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Re: propagation control
Actually, when we were designing the PGP 3.0 data formats, we came up
with an encoding that is linear on both encode and decode, modulo some
fixed-size buffer (default is 4KByte blocks). The encoding adds an
overhead of approximately 1 byte/block. The encoder needs to be able
to buffer up to the block size; the decoder doesn't need to buffer at
More details available upon request.
email@example.com (David P. Kemp) writes:
> > From: "Phillip M. Hallam-Baker" <firstname.lastname@example.org>
> > This is of course very ASN.1, I don't mind ASN.1 except for the DER
> > rules which are completely braindamaged. If it had been made an
> > absolute requirement that every structure should be capable of
> > being encoded and decoded using a simple linear descent the problem
> > would not occur.
> I noticed you were very careful to specify DER, as opposed to BER.
> The Basic Encoding Rules (BER), of course, allow you to specify length of
> an object explicitly at the beginning, or implicitly by sending an
> end token when the object is finished.
> There are two canonical encoding subsets of BER: the Canonical Encoding
> Rules (CER) and Distinguished Encoding Rules (DER). CER has the properties
> you want - the length of an object does not need to be known before
> spitting out the encoded data.
> Whether you prefer DER or CER is largely a matter of whether you do
> encoding or decoding more frequently. In the case of certificates which
> are typically created once on a potentially "large" machine but verified
> many times on "resource-challenged" platforms, a coding format which
> specifies the length of the item at the beginning (allowing memory to
> be allocated before decoding the item) is preferable. DER is such a
> In summary, CER is linear-encode and recursive decode, whereas DER is
> recursive-encode and linear-decode. BER allows it to be done either way.
> These properties are not specific to ASN.1 and BER; they are applicable
> to any transfer format - either the length must be known or filled in at
> the beginning, or it isn't known until the end. I don't know how you
> could have a format that allows both encoding and decoding of arbitrary-
> length nested objects to be done linearly.
> I also think that in the case of certificates, it hardly matters. Doing
> random access or recursive descent over an object that is 1/2 KB or 2KB
> or 5KB long is no problem. It's only when you start dealing with objects
> that don't fit into RAM (such as streaming media) that the difference
> becomes important.
Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
Member, MIT Student Information Processing Board (SIPB)
URL: http://web.mit.edu/warlord/ PP-ASEL N1NWH
warlord@MIT.EDU PGP key available