Hi Andreas, I am not sure that people ignore the redundant length fields on purpose. I think that the strange syntax that TLS uses invites these types of mistakes and I had run into those myself as well.
Ciao Hannes On 10/05/2016 02:03 PM, Andreas Walz wrote: > Hi Olivier, > > your findings are interesting as they pretty much match with what we > have found when studying TLS implementations for embedded systems. In > many implementations there is a tendency to ignore redundant length > fields (or at least to not enforce consistency). There has been some > discussion about this on the list, see thread "Suspicious behaviour of > TLS server implementations". > > We are currently refining our study and in the process of writing a > paper. I hopefully can give more details on results soon. > > Cheers, > Andi > > ___________________________________ > > Andreas Walz > Research Engineer > Institute of reliable Embedded Systems and Communication Electronics (ivESK) > Offenburg University of Applied Sciences, 77652 Offenburg, Germany > > >>>> Olivier Levillain <olivier.levill...@ssi.gouv.fr> 10/04/16 5:51 AM >>> > Hi list, > > I have been working in the labs at ANSSI (the French Network and > Information System Agency) for several years and I just defended my PhD > thesis on the TLS ecosystem (documents are available at > http://paperstreet.picty.org/~yeye/2016/phdthesis-Levillain16/). > >>> On Mon, 2016-09-19 at 10:29 +0200, Nikos Mavrogiannopoulos wrote: >>>> On Mon, 2016-08-08 at 11:28 +0300, Ilari Liusvaara wrote: >>>>> More compact and makes the option where server sends some bad option >>>>> more clear. >>> Note that if we really want to be more compact, we might also observe >>> that there is a redundant length field in the extension as sent by the >>> client: >>> case client_hello: >>> PskIdentity identities<6..2^16-1>; >>> >>> Each extension has at least four bytes on the wire — the extension_type >>> itself, and the length field of the extension_data in a uint16. >>> >>> If I am interpreting the spec correctly, then the data for the >>> PreSharedKeyIdentity extension in the ClientHello then follows that >>> with another uint16, which is *always* a value two lower than the one >>> which immediately precedes it. >>> >>> e.g. >>> >>> 0x00 0x29 // ExtensionType extension_type == 41 >>> 0x00 0x14 // opaque extension_data<0..2^16-1> >>> // This length field is entirely redundant: >>> 0x00 0x12 // PskIdentity identities<6..2^16-1> >>> // First identity: >>> 0x00 0x00 // PskKeyExchangeMode, PskAuthenticationMode >>> 0x00 0x04 // opaque identity<0..2^16-1> >>> 0x44 0x61 0x76 0x65 // "Dave" >>> // Second identity: >>> 0x00 0x00 // PskKeyExchangeMode, PskAuthenticationMode >>> 0x00 0x06 // opaque identity<0..2^16-1)> >>> 0x43 0x68 0x6c 0x6f 0xc3 0xab // "Chloë" >>> >>> Do we care that the '0x00 0x12' bytes on my third line above are >>> entirely redundant on the wire? Or have I interpreted it wrong? >>> >> Not enough to fix it, this is just the way TLS rolls. > > Sorry if I am a little late to the party, but I noticed that even if > this is generally true, I believe it has not always been enforced in TLS > extensions. > > In 2006, the IETF standardised the session tickets extension, allowing > for session resumption without server-side state (RFC 4507). > However, no TLS stack implements the specification correctly: even > if the specification described the _content_ of the extension as > a variable-length object (that is an opaque object prefixed by its > length), every implementation ignores this second (useless) length > field. The RFC 5077, published in 2008, fixes the gap > between the specification and the implementations. > > RFC 4507 : > > The SessionTicket extension has been assigned > the number 35. The format of the SessionTicket > extension is given at the end of this section. > > struct { > opaque ticket<0..2^16-1>; > } SessionTicket; > > 00 23 Ticket Extension type 35 > 01 02 Length of extension contents > 01 00 Length of ticket > FF FF .. .. Actual ticket > > RFC 5077 > > The SessionTicket extension has been assigned > the number 35. The extension_data field of > SessionTicket extension contains the ticket. > > 00 23 Ticket Extension type 35 > 01 00 Length of extension contents (ticket) > FF FF .. .. Actual ticket > > > Best regards, > olivier > > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls > > > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls >
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