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
+	}
+}