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Shimarks are icons generated from the hash of urls and their domains for the
purpose of preventing phishing. The justification is that it is unlikely for an
attacker to be able to create a phishing domain or url that also produces a hash
collision.
There are two types of Shimarks, Domain Shimarks, which are generated from only
the domain of the URL, displayed on the left side of URLs; and URL
Shimarks, which are generated from the entire URL, displayed on the right side
of the URL.
1. Shimarks rely on MD5 hashes. This hash function has been chosen because of
its ubiquity. Although collision attacks have been discovered for MD5, the
relatively short length of URLs and domain names makes it less likely that an
attacker can hide a collision block within a URL. Nonetheless, I am taking
suggestions for a better hash function to use.
2. Shimarks are black and white only, so that they may easily be used in print
and so that they are accesible to the colorblind.
3. Shimarks are 16x16 pixels by default, which allows for 256 bits of
information. Since md5 produces 128 bits, this is then mirrored either
horizontally or vertically depending on the 1st bit of the hash. This mirroring
will hopefully make Shimarks more identifiable.
Therefore, if the bit at 0x00 is 0, then the layout of the ShiMark is:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F
30 31 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F
40 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F
50 51 52 53 54 55 56 57 58 59 5A 5B 5C 5D 5E 5F
60 61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F
70 71 72 73 74 75 76 77 78 79 7A 7B 7C 7D 7E 7F
70 71 72 73 74 75 76 77 78 79 7A 7B 7C 7D 7E 7F
60 61 62 63 64 65 66 67 68 69 6A 6B 6C 6D 6E 6F
50 51 52 53 54 55 56 57 58 59 5A 5B 5C 5D 5E 5F
40 41 42 43 44 45 46 47 48 49 4A 4B 4C 4D 4E 4F
30 31 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F
20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F
10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
Where every two bytes are layed out horizontally and to produce half the
pattern. This is then flipped across the horizontal axis.
Otherwise, it is flipped across the vertical axis:
00 10 20 30 40 50 60 70 70 60 50 40 30 20 10 00
01 11 21 31 41 51 61 71 71 61 51 41 31 21 11 11
02 12 22 32 42 52 62 72 72 62 52 42 32 22 12 22
03 13 23 33 43 53 63 73 73 63 53 43 33 23 13 33
04 14 24 34 44 54 64 74 74 64 54 44 34 24 14 44
05 15 25 35 45 55 65 75 75 65 55 45 35 25 15 55
06 16 26 36 46 56 66 76 76 66 56 46 36 26 16 66
07 17 27 37 47 57 67 77 77 67 57 47 37 27 17 77
08 18 28 38 48 58 68 78 78 68 58 48 38 28 18 88
09 19 29 39 49 59 69 79 79 69 59 49 39 29 19 99
0A 1A 2A 3A 4A 5A 6A 7A 7A 6A 5A 4A 3A 2A 1A AA
0B 1B 2B 3B 4B 5B 6B 7B 7B 6B 5B 4B 3B 2B 1B BB
0C 1C 2C 3C 4C 5C 6C 7C 7C 6C 5C 4C 3C 2C 1C CC
0D 1D 2D 3D 4D 5D 6D 7D 7D 6D 5D 4D 3D 2D 1D DD
0E 1E 2E 3E 4E 5E 6E 7E 7E 6E 5E 4E 3E 2E 1E EE
0F 1F 2F 3F 4F 5F 6F 7F 7F 6F 5F 4F 3F 2F 1F FF
4. For compatibility with text only mediums, Shimarks can be abbreviated by
taking the top left 6x4 pixels and representing them using braille.
Issues with the current specification:
16x16 pixels at icon scale might be difficult to distinguish, therefore
a method for making the difference between hashes more distinctive(a la
identicons), or for using fewer bits, is needed.
Overreliance on abbreviated Shimarks may make collision rates too high.
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