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ckem: pass xof to elliptic.GenerateKey directly #403

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Feb 16, 2023
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ckem: pass xof to elliptic.GenerateKey directly #403

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6 changes: 1 addition & 5 deletions kem/hybrid/ckem.go
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,6 @@ package hybrid
// TODO move over to crypto/ecdh once we can assume Go 1.20.

import (
"bytes"
"crypto/elliptic"
cryptoRand "crypto/rand"
"crypto/subtle"
Expand Down Expand Up @@ -123,10 +122,7 @@ func (sch *cScheme) DeriveKeyPair(seed []byte) (kem.PublicKey, kem.PrivateKey) {
}
h := xof.SHAKE256.New()
_, _ = h.Write(seed)
buf := make([]byte, sch.PrivateKeySize())
_, _ = h.Read(buf)
rnd := bytes.NewReader(buf)
key, x, y, err := elliptic.GenerateKey(sch.curve, rnd)
key, x, y, err := elliptic.GenerateKey(sch.curve, h)
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Is this a bugfix? Previously, only enough randomness was provided for exactly one "private key". After this change, it could sample another key if the first secret was invalid:

func GenerateKey(curve Curve, rand io.Reader) (priv []byte, x, y *big.Int, err error) {
	N := curve.Params().N
	bitSize := N.BitLen()
	byteLen := (bitSize + 7) / 8
	priv = make([]byte, byteLen)

	for x == nil {
		_, err = io.ReadFull(rand, priv)
// ...
		// If the scalar is out of range, sample another random number.
		if new(big.Int).SetBytes(priv).Cmp(N) >= 0 {
			continue
		}

		x, y = curve.ScalarBaseMult(priv)
	}

For reference, sch.PrivateKeySize() ends up with the same size:

func (sch *cScheme) scSize() int {
	return (sch.curve.Params().N.BitLen() + 7) / 8
}
// ...
func (sch *cScheme) PrivateKeySize() int {
	return sch.scSize()
}

if err != nil {
panic(err)
}
Expand Down