Re: reading Public key certificates in Firefox

From: Bodo Moeller (bmoeller_at_acm.org)
Date: 11/17/05 

Date: Thu, 17 Nov 2005 20:30:33 +0000 (UTC)

James Muir <noone@127.0.0.1>:

> In case anyone else is interested, here is a pointer to the official
> ANS.1 standard with rules on how to read the block above:
>
> http://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf
>
> However, I can't seem to locate an official list of ASN.1 tag values.
> For example, I know from looking at some examples in the pdf document
> above that the first byte of the block above, "30", means "SEQUENCE".
> But I can't find an official spec that confirms this.

You'll have to read the above document in conjunction with
http://www.itu.int/ITU-T/studygroups/com17/languages/X.680-0207.pdf

X.690 is not the ASN.1 standard, it just defines (concrete) encodings
for the (abstract) notation. The actual ASN.1 standard is X.680.
Section 24.16 there says

    "All sequence types have a tag which is universal class, number 16."

Section 8.1.2 of X.690 explains how this number is encoded into an octet:
0x30 is 00110000 binary, where the two most significant bits (00)
denote the universal class, the next bit (1) identifies a constructed
rather than primitive encoding, and the final five bits (10000) encode
the number.

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