[strongSwan] nonce Length

[Andreas Steffen]

Hi Jafar,

the mandatory nonce plugin is a nonce generator which returns
the requested number of random bytes. There are many other places in
the strongSwan code where nonces of variable size are needed
(e.g. for the IKE SPI or for the TLS client or server Hello).

Kind regards

Andreas

On 14.09.2017 17:28, Jafar Al-Gharaibeh wrote:
> Hi Andreas,
>
>     Thanks for the quick and thorough answer. I did not find that piece
> of information (nonce size) in the documentation, but as you noted about
> the source code, I did download and dig through the source code
> yesterday and came across the the 32 byte number. Thanks for confirming
> that.
>
>     I also came across nonce plugin configuration:
>     nonce {
>     }
>
> Is there really any thing configurable here or is that just there for
> completeness?
>
> Kind Regards,
> Jafar
>
> On 9/14/2017 1:56 AM, Andreas Steffen wrote:
>> Hi Jafar,
>>
>> section 2.10 of IKEv2 RFC 7296 [1] states that
>>
>>     Nonces used in IKEv2
>>     MUST be randomly chosen, MUST be at least 128 bits in size, and MUST
>>     be at least half the key size of the negotiated pseudorandom function
>>     (PRF).  However, the initiator chooses the nonce before the outcome
>>     of the negotiation is known.  Because of that, the nonce has to be
>>     long enough for all the PRFs being proposed.
>>
>> This is why strongSwan generates nonces with a constant size of 32 bytes
>> (256 bits) as defined in nonce_payloads.h [2]
>>
>>    /**
>>     * Nonce size in bytes for nonces sending to other peer.
>>     */
>>    #define NONCE_SIZE 32
>>
>> Best regards
>>
>> Andreas
>>
>> [1]https://tools.ietf.org/html/rfc7296#section-2.10
>> [2]https://git.strongswan.org/?p=strongswan.git;a=blob;f=src/libcharon/encoding/payloads/nonce_payload.h;h=ee8ad17f789ed4fe6a2e3476fc710b79d74885aa;hb=HEAD#l30
>>
>>
>> On 13.09.2017 20:37, Jafar Al-Gharaibeh wrote:
>>> Hi,
>>>
>>>     What is the default length of the nonce used  to establish and rekey
>>> IKE/Child SAs?  is that based on the DH group? and is the length
>>> configurable?
>>>
>>> Thanks,
>>> Jafar
>> ======================================================================
>> Andreas Steffen                         andreas.steffen at strongswan.org
>> strongSwan - the Open Source VPN Solution!          www.strongswan.org
>> Institute for Networked Solutions
>> HSR University of Applied Sciences Rapperswil
>> CH-8640 Rapperswil (Switzerland)
>> ===========================================================[INS-HSR]==
>>
>

-- 
======================================================================
Andreas Steffen                         andreas.steffen at strongswan.org
strongSwan - the Open Source VPN Solution!          www.strongswan.org
Institute for Networked Solutions
University of Applied Sciences Rapperswil
CH-8640 Rapperswil (Switzerland)
===========================================================[INS-HSR]==

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