Handling large ASN.1 objects such as encrypted CMS files will require
the ability to use the asn1_enter() and asn1_skip() family of
functions on partial object cursors, where a defined additional length
is known to exist after the end of the data buffer pointed to by the
ASN.1 object cursor.
We already have support for partial object cursors in the underlying
asn1_start() operation used by both asn1_enter() and asn1_skip(), and
this is used by the DER image probe routine to check that the
potential DER file comprises a single ASN.1 SEQUENCE object.
Add asn1_enter_partial() to formalise the process of entering an ASN.1
partial object, and refactor the DER image probe routine to use this
instead of open-coding calls to the underlying asn1_start() operation.
There is no need for an equivalent asn1_skip_partial() function, since
only objects that are wholly contained within the partial cursor may
be successfully skipped.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Add DER-encoded ASN.1 as an image format. There is no fixed signature
for DER files. We treat an image as DER if it comprises a single
valid SEQUENCE object covering the entire length of the image.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
