[Cryptography-dev] Low level API for Symmetric Encryption

Thu Aug 8 02:37:39 CEST 2013

That makes sense, InvalidCipher or something.

Alex

On Wed, Aug 7, 2013 at 5:36 PM, Paul Kehrer <paul.l.kehrer at gmail.com> wrote:

> GCM is only defined for block ciphers of 128/192/256 bit block
> lengths. So a construction of GCM(DES()) would have to throw an
> exception since DES uses a 64-bit block size.
>
> On Wed, Aug 7, 2013 at 7:30 PM, Donald Stufft <donald at stufft.io> wrote:
> >
> > On Aug 7, 2013, at 8:16 PM, Alex Gaynor <alex.gaynor at gmail.com> wrote:
> >
> > I guess the fact that different modes have fundamentally different APIs
> > (e.g. GCM needs a MAC, CTR needs the counter, etc.) really points to that
> > they should have different classes.
> >
> > How about keysize, do we need a class for each?
> >
> >
> > AFAIK there's no API differences for key size, it's just the size of the
> key
> > and the name you tell openssl.
> >
> > OTOH now that I think about it more, maybe I'm looking at it backwards,
> I'm
> > thinking about it like AES is the important part of GCM(AES128()) that
> > Jean-Paul mentioned, but really it's the other way around. The
> differences
> > between GCM vs CBC vs Whatever are not specific to key size or cipher.
> >
> > So if GCM/CBC/etc were the driving forces of the API interaction with
> > OpenSSL and there is (I believe) no difference other than parameters you
> > call the API's with for algo/key size instead of the way I was thinking
> of
> > it where AES128() were the driving I think it would work assuming I'm
> right
> > about that.
> >
> >
> >
> > Alex
> >
> >
> > On Wed, Aug 7, 2013 at 5:12 PM, Donald Stufft <donald at stufft.io> wrote:
> >>
> >>
> >> On Aug 7, 2013, at 7:18 PM, Alex Gaynor <alex.gaynor at gmail.com> wrote:
> >>
> >> Another option is to compose them in the constructor: AES(keysize=256,
> >> mode=GCM) or so.
> >>
> >> Alex
> >>
> >>
> >> We'd still essentially be passing AES-256-GCM into OpenSSL afaik. So
> afaik
> >> it'd only look like composing. We're not handling (most) of the
> differences
> >> between the modes (and none of the differences for key size).
> >>
> >> The mode classes could be stuff that handles the slight differences in
> how
> >> you call the API (for instance GCM has an additional call to
> >> EVP_EncryptUpdate to give it the AAD and a different calls to handle
> getting
> >> the MAC data. I'm not sure how well that'd translate? It might work ok.
> >>
> >>
> >>
> >> On Wed, Aug 7, 2013 at 4:12 PM, Donald Stufft <donald at stufft.io> wrote:
> >>>
> >>>
> >>> On Aug 7, 2013, at 6:32 PM, Jean-Paul Calderone
> >>> <jean-paul at hybridcluster.com> wrote:
> >>>
> >>> > On 08/07/2013 06:16 PM, Donald Stufft wrote:
> >>> >> So to kick things off I'd like to get AES-GCM exposed and figured it
> >>> >> could be a good way to start the ball rolling for figuring out how
> we want
> >>> >> to expose symmetric ciphers at the low level API.
> >>> >>
> >>> >> I'm thinking cryptography.primitives.aes which has classes named
> like
> >>> >> AES128GCM, AES256CBC, etc. The obvious naming scheme being
> >>> >> AlgorithmKeysizeMode.
> >>> >>
> >>> >>
> >>> >
> >>> > GCM (CBC, etc) is a mode of operation that is applicable to arbitrary
> >>> > block ciphers.
> >>> >
> >>> > Why should it be tied to "AES128"?  Why wouldn't you GCM(AES128())?
>  If
> >>> > you're talking about primitives, AES128 is more primitive than GCM on
> >>> > AES128.  And GCM isn't specific to AES, so I don't see
> >>> > cryptography.primitives.aes as the proper home for it.
> >>> >
> >>> > I hope these aren't questions with highly obvious answers.
> >>>
> >>> As far as I know (and I could be wrong? I don't know OpenSSL guts that
> >>> well) OpenSSL doesn't do composition like that, in order to get
> AES-128-GCM
> >>> you pass that in.
> >>>
> >>> Is there a way to access openssl where you're composing GCM with
> AES128?
> >>> If not I think we'd be stuck do some sort of "combine variables of the
> >>> classes AES128 and GCM to make the name AES-128-GCM to pass into
> openssl"
> >>> thing which doesn't feel particularly clean to me?
> >>>
> >>>
> >>> -----------------
> >>> Donald Stufft
> >>> PGP: 0x6E3CBCE93372DCFA // 7C6B 7C5D 5E2B 6356 A926 F04F 6E3C BCE9 3372
> >>> DCFA
> >>>
> >>>
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> >>>
> >>
> >>
> >>
> >> --
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> >>
> >> -----------------
> >> Donald Stufft
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> >> DCFA
> >>
> >>
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> >
> >
> >
> > --
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> to
> > say it." -- Evelyn Beatrice Hall (summarizing Voltaire)
> > "The people's good is the highest law." -- Cicero
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> > -----------------
> > Donald Stufft
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> DCFA
> >
> >
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-- 
"I disapprove of what you say, but I will defend to the death your right to
say it." -- Evelyn Beatrice Hall (summarizing Voltaire)
"The people's good is the highest law." -- Cicero
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