The IMAP specification (RFC 2060, 5.1.3. Mailbox International Naming Convention) describes how to handle non-ASCII characters in folder names. It defines a modified UTF-7 encoding:
By convention, international mailbox names are specified using a modified version of the UTF-7 encoding described in [UTF-7]. The purpose of these modifications is to correct the following problems with UTF-7:
UTF-7 uses the "+" character for shifting; this conflicts with the common use of "+" in mailbox names, in particular USENET newsgroup names.
UTF-7's encoding is BASE64 which uses the "/" character; this conflicts with the use of "/" as a popular hierarchy delimiter.
UTF-7 prohibits the unencoded usage of "\"; this conflicts with the use of "\" as a popular hierarchy delimiter.
UTF-7 prohibits the unencoded usage of "~"; this conflicts with the use of "~" in some servers as a home directory indicator.
UTF-7 permits multiple alternate forms to represent the same string; in particular, printable US-ASCII chararacters can be represented in encoded form.
In modified UTF-7, printable US-ASCII characters except for "&" represent themselves; that is, characters with octet values 0x20-0x25 and 0x27-0x7e. The character "&" (0x26) is represented by the two-octet sequence "&-".
All other characters (octet values 0x00-0x1f, 0x7f-0xff, and all Unicode 16-bit octets) are represented in modified BASE64, with a further modification from [UTF-7] that "," is used instead of "/".
Modified BASE64 MUST NOT be used to represent any printing US-ASCII character which can represent itself."&" is used to shift to modified BASE64 and "-" to shift back to US-ASCII. All names start in US-ASCII, and MUST end in US-ASCII (that is, a name that ends with a Unicode 16-bit octet MUST end with a "-").
Before I'll start implementing it, my question: is there some .NET code/library out there (or even in the framework) that does the job? I couldn't find .NET resources (only implementations for other languages/frameworks).
Thank you!
//
// ImapEncoding.cs
//
// Author: Jeffrey Stedfast <jestedfa@microsoft.com>
//
// Copyright (c) 2013-2019 Microsoft Corp. (www.microsoft.com)
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
using System.Text;
namespace MailKit.Net.Imap {
static class ImapEncoding
{
const string utf7_alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,";
static readonly byte[] utf7_rank = {
255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,
255,255,255,255,255,255,255,255,255,255,255, 62, 63,255,255,255,
52, 53, 54, 55, 56, 57, 58, 59, 60, 61,255,255,255,255,255,255,
255, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,255,255,255,255,255,
255, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51,255,255,255,255,255,
};
public static string Decode (string text)
{
var decoded = new StringBuilder ();
bool shifted = false;
int bits = 0, v = 0;
int index = 0;
char c;
while (index < text.Length) {
c = text[index++];
if (shifted) {
if (c == '-') {
// shifted back out of modified UTF-7
shifted = false;
bits = v = 0;
} else if (c > 127) {
// invalid UTF-7
return text;
} else {
byte rank = utf7_rank[(byte) c];
if (rank == 0xff) {
// invalid UTF-7
return text;
}
v = (v << 6) | rank;
bits += 6;
if (bits >= 16) {
char u = (char) ((v >> (bits - 16)) & 0xffff);
decoded.Append (u);
bits -= 16;
}
}
} else if (c == '&' && index < text.Length) {
if (text[index] == '-') {
decoded.Append ('&');
index++;
} else {
// shifted into modified UTF-7
shifted = true;
}
} else {
decoded.Append (c);
}
}
return decoded.ToString ();
}
static void Utf7ShiftOut (StringBuilder output, int u, int bits)
{
if (bits > 0) {
int x = (u << (6 - bits)) & 0x3f;
output.Append (utf7_alphabet[x]);
}
output.Append ('-');
}
public static string Encode (string text)
{
var encoded = new StringBuilder ();
bool shifted = false;
int bits = 0, u = 0;
for (int index = 0; index < text.Length; index++) {
char c = text[index];
if (c >= 0x20 && c < 0x7f) {
// characters with octet values 0x20-0x25 and 0x27-0x7e
// represent themselves while 0x26 ("&") is represented
// by the two-octet sequence "&-"
if (shifted) {
Utf7ShiftOut (encoded, u, bits);
shifted = false;
bits = 0;
}
if (c == 0x26)
encoded.Append ("&-");
else
encoded.Append (c);
} else {
// base64 encode
if (!shifted) {
encoded.Append ('&');
shifted = true;
}
u = (u << 16) | (c & 0xffff);
bits += 16;
while (bits >= 6) {
int x = (u >> (bits - 6)) & 0x3f;
encoded.Append (utf7_alphabet[x]);
bits -= 6;
}
}
}
if (shifted)
Utf7ShiftOut (encoded, u, bits);
return encoded.ToString ();
}
}
}