msgviewer: parse and display authentication results

Parse the Authentication-Results header and display it in the message
viewer (not enabled by default). DKIM, SPF and DMARC authentication
methods are supported. Implement recommendation from RFC 7601 Sec 7.1 to
have an explicit list of trustworthy hostnames before displaying the
authentication results. Be aware that the authentication headers can be
forged.

To display the results for a specific authentication method, add the
corresponding name to the layout of headers in the viewer section of
aerc.conf, e.g. to display all three, use:

header-layout = From|To,Cc|Bcc,Date,Subject,DKIM|SPF|DMARC

More information will be displayed when "+" is appended to the
authentication method name, e.g. DKIM+ or SPF+ or DMARC+.

Also, add the trustworthy hosts per account with the trusted-authres
parameter, e.g.

trusted-authres = *

to trust every host or use regular expressions for a finer control.
Multiple hosts can be entered as a comma-separated list. Authentication
results will only be displayed when the host is listed in the
trusted-authres list.

Link: https://datatracker.ietf.org/doc/html/rfc7601
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Tested-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Robin Jarry <robin@jarry.cc>

1 files changed, 145 insertions, 0 deletions

diff --git a/lib/auth/auth.go b/lib/auth/auth.go
new file mode 100644
index 0000000..8a0a40f
--- /dev/null
+++ b/lib/auth/auth.go
@@ -0,0 +1,145 @@
+package auth
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+
+	"github.com/emersion/go-message/mail"
+	"github.com/emersion/go-msgauth/authres"
+)
+
+const (
+	AuthHeader = "Authentication-Results"
+)
+
+type Method string
+
+const (
+	DKIM  Method = "dkim"
+	SPF   Method = "spf"
+	DMARC Method = "dmarc"
+)
+
+type Result string
+
+const (
+	ResultNone    Result = "none"
+	ResultPass    Result = "pass"
+	ResultFail    Result = "fail"
+	ResultNeutral Result = "neutral"
+	ResultPolicy  Result = "policy"
+)
+
+type Details struct {
+	Results []Result
+	Infos   []string
+	Reasons []string
+	Err     error
+}
+
+func (d *Details) add(r Result, info string, reason string) {
+	d.Results = append(d.Results, r)
+	d.Infos = append(d.Infos, info)
+	d.Reasons = append(d.Reasons, reason)
+}
+
+type ParserFunc func(*mail.Header, []string) (*Details, error)
+
+func New(s string) ParserFunc {
+	if i := strings.IndexRune(s, '+'); i > 0 {
+		s = s[:i]
+	}
+	m := Method(strings.ToLower(s))
+	switch m {
+	case DKIM, SPF, DMARC:
+		return CreateParser(m)
+	}
+	return nil
+}
+
+func trust(s string, trusted []string) bool {
+	for _, t := range trusted {
+		if matched, _ := regexp.MatchString(t, s); matched || t == "*" {
+			return true
+		}
+	}
+	return false
+}
+
+var cleaner = regexp.MustCompile(`(\(.*);(.*\))`)
+
+func CreateParser(m Method) func(*mail.Header, []string) (*Details, error) {
+	return func(header *mail.Header, trusted []string) (*Details, error) {
+		details := &Details{}
+		found := false
+
+		hf := header.FieldsByKey(AuthHeader)
+		for hf.Next() {
+			headerText, err := hf.Text()
+			if err != nil {
+				return nil, err
+			}
+
+			identifier, results, err := authres.Parse(headerText)
+			if err != nil && err.Error() == "msgauth: unsupported version" {
+				// Some MTA write their authres header without an identifier
+				// which does not conform to RFC but still exists in the wild
+				identifier, results, err = authres.Parse("unknown;" + headerText)
+				if err != nil {
+					return nil, err
+				}
+			} else if err != nil && err.Error() == "msgauth: malformed authentication method and value" {
+				// the go-msgauth parser doesn't like semi-colons in the comments
+				// as a work-around we remove those
+				cleanHeader := cleaner.ReplaceAllString(headerText, "${1}${2}")
+				identifier, results, err = authres.Parse(cleanHeader)
+				if err != nil {
+					return nil, err
+				}
+			} else if err != nil {
+				return nil, err
+			}
+
+			// implements recommendation from RFC 7601 Sec 7.1 to
+			// have an explicit list of trustworthy hostnames
+			// before displaying AuthRes results
+			if !trust(identifier, trusted) {
+				return nil, fmt.Errorf("%s is not trusted", identifier)
+			}
+
+			for _, result := range results {
+				switch r := result.(type) {
+				case *authres.DKIMResult:
+					if m == DKIM {
+						info := r.Identifier
+						if info == "" && r.Domain != "" {
+							info = r.Domain
+						}
+						details.add(Result(r.Value), info, r.Reason)
+						found = true
+					}
+				case *authres.SPFResult:
+					if m == SPF {
+						info := r.From
+						if info == "" && r.Helo != "" {
+							info = r.Helo
+						}
+						details.add(Result(r.Value), info, r.Reason)
+						found = true
+					}
+				case *authres.DMARCResult:
+					if m == DMARC {
+						details.add(Result(r.Value), r.From, r.Reason)
+						found = true
+					}
+				}
+			}
+		}
+
+		if !found {
+			details.add(ResultNone, "", "")
+		}
+		return details, nil
+	}
+}