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"""
Encode and decode UTF-7 string, as described in the RFC 3501
There are variations, specific to IMAP4rev1, therefore the built-in python UTF-7 codec can't be used.
The main difference is the shift character, used to switch from ASCII to base64 encoding context.
This is "&" in that modified UTF-7 convention, since "+" is considered as mainly used in mailbox names.
Full description at RFC 3501, section 5.1.3.
"""
import binascii
from typing import MutableSequence
AMPERSAND_ORD = ord('&')
HYPHEN_ORD = ord('-')
# ENCODING
# --------
def _modified_base64(value: str) -> bytes:
return binascii.b2a_base64(value.encode('utf-16be')).rstrip(b'\n=').replace(b'/', b',')
def _do_b64(_in: MutableSequence[str], r: MutableSequence[bytes]):
if _in:
r.append(b'&' + _modified_base64(''.join(_in)) + b'-')
_in.clear()
def utf7_encode(value: str) -> bytes:
res = []
_in = []
for char in value:
ord_c = ord(char)
if 0x20 <= ord_c <= 0x25 or 0x27 <= ord_c <= 0x7e:
_do_b64(_in, res)
res.append(char.encode())
elif char == '&':
_do_b64(_in, res)
res.append(b'&-')
else:
_in.append(char)
_do_b64(_in, res)
return b''.join(res)
# DECODING
# --------
def _modified_unbase64(value: bytearray) -> str:
return binascii.a2b_base64(value.replace(b',', b'/') + b'===').decode('utf-16be')
def utf7_decode(value: bytes) -> str:
res = []
encoded_chars = bytearray()
for char in value:
if char == AMPERSAND_ORD and not encoded_chars:
encoded_chars.append(AMPERSAND_ORD)
elif char == HYPHEN_ORD and encoded_chars:
if len(encoded_chars) == 1:
res.append('&')
else:
res.append(_modified_unbase64(encoded_chars[1:]))
encoded_chars = bytearray()
elif encoded_chars:
encoded_chars.append(char)
else:
res.append(chr(char))
if encoded_chars:
res.append(_modified_unbase64(encoded_chars[1:]))
return ''.join(res)
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