dkim: Implement internal dkim signing and verification

This patch implements internal DKIM signing and verification.

internal/dkim/message.go

@@ -0,0 +1,77 @@
+package dkim
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+type header struct {
+	Name   string
+	Value  string
+	Source string
+}
+
+type headers []header
+
+// FindAll the headers with the given name, in order of appearance.
+func (h headers) FindAll(name string) headers {
+	hs := make(headers, 0)
+	for _, header := range h {
+		if strings.EqualFold(header.Name, name) {
+			hs = append(hs, header)
+		}
+	}
+	return hs
+}
+
+var errInvalidHeader = errors.New("invalid header")
+
+// Parse a RFC822 message, return the headers, body, and error if any.
+// We expect it to only contain CRLF line endings.
+// Does NOT touch whitespace, this is important to preserve the original
+// message and headers, which is required for the signature.
+func parseMessage(message string) (headers, string, error) {
+	headers := make(headers, 0)
+	lines := strings.Split(message, "\r\n")
+	eoh := 0
+	for i, line := range lines {
+		if line == "" {
+			eoh = i
+			break
+		}
+
+		if strings.HasPrefix(line, " ") || strings.HasPrefix(line, "\t") {
+			// Continuation of the previous header.
+			if len(headers) == 0 {
+				return nil, "", fmt.Errorf(
+					"%w: bad continuation", errInvalidHeader)
+			}
+			headers[len(headers)-1].Value += "\r\n" + line
+			headers[len(headers)-1].Source += "\r\n" + line
+		} else {
+			// New header.
+			h, err := parseHeader(line)
+			if err != nil {
+				return nil, "", err
+			}
+
+			headers = append(headers, h)
+		}
+	}
+
+	return headers, strings.Join(lines[eoh+1:], "\r\n"), nil
+}
+
+func parseHeader(line string) (header, error) {
+	name, value, found := strings.Cut(line, ":")
+	if !found {
+		return header{}, fmt.Errorf("%w: no colon", errInvalidHeader)
+	}
+
+	return header{
+		Name:   name,
+		Value:  value,
+		Source: line,
+	}, nil
+}