UTF-8 decoder capability and stress test

----------------------------------------

Markus Kuhn <http://www.cl.cam.ac.uk/~mgk25/> - 2003-02-19

This test file can help you examine, how your UTF-8 decoder handles

various types of correct, malformed, or otherwise interesting UTF-8

sequences. This file is not meant to be a conformance test. It does

not prescribes any particular outcome and therefore there is no way to

"pass" or "fail" this test file, even though the texts suggests a

preferable decoder behaviour at some places. The aim is instead to

help you think about and test the behaviour of your UTF-8 on a

systematic collection of unusual inputs. Experience so far suggests

that most first-time authors of UTF-8 decoders find at least one

serious problem in their decoder by using this file.

The test lines below cover boundary conditions, malformed UTF-8

sequences as well as correctly encoded UTF-8 sequences of Unicode code

points that should never occur in a correct UTF-8 file.

According to ISO 10646-1:2000, sections D.7 and 2.3c, a device

receiving UTF-8 shall interpret a "malformed sequence in the same way

that it interprets a character that is outside the adopted subset" and

"characters that are not within the adopted subset shall be indicated

to the user" by a receiving device. A quite commonly used approach in

UTF-8 decoders is to replace any malformed UTF-8 sequence by a

replacement character (U+FFFD), which looks a bit like an inverted

question mark, or a similar symbol. It might be a good idea to

visually distinguish a malformed UTF-8 sequence from a correctly

encoded Unicode character that is just not available in the current

font but otherwise fully legal, even though ISO 10646-1 doesn't

mandate this. In any case, just ignoring malformed sequences or

unavailable characters does not conform to ISO 10646, will make

debugging more difficult, and can lead to user confusion.

Please check, whether a malformed UTF-8 sequence is (1) represented at

all, (2) represented by exactly one single replacement character (or

equivalent signal), and (3) the following quotation mark after an

illegal UTF-8 sequence is correctly displayed, i.e. proper

resynchronization takes place immageately after any malformed

sequence. This file says "THE END" in the last line, so if you don't

see that, your decoder crashed somehow before, which should always be

cause for concern.

All lines in this file are exactly 79 characters long (plus the line

feed). In addition, all lines end with "|", except for the two test

lines 2.1.1 and 2.2.1, which contain non-printable ASCII controls

U+0000 and U+007F. If you display this file with a fixed-width font,

these "|" characters should all line up in column 79 (right margin).

This allows you to test quickly, whether your UTF-8 decoder finds the

correct number of characters in every line, that is whether each

malformed sequences is replaced by a single replacement character.

Note that as an alternative to the notion of malformed sequence used

here, it is also a perfectly acceptable (and in some situations even

preferable) solution to represent each individual byte of a malformed

sequence by a replacement character. If you follow this strategy in

your decoder, then please ignore the "|" column.

Here come the tests:                                                          |

                                                                              |

1  Some correct UTF-8 text                                                    |

                                                                              |

(The codepoints for this test are:                                            |

  U+03BA U+1F79 U+03C3 U+03BC U+03B5  --ryan.)                                |

                                                                              |

You should see the Greek word 'kosme':       "κόσμε"                          |

                                                                              |

                                                                              |

2  Boundary condition test cases                                              |

                                                                              |

2.1  First possible sequence of a certain length                              |

                                                                              |

(byte zero skipped...there's a null added at the end of the test. --ryan.)    |

                                                                              |

2.1.2  2 bytes (U-00000080):        "€"                                       |

2.1.3  3 bytes (U-00000800):        "ࠀ"                                       |

2.1.4  4 bytes (U-00010000):        "𐀀"                                       |

                                                                              |

(5 and 6 byte sequences were made illegal in rfc3629. --ryan.)                |

2.1.5  5 bytes (U-00200000):        "�����"                                       |

2.1.6  6 bytes (U-04000000):        "������"                                       |

                                                                              |

2.2  Last possible sequence of a certain length                               |

                                                                              |

2.2.1  1 byte  (U-0000007F):        ""                                       |

2.2.2  2 bytes (U-000007FF):        "߿"                                       |

                                                                              |

(Section 5.3.2 below calls this illegal. --ryan.)                             |

2.2.3  3 bytes (U-0000FFFF):        "￿"                                       |

                                                                              |

(5 and 6 bytes sequences, and 4 bytes sequences > 0x10FFFF were made illegal  |

 in rfc3629, so these next three should be replaced with a invalid            |

 character codepoint. --ryan.)                                                |

2.2.4  4 bytes (U-001FFFFF):        "����"                                       |

2.2.5  5 bytes (U-03FFFFFF):        "�����"                                       |

2.2.6  6 bytes (U-7FFFFFFF):        "������"                                       |

                                                                              |

2.3  Other boundary conditions                                                |

                                                                              |

2.3.1  U-0000D7FF = ed 9f bf = "퟿"                                            |

2.3.2  U-0000E000 = ee 80 80 = ""                                            |

2.3.3  U-0000FFFD = ef bf bd = "�"                                            |

2.3.4  U-0010FFFF = f4 8f bf bf = "􏿿"                                         |

                                                                              |

(This one is bogus in rfc3629. --ryan.)                                       |

2.3.5  U-00110000 = f4 90 80 80 = "����"                                         |

                                                                              |

3  Malformed sequences                                                        |

                                                                              |

3.1  Unexpected continuation bytes                                            |

                                                                              |

Each unexpected continuation byte should be separately signalled as a         |

malformed sequence of its own.                                                |

                                                                              |

3.1.1  First continuation byte 0x80: "�"                                      |

3.1.2  Last  continuation byte 0xbf: "�"                                      |

                                                                              |

3.1.3  2 continuation bytes: "��"                                             |

3.1.4  3 continuation bytes: "���"                                            |

3.1.5  4 continuation bytes: "����"                                           |

3.1.6  5 continuation bytes: "�����"                                          |

3.1.7  6 continuation bytes: "������"                                         |

3.1.8  7 continuation bytes: "�������"                                        |

                                                                              |

3.1.9  Sequence of all 64 possible continuation bytes (0x80-0xbf):            |

                                                                              |

   "����������������                                                          |

    ����������������                                                          |

    ����������������                                                          |

    ����������������"                                                         |

                                                                              |

3.2  Lonely start characters                                                  |

                                                                              |

3.2.1  All 32 first bytes of 2-byte sequences (0xc0-0xdf),                    |

       each followed by a space character:                                    |

                                                                              |

   "� � � � � � � � � � � � � � � �                                           |

    � � � � � � � � � � � � � � � � "                                         |

                                                                              |

3.2.2  All 16 first bytes of 3-byte sequences (0xe0-0xef),                    |

       each followed by a space character:                                    |

                                                                              |

   "� � � � � � � � � � � � � � � � "                                         |

                                                                              |

3.2.3  All 8 first bytes of 4-byte sequences (0xf0-0xf7),                     |

       each followed by a space character:                                    |

                                                                              |

   "� � � � � � � � "                                                         |

                                                                              |

3.2.4  All 4 first bytes of 5-byte sequences (0xf8-0xfb),                     |

       each followed by a space character:                                    |

                                                                              |

   "� � � � "                                                                 |

                                                                              |

3.2.5  All 2 first bytes of 6-byte sequences (0xfc-0xfd),                     |

       each followed by a space character:                                    |

                                                                              |

   "� � "                                                                     |

                                                                              |

3.3  Sequences with last continuation byte missing                            |

                                                                              |

All bytes of an incomplete sequence should be signalled as a single           |

malformed sequence, i.e., you should see only a single replacement            |

character in each of the next 10 tests. (Characters as in section 2)          |

                                                                              |

3.3.1  2-byte sequence with last byte missing (U+0000):     "�"               |

3.3.2  3-byte sequence with last byte missing (U+0000):     "��"               |

3.3.3  4-byte sequence with last byte missing (U+0000):     "���"               |

3.3.4  5-byte sequence with last byte missing (U+0000):     "����"               |

3.3.5  6-byte sequence with last byte missing (U+0000):     "�����"               |

3.3.6  2-byte sequence with last byte missing (U-000007FF): "�"               |

3.3.7  3-byte sequence with last byte missing (U-0000FFFF): "�"               |

3.3.8  4-byte sequence with last byte missing (U-001FFFFF): "���"               |

3.3.9  5-byte sequence with last byte missing (U-03FFFFFF): "����"               |

3.3.10 6-byte sequence with last byte missing (U-7FFFFFFF): "�����"               |

                                                                              |

3.4  Concatenation of incomplete sequences                                    |

                                                                              |

All the 10 sequences of 3.3 concatenated, you should see 10 malformed         |

sequences being signalled:                                                    |

                                                                              |

   "�����������������������������"                                                               |

                                                                              |

3.5  Impossible bytes                                                         |

                                                                              |

The following two bytes cannot appear in a correct UTF-8 string               |

                                                                              |

3.5.1  fe = "�"                                                               |

3.5.2  ff = "�"                                                               |

3.5.3  fe fe ff ff = "����"                                                   |

                                                                              |

4  Overlong sequences                                                         |

                                                                              |

The following sequences are not malformed according to the letter of          |

the Unicode 2.0 standard. However, they are longer then necessary and         |

a correct UTF-8 encoder is not allowed to produce them. A "safe UTF-8         |

decoder" should reject them just like malformed sequences for two             |

reasons: (1) It helps to debug applications if overlong sequences are         |

not treated as valid representations of characters, because this helps        |

to spot problems more quickly. (2) Overlong sequences provide                 |

alternative representations of characters, that could maliciously be          |

used to bypass filters that check only for ASCII characters. For              |

instance, a 2-byte encoded line feed (LF) would not be caught by a            |

line counter that counts only 0x0a bytes, but it would still be               |

processed as a line feed by an unsafe UTF-8 decoder later in the              |

pipeline. From a security point of view, ASCII compatibility of UTF-8         |

sequences means also, that ASCII characters are *only* allowed to be          |

represented by ASCII bytes in the range 0x00-0x7f. To ensure this             |

aspect of ASCII compatibility, use only "safe UTF-8 decoders" that            |

reject overlong UTF-8 sequences for which a shorter encoding exists.          |

                                                                              |

4.1  Examples of an overlong ASCII character                                  |

                                                                              |

With a safe UTF-8 decoder, all of the following five overlong                 |

representations of the ASCII character slash ("/") should be rejected         |

like a malformed UTF-8 sequence, for instance by substituting it with         |

a replacement character. If you see a slash below, you do not have a          |

safe UTF-8 decoder!                                                           |

                                                                              |

4.1.1 U+002F = c0 af             = "��"                                        |

4.1.2 U+002F = e0 80 af          = "���"                                        |

4.1.3 U+002F = f0 80 80 af       = "����"                                        |

4.1.4 U+002F = f8 80 80 80 af    = "�����"                                        |

4.1.5 U+002F = fc 80 80 80 80 af = "������"                                        |

                                                                              |

4.2  Maximum overlong sequences                                               |

                                                                              |

Below you see the highest Unicode value that is still resulting in an         |

overlong sequence if represented with the given number of bytes. This         |

is a boundary test for safe UTF-8 decoders. All five characters should        |

be rejected like malformed UTF-8 sequences.                                   |

                                                                              |

4.2.1  U-0000007F = c1 bf             = "��"                                   |

4.2.2  U-000007FF = e0 9f bf          = "���"                                   |

4.2.3  U-0000FFFF = f0 8f bf bf       = "����"                                   |

4.2.4  U-001FFFFF = f8 87 bf bf bf    = "�����"                                   |

4.2.5  U-03FFFFFF = fc 83 bf bf bf bf = "������"                                   |

                                                                              |

4.3  Overlong representation of the NUL character                             |

                                                                              |

The following five sequences should also be rejected like malformed           |

UTF-8 sequences and should not be treated like the ASCII NUL                  |

character.                                                                    |

                                                                              |

4.3.1  U+0000 = c0 80             = "��"                                       |

4.3.2  U+0000 = e0 80 80          = "���"                                       |

4.3.3  U+0000 = f0 80 80 80       = "����"                                       |

4.3.4  U+0000 = f8 80 80 80 80    = "�����"                                       |

4.3.5  U+0000 = fc 80 80 80 80 80 = "������"                                       |

                                                                              |

5  Illegal code positions                                                     |

                                                                              |

The following UTF-8 sequences should be rejected like malformed               |

sequences, because they never represent valid ISO 10646 characters and        |

a UTF-8 decoder that accepts them might introduce security problems           |

comparable to overlong UTF-8 sequences.                                       |

                                                                              |

5.1 Single UTF-16 surrogates                                                  |

                                                                              |

5.1.1  U+D800 = ed a0 80 = "���"                                                |

5.1.2  U+DB7F = ed ad bf = "���"                                                |

5.1.3  U+DB80 = ed ae 80 = "���"                                                |

5.1.4  U+DBFF = ed af bf = "���"                                                |

5.1.5  U+DC00 = ed b0 80 = "���"                                                |

5.1.6  U+DF80 = ed be 80 = "���"                                                |

5.1.7  U+DFFF = ed bf bf = "���"                                                |

                                                                              |

5.2 Paired UTF-16 surrogates                                                  |

                                                                              |

5.2.1  U+D800 U+DC00 = ed a0 80 ed b0 80 = "������"                               |

5.2.2  U+D800 U+DFFF = ed a0 80 ed bf bf = "������"                               |

5.2.3  U+DB7F U+DC00 = ed ad bf ed b0 80 = "������"                               |

5.2.4  U+DB7F U+DFFF = ed ad bf ed bf bf = "������"                               |

5.2.5  U+DB80 U+DC00 = ed ae 80 ed b0 80 = "������"                               |

5.2.6  U+DB80 U+DFFF = ed ae 80 ed bf bf = "������"                               |

5.2.7  U+DBFF U+DC00 = ed af bf ed b0 80 = "������"                               |

5.2.8  U+DBFF U+DFFF = ed af bf ed bf bf = "������"                               |

                                                                              |

5.3 Other illegal code positions                                              |

                                                                              |

5.3.1  U+FFFE = ef bf be = "￾"                                                |

5.3.2  U+FFFF = ef bf bf = "￿"                                                |

                                                                              |

THE END                                                                       |