%% You should probably cite draft-levine-dkim-conditional-04 instead of this revision.
@techreport{levine-dkim-conditional-02,
    number =    {draft-levine-dkim-conditional-02},
    type =      {Internet-Draft},
    institution =   {Internet Engineering Task Force},
    publisher = {Internet Engineering Task Force},
    note =      {Work in Progress},
    url =       {https://datatracker.ietf.org/doc/draft-levine-dkim-conditional/02/},
    author =    {John R. Levine},
    title =     {{Mandatory Tags for DKIM Signatures}},
    pagetotal = 5,
    year =      2015,
    month =     sep,
    day =       29,
    abstract =  {The DKIM protocol applies a cryptographic signature to an e-mail message. This specification extends DKIM to allow new signature tags that validators are required to evaluate. The first such tag specifies a second signature that must be present for a signature to be valid.},
}