1 | A Fast Method for Identifying Plain Text Files |
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2 | ============================================== |
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3 | |
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4 | |
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5 | Introduction |
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6 | ------------ |
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7 | |
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8 | Given a file coming from an unknown source, it is sometimes desirable |
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9 | to find out whether the format of that file is plain text. Although |
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10 | this may appear like a simple task, a fully accurate detection of the |
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11 | file type requires heavy-duty semantic analysis on the file contents. |
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12 | It is, however, possible to obtain satisfactory results by employing |
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13 | various heuristics. |
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14 | |
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15 | Previous versions of PKZip and other zip-compatible compression tools |
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16 | were using a crude detection scheme: if more than 80% (4/5) of the bytes |
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17 | found in a certain buffer are within the range [7..127], the file is |
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18 | labeled as plain text, otherwise it is labeled as binary. A prominent |
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19 | limitation of this scheme is the restriction to Latin-based alphabets. |
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20 | Other alphabets, like Greek, Cyrillic or Asian, make extensive use of |
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21 | the bytes within the range [128..255], and texts using these alphabets |
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22 | are most often misidentified by this scheme; in other words, the rate |
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23 | of false negatives is sometimes too high, which means that the recall |
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24 | is low. Another weakness of this scheme is a reduced precision, due to |
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25 | the false positives that may occur when binary files containing large |
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26 | amounts of textual characters are misidentified as plain text. |
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27 | |
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28 | In this article we propose a new, simple detection scheme that features |
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29 | a much increased precision and a near-100% recall. This scheme is |
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30 | designed to work on ASCII, Unicode and other ASCII-derived alphabets, |
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31 | and it handles single-byte encodings (ISO-8859, MacRoman, KOI8, etc.) |
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32 | and variable-sized encodings (ISO-2022, UTF-8, etc.). Wider encodings |
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33 | (UCS-2/UTF-16 and UCS-4/UTF-32) are not handled, however. |
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34 | |
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35 | |
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36 | The Algorithm |
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37 | ------------- |
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38 | |
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39 | The algorithm works by dividing the set of bytecodes [0..255] into three |
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40 | categories: |
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41 | - The white list of textual bytecodes: |
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42 | 9 (TAB), 10 (LF), 13 (CR), 32 (SPACE) to 255. |
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43 | - The gray list of tolerated bytecodes: |
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44 | 7 (BEL), 8 (BS), 11 (VT), 12 (FF), 26 (SUB), 27 (ESC). |
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45 | - The black list of undesired, non-textual bytecodes: |
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46 | 0 (NUL) to 6, 14 to 31. |
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47 | |
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48 | If a file contains at least one byte that belongs to the white list and |
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49 | no byte that belongs to the black list, then the file is categorized as |
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50 | plain text; otherwise, it is categorized as binary. (The boundary case, |
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51 | when the file is empty, automatically falls into the latter category.) |
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52 | |
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53 | |
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54 | Rationale |
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55 | --------- |
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56 | |
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57 | The idea behind this algorithm relies on two observations. |
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58 | |
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59 | The first observation is that, although the full range of 7-bit codes |
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60 | [0..127] is properly specified by the ASCII standard, most control |
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61 | characters in the range [0..31] are not used in practice. The only |
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62 | widely-used, almost universally-portable control codes are 9 (TAB), |
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63 | 10 (LF) and 13 (CR). There are a few more control codes that are |
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64 | recognized on a reduced range of platforms and text viewers/editors: |
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65 | 7 (BEL), 8 (BS), 11 (VT), 12 (FF), 26 (SUB) and 27 (ESC); but these |
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66 | codes are rarely (if ever) used alone, without being accompanied by |
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67 | some printable text. Even the newer, portable text formats such as |
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68 | XML avoid using control characters outside the list mentioned here. |
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69 | |
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70 | The second observation is that most of the binary files tend to contain |
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71 | control characters, especially 0 (NUL). Even though the older text |
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72 | detection schemes observe the presence of non-ASCII codes from the range |
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73 | [128..255], the precision rarely has to suffer if this upper range is |
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74 | labeled as textual, because the files that are genuinely binary tend to |
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75 | contain both control characters and codes from the upper range. On the |
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76 | other hand, the upper range needs to be labeled as textual, because it |
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77 | is used by virtually all ASCII extensions. In particular, this range is |
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78 | used for encoding non-Latin scripts. |
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79 | |
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80 | Since there is no counting involved, other than simply observing the |
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81 | presence or the absence of some byte values, the algorithm produces |
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82 | consistent results, regardless what alphabet encoding is being used. |
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83 | (If counting were involved, it could be possible to obtain different |
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84 | results on a text encoded, say, using ISO-8859-16 versus UTF-8.) |
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85 | |
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86 | There is an extra category of plain text files that are "polluted" with |
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87 | one or more black-listed codes, either by mistake or by peculiar design |
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88 | considerations. In such cases, a scheme that tolerates a small fraction |
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89 | of black-listed codes would provide an increased recall (i.e. more true |
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90 | positives). This, however, incurs a reduced precision overall, since |
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91 | false positives are more likely to appear in binary files that contain |
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92 | large chunks of textual data. Furthermore, "polluted" plain text should |
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93 | be regarded as binary by general-purpose text detection schemes, because |
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94 | general-purpose text processing algorithms might not be applicable. |
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95 | Under this premise, it is safe to say that our detection method provides |
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96 | a near-100% recall. |
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97 | |
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98 | Experiments have been run on many files coming from various platforms |
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99 | and applications. We tried plain text files, system logs, source code, |
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100 | formatted office documents, compiled object code, etc. The results |
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101 | confirm the optimistic assumptions about the capabilities of this |
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102 | algorithm. |
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103 | |
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104 | |
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105 | -- |
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106 | Cosmin Truta |
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107 | Last updated: 2006-May-28 |
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