tahoe-lafs/trac-2024-07-25
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add example of 306-bit (2K+T) caps 

[Imported from Trac: page NewMutableEncodingDesign, version 23]
davidsarah 2012-01-11 21:51:12 +00:00
parent ab0139cc1c
commit 09436d4df6
1 changed files with 10 additions and 7 deletions
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17
NewMutableEncodingDesign.md

@ -62,17 +62,20 @@ A likely security parameter K (=kappa) would be 128 bits.
Assuming a `lafs:` prefix and no additional metadata, here's what
various lengths of base62-encoded filecaps would look like:
* K+T:
* 128+50 `lafs:3zusfONRfEOp17B0ad2TDDSJxtBpvH`
* 2*K:
* 256 `lafs:fZeioazoWrO62reiAjzUAyV0uz3ssh6Hnanv8cKMClY`
* 3*K:
* 384 `lafs:3a31SqUbf8fpWE1opRCT3coDhRqTU7bDU2AvC3RQJBu6ZNFhVscyxA9slYtPVT79x`
* K + T = 178:
* `lafs:3zusfONRfEOp17B0ad2TDDSJxtBpvH`
* 2*K = 256:
* `lafs:fZeioazoWrO62reiAjzUAyV0uz3ssh6Hnanv8cKMClY`
* 2*K + T = 306:
* `lafs:2s1SqUbf8fpsh6HnThRqTUtBpvH7D8fpWEDSfZeioazoWrO62rei`
* 3*K = 384:
* `lafs:3a31SqUbf8fpWE1opRCT3coDhRqTU7bDU2AvC3RQJBu6ZNFhVscyxA9slYtPVT79x`
Adding 2 metadata characters and a clear separator gives us:
* 128+50 `lafs:DW-3zusfONRfEOp17B0ad2TDDSJxtBpvH`
* 178 `lafs:DW-3zusfONRfEOp17B0ad2TDDSJxtBpvH`
* 256: `lafs:DR-fZeioazoWrO62reiAjzUAyV0uz3ssh6Hnanv8cKMClY`
* 306: `lafs:DR-2s1SqUbf8fpsh6HnThRqTUtBpvH7D8fpWEDSfZeioazoWrO62rei`
* 384: `lafs:DV-3a31SqUbf8fpWE1opRCT3coDhRqTU7bDU2AvC3RQJBu6ZNFhVscyxA9slYtPVT79x`
[#217:c44]comment:44:[/tahoe-lafs/trac-2024-07-25/issues/7722](/tahoe-lafs/trac-2024-07-25/issues/7722) says that, if we don't need to prevent collisions, then we can use a K-bit hash for K-bit second-pre-image resistance. However, see [#882:c6]comment:6:[/tahoe-lafs/trac-2024-07-25/issues/8387](/tahoe-lafs/trac-2024-07-25/issues/8387) for a counterargument saying that 50 extra bits or so are needed to be secure against multi-target attacks (i.e. T = 50). This page has now been updated assuming the counterargument is correct.