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PRECIS-i18n: Internationalized Usernames and Passwords

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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 3.3 or later.


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')
>>> username.enforce('\u212Aevin')
>>> username.enforce('\uFF2Bevin')
>>> 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')
>>> '\u212Aevin'.encode('UsernameCasePreserved')
>>> '\uFF2Bevin'.encode('UsernameCasePreserved')
>>> '\u212Aevin'.encode('UsernameCaseMapped')
>>> '\uFF2Bevin'.encode('OpaqueString')
>>> '\U0001F17Aevin'.encode('UsernameCasePreserved')
Traceback (most recent call last):
UnicodeEncodeError: 'UsernameCasePreserved' codec can't encode character '\U0001f17a' in position 0: DISALLOWED/symbols

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 verions 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

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)

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