+.new
+.vitem "&*${certextract{*&<&'field'&>&*}{*&<&'certificate'&>&*}&&&
+ {*&<&'string2'&>&*}{*&<&'string3'&>&*}}*&"
+.cindex "expansion" "extracting cerificate fields"
+.cindex "&%certextract%&" "certificate fields"
+.cindex "certificate" "extracting fields"
+The <&'certificate'&> must be a variable of type certificate.
+The field name is expanded and used to retrive the relevant field from
+the certificate. Supported fields are:
+.display
+&`version `&
+&`serial_number `&
+&`subject `& RFC4514 DN
+&`issuer `& RFC4514 DN
+&`notbefore `& time
+&`notafter `& time
+&`sig_algorithm `&
+&`signature `&
+&`subj_altname `& tagged list
+&`ocsp_uri `& list
+&`crl_uri `& list
+.endd
+If the field is found,
+<&'string2'&> is expanded, and replaces the whole item;
+otherwise <&'string3'&> is used. During the expansion of <&'string2'&> the
+variable &$value$& contains the value that has been extracted. Afterwards, it
+is restored to any previous value it might have had.
+
+If {<&'string3'&>} is omitted, the item is replaced by an empty string if the
+key is not found. If {<&'string2'&>} is also omitted, the value that was
+extracted is used.
+
+Some field names take optional modifiers, appended and separated by commas.
+
+The field selectors marked as "RFC4514" above
+output a Distinguished Name string which is
+not quite
+parseable by Exim as a comma-separated tagged list
+(the exceptions being elements containin commas).
+RDN elements of a single type may be selected by
+a modifier of the type label; if so the expansion
+result is a list (newline-separated by default).
+The separator may be changed by another modifer of
+a right angle-bracket followed immediately by the new separator.
+Recognised RDN type labels include "CN", "O", "OU" and "DC".
+
+The field selectors marked as "time" above
+may output a number of seconds since epoch
+if the modifier "int" is used.
+
+The field selectors marked as "list" above return a list,
+newline-separated by default,
+(embedded separator characters in elements are doubled).
+The separator may be changed by a modifier of
+a right angle-bracket followed immediately by the new separator.
+
+The field selectors marked as "tagged" above
+prefix each list element with a type string and an equals sign.
+Elements of only one type may be selected by a modifier
+which is one of "dns", "uri" or "mail";
+if so the elenment tags are omitted.
+
+If not otherwise noted field values are presented in human-readable form.
+.wen
+