Introduction

Postquantum Preshared Keys (PPK) are an addition to the IKEv2 protocol to ensure that currently encrypted traffic which is stored is safe against against future quantum computer decryption. A PPK is shared securely out-of-band and is used as an input into the SKEYSEED generation. This means that even if the IKEv2 DiffieHellman is compromised, an attacker cannot obtain the key material (KEYMAT). The PPK method is specified in RFC8784.

Implementation

To allow the use of PPK’s in libreswan, the following options were added:

• The values 16435, 16436 and 16437 were picked as numbers for PPK_USE, PPK_IDENTITY and NO_PPK_AUTH Notify messages, respectively. The numbers were assigned by IANA (on 7th February 2018).

• Changes were made in source files (programs/pluto/ikev2_*.c) where IKEv2 is implemented.

• The secret reading/handling code in lib/libswan was updated to support PPKs.

• New test cases were added. These can be found as ikev2-ppk-* folders in the testing/pluto/ directory.

Configuring a PPK and connection in libreswan

Each PPK has its own PPK_ID - a unique string that identifies which PPK to use.

In libreswan PPK are stored in the secrets file (eg /etc/ipsec.secrets)

Two types of PPK are supported: static and dynamic.

Important note: Dynamic PPK will be removed in version 3.24., and re-added in later versions.

1. Static PPK is represented with two strings separated by spaces. The first string is PPK_ID and the second one PPK. The keyword for .secrets file in libreswan is PPKS.

e.g. 10.1.0.1 10.2.0.1 : PPKS “lsw1” “very_long_string_that_is_a_postquantum_preshared_key”

2. Dynamic PPK (work in progress) is represented with two strings separated by spaces. The first string is PPK_ID and the second one is a path to another file which the PPKs will be taken from. This file should also have only two strings in it. The first one would be offset and the second one a long string that will be used as a one-time pad. Each time when a PPK should be loaded into the libreswan, the string of fixed length will be taken from a second string with an offset from first string. After this PPK has been used, libreswan overrides that part of a string with zeros (0x30) and updates the offset to offset+PPK size. This way each time two peers establishing an IPSEC tunnel will use a different PPK for it. ‘’

The ppk option is specified in the connection configuration. Accepted values are never/no, propose/yes and insist.

The PPK option is only valid when using IKEv2.

It is recommended that the PPK has length of at least 256 bits, in order to provide real security against quantum computer attacks.

Source code

The code is commited into libreswan master branch, and PPK feature is available in libreswan as of release 3.23.

Code commit:

https://github.com/libreswan/libreswan/commit/d8e0c68c0dc19d95dfcf19fec934e9dc69c293ac

The main developer of this feature is Vukasin Karadzic, and this has been done as a summer project under the mentorship of Paul Wouters and Tuomo Soini.

For any questions, found bugs or suggestions feel free to contact him on vukasin.karadzic@gmail.com or send an e-mail to swan-dev@lists.libreswan.org mailing list.