dkim: Implement internal dkim signing and verification

This patch implements internal DKIM signing and verification.

internal/dkim/canonicalize.go

@@ -0,0 +1,158 @@
+package dkim
+
+import (
+	"errors"
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+var (
+	errNoBody = errors.New("no body found")
+
+	errUnknownCanonicalization = errors.New("unknown canonicalization")
+)
+
+type canonicalization string
+
+var (
+	simpleCanonicalization  canonicalization = "simple"
+	relaxedCanonicalization canonicalization = "relaxed"
+)
+
+func (c canonicalization) body(b string) string {
+	switch c {
+	case simpleCanonicalization:
+		return simpleBody(b)
+	case relaxedCanonicalization:
+		return relaxBody(b)
+	default:
+		panic("unknown canonicalization")
+	}
+}
+
+func (c canonicalization) headers(hs headers) headers {
+	switch c {
+	case simpleCanonicalization:
+		return hs
+	case relaxedCanonicalization:
+		return relaxHeaders(hs)
+	default:
+		panic("unknown canonicalization")
+	}
+}
+
+func (c canonicalization) header(h header) header {
+	switch c {
+	case simpleCanonicalization:
+		return h
+	case relaxedCanonicalization:
+		return relaxHeader(h)
+	default:
+		panic("unknown canonicalization")
+	}
+}
+
+func stringToCanonicalization(s string) (canonicalization, error) {
+	switch s {
+	case "simple":
+		return simpleCanonicalization, nil
+	case "relaxed":
+		return relaxedCanonicalization, nil
+	default:
+		return "", fmt.Errorf("%w: %s", errUnknownCanonicalization, s)
+	}
+}
+
+// Notes on whitespace reduction:
+// https://datatracker.ietf.org/doc/html/rfc6376#section-2.8
+// There are only 3 forms of whitespace:
+//  - WSP  =  SP / HTAB
+//    Simple whitespace: space or tab.
+//  - LWSP =  *(WSP / CRLF WSP)
+//    Linear whitespace: any number of { simple whitespace OR CRLF followed by
+//    simple whitespace }.
+//  - FWS  =  [*WSP CRLF] 1*WSP
+//    Folding whitespace: optional { simple whitespace OR CRLF } followed by
+//    one or more simple whitespace.
+
+func simpleBody(body string) string {
+	// https://datatracker.ietf.org/doc/html/rfc6376#section-3.4.3
+	// Replace repeated CRLF at the end of the body with a single CRLF.
+	body = repeatedCRLFAtTheEnd.ReplaceAllString(body, "\r\n")
+
+	// Ensure a non-empty body ends with a single CRLF.
+	// All bodies (including an empty one) must end with a CRLF.
+	if !strings.HasSuffix(body, "\r\n") {
+		body += "\r\n"
+	}
+
+	return body
+}
+
+var (
+	// Continued header: WSP after CRLF.
+	continuedHeader = regexp.MustCompile(`\r\n[ \t]+`)
+
+	// WSP before CRLF.
+	wspBeforeCRLF = regexp.MustCompile(`[ \t]+\r\n`)
+
+	// Repeated WSP.
+	repeatedWSP = regexp.MustCompile(`[ \t]+`)
+
+	// Empty lines at the end of the body.
+	repeatedCRLFAtTheEnd = regexp.MustCompile(`(\r\n)+$`)
+)
+
+func relaxBody(body string) string {
+	// https://datatracker.ietf.org/doc/html/rfc6376#section-3.4.4
+	body = wspBeforeCRLF.ReplaceAllLiteralString(body, "\r\n")
+	body = repeatedWSP.ReplaceAllLiteralString(body, " ")
+	body = repeatedCRLFAtTheEnd.ReplaceAllLiteralString(body, "\r\n")
+
+	// Ensure a non-empty body ends with a single CRLF.
+	if len(body) >= 1 && !strings.HasSuffix(body, "\r\n") {
+		body += "\r\n"
+	}
+
+	return body
+}
+
+func relaxHeader(h header) header {
+	// https://datatracker.ietf.org/doc/html/rfc6376#section-3.4.2
+	// Convert all header field names to lowercase.
+	name := strings.ToLower(h.Name)
+
+	// Remove WSP before the ":" separating the name and value.
+	name = strings.TrimRight(name, " \t")
+
+	// Unfold continuation lines in values.
+	value := continuedHeader.ReplaceAllString(h.Value, " ")
+
+	// Reduce all sequences of WSP to a single SP.
+	value = repeatedWSP.ReplaceAllLiteralString(value, " ")
+
+	// Delete all WSP at the end of each unfolded header field value.
+	value = strings.TrimRight(value, " \t")
+
+	// Remove WSP after the ":" separating the name and value.
+	value = strings.TrimLeft(value, " \t")
+
+	return header{
+		Name:  name,
+		Value: value,
+
+		// The "source" is the relaxed field: name, colon, and value (with
+		// no space around the colon).
+		Source: name + ":" + value,
+	}
+}
+
+func relaxHeaders(hs headers) headers {
+	rh := make(headers, 0, len(hs))
+	for _, h := range hs {
+		rh = append(rh, relaxHeader(h))
+	}
+
+	return rh
+}