On 01/22/2016 06:43 PM, =JeffH wrote: > ON Fri, 22 Jan 2016 15:53:27 Benjamin Kaduk noted: > > On 01/22/2016 03:26 PM, =JeffH wrote: > > > On 01/22/2016 12:29 PM, =JeffH wrote: > > >> [ fixed pitch font advised here ] > > > > > > the below is corrected to use "byte count" rather than "index" or > > > "indicies" (and to ditch the tabs).. > > > > > > > > > > On 01/22/2016 09:42 AM, =JeffH wrote: > > > > > [ resending from different account - my work addr ends up in spam > > > > > bucket for many it seems ] > > > > > > > > > > On 1/20/16, 11:01 AM, "Benjamin Kaduk" <bka...@akamai.com> wrote: > > > > > >On 01/20/2016 12:47 PM, Hodges, Jeff wrote: > > > > > >> On 1/13/16, 12:53 PM, "Benjamin Kaduk" <bka...@akamai.com> > wrote: > > > > > >>> On 01/13/2016 02:44 PM, Jong-Shian Wu wrote: > >>>>>>>> I have a question about the even-vs-odd restrictions on the > >>>>>>>> length of a valid variable-length vector defined in TLS > >>>>>>>> specification after reading the section 4.3 of RFC 5246 > >>>>>>>> [1] which states that: "The length of an encoded vector > >>>>>>>> must be an even multiple of the length of a single element > >>>>>>>> (for example, a 17-byte vector of uint16 would be > >>>>>>>> illegal)." > >>>>>>>> > >>>>>>> It means "whole-number" as opposed to fractional, i.e., there > >>>>>>> should not be unused "junk bytes" at the end. > >>>>>> > >>>>>> In case it's helpful, here's a suggested re-write of that > >>>>>> quoted sentence above.. > > > > > >> > > > > > >> The length of an encoded variable-length vector must be an > > > > > >> exact multiple of the length of a single element. For example, > > > > > >> an encoded 17-byte vector of uint16 would be illegal, and an > > > > > >> encoded variable-length vector of four 32 byte elements, > > > > > >> having a ceiling of 2^16-1, will be 130 bytes long overall > > > > > >> (2 byte length field followed by 128 bytes of data). > > > > > > > > > > > >Wouldn't the ceiling more properly be 2^16-4 in that case? > > > > > > > > > > hm, I'm not sure -- what would be the rationale? The exact > multiple > > > > > criteria? but 2^16 / 32 = 2048 while (2^16-4) / 32 = 2047.875 > > > > > > > > Ah, I seem to have conflated bits and bytes due to reading too > quickly > > > > and should have said (2^16-32), as Ilari alluded to with "or > rounding > > > > thereof to integral multiple of > > > > elements". > > > > > > hm, but in this case it seems that a variable-length vector declared > > > with a length range of <0..2^16-1> would exactly accommodate up to > > > 2048 32-byte elements.. > > > > > > opaque Foo[32] ; > > > > > > Foo fooSequence<0..2^16-1>; /* will accommodate up to 2048 > > > Foo instances */ > > > > > > ..because it has a zero-based byte count, as in this example.. > > > > > > > > > opaque Array<0..2^2-1> ; /* should accommodate > > > 2^2 = 4 1-byte elements */ > > > > > > /* > > > Array with 4 > > > elements in memory: [ xx xx xx xx ] > > > byte count (hex): 0 1 2 3 3 = 2^2-1 > > > > > > byte count (binary): 00 01 10 11 > > > > > > */ > > > > > > > > > ..yes? or am I missing something? > > > > You are missing something. > > > > The encoded length represents the actual number of bytes that will > > follow, so that a zero-length array with maximum length 2^16-1 is > > encoded as just 00 00. So, even though the index into the array starts > > at zero, the actual length "starts at" 1, just like in C. > > Ok, thanks, yes, I was confusing byte count with indices. > > To test my understanding: > > Given.. > > opaque Foo[32] ; > > Foo fooSequence<0..2^16-32> ; > > > ..fooSequence will accommodate up to 2047 Foo instances, and the > length prefix of fooSequence will itself be two bytes long, yes? >
Yes. > > AFAICT, though, from [1] (and prior TLS specs), it seems legit to > declare fooSequence like so.. > > Foo fooSequence<0..2^16-1> ; > > ..with the expectation that a correct implementation will allow only > 2047 Foo instances to be encoded into a fooSequence on the wire, yes ? > I believe a correct implementation would do so, yes, though I am not quite enough of a spec lawyer to be able to state that authoritatively. The <0..2^16-32> form is perhaps friendlier to the reader. > > Also, if one actually needs to be able to accommodate up to _2048_ Foo > instances, and can tolerate having the length prefix be three bytes > long, then one could declare fooSequence as.. > > Foo fooSequence<0..2^16>; > > ..or.. > > Foo fooSequence<0..2^24-1>; > > ..yes? Yes. (The latter form might permit an attacker to force a peer to consume a lot of memory processing a request, of course.) -Ben _______________________________________________ TLS mailing list TLS@ietf.org https://www.ietf.org/mailman/listinfo/tls
