[ietf-dkim] new issue: more details of key record format in base

[Stephen Farrell]

Just so's we don't lose this (the other thread is called "editorials
and nits":-).

We should fix base to match the information below supplied by EKR,
which is, I assume, what's implemented and which is also correct.
(And get rid of the hardcoded 65537 stuff.)

Stephen.

Here's the relevant text:

    k=   Key type (plain-text; OPTIONAL, default is "rsa").  Signers and
        verifiers MUST support the "rsa" key type.  The "rsa" key type
        indicates that an RSA public key, as defined in [RFC3447],
        sections 3.1 and A.1.1, is being used in the p= tag.  (Note:  the
        p= tag further encodes the value using the base64 algorithm.)

And here's what's in A.1.1:

    An RSA public key should be represented with the ASN.1 type
    RSAPublicKey:

       RSAPublicKey ::= SEQUENCE {
           modulus           INTEGER,  -- n
           publicExponent    INTEGER   -- e
       }

    The fields of type RSAPublicKey have the following meanings:

     * modulus is the RSA modulus n.

     * publicExponent is the RSA public exponent e.

The only ASN.1 definition here is for the full public key.