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But I don't know of any mainstream hash where this theoretical risk actually applies (e.g. sha3 prepended by zeros is still safe, but obviously not if truncate the sha3-provided bits off). Edit: just to be clear, I'm not suggesting anyone actally use LPMAC-sha1 given the current state of sha1.įor another example in general it's unsafe to truncate a "secure" hash - hashes that satisfy most security requirements can be constructed that are not safe when truncated (e.g. this paper: which proposes an attack on length-and-key prefixed messages, using some sha1 weaknesses and merely over 2^84 memory and 2^154 queries - color me impressed, but not scared). HMAC is a neat trick to avoid length extension attacks (and other issues) in a generalized fashion, but that doesn't mean those risks actually apply in practice.
#PDF INSUFFICIENT DATA FOR AN IMAGE 2017 SOFTWARE#
If your format has a length prefix (really common) then you may well be "vulnerable" in the sense that appending arbitrary data is "valid", but a canonical form without the appended data is trivial to construct and indeed most software would likely completely ignore that extra data.
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People sometimes overstate the impact of length extension attacks. Video and image formats use multiple encodings to give encoders the room to make time-space trade-offs. If we want to redundantly represent some bytes redundantly in case of data loss, then there must be multiple representations of those bytes that are all acceptable for the reader for this to work. That might be very challenging in practice, because a more expressive language directly allows a more compressed/efficient encoding of the same information, but at the cost of being more difficult (or impossible) to create a canonical representation.Īlso, data formats that are purposefully redundant for error tolerance all basically have the property that readers should be tolerant of non-canonical forms. > Constrained formats, and formats where the set of 'essential information' can be canonicalized into a particular representation should be the norm, rather than the exotic exception, especially in situations where minute inessential differences can be cascaded to drastically alter the result. Constrained formats, and formats where the set of 'essential information' can be canonicalized into a particular representation should be the norm, rather than the exotic exception, especially in situations where minute inessential differences can be cascaded to drastically alter the result.
#PDF INSUFFICIENT DATA FOR AN IMAGE 2017 PDF#
For general-purpose programming languages like PostScript, of which PDF is a subset, this is essentially an unfulfillable requirement, as any number of input "source code" can produce observationally "same" results. This shows the importance of techniques like canonicalization and determinism, which ensure that given a particular knowledge set, that result could only have been arrived at given exactly one input. Selecting a region changes the language and/or content on didn't get a chance to make this point in that other thread, because the thread of its follow-ups quickly morphed from promising to meandering, but the combination of lax formats (PDF and JPEG in this instance) makes this style of collision particularly reductive, and in a sense, a cheapshot, if still devastatingly damaging given both PDF's and JPEG's ubiquity - both separately and together - in document storage and archival. The optimized document looks like the original PDF but doesn’t contain any layer information. This does not affect the functionality of the PDF, but it does decrease the file size.ĭiscard Hidden Layer Content And Flatten Visible Layersĭecreases file size. Strips information from a PDF document that is useful only to the application that created the document. Links that jump to other locations within the PDF are not removed.ĭiscard Private Data Of Other Applications ( PDF Optimizer doesn’t optimize attached files.) Removes all file attachments, including attachments added to the PDF as comments. (Use the Save As command to restore metadata streams to a copy of the PDF.) Removes information in the document information dictionary and all metadata streams. Removes all comments, forms, form fields, and multimedia from the PDF.ĭiscard Document Information And Metadata You can locate hidden text and user-related information by using the Examine Documentcommand ( Tools > Redact > Sanitize Document, and then choose to Remove Hidden Information).ĭiscard All Comments, Forms And Multimedia If you’re unable to find personal information, it may be hidden. Use the Discard User Data panel to remove any personal information that you don’t want to distribute or share with others.