Encoding for transport-safe

"uuencode" has been used for a long time to transport binary. It encodes three 8-bit characters into four 6-bit characters, however, the result contains many kinds of symbols. Some of them have special meanings in header so they cannot be used to extent header functionality.

Space character bothers the transport system. Space character cannot exist in end of line of the file system of BITNET. Suppose that an encoded object with uuencode contains space character in end of line. When a message gateway BITNET received this kind of message, it removes the space character, of course. In the result, receivers cannot decode and extract the original object.

MIME specified 2 encoding methods for body.

Base64 encoding

Encode three 8-bit characters into four 6-bit characters with 64 letters, "0-9A-Za-z/+". PEM originates it.

Quoted-Printable encoding

Represent non-printable characters in hexagonal preceded by "=".

Encoding is specified by Content-Transfer-Encoding:(CTE:) in content-header. The candidate values are as follows:

7bit

No encoding is applied. The content consists of 7 bit lines.

8bit

No encoding is applied. The content consists of 8 bit lines.

binary

No encoding is applied. The content is 8 bit stream.

base64

Encoded with Base64. The content consists of 7 bit lines.

quoted-printable

Encoded with Quoted-Printable. The content consists of 7 bit lines.

If CTE: is omitted, it is treated as `7bit'.

Since ISO-2022-JP is 7bit character set, CTE: is 7bit. That it, CTE: can be omitted. You may encode it with base64 or quoted-printable, of course. However, you cannot read messages in folder directly with such a encoding, I don't recommend.