Introduction to TLS and SSL

Introduction to TLS and SSL

There are situations where there is a security requirement to assure the confidentiality or integrity of network traffic in an OpenStack deployment. This is generally achieved using cryptographic measures, such as the Transport Layer Security (TLS) protocol.

In a typical deployment all traffic transmitted over public networks is secured, but security best practice dictates that internal traffic must also be secured. It is insufficient to rely on security domain separation for protection. If an attacker gains access to the hypervisor or host resources, compromises an API endpoint, or any other service, they must not be able to easily inject or capture messages, commands, or otherwise affect the management capabilities of the cloud.

All domains should be secured with TLS, including the management domain services and intra-service communications. TLS provides the mechanisms to ensure authentication, non-repudiation, confidentiality, and integrity of user communications to the OpenStack services and between the OpenStack services themselves.

Due to the published vulnerabilities in the Secure Sockets Layer (SSL) protocols, we strongly recommend that TLS is used in preference to SSL, and that SSL is disabled in all cases, unless compatibility with obsolete browsers or libraries is required.

Public Key Infrastructure (PKI) is the framework for securing communication in a network. It consists of a set of systems and processes to ensure traffic can be sent securely while validating the identity of the parties. The PKI profile described here is the Internet Engineering Task Force (IETF) Public Key Infrastructure (PKIX) profile developed by the PKIX working group. The core components of PKI are:

Digital Certificates

Signed public key certificates are data structures that have verifiable data of an entity, its public key along with some other attributes. These certificates are issued by a Certificate Authority (CA). As the certificates are signed by a CA that is trusted, once verified, the public key associated with the entity is guaranteed to be associated with the said entity. The most common standard used to define these certificates is the X.509 standard. The X.509 v3 which is the current standard is described in detail in RFC5280. Certificates are issued by CAs as a mechanism to prove the identity of online entities. The CA digitally signs the certificate by creating a message digest from the certificate and encrypting the digest with its private key.

End entity

User, process, or system that is the subject of a certificate. The end entity sends its certificate request to a Registration Authority (RA) for approval. If approved, the RA forwards the request to a Certification Authority (CA). The Certification Authority verifies the request and if the information is correct, a certificate is generated and signed. This signed certificate is then send to a Certificate Repository.

Relying party

The endpoint that receives the digitally signed certificate that is verifiable with reference to the public key listed on the certificate. The relying party should be in a position to verify the certificate up the chain, ensure that it is not present in the CRL and also must be able to verify the expiry date on the certificate.

Certification Authority (CA)

CA is a trusted entity, both by the end party and the party that relies upon the certificate for certification policies, management handling, and certificate issuance.

Registration Authority (RA)

An optional system to which a CA delegates certain management functions, this includes functions such as, authentication of end entities before they are issued a certificate by a CA.

Certificate Revocation Lists (CRL)

A Certificate Revocation List (CRL) is a list of certificate serial numbers that have been revoked. End entities presenting these certificates should not be trusted in a PKI model. Revocation can happen because of several reasons for example, key compromise, CA compromise.

CRL issuer

An optional system to which a CA delegates the publication of certificate revocation lists.

Certificate Repository

Where the end entity certificates and certificate revocation lists are stored and looked up - sometimes referred to as the certificate bundle.

PKI builds the framework on which to provide encryption algorithms, cipher modes, and protocols for securing data and authentication. We strongly recommend securing all services with Public Key Infrastructure (PKI), including the use of TLS for API endpoints. It is impossible for the encryption or signing of transports or messages alone to solve all these problems. Hosts themselves must be secure and implement policy, namespaces, and other controls to protect their private credentials and keys. However, the challenges of key management and protection do not reduce the necessity of these controls, or lessen their importance.

Certification authorities

Many organizations have an established Public Key Infrastructure with their own Certification Authority (CA), certificate policies, and management for which they should use to issue certificates for internal OpenStack users or services. Organizations in which the public security domain is Internet facing will additionally need certificates signed by a widely recognized public CA. For cryptographic communications over the management network, it is recommended one not use a public CA. Instead, we expect and recommend most deployments deploy their own internal CA.

It is recommended that the OpenStack cloud architect consider using separate PKI deployments for internal systems and customer facing services. This allows the cloud deployer to maintain control of their PKI infrastructure and among other things makes requesting, signing and deploying certificates for internal systems easier. Advanced configurations may use separate PKI deployments for different security domains. This allows deployers to maintain cryptographic separation of environments, ensuring that certificates issued to one are not recognized by another.

Certificates used to support TLS on internet facing cloud endpoints (or customer interfaces where the customer is not expected to have installed anything other than standard operating system provided certificate bundles) should be provisioned using Certificate Authorities that are installed in the operating system certificate bundle. Typical well known vendors include Let’s Encrypt, Verisign and Thawte but many others exist.

There are management, policy, and technical challenges around creating and signing certificates. This is an area where cloud architects or operators may wish to seek the advice of industry leaders and vendors in addition to the guidance recommended here.

TLS libraries

Components, services, and applications within the OpenStack ecosystem or dependencies of OpenStack are implemented or can be configured to use TLS libraries. The TLS and HTTP services within OpenStack are typically implemented using OpenSSL which has a module that has been validated for FIPS 140-2. However, keep in mind that each application or service can still introduce weaknesses in how they use the OpenSSL libraries.

Cryptographic algorithms, cipher modes, and protocols

We recommend that only TLS 1.2 is used. Other versions such as TLS 1.0 and 1.1 are vulnerable to multiple attacks. TLS 1.0 should be disabled in your environment. TLS 1.1 may be used for broad client compatibility, however exercise caution when enabling this protocol. Only enable TLS version 1.1 if there is a mandatory compatibility requirement and you are aware of the risks involved. All versions of SSL, the predecessor to TLS, must not be used due to multiple public vulnerabilities.

When you are using TLS 1.2 and control both the clients and the server, the cipher suite should be limited to ECDHE-ECDSA-AES256-GCM-SHA384. In circumstances where you do not control both endpoints and are using TLS 1.1 or 1.2 the more general HIGH:!aNULL:!eNULL:!DES:!3DES:!SSLv3:!TLSv1:!CAMELLIA is a reasonable cipher selection.

However, as this book does not intend to be a thorough reference on cryptography we do not wish to be prescriptive about what specific algorithms or cipher modes you should enable or disable in your OpenStack services. There are some authoritative references we would like to recommend for further information:

Summary

Given the complexity of the OpenStack components and the number of deployment possibilities, you must take care to ensure that each component gets the appropriate configuration of TLS certificates, keys, and CAs. Subsequent sections discuss the following services:

  • Compute API endpoints

  • Identity API endpoints

  • Networking API endpoints

  • Storage API endpoints

  • Messaging server

  • Database server

  • Dashboard

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