PRECIS-i18n: Internationalized Usernames and Passwords
Project description
PRECIS-i18n: Internationalized Usernames and Passwords
If you want your application to accept Unicode user names and passwords, you must be careful in how you validate and compare them. The PRECIS framework makes internationalized user names and passwords safer for use by applications. PRECIS profiles transform Unicode strings into a canonical form, suitable for comparison.
This module implements the PRECIS Framework as described in:
- PRECIS Framework: Preparation, Enforcement, and Comparison of Internationalized Strings in Application Protocols (RFC 8264)
- Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords (RFC 8265)
- Preparation, Enforcement, and Comparison of Internationalized Strings Representing Nicknames (RFC 8266)
Requires Python 3.5 or later.
Usage
Use the get_profile
function to obtain a profile object, then use its
enforce
method. The enforce
method returns a Unicode string.
>>> from precis_i18n import get_profile
>>> username = get_profile('UsernameCaseMapped')
>>> username.enforce('Kevin')
'kevin'
>>> username.enforce('\u212Aevin')
'kevin'
>>> username.enforce('\uFF2Bevin')
'kevin'
>>> username.enforce('\U0001F17Aevin')
Traceback (most recent call last):
...
UnicodeEncodeError: 'UsernameCaseMapped' codec can't encode character '\U0001f17a' in position 0: DISALLOWED/symbols
Alternatively, you can use the Python str.encode
API. Import the
precis_i18n.codec
module to register the PRECIS codec names. Now you
can use the str.encode
method with any Unicode string. The result will
be a UTF-8 encoded byte string or a UnicodeEncodeError
if the string
is disallowed.
>>> import precis_i18n.codec
>>> 'Kevin'.encode('UsernameCasePreserved')
b'Kevin'
>>> '\u212Aevin'.encode('UsernameCasePreserved')
b'Kevin'
>>> '\uFF2Bevin'.encode('UsernameCasePreserved')
b'Kevin'
>>> '\u212Aevin'.encode('UsernameCaseMapped')
b'kevin'
>>> '\uFF2Bevin'.encode('OpaqueString')
b'\xef\xbc\xabevin'
>>> '\U0001F17Aevin'.encode('UsernameCasePreserved')
Traceback (most recent call last):
...
UnicodeEncodeError: 'UsernameCasePreserved' codec can't encode character '\U0001f17a' in position 0: DISALLOWED/symbols
Alternative Unicode Versions
The get_profile
function uses whatever version of unicodedata
is
provided by the Python runtime. The Unicode version is usually tied to
the major version of the Python runtime. Python 3.7.x uses Unicode 11.0.
Python 3.6.x uses Unicode 10.0.
To use an alternative unicodedata
implementation, pass the
unicodedata
keyword argument to get_profile
.
For example, you could separately install version 12.0 of the
unicodedata2
module from PyPI. Then, pass it to get_profile to
retrieve a profile that uses Unicode 12.0.
>>> import unicodedata2
>>> from precis_i18n import get_profile
>>> username = get_profile('UsernameCaseMapped', unicodedata=unicodedata2)
>>> username.enforce('Kevin')
'kevin'
Supported Profiles and Codecs
Each PRECIS profile has a corresponding codec name. The CaseMapped
variant converts the string to lower case for implementing
case-insensitive comparison.
- UsernameCasePreserved
- UsernameCaseMapped
- OpaqueString
- NicknameCasePreserved
- NicknameCaseMapped
The CaseMapped
profiles use Unicode ToLower
per the latest RFC.
Previous versions of this package used Unicode Default Case Folding.
There are CaseMapped variants for different case transformations. These
profile names are deprecated:
- UsernameCaseMapped:ToLower
- UsernameCaseMapped:CaseFold
- NicknameCaseMapped:ToLower
- NicknameCaseMapped:CaseFold
The PRECIS base string classes are also available as codecs:
- IdentifierClass
- FreeFormClass
Userparts and Space Delimited Usernames
The Username profiles in this implementation do not allow spaces. The Username profiles correspond to the definition of "userparts" in RFC 8265. If you want to allow spaces in your application's user names, you must split the string first.
def enforce_app_username(name):
profile = precis_i18n.get_profile('UsernameCasePreserved')
userparts = [profile.enforce(userpart) for userpart in name.split(' ')]
return ' '.join(userparts)
Be aware that a username constructed this way can contain bidirectional text in the separate userparts.
Error Messages
A PRECIS profile raises a UnicodeEncodeError
exception if a string is
disallowed. The reason
field specifies the kind of error.
Reason | Explanation |
---|---|
DISALLOWED/arabic_indic | Arabic-Indic digits cannot be mixed with Extended Arabic-Indic Digits. (Context) |
DISALLOWED/bidi_rule | Right-to-left string cannot contain left-to-right characters due to the "Bidi" rule. (Context) |
DISALLOWED/controls | Control character is not allowed. |
DISALLOWED/empty | After applying the profile, the result cannot be empty. |
DISALLOWED/exceptions | Exception character is not allowed. |
DISALLOWED/extended_arabic_indic | Extended Arabic-Indic digits cannot be mixed with Arabic-Indic Digits. (Context) |
DISALLOWED/greek_keraia | Greek keraia must be followed by a Greek character. (Context) |
DISALLOWED/has_compat | Compatibility characters are not allowed. |
DISALLOWED/hebrew_punctuation | Hebrew punctuation geresh or gershayim must be preceded by Hebrew character. (Context) |
DISALLOWED/katakana_middle_dot | Katakana middle dot must be accompanied by a Hiragana, Katakana, or Han character. (Context) |
DISALLOWED/middle_dot | Middle dot must be surrounded by the letter 'l'. (Context) |
DISALLOWED/not_idempotent | After reapplying the profile, the result is not stable. |
DISALLOWED/old_hangul_jamo | Conjoining Hangul Jamo is not allowed. |
DISALLOWED/other | Other character is not allowed. |
DISALLOWED/other_letter_digits | Non-traditional letter or digit is not allowed. |
DISALLOWED/precis_ignorable_properties | Default ignorable or non-character is not allowed. |
DISALLOWED/punctuation | Non-ASCII punctuation character is not allowed. |
DISALLOWED/spaces | Space character is not allowed. |
DISALLOWED/symbols | Non-ASCII symbol character is not allowed. |
DISALLOWED/unassigned | Unassigned Unicode character is not allowed. |
DISALLOWED/zero_width_joiner | Zero width joiner must immediately follow a combining virama. (Context) |
DISALLOWED/zero_width_nonjoiner | Zero width non-joiner must immediately follow a combining virama, or appear where it breaks a cursive connection in a formally cursive script. (Context) |
The Nickname Profile and White Space
When PRECIS processes a string using the Nickname
profile, one of the
enforcement steps silently removes leading and trailing white space.
Starting with version 1.1, this library uses a more restrictive
definition of white space in the Nickname
profile.
- 1.1 and later only include Unicode category
Zs
. If you try to enforce a Nickname that contains white space characters like'\n'
, you will get a UnicodeEncodeErrorDISALLOWED/controls
. - 1.0.5 and earlier included control characters such as
'\n'
,'\t'
, and'\r'
when removing leading/trailing white space from Nicknames. The result treated these legacy white space characters the same asZs
and stripped them. - In all versions, internal white space (not leading or trailing)
matches Unicode category
Zs
only.
The trimming of white space is specific to the Nickname profile only. Here is an example of the current behavior:
>>> from precis_i18n import get_profile
>>> nickname = get_profile('NicknameCaseMapped')
>>> nickname.enforce('Kevin\n')
Traceback (most recent call last):
...
UnicodeEncodeError: 'NicknameCaseMapped' codec can't encode character '\x0a' in position 5: DISALLOWED/controls
In version 1.0.5 and earlier, the NicknameCaseMapped
profile enforced "Kevin\n"
as "kevin"
.
Unicode Version Update Procedure
When Unicode releases a new version, take the following steps to update internal tables and pass unit tests:
- Under a version of Python that supports the new Unicode version, run
the tests using
python -m unittest discover
and check that thetest_derived_props
test FAILS due to a missing file. - Generate a new
derived-props
file by runningPYTHONPATH=. python test/test_derived_props.py > derived-props-VERSION.txt
. Rename the file using the Unicode version, and re-run the tests. The unit tests will further check that no derived properties in the new file contradict the previous values. - Check for changes to internal tables used for context rules by
running
PYTHONPATH=. python tools/check_codepoints.py
. Update the corresponding tables in precis_i18n/unicode.py if necessary.
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