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Use a hash to detect changes to Certificate resource.
Signed-off-by: Tim Ramlot <42113979+inteon@users.noreply.github.com>
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,31 @@ | ||
| # infohash | ||
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| A hash & comparator that tells you why the hash did not match. | ||
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| For example if you edit the Name field in a struct, this library will tell | ||
| you that the hash did not match because the Name field was changed. | ||
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| ## Space Complexity | ||
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| Per struct, we store a "global" hash and a "local" hash for each field. | ||
| The global hash is 64 bits long, and the local hashes are 32 bits long. | ||
| We use hamming codes to detect which "local" hash has changed. This requires | ||
| us to store log2(nr_fields+1) 32 bit parity hashes instead of nr_fields 32 bit | ||
| hashes. The total space complexity is therefore: | ||
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| ``` | ||
| 64 + log2(nr_fields+1) * 32 bits | ||
| = 8 + log2(nr_fields+1) * 4 bytes | ||
| = 16 + log2(nr_fields+1) * 8 hex characters | ||
| ``` | ||
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| As a rule of thumb, this library is only useful if you have at least +-7 fields, | ||
| otherwise you can just store the hashes of each field individually. | ||
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| ## Safety | ||
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| This library will detect any changes to the struct (with collision chance of 1/2^32, accounting for birthday attack). | ||
| However, accurately detecting which field changed is only possible if only one field changed. | ||
| If multiple fields changed, we can only detect that at least one field changed, but not which one. | ||
| This is because we use hamming codes to detect which field changed, and hamming codes can only detect | ||
| single field errors. |
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,58 @@ | ||
| /* | ||
| Copyright 2020 The cert-manager Authors. | ||
| Licensed under the Apache License, Version 2.0 (the "License"); | ||
| you may not use this file except in compliance with the License. | ||
| You may obtain a copy of the License at | ||
| http://www.apache.org/licenses/LICENSE-2.0 | ||
| Unless required by applicable law or agreed to in writing, software | ||
| distributed under the License is distributed on an "AS IS" BASIS, | ||
| WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| See the License for the specific language governing permissions and | ||
| limitations under the License. | ||
| */ | ||
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| package infohash | ||
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| func calculateHammingCode(fields []uint32) []uint32 { | ||
| log2NumberOfFieldsPlusOne := log2OfXPlusOne(uint32(len(fields))) | ||
| parityCodes := make([]uint32, log2NumberOfFieldsPlusOne) | ||
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| for id, field := range fields { | ||
| for i := range parityCodes { | ||
| if (id+1)&(1<<i) != 0 { | ||
| parityCodes[i] ^= field | ||
| } | ||
| } | ||
| } | ||
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| return parityCodes | ||
| } | ||
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| func findErrorLocation(fields []uint32, hammingCode []uint32) (bool, uint32) { | ||
| expectedCode := calculateHammingCode(fields) | ||
| var errorLocation uint32 | ||
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| for i := range hammingCode { | ||
| if hammingCode[i] != expectedCode[i] { | ||
| errorLocation |= 1 << i | ||
| } | ||
| } | ||
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| if errorLocation == 0 || errorLocation > uint32(len(fields)) { | ||
| return false, 0 | ||
| } | ||
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| return true, errorLocation - 1 | ||
| } | ||
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| func log2OfXPlusOne(x uint32) uint32 { | ||
| var r uint32 | ||
| for x > 0 { | ||
| x >>= 1 | ||
| r += 1 | ||
| } | ||
| return r | ||
| } |
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