the highest bit of the last byte of the peer-sent X25519 pub

Error message

the highest bit of the last byte of the peer-sent X25519 public key is not 0

What it means

Error "the highest bit of the last byte of the peer-sent X25519 public key is not 0" thrown in XTLS/Xray-core.

Source

Thrown at proxy/vless/encryption/server.go:151

	var lastCTR cipher.Stream
	for j, k := range i.NfsSKeys {
		if lastCTR != nil {
			lastCTR.XORKeyStream(relays, relays[:32]) // recover this relay
		}
		index := 32
		if _, ok := k.(*mlkem.DecapsulationKey768); ok {
			index = 1088
		}
		if i.XorMode > 0 {
			NewCTR(i.NfsPKeysBytes[j], iv).XORKeyStream(relays, relays[:index]) // we don't use buggy elligator2, because we have PSK :)
		}
		if k, ok := k.(*ecdh.PrivateKey); ok {
			publicKey, err := ecdh.X25519().NewPublicKey(relays[:index])
			if err != nil {
				return nil, err
			}
			if publicKey.Bytes()[31] > 127 { // we just don't want the observer can change even one bit without breaking the connection, though it has nothing to do with security
				return nil, errors.New("the highest bit of the last byte of the peer-sent X25519 public key is not 0")
			}
			nfsKey, err = k.ECDH(publicKey)
			if err != nil {
				return nil, err
			}
		}
		if k, ok := k.(*mlkem.DecapsulationKey768); ok {
			var err error
			nfsKey, err = k.Decapsulate(relays[:index])
			if err != nil {
				return nil, err
			}
		}
		if j == len(i.NfsSKeys)-1 {
			break
		}
		relays = relays[index:]
		lastCTR = NewCTR(nfsKey, iv)

View on GitHub (pinned to 7d214f8b09)

When it happens

Trigger: Thrown at proxy/vless/encryption/server.go:151 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of XTLS/Xray-core@7d214f8b09 (2026-08-15). Data as JSON: /api/errors/0c32cf39879ad13d. Report an issue: GitHub.