Crate charset[][src]

Expand description

charset is a wrapper around encoding_rs that provides (non-streaming) decoding for character encodings that occur in email by providing decoding for UTF-7 in addition to the encodings defined by the Encoding Standard (and provided by encoding_rs).

Note: Do not use this crate for consuming Web content. For security reasons, consumers of Web content are prohibited from supporting UTF-7. Use encoding_rs directly when consuming Web content.

The set of encodings consisting of UTF-7 and the encodings defined in the Encoding Standard is believed to be appropriate for consuming email, because that’s the set of encodings supported by Thunderbird. Furthermore, UTF-7 support is believed to be necessary based on the experience of the Firefox OS email client. In fact, while the UTF-7 implementation in this crate is independent of Thunderbird’s UTF-7 implementation, Thunderbird uses encoding_rs to decode the other encodings. The set of labels/aliases recognized by this crate matches those recognized by Thunderbird.

Known compatibility limitations (shared with Thunderbird and known from Thunderbird bug reports):

  • JavaMail may use non-standard labels for legacy encodings such that the labels aren’t recognized by this crate even if the encodings themselves would be supported.
  • Some ancient Usenet posting in Chinese may not be decodable, because this crate does not support HZ.
  • Some emails sent in Chinese by Sun’s email client for CDE on Solaris around the turn of the millennium may not decodable, because this crate does not support ISO-2022-CN.
  • Some emails sent in Korean by IBM/Lotus Notes may not be decodable, because this crate does not support ISO-2022-KR.

This crate intentionally does not support encoding content into legacy encodings. When sending email, always use UTF-8. This is, just call .as_bytes() on &str and label the content as UTF-8.

Security considerations

Again, this crate is for email. Please do NOT use it for Web content.

Never try to perform any security analysis on the undecoded data in ASCII-incompatible encodings and in UTF-7 in particular. Always decode first and analyze after. UTF-7 allows even characters that don’t have to be represeted as base64 to be represented as base64. Also, for consistency with Thunderbird, the UTF-7 decoder in this crate allows e.g. ASCII controls to be represented without base64 encoding even when the spec says they should be base64-encoded.

This implementation is non-constant-time by design. An attacker who can observe input length and the time it takes to decode it can make guesses about relative proportions of characters from different ranges. Guessing the proportion of ASCII vs. non-ASCII should be particularly feasible.


A character encoding suitable for decoding email.


The UTF-7 encoding.


Converts ASCII to UTF-8 with non-ASCII bytes replaced with the REPLACEMENT CHARACTER.

Converts bytes whose unsigned value is interpreted as Unicode code point (i.e. U+0000 to U+00FF, inclusive) to UTF-8.