Post-quantum TLS now supported in AWS KMS

AWS Key Management Service (AWS KMS) now supports post-quantum hybrid key exchange for the Transport Layer Security (TLS) network encryption protocol that is used when connecting to KMS API endpoints. In this post, I’ll tell you what post-quantum TLS is, what hybrid key exchange is, why it’s important, how to take advantage of this new feature, and how to give us feedback.

What is post-quantum TLS?

Post-quantum TLS is a feature that adds new, post-quantum cipher suites to the protocol. AWS implements TLS using s2n, a streamlined open source implementation of TLS. In June, 2019, AWS introduced post-quantum s2n, which implements two proposed post-quantum hybrid cipher suites specified in this IETF draft. The cipher suites specify a key exchange that provides the security protections of both the classical and post-quantum schemes.

Why is this important?

A large-scale quantum computer would break the current public key cryptography that is used for key exchange in every TLS connection. While a large-scale quantum computer is not available today, it’s still important to think about and plan for your long-term security needs. TLS traffic recorded today could be decrypted by a large-scale quantum computer in the future. If you’re developing applications that rely on the long-term confidentiality of data passed over a TLS connection, you should consider a plan to migrate to post-quantum cryptography before a large-scale quantum computer is available for use by potential adversaries. AWS is working to prepare for this future, and we want you to be well-prepared, too.

We’re offering this feature now instead of waiting so you’ll have a way to measure the potential performance impact to your applications, and you’ll have the additional benefit of the protection afforded by the proposed post-quantum schemes today. While we believe the use of this feature raises the already high security bar for connecting to KMS endpoints, these new cipher suites will have an impact on bandwidth utilization, latency, and could also create issues for intermediate systems that proxy TLS connections. We’d like to get feedback from you on the effectiveness of our implementation so we can improve it over time.

Some background on post-quantum TLS

Today, all requests to AWS KMS use TLS with one of two key exchange schemes:

FFDHE and ECDHE are industry standards for secure key exchange. KMS uses only ephemeral keys for TLS key negotiation; this ensures every connection uses a unique key and the compromise of one connection does not affect the security of another connection. They are secure today against known cryptanalysis techniques which use classic computers; however, they’re not secure against known attacks which use a large-scale quantum computer. In the future a sufficiently capable large-scale quantum computer could run Shor’s Algorithm to recover the TLS session key of a recorded session, and therefore gain access to the data inside. Protecting against a large-scale quantum computer requires using a post-quantum key exchange algorithm during the TLS handshake.

The possibility of large-scale quantum computing has spurred the development of new quantum-resistant cryptographic algorithms. The National Institute for Science and Technology (NIST) has started the process of standardizing post-quantum cryptographic algorithms. AWS contributed to two NIST submissions:

BIKE and SIKE are Key Encapsulation Mechanisms (KEMs); a KEM is a type of key exchange used to establish a shared symmetric key. Post-quantum s2n only uses ephemeral BIKE and SIKE keys.

The NIST standardization process isn’t expected to complete until 2024. Until then, there is a risk that the exclusive use of proposed algorithms like BIKE and SIKE could expose data in TLS connections to security vulnerabilities not yet discovered. To mitigate this risk and use these new post-quantum schemes safely today, we need a way to combine classical algorithms with the expected post-quantum security of the new algorithms submitted to NIST. The Hybrid Post-Quantum Key Encapsulation Methods for Transport Layer Security 1.2 IETF draft describes how to combine BIKE and SIKE with ECDHE to create two new cipher suites for TLS.

These two cipher suites use a hybrid key exchange that performs two independent key exchanges during the TLS handshake and then cryptographically combines the keys into a single TLS session key. This strategy combines the high assurance of a classical key exchange with the security of the proposed post-quantum key exchanges.

The effect of hybrid post-quantum TLS on performance

Post-quantum cipher suites have a different performance profile and bandwidth requirements than traditional cipher suites. We measured the latency and bandwidth for a single handshake on an EC2 C5 2x.large. This provides a baseline for what to expect when you connect to KMS with the SDK. Your exact results will depend on your hardware (CPU speed and number of cores), existing workloads (how often you call KMS and what other work your application performs), and your network (location and capacity).

BIKE and SIKE have different performance tradeoffs: BIKE has faster computations and large keys, and SIKE has slower computations and smaller keys. The tables below show the results of the AWS measurements. ECDHE, a classic cryptographic key exchange algorithm, is included by itself for comparison.

Table 1
TLS Message ECDHE (bytes) ECDHE w/ BIKE (bytes) ECDHE w/SIKE (bytes)
ClientHello 139 147 147
ServerKeyExchange 329 2,875 711
ClientKeyExchange 66 2,610 470

Table 1 shows the amount of data (in bytes) sent in each TLS message. The ClientHello message is larger for post-quantum cipher suites because they include a new ClientHello extension. The key exchange messages are larger because they include BIKE or SIKE messages.

Table 2
Item ECDHE (ms) ECDHE w/ BIKE (ms) ECDHE w/SIKE (ms)
Server processing time 0.11 20.26 95.53
Client processing time 0.10 0.39 57.05
Total handshake time 1.19 25.58 155.08

Table 2 shows the time (in milliseconds) a client and server in the same region take to complete a handshake. Server processing time includes: key generation, signing the server key exchange message, and processing the client key exchange message. The client processing time includes: verifying the server’s certificate, processing the server key exchange message, and generating the client key exchange message. The total time was measured on the client from the start of the handshake to the end and includes network transfer time. All connections used RSA authentication with a 2048-bit key, and ECDHE used the secp256r1 curve. The BIKE test used the BIKE-1 Level 1 parameter and the SIKE test used the SIKEp503 parameter.

A TLS handshake is only performed once to setup a new connection. The SDK will reuse connections for multiple KMS requests when possible. This means that you don’t want to include measurements of subsequent round-trips under an existing TLS session, otherwise you will skew your performance data.

How to use hybrid post-quantum cipher suites

Note: The “AWS CRT HTTP Client” in the aws-crt-dev-preview branch of the aws-sdk-java-v2 repository is a beta release. This beta release and your use are subject to Section 1.10 (“Beta Service Participation”) of the AWS Service Terms.

To use the post-quantum cipher suites with AWS KMS, you’ll need the Developer Previews of the Java SDK 2.0 and the AWS Common Runtime. You’ll need to configure the AWS Common Runtime HTTP client to use s2n’s post-quantum hybrid cipher suites, and configure the AWS Java SDK 2.0 to use that HTTP client. This client can then be used when connecting to any KMS endpoints, but only those endpoints that are not using FIPS 140-2 validated crypto for the TLS termination. For example, kms.<region>.amazonaws.com supports the use of post-quantum cipher suites, while kms-fips.<region>.amazonaws.com does not.

To see a complete example of everything setup check out the example application here.
 

Figure 1: GitHub and package layout

Figure 1: GitHub and package layout

Figure 1 shows the GitHub and package layout. The steps below will walk you through building and configuring the SDK.

  1. Download the Java SDK v2 Common Runtime Developer Preview:
  2. Build the aws-crt-client JAR:
    
    $ mvn install -Pquick
    
  3. In your project add the AWS Common Runtime client to your Maven Dependencies:
    
    <dependency> <groupId>software.amazon.awssdk</groupId> <artifactId>aws-crt-client</artifactId> <version>2.10.7-SNAPSHOT</version>
    </dependency>
    
  4. Configure the new SDK and cipher suite in your application’s existing initialization code:
    
    if(!TLS_CIPHER_KMS_PQ_TLSv1_0_2019_06.isSupported()){ throw new RuntimeException("Post Quantum Ciphers not supported on this Platform");
    }
    SdkAsyncHttpClient awsCrtHttpClient = AwsCrtAsyncHttpClient.builder() .tlsCipherPreference(TLS_CIPHER_KMS_PQ_TLSv1_0_2019_06) .build();
    KmsAsyncClient kms = KmsAsyncClient.builder() .httpClient(awsCrtHttpClient) .build();
    ListKeysResponse response = kms.listKeys().get();
    

Now, all connections made to AWS KMS in supported regions will use the new hybrid post-quantum cipher suites.

Things to try

Here are some ideas about how to use this post-quantum-enabled client:

  • Run load tests and benchmarks. These new cipher suites perform differently than traditional key exchange algorithms. You might need to adjust your connection timeouts to allow for the longer handshake times or, if you’re running inside an AWS Lambda function, extend the execution timeout setting.
  • Try connecting from different locations. Depending on the network path your request takes, you might discover that intermediate hosts, proxies, or firewalls with deep packet inspection (DPI) block the request. This could be due to the new cipher suites in the ClientHello or the larger key exchange messages. If this is the case, you might need to work with your Security team or IT administrators to update the relevant configuration to unblock the new TLS cipher suites. We’d like to hear from your about how your infrastructure interacts with this new variant of TLS traffic.

More info

If you’re interested to learn more about post-quantum cryptography check out:

Conclusion

In this blog post, I introduced you to the topic of post-quantum security and covered what AWS and NIST are doing to address the issue. I also showed you how to begin experimenting with hybrid post-quantum key exchange algorithms for TLS when connecting to KMS endpoints.

If you have feedback about this blog post, submit comments in the Comments section below. If you have questions about how to configure the HTTP client or its interaction with KMS endpoints, please start a new thread on the AWS KMS discussion forum.