Re: [IPsec] Question about RFC 5114

> -----Original Message-----
> From: ipsec-bounces@ietf.org [mailto:ipsec-bounces@ietf.org] On Behalf
> Of Kaz Kobara
> Sent: Friday, March 26, 2010 6:26 PM
> To: latten@austin.ibm.com; mlepinski@bbn.com; kent@bbn.com
> Cc: ipsec@ietf.org; avagarwa@redhat.com
> Subject: Re: [IPsec] Question about RFC 5114
> 
> Hi Joy
> 
> When one uses a subgroup like defined in RFC 5114, q (and (p-1)/2q )
> must be chosen carefully.
> 
> Precisely:
> 1. q must be a prime number of 2k or more bits where k is a security
> parameter.
> 2. q must be a divisor of ((p - 1) / 2).
> 3. Every factors of (p - 1) / (2q) must also be primes comparable to or
> greater than q in size.

I must point out that the MODP groups defined in RFC 5114 do not meet criteria 3.

> 
> p corresponding such q is called a "secure prime."
> 
> X is simply to shift the range of 0 to q-2 to 1 to q-1 to exclude 0
> (since g^0 mod p = 1).
> 
> Kaz
> 
> > -----Original Message-----
> > From: ipsec-bounces@ietf.org [mailto:ipsec-bounces@ietf.org] On
> Behalf Of
> > Joy Latten
> > Sent: Saturday, March 27, 2010 6:25 AM
> > To: mlepinski@bbn.com; kent@bbn.com
> > Cc: ipsec@ietf.org; avagarwa@redhat.com
> > Subject: [IPsec] Question about RFC 5114
> >
> > Hi,
> >
> > I am looking to implement modp groups 22, 23, and 24 into IKE but
> have a
> > question.
> >
> > RFC 5114 gives the prime, p, the generator, g and a subgroup, q, with
> a
> > specific size...
> >
> > Because prior rfcs for modp groups did not specify a "q", I was not
> sure
> > if this was a new constant or just stating a size requirement?
> > So I took a look at NIST 800-56A. In particular,
> >
> > 5.6.1 Private/Public Key Pair Generation
> >
> > 5.6.1.1 FFC Key Pair Generation
> > For the FFC schemes, each static and ephemeral private key and public
> > key shall be generated using an Approved method and the selected
> valid
> > domain parameters (p, q, g{, SEED,pgenCounter}) (see Appendix B of
> FIPS
> > 186-3).
> > ...
> >
> > I then took a look at FIPS 186-3, Appendix B, which documents 2
> methods
> > for finite field cryptography (FFC) key pair generation.
> > For example, one method is "Key Pair Generation Using Extra Random
> > Bits". It actually states that "q" is an input and it is used to do
> an
> > additional computation to compute "x".
> >
> > I am somewhat confused, are the modp groups 22, 23 & 24 suppose to
> use
> > one of these new methods and that is why "q" is given in rfc 5114?
> > Or am I to ignore this and just continue with existing way
> > where "q" is not used and there aren't any additional computations
> > to compute x.
> >
> > I am not even sure this is correct place to ask, but any advice
> > would be welcome.
> >
> > regards,
> > Joy
> >
> >
> > (Cut-n-paste from FIPs 186-3 below to show input and process)
> >
> >  Input:
> >     (p, q, g)      The subset of the domain parameters that are used
> >                    for this process. p, q and g shall either be
> >                    provided as integers during input, or shall be
> >                    converted to integers prior to use.
> >
> > Process:
> > 1. N = len(q); L = len(p).    Comment: Check that the (L, N) pair
> >                               is specified in Section 4.2.
> > 2. If the (L, N) pair is invalid, then return an ERROR indicator,
> >    Invalid_x, and Invalid_y.
> > 3. requested_security_strength = the security strength associated
> >    with the (L, N) pair;      see SP 800-57.
> > 4. Obtain a string of N+64 returned_bits from an RBG with a security
> >    strength of requested_security_strength or more. If an ERROR
> >    indication is returned, then return an ERROR indication,
> >    Invalid_x, and Invalid_y.
> > 5. Convert returned_bits to the (non-negative) integer c (see
> >    Appendix C.2.1).
> > 6. x = (c mod (q–1)) + 1.       Comment: 0 ≤ c mod (q–1) ≤ q–2 and
> >                                 implies that 1 ≤ x ≤ q–1.
> > 7. y = gx mod p.
> > 8. Return SUCCESS, x, and y.
> >
> > _______________________________________________
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> 
> 
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