Amazon Web Services (AWS) prioritizes the security, privacy, and performance of its services. AWS is responsible for the security of the cloud and the services it offers, and customers own the security of the hosts, applications, and services they deploy in the cloud. AWS has also been introducing quantum-resistant key exchange in common transport protocols used by our customers in order to provide long-term confidentiality. In this blog post, we elaborate how customer compliance and security configuration responsibility will operate in the post-quantum migration of secure connections to the cloud. We explain how customers are responsible for enabling quantum-resistant algorithms or having these algorithms enabled by default in their applications that connect to AWS. We also discuss how AWS will honor and choose these algorithms (if they are supported on the server side) even if that means the introduction of a small delay to the connection.

Secure connectivity

Security and compliance is a shared responsibility between AWS and the customer. This Shared Responsibility Model can help relieve the customer’s operational burden as AWS operates, manages, and controls the components from the host operating system and virtualization layer down to the physical security of the facilities in which the service operates. The customer assumes responsibility and management of the guest operating system and other associated application software, as well as the configuration of the AWS provided security group firewall. AWS has released Customer Compliance Guides (CCGs) to support customers, partners, and auditors in their understanding of how compliance requirements from leading frameworks map to AWS service security recommendations.

In the context of secure connectivity, AWS makes available secure algorithms in encryption protocols (for example, TLS, SSH, and VPN) for customers that connect to its services. That way AWS is responsible for enabling and prioritizing modern cryptography in connections to the AWS Cloud. Customers, on the other hand, use clients that enable such algorithms and negotiate cryptographic ciphers when connecting to AWS. It is the responsibility of the customer to configure or use clients that only negotiate the algorithms the customer prefers and trusts when connecting.

Prioritizing quantum-resistance or performance?

AWS has been in the process of migrating to post-quantum cryptography in network connections to AWS services. New cryptographic algorithms are designed to protect against a future cryptanalytically relevant quantum computer (CRQC) which could threaten the algorithms we use today. Post-quantum cryptography involves introducing post-quantum (PQ) hybrid key exchanges in protocols like TLS 1.3 or SSH/SFTP. Because both classical and PQ-hybrid exchanges need to be supported for backwards compatibility, AWS will prioritize PQ-hybrid exchanges for clients that support it and classical for clients that have not been upgraded yet. We don’t want to switch a client to classical if it advertises support for PQ.

PQ-hybrid key establishment leverages quantum-resistant key encapsulation mechanisms (KEMs) used in conjunction with classical key exchange. The client and server still do an ECDH key exchange, which gets combined with the KEM shared secret when deriving the symmetric key. For example, clients could perform an ECDH key exchange with curve P256 and post-quantum Kyber-768 from NIST’s PQC Project Round 3 (TLS group identifier X25519Kyber768Draft00) when connecting to AWS Certificate Manager (ACM), AWS Key Management Service (AWS KMS), and AWS Secrets Manager. This strategy combines the high assurance of a classical key exchange with the quantum-resistance of the proposed post-quantum key exchanges, to help ensure that the handshakes are protected as long as the ECDH or the post-quantum shared secret cannot be broken. The introduction of the ML-KEM algorithm adds more data (2.3 KB) to be transferred and slightly more processing overhead. The processing overhead is comparable to the existing ECDH algorithm, which has been used in most TLS connections for years. As shown in the following table, the total overhead of hybrid key exchanges has been shown to be immaterial in typical handshakes over the Internet. (Sources: Blog posts How to tune TLS for hybrid post-quantum cryptography with Kyber and The state of the post-quantum Internet)

The new key exchanges introduce some unique conceptual choices that we didn’t have before, which could lead to the peers negotiating classical-only algorithms. In the past, our cryptographic protocol configurations involved algorithms that were widely trusted to be secure. The client and server configured a priority for their algorithms of choice and they picked the more appropriate ones from their negotiated prioritized order. Now, the industry has two families of algorithms, the “trusted classical” and the “trusted post-quantum” algorithms. Given that a CRQC is not available, both classical and post-quantum algorithms are considered secure. Thus, there is a paradigm shift that calls for a decision in the priority vendors should enforce on the client and server configurations regarding the “secure classical” or “secure post-quantum” algorithms.

Figure 1 shows a typical PQ-hybrid key exchange in TLS.

Figure 1: A typical TLS 1.3 handshake

In the example in Figure 1, the client advertises support for PQ-hybrid algorithms with ECDH curve P256 and quantum-resistant ML-KEM-768, ECDH curve P256 and quantum-resistant Kyber-512 Round 3, and classical ECDH with P256. The client also sends a Keyshare value for classical ECDH with P256 and for PQ-hybrid P256+MLKEM768. The Keyshare values include the client’s public keys. The client does not include a Keyshare for P256+Kyber512, because that would increase the size of the ClientHello unnecessarily and because ML-KEM-768 is the ratified version of Kyber Round 3, and so the client chose to only generate and send a P256+MLKEM768 public key. Now let’s say that the server supports ECDH curve P256 and PQ-hybrid P256+Kyber512, but not P256+MLKEM768. Given the groups and the Keyshare values the client included in the ClientHello, the server has the following two options:

1. Use the client P256 Keyshare to negotiate a classical key exchange, as shown in Figure 1. Although one might assume that the P256+Kyber512 Keyshare could have been used for a quantum-resistant key exchange, the server can pick to negotiate only classical ECDH key exchange with P256, which is not resistant to a CRQC.
2. Send a Hello Retry Request (HRR) to tell the client to send a PQ-hybrid Keyshare for P256+Kyber512 in a new ClientHello (Figure 2). This introduces a round trip, but it also forces the peers to negotiate a quantum-resistant symmetric key.

Note: A round-trip could take 30-50 ms in typical Internet connections.

Previously, some servers were using the Keyshare value to pick the key exchange algorithm (option 1 above). This generally allowed for faster TLS 1.3 handshakes that did not require an extra round-trip (HRR), but in the post-quantum scenario described earlier, it would mean the server does not negotiate a quantum-resistant algorithm even though both peers support it.

Such scenarios could arise in cases where the client and server don’t deploy the same version of a new algorithm at the same time. In the example in Figure 1, the server could have been an early adopter of the post-quantum algorithm and added support for P256+Kyber512 Round 3. The client could subsequently have upgraded to the ratified post-quantum algorithm with ML-KEM (P256+MLKEM768). AWS doesn’t always control both the client and the server. Some AWS services have adopted the earlier versions of Kyber and others will deploy ML-KEM-768 from the start. Thus, such scenarios could arise while AWS is in the post-quantum migration phase.

Note: In these cases, there won’t be a connection failure; the side-effect is that the connection will use classical-only algorithms although it could have negotiated PQ-hybrid.

These intricacies are not specific to AWS. Other industry peers have been thinking about these issues, and they have been a topic of discussion in the Internet Engineering Task Force (IETF) TLS Working Group. The issue of potentially negotiating a classical key exchange although the client and server support quantum-resistant ones is discussed in the Security Considerations of the TLS Key Share Prediction draft (draft-davidben-tls-key-share-prediction). To address some of these concerns, the Transport Layer Security (TLS) Protocol Version 1.3 draft (draft-ietf-tls-rfc8446bis), which is the draft update of TLS 1.3 (RFC 8446), introduces text about client and server behavior when choosing key exchange groups and the use of Keyshare values in Section 4.2.8. The TLS Key Share Prediction draft also tries to address the issue by providing DNS as a mechanism for the client to use a proper Keyshare that the server supports.

Prioritizing quantum resistance

In a typical TLS 1.3 handshake, the ClientHello includes the client’s key exchange algorithm order of preferences. Upon receiving the ClientHello, the server responds by picking the algorithms based on its preferences.

Figure 2 shows how a server can send a HelloRetryRequest (HRR) to the client in the previous scenario (Figure 1) in order to request the negotiation of quantum-resistant keys by using P256+Kyber512. This approach introduces an extra round trip to the handshake.

Figure 2: An HRR from the server to request the negotiation of mutually supported quantum-resistant keys with the client

AWS services that terminate TLS 1.3 connections will take this approach. They will prioritize quantum resistance for clients that advertise support for it. If the AWS service has added quantum-resistant algorithms, it will honor a client-supported post-quantum key exchange even if that means that the handshake will take an extra round trip and the PQ-hybrid key exchange will include minor processing overhead (ML-KEM is almost performant as ECDH). A typical round trip in regionalized TLS connections today is usually under 50 ms and won’t have material impact to the connection performance. In the post-quantum transition, we consider clients that advertise support for quantum-resistant key exchange to be clients that take the CRQC risk seriously. Thus, the AWS server will honor that preference if the server supports the algorithm.

Pull Request 4526 introduces this behavior in s2n-tls, the AWS open source, efficient TLS library built over other crypto libraries like OpenSSL libcrypto or AWS libcrypto (AWS-LC). When built with s2n-tls, s2n-quic handshakes will also inherit the same behavior. s2n-quic is the AWS open source Rust implementation of the QUIC protocol.

What AWS customers can do to verify post-quantum key exchanges

AWS services that have already adopted the behavior described in this post include AWS KMS, ACM, and Secrets Manager TLS endpoints, which have been supporting post-quantum hybrid key exchange for a few years already. Other endpoints that will deploy quantum-resistant algorithms will inherit the same behavior.

AWS customers that want to take advantage of new quantum-resistant algorithms introduced in AWS services are expected to enable them on the client side or the server side of a customer-managed endpoint. For example, if you are using the AWS Common Runtime (CRT) HTTP client in the AWS SDK for Java v2, you would need to enable post-quantum hybrid TLS key exchanges with the following.

SdkAsyncHttpClient awsCrtHttpClient = AwsCrtAsyncHttpClient.builder()
            .postQuantumTlsEnabled(true)
            .build();

The AWS KMS and Secrets Manager documentation includes more details for using the AWS SDK to make HTTP API calls over quantum-resistant connections to AWS endpoints that support post-quantum TLS.

To confirm that a server endpoint properly prioritizes and enforces the PQ algorithms, you can use an “old” client that sends a PQ-hybrid Keyshare value that the PQ-enabled server does not support. For example, you could use s2n-tls built with AWS-LC (which supports the quantum-resistant KEMs). You could use a client TLS policy (PQ-TLS-1-3-2023-06-01) that is newer than the server’s policy (PQ-TLS-1-0-2021-05-24). That will lead the server to request the client by means of an HRR to send a new ClientHello that includes P256+MLKEM768, as shown following.

./bin/s2nd -c PQ-TLS-1-0-2021-05-24 localhost 4444
sudo tcpdump port 4444 -w hrr-capture.pcap
./bin/s2nc localhost 4444 -c PQ-TLS-1-3-2023-06-01 -i

The hrr-capture.pcap packet capture will show the negotiation and the HRR from the server.

To confirm that a server endpoint properly implements the post-quantum hybrid key exchanges, you can use a modern client that supports the key exchange and connect against the endpoint. For example, using the s2n-tls client built with AWS-LC (which supports the quantum-resistant KEMs), you could try connecting to a Secrets Manager endpoint by using a post-quantum TLS policy (for example, PQ-TLS-1-2-2023-12-15) and observe the PQ hybrid key exchange used in the output, as shown following.

./bin/s2nc -c PQ-TLS-1-2-2023-12-15 secretsmanager.us-east-1.amazonaws.com 443
CONNECTED:
Handshake: NEGOTIATED|FULL_HANDSHAKE|MIDDLEBOX_COMPAT
Client hello version: 33
Client protocol version: 34
Server protocol version: 34
Actual protocol version: 34
Server name: secretsmanager.us-east-1.amazonaws.com
Curve: NONE
KEM: NONE
KEM Group: SecP256r1Kyber768Draft00
Cipher negotiated: TLS_AES_128_GCM_SHA256
Server signature negotiated: RSA-PSS-RSAE+SHA256
Early Data status: NOT REQUESTED
Wire bytes in: 6699
Wire bytes out: 1674
s2n is ready
Connected to secretsmanager.us-east-1.amazonaws.com:443

An alternative would be using the Open Quantum Safe (OQS) for OpenSSL client to do the same.

As another example, if you want to transfer a file over a quantum-resistant SFTP connection with AWS Transfer Family, you would need to configure a PQ cryptography SSH security policy on your AWS File Transfer SFTP endpoint (for example, TransferSecurityPolicy-2024-01) and enable quantum-resistant SSH key exchange in the SFTP client. Note that in SSH/SFTP, although the AWS server side will advertise the quantum-resistant schemes as higher priority, the client picks the key exchange algorithm. So, a client that supports PQ cryptography would need to have the PQ algorithms configured with higher priority (as described in the Shared Responsibility Model). For more details, refer to the AWS Transfer Family documentation.

Conclusion

Cryptographic migrations can introduce intricacies to cryptographic negotiations between clients and servers. During the migration phase, AWS services will mitigate the risks of these intricacies by prioritizing post-quantum algorithms for customers that advertise support for these algorithms—even if that means a small slowdown in the initial negotiation phase. While in the post-quantum migration phase, customers who choose to enable quantum resistance have made a choice which shows that they consider the CRQC risk as important. To mitigate this risk, AWS will honor the customer’s choice, assuming that quantum resistance is supported on the server side.

If you have feedback about this post, submit comments in the Comments section below. If you have questions about this post, start a new thread on the AWS Security, Identity, & Compliance re:Post or contact AWS Support. For more details regarding AWS PQC efforts, refer to our PQC page.