dkim: Implement internal dkim signing and verification

This patch implements internal DKIM signing and verification.

internal/dkim/context.go

@@ -0,0 +1,56 @@
+package dkim
+
+import (
+	"context"
+	"net"
+)
+
+type contextKey string
+
+const traceKey contextKey = "trace"
+
+func trace(ctx context.Context, f string, args ...interface{}) {
+	traceFunc, ok := ctx.Value(traceKey).(TraceFunc)
+	if !ok {
+		return
+	}
+	traceFunc(f, args...)
+}
+
+type TraceFunc func(f string, a ...interface{})
+
+func WithTraceFunc(ctx context.Context, trace TraceFunc) context.Context {
+	return context.WithValue(ctx, traceKey, trace)
+}
+
+const lookupTXTKey contextKey = "lookupTXT"
+
+func lookupTXT(ctx context.Context, domain string) ([]string, error) {
+	lookupTXTFunc, ok := ctx.Value(lookupTXTKey).(lookupTXTFunc)
+	if !ok {
+		return net.LookupTXT(domain)
+	}
+	return lookupTXTFunc(ctx, domain)
+}
+
+type lookupTXTFunc func(ctx context.Context, domain string) ([]string, error)
+
+func WithLookupTXTFunc(ctx context.Context, lookupTXT lookupTXTFunc) context.Context {
+	return context.WithValue(ctx, lookupTXTKey, lookupTXT)
+}
+
+const maxHeadersKey contextKey = "maxHeaders"
+
+func WithMaxHeaders(ctx context.Context, maxHeaders int) context.Context {
+	return context.WithValue(ctx, maxHeadersKey, maxHeaders)
+}
+
+func maxHeaders(ctx context.Context) int {
+	maxHeaders, ok := ctx.Value(maxHeadersKey).(int)
+	if !ok {
+		// By default, cap the number of headers to 5 (arbitrarily chosen, may
+		// be adjusted in the future).
+		return 5
+	}
+	return maxHeaders
+}