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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 +1,35 @@ | ||
| # infohash | ||
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| Am extendable 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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| The hash is extendable, so you can add fields to the struct without breaking | ||
| the hash. | ||
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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. To store the number of fields, we use 16 bits. The total space complexity | ||
| is therefore: | ||
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| ``` | ||
| (16 + 64 + log2(nr_fields+1) * 32) bits | ||
| = (10 + log2(nr_fields+1) * 4) bytes | ||
| = (20 + 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 |
|---|---|---|
| @@ -0,0 +1,30 @@ | ||
| package infohash | ||
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| import "fmt" | ||
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| // ErrInvalidHash is returned by Compare if the hash is invalid. | ||
| var ErrInvalidHash = fmt.Errorf("invalid hash") | ||
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| // FieldChangedError is returned by Compare if the hashes indicate | ||
| // that the fields have changed. | ||
| type FieldChangedError struct { | ||
| // Change contains the field that changed and its new value. | ||
| // If we do know which field changed (eg. because there were | ||
| // multiple changes), Change will be nil. | ||
| Change *FieldChange | ||
| } | ||
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| type FieldChange struct { | ||
| FieldName string | ||
| NewValue interface{} | ||
| } | ||
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| func (e FieldChangedError) Error() string { | ||
| if e.Change == nil { | ||
| return "field changed" | ||
| } | ||
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| newValue := prettyPrintConfigForHash.Sprintf("%#v", e.Change.NewValue) | ||
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| return "field changed: " + e.Change.FieldName + " to " + newValue | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,47 @@ | ||
| package infohash | ||
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| func calculateParityCodes(values []uint32) []uint32 { | ||
| log2NumberOfFieldsPlusOne := log2OfXPlusOne(uint32(len(values))) | ||
| parityCodes := make([]uint32, log2NumberOfFieldsPlusOne) | ||
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| for id, field := range values { | ||
| 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(values []uint32, parityCodes []uint32) (int, error) { | ||
| expectedCode := calculateParityCodes(values) | ||
| var errorLocation uint32 | ||
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| if len(parityCodes) != len(expectedCode) { | ||
| return 0, ErrInvalidHash | ||
| } | ||
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| for i := range parityCodes { | ||
| if parityCodes[i] != expectedCode[i] { | ||
| errorLocation |= 1 << i | ||
| } | ||
| } | ||
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| if errorLocation == 0 || errorLocation > uint32(len(values)) { | ||
| // Could not find the error location (there is probably more than one error) | ||
| return -1, nil | ||
| } | ||
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| return int(errorLocation - 1), nil | ||
| } | ||
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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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