Protected predicates
--------------------

(no local declarations; see entity ancestors if any)

Private predicates
------------------

(no local declarations; see entity ancestors if any)

Operators
---------

(none)