Once upon a time you could provide XSL style sheets for canonicalization. And then they added JS to XSL.
To validate my signature, run my code. Yup On Tuesday, August 25, 2015, <travis+ml-lang...@subspacefield.org> wrote: > > http://www.contextis.com/documents/33/Exploiting_XML_Digital_Signature_Implementations-HITBKL20131.pdf > > By signing XML content, rather than the raw bytes of an XML > document, the W3C were faced with a problem, specifically the > possibility that intermediate XML processors might modify the > document's physical structure without changing the meaning. > > At this point you are permitted to start chuckling, privately. > > An obvious example is text encodings. As long as the content > is the same there is no reason why an XML file stored as UTF-8 > should not have the same signature value as one stored as > UTF-16. There are other changes which could occur which don't > affect the meaning of the XML but would affect its physical > representation, such as the order of attributes, as the XML > specification does not mandate how a processor should > serialize content. > > Eyebrows raised. > > With this problem in mind the W3C devised the canonical XML > specification which defines a series of processing rules which > can be applied to parsed XML content to create a known > canonical binary representation. For example, it specifies the > ordering of attributes, and mandates the use of UTF-8 as the > only text encoding scheme. > > Summary: We won't specify how you serialize it, only how you serialize > it to validate the signature. As a result, you have to parse the > untrusted message and expose parsing and canonicalization to the > anonymous attack surface before determining the signature is invalid, > assuming you even managed to check that properly: > > > https://lists.w3.org/Archives/Public/public-xmlsec/2009Nov/att-0019/Camera-Ready.pdf > http://www.slideshare.net/44Con/the-forgers-artjamesforshaw44con2k13 > > https://www.owasp.org/images/5/5a/07A_Breaking_XML_Signature_and_Encryption_-_Juraj_Somorovsky.pdf > > https://www.usenix.org/system/files/conference/usenixsecurity12/sec12-final91.pdf > > Countermeasures: > > http://arxiv.org/pdf/1401.7483.pdf > > Proposed that the anonymous attack surface be required to do minimum > processing on untrusted input before authentication/authorization. > That means no parsing, nothing more complicated than slicing off a > signature and validating it. Proposed that this not just encourages > security in the non-authenticated case, it also minimizes the work to > validate the security of the anonymous attack surface. > > Open question: how much flexibility in cipher negotiation or choices > and serialization can be done safely during this stage. Compare > OpenSSL. Considered that flexibilty (which requires more complex > pre-auth logic) comes with risk, but if chosen carefully can be > minimized. > -- > http://www.subspacefield.org/~travis/ | if spammer then > j...@subspacefield.org <javascript:;> > "Computer crime, the glamor crime of the 1970s, will become in the > 1980s one of the greatest sources of preventable business loss." > John M. Carroll, "Computer Security", first edition cover flap, 1977 >
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