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