public boolean next()

in sshd-core/src/main/java/org/apache/sshd/client/kex/DHGClient.java [134:237]
92 lines of code
14 McCabe index (conditional complexity)

    public boolean next(int cmd, Buffer buffer) throws Exception {
        AbstractClientSession session = getClientSession();
        if (log.isDebugEnabled()) {
            log.debug("next({})[{}] process command={}",
                    this, session, KeyExchange.getSimpleKexOpcodeName(cmd));
        }

        if (cmd != SshConstants.SSH_MSG_KEXDH_REPLY) {
            throw new SshException(SshConstants.SSH2_DISCONNECT_KEY_EXCHANGE_FAILED,
                    "Protocol error: expected packet SSH_MSG_KEXDH_REPLY, got " + KeyExchange.getSimpleKexOpcodeName(cmd));
        }

        byte[] k_s = buffer.getBytes();
        byte[] f = updateF(buffer);
        byte[] sig = buffer.getBytes();

        if (kemClient == null) {
            dh.setF(f);
            k = normalize(dh.getK());
        } else {
            try {
                int l = kemClient.getEncapsulationLength();
                if (dh instanceof CurveSizeIndicator) {
                    int expectedLength = l + ((CurveSizeIndicator) dh).getByteLength();
                    if (f.length != expectedLength) {
                        throw new SshException(SshConstants.SSH2_DISCONNECT_KEY_EXCHANGE_FAILED,
                                "Wrong F length (should be " + expectedLength + " bytes): " + f.length);
                    }
                } else if (f.length <= l) {
                    throw new SshException(SshConstants.SSH2_DISCONNECT_KEY_EXCHANGE_FAILED,
                            "Strange F length: " + f.length + "  <= " + l);
                }
                dh.setF(Arrays.copyOfRange(f, l, f.length));
                Digest keyHash = dh.getHash();
                keyHash.init();
                keyHash.update(kemClient.extractSecret(Arrays.copyOf(f, l)));
                keyHash.update(dh.getK());
                k = keyHash.digest();
            } catch (IllegalArgumentException ex) {
                log.error("Key encapsulation error", ex);
                throw new SshException(SshConstants.SSH2_DISCONNECT_KEY_EXCHANGE_FAILED,
                        "Key encapsulation error: " + ex.getMessage());
            }
        }

        String keyAlg = session.getNegotiatedKexParameter(KexProposalOption.SERVERKEYS);

        boolean wantCert = KeyUtils.isCertificateAlgorithm(keyAlg);

        buffer = new ByteArrayBuffer(k_s);
        PublicKey serverKey = buffer.getRawPublicKey();
        PublicKey serverPublicHostKey = serverKey;

        if (serverKey instanceof OpenSshCertificate) {
            OpenSshCertificate openSshKey = (OpenSshCertificate) serverKey;
            if (!wantCert) {
                log.error("Got a server key certificate, but negotiated algorithm is {}", keyAlg);
                throw new SshException(SshConstants.SSH2_DISCONNECT_KEY_EXCHANGE_FAILED,
                        "OpenSshCertificate found with KEX algorithm " + keyAlg);
            }
            serverPublicHostKey = openSshKey.getCertPubKey();

            try {
                verifyCertificate(session, openSshKey);
            } catch (SshException e) {
                if (CoreModuleProperties.ABORT_ON_INVALID_CERTIFICATE.getRequired(session)) {
                    throw e;
                } else {
                    // ignore certificate
                    serverKey = openSshKey.getCertPubKey();
                    log.info("Ignoring invalid certificate {}", openSshKey.getId(), e);
                }
            }
        } else if (wantCert) {
            log.error("Got a plain public key (not a certificate) for negotiated algorithm {}", keyAlg);
            throw new SshException(SshConstants.SSH2_DISCONNECT_KEY_EXCHANGE_FAILED,
                    "Server did not send a certificate with KEX algorithm " + keyAlg);
        }

        buffer = new ByteArrayBuffer();
        buffer.putBytes(v_c);
        buffer.putBytes(v_s);
        buffer.putBytes(i_c);
        buffer.putBytes(i_s);
        buffer.putBytes(k_s);
        dh.putE(buffer, getE());
        dh.putF(buffer, f);
        buffer.putBytes(k);
        hash.update(buffer.array(), 0, buffer.available());
        h = hash.digest();

        Signature verif = ValidateUtils.checkNotNull(
                NamedFactory.create(session.getSignatureFactories(), keyAlg),
                "No verifier located for algorithm=%s", keyAlg);
        verif.initVerifier(session, serverPublicHostKey);
        verif.update(session, h);
        if (!verif.verify(session, sig)) {
            throw new SshException(SshConstants.SSH2_DISCONNECT_KEY_EXCHANGE_FAILED,
                    "KeyExchange signature verification failed for key type=" + keyAlg);
        }

        session.setServerKey(serverKey);
        return true;
    }