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Re: X509SerialNumber schema

From: Thomas Maslen <maslen@dstc.edu.au>
Date: Fri, 16 Mar 2001 20:24:57 +1000
Message-Id: <200103161024.f2GAOMJ21749@piglet.dstc.edu.au>
To: "Donald E. Eastlake 3rd" <dee3@torque.pothole.com>
cc: w3c-ietf-xmldsig@w3.org
> X.509v3 defines CertificateSerialNumber as INTEGER.  


A number (no pun intended) of other ASN.1 INTEGER values from X.509 are also 
mapped into xmldsig constructs, including:

    6.4.1:  DSAKeyValue P, Q, G, Y, J, Seed and PgenCounter are all Base64

    6.4.2:  PKCS#1 SignatureValue.

    6.4.2:  RSAKeyValue Modulus and Exponent are both Base64

Section 6.4.2 also contains a paragraph that reads:

        Arbitrary-length integers (e.g. "bignums" such as RSA modulii) are 
        represented in XML as octet strings.  The integer value is first 
        converted to a "big endian" bitstring. The bitstring is then padded 
        with leading zero bits so that the total number of bits == 0 mod 8 
        (so that there are an even number of bytes). If the bitstring 
        contains entire leading bytes that are zero, these are removed (so 
        the high-order byte is always non-zero). This octet string is then 
        base64 [MIME] encoded. (The conversion from integer to octet string
        is equivalent to IEEE 1363's I2OSP [1363] with minimal length).

Given this, encoding the X509SerialNumber element as an xml-schema integer 
seems pretty inconsistent with the rest of the xmldsig spec.

> But it isn't our fault that instead of just numbering their certificates 
> 1, 2, 3, ... as was presumably the original concept, some CA's seem to want 
> to encode lots of private extension information and the kitchen sink into 
> this field or use a hash or whatever.

We may not like it, but for better or worse it is an arbitrary-length integer
and has to be treated as one.

> We need feedback from implementors.

An xmldsig implementation already needs code to handle Base64 (simple);
requiring (more complex) bignum-to-decimal and decimal-to-bignum code as well,
just to make small-integer certificate serial numbers look nice in the XML,
would be unfortunate.  Not a big deal on a desktop machine, where code bloat
is the order of the day, but possibly the last straw for trying to make 
xmldsig useful on a PDA.

By the way, is that paragraph from section 6.4.2 intended to apply to all 
bignums, e.g. the DSA ones in section 6.4.1?

Thomas Maslen
Received on Friday, 16 March 2001 05:25:14 UTC

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