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Randomizing x coordinate in shamir split #2621

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merged 4 commits into from
Apr 28, 2017
Merged

Randomizing x coordinate in shamir split #2621

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78ad36b
Randomizing x coordinate in shamir split
Apr 20, 2017
754c8dd
Only generate xCoordinates for the number of parts
Apr 20, 2017
d33b4b6
Reverting
Apr 20, 2017
8bda5a6
Seeding math/rand with current time
Apr 20, 2017
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10 changes: 8 additions & 2 deletions shamir/shamir.go
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,8 @@ import (
"crypto/rand"
"crypto/subtle"
"fmt"
mathrand "math/rand"
"time"
)

const (
Expand Down Expand Up @@ -166,13 +168,17 @@ func Split(secret []byte, parts, threshold int) ([][]byte, error) {
return nil, fmt.Errorf("cannot split an empty secret")
}

// Generate random list of x coordinates
mathrand.Seed(time.Now().UnixNano())
xCoordinates := mathrand.Perm(255)

// Allocate the output array, initialize the final byte
// of the output with the offset. The representation of each
// output is {y1, y2, .., yN, x}.
out := make([][]byte, parts)
for idx := range out {
out[idx] = make([]byte, len(secret)+1)
out[idx][len(secret)] = uint8(idx) + 1
out[idx][len(secret)] = uint8(xCoordinates[idx]) + 1
}

// Construct a random polynomial for each byte of the secret.
Expand All @@ -189,7 +195,7 @@ func Split(secret []byte, parts, threshold int) ([][]byte, error) {
// We cheat by encoding the x value once as the final index,
// so that it only needs to be stored once.
for i := 0; i < parts; i++ {
x := uint8(i) + 1
x := uint8(xCoordinates[i]) + 1
y := p.evaluate(x)
out[i][idx] = y
}
Expand Down