Source : Free On-Line Dictionary of Computing

Canonical Encoding Rules
     
         (CER) A restricted variant of {BER} for
        producing unequivocal {transfer syntax} for data structures
        described by {ASN.1}.
     
        Whereas {BER} gives choices as to how data values may be
        encoded, CER and {DER} select just one encoding from those
        allowed by the basic encoding rules, eliminating all of the
        options.  They are useful when the encodings must be
        preserved, e.g. in security exchanges.
     
        CER and {DER} differ in the set of restrictions that they
        place on the encoder.  The basic difference between CER and
        {DER} is that {DER} uses definitive length form and CER uses
        indefinite length form.
     
        Documents: {ITU-T} X.690, {ISO} 8825-1.
     
        See also {PER}.
     
        (1998-05-19)