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add documentation of ASN.1 context-specific decoding

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Steffen Jaeckel 2014-10-09 22:39:39 +02:00
parent 3233e51a23
commit 627aef962c
1 changed files with 16 additions and 7 deletions
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23
crypt.tex
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@ -5058,7 +5058,9 @@ encoded in a SEQUENCE are stored as a child element.
When a SEQUENCE or SET has been encountered a SEQUENCE (or SET resp.) item will be added as a sibling (e.g. list.type == LTC\_ASN1\_SEQUENCE) and the child
pointer points to a new list of items contained within the object.
\index{der\_decode\_sequence\_flexi()}\index{LTC\_ASN1\_CONSTRUCTED}
\index{der\_decode\_sequence\_flexi()}
\index{LTC\_ASN1\_CONSTRUCTED}
\index{LTC\_ASN1\_CONTEXT\_SPECIFIC}
\begin{verbatim}
int der_decode_sequence_flexi(const unsigned char *in,
unsigned long *inlen,
@ -5073,16 +5075,23 @@ normally. The decoded list \textit{out} will point to the very first element of
An invalid decoding will terminate the process, and free the allocated memory automatically.
As of v1.18 the flexi decoder will also decode arbitrary constructed types
other than SEQUENCE and SET. The \textit{type} field will be set to
\textbf{LTC\_ASN1\_CONSTRUCTED} and the plain identifier that was indicated in the ASN.1
encoding is stored in the \textit{used} field. Further decoding is done in the
same way as if it were a SEQUENCE or SET.
Also as of v1.18 the flexi decoder is capable to handle
\textit{context-specific} encodings. The \textit{type} field will be set to
\textbf{LTC\_ASN1\_CONTEXT\_SPECIFIC} and the plain identifier that was indicated
in the ASN.1 encoding is stored in the \textit{used} field. Encapsulated data
in the \textit{context-specific} encoding is copied to newly allocated memory
and is accessible through the \textit{data} field.
\textbf{Note:} the list decoded by this function is \textbf{NOT} in the correct form for der\_encode\_sequence() to use directly. You will first
have to convert the list by first storing all of the siblings in an array then storing all the children as sub-lists of a sequence using the \textit{.data}
pointer. Currently no function in LibTomCrypt provides this ability.
\textbf{Note 2:} the flexi decoder will also decode arbitrary constructed types
other than SEQUENCE and SET. The \textit{type} field will be set to
\textbf{LTC\_ASN1\_CONSTRUCTED} and the plain type that was indicated in the ASN.1
encoding is stored in the \textit{used} field. Further decoding is done in the
same way as if it were a SEQUENCE or SET.
\subsubsection{Sample Decoding}
Suppose we decode the following structure:
\begin{small}