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package x25519
import (
"crypto/subtle"
fp "github.com/cloudflare/circl/math/fp25519"
)
// Size is the length in bytes of a X25519 key.
const Size = 32
// Key represents a X25519 key.
type Key [Size]byte
func (k *Key) clamp(in *Key) *Key {
*k = *in
k[0] &= 248
k[31] = (k[31] & 127) | 64
return k
}
// isValidPubKey verifies if the public key is not a low-order point.
func (k *Key) isValidPubKey() bool {
fp.Modp((*fp.Elt)(k))
isLowOrder := false
for _, P := range lowOrderPoints {
isLowOrder = isLowOrder || subtle.ConstantTimeCompare(P[:], k[:]) != 0
}
return !isLowOrder
}
// KeyGen obtains a public key given a secret key.
func KeyGen(public, secret *Key) {
ladderJoye(public.clamp(secret))
}
// Shared calculates Alice's shared key from Alice's secret key and Bob's
// public key returning true on success. A failure case happens when the public
// key is a low-order point, thus the shared key is all-zeros and the function
// returns false.
func Shared(shared, secret, public *Key) bool {
validPk := *public
validPk[31] &= (1 << (255 % 8)) - 1
ok := validPk.isValidPubKey()
ladderMontgomery(shared.clamp(secret), &validPk)
return ok
}
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