tahoe-lafs/trac-2024-07-25
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added cap examples for K=160, and link to argument about multi-target attacks 

[Imported from Trac: page NewMutableEncodingDesign, version 13]
davidsarah 2010-01-08 02:33:56 +00:00
parent 854b649f2e
commit 9e40b833b8
1 changed files with 13 additions and 4 deletions
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17
NewMutableEncodingDesign.md

@ -46,8 +46,8 @@ directly in the filecap, simplifying the design considerably.
## Filecap Length ## Filecap Length
A likely security parameter K (=kappa) would be 96 or 128 bits, and most of A likely security parameter K (=kappa) would be 96, 128, or 160 bits, and most of
the filecaps will be some multiple of K. the filecaps will be some multiple of K. [96 bits is too short IMHO --David-Sarah]
Assuming a `tahoe:` prefix and no additional metadata, here's what Assuming a `tahoe:` prefix and no additional metadata, here's what
various lengths of base62-encoded filecaps would look like: various lengths of base62-encoded filecaps would look like:
@ -55,25 +55,34 @@ various lengths of base62-encoded filecaps would look like:
* 1*K: * 1*K:
* 96 `tahoe:14efs6T5YNim0vDVV` * 96 `tahoe:14efs6T5YNim0vDVV`
* 128 `tahoe:4V2uIYVX0PcHu9fQrJ3GSH` * 128 `tahoe:4V2uIYVX0PcHu9fQrJ3GSH`
* 160 `tahoe:8gdR7Epld72UvkF6Pe9hhT8NQx3`
* 2*K: * 2*K:
* 192 `tahoe:072Og6e75IOP9ZZsbR1Twjs6X5xXJnBAF` * 192 `tahoe:072Og6e75IOP9ZZsbR1Twjs6X5xXJnBAF`
* 256 `tahoe:fZeioazoWrO62reiAjzUAyV0uz3ssh6Hnanv8cKMClY` * 256 `tahoe:fZeioazoWrO62reiAjzUAyV0uz3ssh6Hnanv8cKMClY`
* 320 `tahoe:j6Re0BbWp7DqJtgd9fUXl4pWiD5kr1mjT9DtudJ72o0vhPen83Utza`
* 3*K: * 3*K:
* 288 `tahoe:11DriaxD9nipA10ueBvv5uoMoehvxgPerpQiXyvMPeiUUdtf6` * 288 `tahoe:11DriaxD9nipA10ueBvv5uoMoehvxgPerpQiXyvMPeiUUdtf6`
* 384 `tahoe:3a31SqUbf8fpWE1opRCT3coDhRqTU7bDU2AvC3RQJBu6ZNFhVscyxA9slYtPVT79x` * 384 `tahoe:3a31SqUbf8fpWE1opRCT3coDhRqTU7bDU2AvC3RQJBu6ZNFhVscyxA9slYtPVT79x`
* 480 `tahoe:P6rGeI6CwlG4i8W2l6haSoC9rfPjw0KHBi8xAk4F8vdhnRhL8nwE27CitJ9Rld06HuKt8VucypJRcbGFo`
Adding 2 metadata characters and a clear separator gives us: Adding 2 metadata characters and a clear separator gives us:
* 96: `tahoe:MW-14efs6T5YNim0vDVV` * 96: `tahoe:MW-14efs6T5YNim0vDVV`
* 128: `tahoe:DW-4V2uIYVX0PcHu9fQrJ3GSH` * 128: `tahoe:DW-4V2uIYVX0PcHu9fQrJ3GSH`
* 160: `tahoe:DW-8gdR7Epld72UvkF6Pe9hhT8NQx3`
* 192: `tahoe:MR-072Og6e75IOP9ZZsbR1Twjs6X5xXJnBAF` * 192: `tahoe:MR-072Og6e75IOP9ZZsbR1Twjs6X5xXJnBAF`
* 256: `tahoe:DR-fZeioazoWrO62reiAjzUAyV0uz3ssh6Hnanv8cKMClY` * 256: `tahoe:DR-fZeioazoWrO62reiAjzUAyV0uz3ssh6Hnanv8cKMClY`
* 288: `tahoe:MR-11DriaxD9nipA10ueBvv5uoMoehvxgPerpQiXyvMPeiUUdtf6` * 288: `tahoe:MR-11DriaxD9nipA10ueBvv5uoMoehvxgPerpQiXyvMPeiUUdtf6`
* 384: `tahoe:MR-3a31SqUbf8fpWE1opRCT3coDhRqTU7bDU2AvC3RQJBu6ZNFhVscyxA9slYtPVT79x` * 320: `tahoe:MV-j6Re0BbWp7DqJtgd9fUXl4pWiD5kr1mjT9DtudJ72o0vhPen83Utza`
* 384: `tahoe:DV-3a31SqUbf8fpWE1opRCT3coDhRqTU7bDU2AvC3RQJBu6ZNFhVscyxA9slYtPVT79x`
* 480: `tahoe:MV-P6rGeI6CwlG4i8W2l6haSoC9rfPjw0KHBi8xAk4F8vdhnRhL8nwE27CitJ9Rld06HuKt8VucypJRcbGFo`
[#217:c44](http://allmydata.org/trac/tahoe/ticket/217#comment:44) says that, [#217:c44](http://allmydata.org/trac/tahoe/ticket/217#comment:44) says that,
if we don't need to prevent collisions, then we can use a K-bit hash for if we don't need to prevent collisions, then we can use a K-bit hash for
K-bit second-pre-image resistance. K-bit second-pre-image resistance. However, see
[#882:c6](http://allmydata.org/trac/tahoe/ticket/882#comment:6) for a
counterargument saying that 50 extra bits or so are needed to be secure
against multi-target attacks.
# Design Proposals # Design Proposals