UTF-8 decoder capability and stress test |
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Markus Kuhn <mkuhn@acm.org> - 1999-11-07 |
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This test text examines, how UTF-8 decoders handle various types of |
corrupted or otherwise interesting UTF-8 sequences. |
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According to ISO 10646-1, sections R.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". This means |
usually that the malformed UTF-8 sequence is replaced 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. For both cases, a clearly recognisable symbol should be used. |
Just ignoring malformed sequences or unavailable characters will make |
debugging more difficult and can lead to user confusion. |
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Test sequences: |
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Check, whether a malformed UTF-8 sequence is (1) represented at all, |
(2) represented by exactly one 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. This file says "THE END" in the last line, so if you don't see |
that, your decoder crashed somehow before, which is also not nice. |
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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. |
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Here come the tests: |
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1 Some correct UTF-8 text |
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You should see the Greek word 'kosme': "κόσμε" |
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2 Minimum and maximum valid values for every UTF-8 sequence length |
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You should see a correctly encoded character each time (which is not |
necessarily an existing character). |
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2.1 First possible sequence of a certain length |
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2.1.1 1 byte (U-00000000): "�"
2.1.2 2 bytes (U-00000080): "" |
2.1.3 3 bytes (U-00000800): "ࠀ" |
2.1.4 4 bytes (U-00010000): "𐀀" |
2.1.5 5 bytes (U-00200000): "�����" |
2.1.6 6 bytes (U-04000000): "������" |
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2.2 Last possible sequence of a certain length |
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2.2.1 1 byte (U-0000007f): ""
2.2.2 2 bytes (U-000007ff): "߿" |
2.2.3 3 bytes (U-0000ffff): "" |
2.2.4 4 bytes (U-001fffff): "����" |
2.2.5 5 bytes (U-03ffffff): "�����" |
2.2.6 6 bytes (U-7fffffff): "������" |
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3 Malformed sequences |
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3.1 Unexpected continuation bytes |
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Each unexpected continuation byte should be separately signalled as a |
malformed sequence of its own. |
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3.1.1 First continuation byte 0x80: "�" |
3.1.2 Last continuation byte 0xbf: "�" |
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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: "�������" |
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3.1.9 Sequence of all 64 possible continuation bytes (0x80-0xbf): |
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"���������������� |
���������������� |
���������������� |
����������������" |
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3.2 Lonely start characters |
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3.2.1 All 32 first bytes of 2-byte sequences (0xc0-0xdf), |
each followed by a space character: |
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"� � � � � � � � � � � � � � � � |
� � � � � � � � � � � � � � � � " |
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3.2.2 All 16 first bytes of 3-byte sequences (0xe0-0xef), |
each followed by a space character: |
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"� � � � � � � � � � � � � � � � " |
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3.2.3 All 8 first bytes of 4-byte sequences (0xf0-0xf7), |
each followed by a space character: |
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"� � � � � � � � " |
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3.2.4 All 4 first bytes of 5-byte sequences (0xf8-0xfb), |
each followed by a space character: |
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"� � � � " |
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3.2.5 All 2 first bytes of 6-byte sequences (0xfc-0xfd), |
each followed by a space character: |
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"� � " |
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3.3 Sequences with last continuation byte missing |
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All bytes of an incomplete sequence should be signalled as a single |
malformed sequence, i.e., you should see only a single replacement |
characters in each of the next 10 tests. (Characters as in section 2) |
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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): "�����" |
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3.4 Concatenation of incomplete sequences |
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All the 10 sequences of 3.3 concatenated, you should see 10 malformed |
sequences being signalled: |
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"�����������������������������" |
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3.5 Impossible bytes |
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The following two bytes cannot appear in a correct UTF-8 string |
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3.5.1 fe = "�" |
3.5.2 ff = "�" |
3.5.3 fe fe ff ff = "����" |
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4 Overlong sequences |
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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. |
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4.1 Examples of an overlong ASCII character |
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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! |
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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 = "������" |
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4.2 Maximum overlong sequences |
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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. |
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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 = "������" |
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4.3 Overlong representation of the NUL character |
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The following five sequences should also be rejected like malformed |
UTF-8 sequences and should not be treated like the ASCII NUL |
character. |
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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 = "������" |
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THE END |