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// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package mime
import (
"bytes"
"encoding/base64"
"errors"
"fmt"
"io"
"strings"
"unicode"
"unicode/utf8"
)
// A WordEncoder is an RFC 2047 encoded-word encoder.
type WordEncoder byte
const (
// BEncoding represents Base64 encoding scheme as defined by RFC 2045.
BEncoding = WordEncoder('b')
// QEncoding represents the Q-encoding scheme as defined by RFC 2047.
QEncoding = WordEncoder('q')
)
var (
errInvalidWord = errors.New("mime: invalid RFC 2047 encoded-word")
)
// Encode returns the encoded-word form of s. If s is ASCII without special
// characters, it is returned unchanged. The provided charset is the IANA
// charset name of s. It is case insensitive.
func (e WordEncoder) Encode(charset, s string) string {
if !needsEncoding(s) {
return s
}
return e.encodeWord(charset, s)
}
func needsEncoding(s string) bool {
for _, b := range s {
if (b < ' ' || b > '~') && b != '\t' {
return true
}
}
return false
}
// encodeWord encodes a string into an encoded-word.
func (e WordEncoder) encodeWord(charset, s string) string {
var buf strings.Builder
// Could use a hint like len(s)*3, but that's not enough for cases
// with word splits and too much for simpler inputs.
// 48 is close to maxEncodedWordLen/2, but adjusted to allocator size class.
buf.Grow(48)
e.openWord(&buf, charset)
if e == BEncoding {
e.bEncode(&buf, charset, s)
} else {
e.qEncode(&buf, charset, s)
}
closeWord(&buf)
return buf.String()
}
const (
// The maximum length of an encoded-word is 75 characters.
// See RFC 2047, section 2.
maxEncodedWordLen = 75
// maxContentLen is how much content can be encoded, ignoring the header and
// 2-byte footer.
maxContentLen = maxEncodedWordLen - len("=?UTF-8?q?") - len("?=")
)
var maxBase64Len = base64.StdEncoding.DecodedLen(maxContentLen)
// bEncode encodes s using base64 encoding and writes it to buf.
func (e WordEncoder) bEncode(buf *strings.Builder, charset, s string) {
w := base64.NewEncoder(base64.StdEncoding, buf)
// If the charset is not UTF-8 or if the content is short, do not bother
// splitting the encoded-word.
if !isUTF8(charset) || base64.StdEncoding.EncodedLen(len(s)) <= maxContentLen {
io.WriteString(w, s)
w.Close()
return
}
var currentLen, last, runeLen int
for i := 0; i < len(s); i += runeLen {
// Multi-byte characters must not be split across encoded-words.
// See RFC 2047, section 5.3.
_, runeLen = utf8.DecodeRuneInString(s[i:])
if currentLen+runeLen <= maxBase64Len {
currentLen += runeLen
} else {
io.WriteString(w, s[last:i])
w.Close()
e.splitWord(buf, charset)
last = i
currentLen = runeLen
}
}
io.WriteString(w, s[last:])
w.Close()
}
// qEncode encodes s using Q encoding and writes it to buf. It splits the
// encoded-words when necessary.
func (e WordEncoder) qEncode(buf *strings.Builder, charset, s string) {
// We only split encoded-words when the charset is UTF-8.
if !isUTF8(charset) {
writeQString(buf, s)
return
}
var currentLen, runeLen int
for i := 0; i < len(s); i += runeLen {
b := s[i]
// Multi-byte characters must not be split across encoded-words.
// See RFC 2047, section 5.3.
var encLen int
if b >= ' ' && b <= '~' && b != '=' && b != '?' && b != '_' {
runeLen, encLen = 1, 1
} else {
_, runeLen = utf8.DecodeRuneInString(s[i:])
encLen = 3 * runeLen
}
if currentLen+encLen > maxContentLen {
e.splitWord(buf, charset)
currentLen = 0
}
writeQString(buf, s[i:i+runeLen])
currentLen += encLen
}
}
// writeQString encodes s using Q encoding and writes it to buf.
func writeQString(buf *strings.Builder, s string) {
for i := 0; i < len(s); i++ {
switch b := s[i]; {
case b == ' ':
buf.WriteByte('_')
case b >= '!' && b <= '~' && b != '=' && b != '?' && b != '_':
buf.WriteByte(b)
default:
buf.WriteByte('=')
buf.WriteByte(upperhex[b>>4])
buf.WriteByte(upperhex[b&0x0f])
}
}
}
// openWord writes the beginning of an encoded-word into buf.
func (e WordEncoder) openWord(buf *strings.Builder, charset string) {
buf.WriteString("=?")
buf.WriteString(charset)
buf.WriteByte('?')
buf.WriteByte(byte(e))
buf.WriteByte('?')
}
// closeWord writes the end of an encoded-word into buf.
func closeWord(buf *strings.Builder) {
buf.WriteString("?=")
}
// splitWord closes the current encoded-word and opens a new one.
func (e WordEncoder) splitWord(buf *strings.Builder, charset string) {
closeWord(buf)
buf.WriteByte(' ')
e.openWord(buf, charset)
}
func isUTF8(charset string) bool {
return strings.EqualFold(charset, "UTF-8")
}
const upperhex = "0123456789ABCDEF"
// A WordDecoder decodes MIME headers containing RFC 2047 encoded-words.
type WordDecoder struct {
// CharsetReader, if non-nil, defines a function to generate
// charset-conversion readers, converting from the provided
// charset into UTF-8.
// Charsets are always lower-case. utf-8, iso-8859-1 and us-ascii charsets
// are handled by default.
// One of the CharsetReader's result values must be non-nil.
CharsetReader func(charset string, input io.Reader) (io.Reader, error)
}
// Decode decodes an RFC 2047 encoded-word.
func (d *WordDecoder) Decode(word string) (string, error) {
// See https://tools.ietf.org/html/rfc2047#section-2 for details.
// Our decoder is permissive, we accept empty encoded-text.
if len(word) < 8 || !strings.HasPrefix(word, "=?") || !strings.HasSuffix(word, "?=") || strings.Count(word, "?") != 4 {
return "", errInvalidWord
}
word = word[2 : len(word)-2]
// split word "UTF-8?q?text" into "UTF-8", 'q', and "text"
charset, text, _ := strings.Cut(word, "?")
if charset == "" {
return "", errInvalidWord
}
encoding, text, _ := strings.Cut(text, "?")
if len(encoding) != 1 {
return "", errInvalidWord
}
content, err := decode(encoding[0], text)
if err != nil {
return "", err
}
var buf strings.Builder
if err := d.convert(&buf, charset, content); err != nil {
return "", err
}
return buf.String(), nil
}
// DecodeHeader decodes all encoded-words of the given string. It returns an
// error if and only if CharsetReader of d returns an error.
func (d *WordDecoder) DecodeHeader(header string) (string, error) {
// If there is no encoded-word, returns before creating a buffer.
i := strings.Index(header, "=?")
if i == -1 {
return header, nil
}
var buf strings.Builder
buf.WriteString(header[:i])
header = header[i:]
betweenWords := false
for {
start := strings.Index(header, "=?")
if start == -1 {
break
}
cur := start + len("=?")
i := strings.Index(header[cur:], "?")
if i == -1 {
break
}
charset := header[cur : cur+i]
cur += i + len("?")
if len(header) < cur+len("Q??=") {
break
}
encoding := header[cur]
cur++
if header[cur] != '?' {
break
}
cur++
j := strings.Index(header[cur:], "?=")
if j == -1 {
break
}
text := header[cur : cur+j]
end := cur + j + len("?=")
content, err := decode(encoding, text)
if err != nil {
betweenWords = false
buf.WriteString(header[:start+2])
header = header[start+2:]
continue
}
// Write characters before the encoded-word. White-space and newline
// characters separating two encoded-words must be deleted.
if start > 0 && (!betweenWords || hasNonWhitespace(header[:start])) {
buf.WriteString(header[:start])
}
if err := d.convert(&buf, charset, content); err != nil {
return "", err
}
header = header[end:]
betweenWords = true
}
if len(header) > 0 {
buf.WriteString(header)
}
return buf.String(), nil
}
func decode(encoding byte, text string) ([]byte, error) {
switch encoding {
case 'B', 'b':
return base64.StdEncoding.DecodeString(text)
case 'Q', 'q':
return qDecode(text)
default:
return nil, errInvalidWord
}
}
func (d *WordDecoder) convert(buf *strings.Builder, charset string, content []byte) error {
switch {
case strings.EqualFold("utf-8", charset):
buf.Write(content)
case strings.EqualFold("iso-8859-1", charset):
for _, c := range content {
buf.WriteRune(rune(c))
}
case strings.EqualFold("us-ascii", charset):
for _, c := range content {
if c >= utf8.RuneSelf {
buf.WriteRune(unicode.ReplacementChar)
} else {
buf.WriteByte(c)
}
}
default:
if d.CharsetReader == nil {
return fmt.Errorf("mime: unhandled charset %q", charset)
}
r, err := d.CharsetReader(strings.ToLower(charset), bytes.NewReader(content))
if err != nil {
return err
}
if _, err = io.Copy(buf, r); err != nil {
return err
}
}
return nil
}
// hasNonWhitespace reports whether s (assumed to be ASCII) contains at least
// one byte of non-whitespace.
func hasNonWhitespace(s string) bool {
for _, b := range s {
switch b {
// Encoded-words can only be separated by linear white spaces which does
// not include vertical tabs (\v).
case ' ', '\t', '\n', '\r':
default:
return true
}
}
return false
}
// qDecode decodes a Q encoded string.
func qDecode(s string) ([]byte, error) {
dec := make([]byte, len(s))
n := 0
for i := 0; i < len(s); i++ {
switch c := s[i]; {
case c == '_':
dec[n] = ' '
case c == '=':
if i+2 >= len(s) {
return nil, errInvalidWord
}
b, err := readHexByte(s[i+1], s[i+2])
if err != nil {
return nil, err
}
dec[n] = b
i += 2
case (c <= '~' && c >= ' ') || c == '\n' || c == '\r' || c == '\t':
dec[n] = c
default:
return nil, errInvalidWord
}
n++
}
return dec[:n], nil
}
// readHexByte returns the byte from its quoted-printable representation.
func readHexByte(a, b byte) (byte, error) {
var hb, lb byte
var err error
if hb, err = fromHex(a); err != nil {
return 0, err
}
if lb, err = fromHex(b); err != nil {
return 0, err
}
return hb<<4 | lb, nil
}
func fromHex(b byte) (byte, error) {
switch {
case b >= '0' && b <= '9':
return b - '0', nil
case b >= 'A' && b <= 'F':
return b - 'A' + 10, nil
// Accept badly encoded bytes.
case b >= 'a' && b <= 'f':
return b - 'a' + 10, nil
}
return 0, fmt.Errorf("mime: invalid hex byte %#02x", b)
}