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authorBjörn Gustavsson <[email protected]>2013-02-28 11:42:47 +0100
committerBjörn Gustavsson <[email protected]>2013-05-31 14:52:16 +0200
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PER/UPER: Correct encoding of a length outside the root range
Consider a type with a size constraint with an extension marker such as: S ::= OCTET STRING (SIZE (0..10, ...)) For a length outside the root range (e.g. 42), the PER/UPER encoder will encode the length field in the same way as it would the type INTEGER (0..MAX) (i.e., as semi-constrained whole number), while the decoder would decode the length in the same way as length field without any constraint. Clearly, either the encoder or the decoder is wrong. But which one? Dubuisson's [1] book (page 442) says that the length should be encoded as a semi-constrained whole number if the length is outside the root range. The X.691 standard document [2] also says (e.g. in 15.11) that length fields should be a semi-constrained number, but gives a reference to section gives a reference to section 10.9, "General rules for encoding a length determinant", and not to to 10.7, "Encoding of a semi-constrained whole number". Reading the standard that way should imply that a length outside the root range should be encoded in the same way as an unconstrained length, and that the decoder does the right thing. Further support for that interpretation: - Larmouth's book [3], page 303. - The ASN.1 playground. [4] References: [1] http://www.oss.com/asn1/resources/books-whitepapers-pubs/dubuisson-asn1-book.PDF [2] http://www.itu.int/ITU-T/studygroups/com17/languages/X.691-0207.pdf [3] http://www.oss.com/asn1/resources/books-whitepapers-pubs/larmouth-asn1-book.pdf [4] http://asn1-playground.oss.com
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