1 files changed, 147 insertions, 0 deletions
diff --git a/internal/compose/mime.go b/internal/compose/mime.go
new file mode 100644
index 0000000..da68bd8
--- /dev/null
+++ b/internal/compose/mime.go
@@ -0,0 +1,147 @@
+// Package compose builds standards-based MIME bodies for Usenet posts.
+package compose
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+	"errors"
+	"fmt"
+	"mime"
+	"mime/multipart"
+	"net/textproto"
+	"path/filepath"
+	"strings"
+	"unicode/utf8"
+)
+
+const maxAttachmentBytes = 8 << 20
+
+type Attachment struct {
+	Filename    string
+	ContentType string
+	Data        []byte
+}
+
+func BuildMixed(text string, attachments []Attachment) (contentType, transferEncoding, body string, err error) {
+	if !utf8.ValidString(text) {
+		return "", "", "", errors.New("MIME text is not valid UTF-8")
+	}
+	for _, attachment := range attachments {
+		if strings.TrimSpace(attachment.Filename) == "" {
+			return "", "", "", errors.New("attachment filename is required")
+		}
+		if strings.ContainsAny(attachment.Filename, "\r\n") || len(attachment.Data) > maxAttachmentBytes {
+			return "", "", "", errors.New("attachment filename or size is invalid")
+		}
+	}
+	boundary, err := boundary()
+	if err != nil {
+		return "", "", "", err
+	}
+	var builder strings.Builder
+	writer := multipart.NewWriter(&builder)
+	if err := writer.SetBoundary(boundary); err != nil {
+		return "", "", "", err
+	}
+	textHeader := make(textproto.MIMEHeader)
+	textHeader.Set("Content-Type", "text/plain; charset=UTF-8")
+	textHeader.Set("Content-Transfer-Encoding", "8bit")
+	part, err := writer.CreatePart(textHeader)
+	if err != nil {
+		return "", "", "", err
+	}
+	if _, err := part.Write([]byte(normalizeCRLF(text))); err != nil {
+		return "", "", "", err
+	}
+	for _, attachment := range attachments {
+		header := make(textproto.MIMEHeader)
+		contentType := strings.TrimSpace(attachment.ContentType)
+		if contentType == "" {
+			contentType = "application/octet-stream"
+		}
+		if _, _, err := mime.ParseMediaType(contentType); err != nil {
+			return "", "", "", fmt.Errorf("invalid attachment content type: %w", err)
+		}
+		header.Set("Content-Type", contentType+`; name="`+escapeParameter(attachment.Filename)+`"`)
+		header.Set("Content-Disposition", `attachment; filename="`+escapeParameter(filepath.Base(attachment.Filename))+`"`)
+		header.Set("Content-Transfer-Encoding", "base64")
+		part, err := writer.CreatePart(header)
+		if err != nil {
+			return "", "", "", err
+		}
+		encoded := make([]byte, base64.StdEncoding.EncodedLen(len(attachment.Data)))
+		base64.StdEncoding.Encode(encoded, attachment.Data)
+		for len(encoded) > 0 {
+			lineLen := 76
+			if len(encoded) < lineLen {
+				lineLen = len(encoded)
+			}
+			if _, err := part.Write(append(encoded[:lineLen], '\r', '\n')); err != nil {
+				return "", "", "", err
+			}
+			encoded = encoded[lineLen:]
+		}
+	}
+	if err := writer.Close(); err != nil {
+		return "", "", "", err
+	}
+	return `multipart/mixed; boundary="` + boundary + `"`, "7bit", builder.String(), nil
+}
+
+func BuildOpenPGPMIME(armoredCiphertext string) (contentType, transferEncoding, body string, err error) {
+	if strings.TrimSpace(armoredCiphertext) == "" || strings.Contains(armoredCiphertext, "\x00") {
+		return "", "", "", errors.New("OpenPGP ciphertext is required")
+	}
+	boundary, err := boundary()
+	if err != nil {
+		return "", "", "", err
+	}
+	var builder strings.Builder
+	writer := multipart.NewWriter(&builder)
+	if err := writer.SetBoundary(boundary); err != nil {
+		return "", "", "", err
+	}
+	versionHeader := make(textproto.MIMEHeader)
+	versionHeader.Set("Content-Type", "application/pgp-encrypted")
+	versionHeader.Set("Content-Description", "PGP/MIME version identification")
+	part, err := writer.CreatePart(versionHeader)
+	if err != nil {
+		return "", "", "", err
+	}
+	if _, err := part.Write([]byte("Version: 1\r\n")); err != nil {
+		return "", "", "", err
+	}
+	cipherHeader := make(textproto.MIMEHeader)
+	cipherHeader.Set("Content-Type", "application/octet-stream; name=encrypted.asc")
+	cipherHeader.Set("Content-Disposition", `inline; filename="encrypted.asc"`)
+	cipherHeader.Set("Content-Description", "OpenPGP encrypted message")
+	part, err = writer.CreatePart(cipherHeader)
+	if err != nil {
+		return "", "", "", err
+	}
+	if _, err := part.Write([]byte(normalizeCRLF(armoredCiphertext))); err != nil {
+		return "", "", "", err
+	}
+	if err := writer.Close(); err != nil {
+		return "", "", "", err
+	}
+	return `multipart/encrypted; protocol="application/pgp-encrypted"; boundary="` + boundary + `"`, "7bit", builder.String(), nil
+}
+
+func boundary() (string, error) {
+	random := make([]byte, 12)
+	if _, err := rand.Read(random); err != nil {
+		return "", fmt.Errorf("generate MIME boundary: %w", err)
+	}
+	return "aegis-" + base64.RawURLEncoding.EncodeToString(random), nil
+}
+
+func escapeParameter(value string) string {
+	return strings.NewReplacer("\\", "_", `"`, "'", "\r", "_", "\n", "_").Replace(value)
+}
+
+func normalizeCRLF(value string) string {
+	value = strings.ReplaceAll(value, "\r\n", "\n")
+	value = strings.ReplaceAll(value, "\r", "\n")
+	return strings.ReplaceAll(value, "\n", "\r\n")
+}