]> TLS 1.3 Authentication and Integrity only Ciphersuites Cisco Systems
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The ciphersuites defined in this document are intended for a small limited set of applications where confidentiality requirements are relaxed and the need to minimize the cryptographic algorithms are prioritized. This section describes some of those applicable use cases. Use cases in the industrial automation industry, while requiring data integrity, relax the confidential communications requirement. Mainly, information communicated to unmanned machines to execute repetitive tasks do not convey private information. For example, there could be a system with a robotic arm that is doing high speed pick-and-place of materials. The position synchronization data and motion commands are required to have very low latency, as the process needs to be done at high speed on a compute and memory constrained device. However, information such as the position, speed, acceleration of the robotic arm or other material in the system is not confidential. That is, while an attacker can determine the behavioral aspects and task of the device; no intellectual property concerns or data privacy concerns exist for these communications. However, data integrity is required as being able to modify this data would be a threat that an attacker might seek to exploit with serious consequences; the attacker could modify the motion information in order to cause physical damage to the equipment. Another use case which is closely related is that of fine grained time updates. Motion systems often rely on time synchronization to ensure proper execution. Time updates are essentially public, there is no threat from an attacker knowing the time update information. This should make intuitive sense to those not familiar with these applications; rarely if ever does time information present a serious attack surface dealing with privacy. However the authenticity is still quite important. Modification of the data can at best lead to a denial-of-service attack, although a more intelligent threat actor might be able to cause actual physical damage. As these time synchronization updates are very fine-grained, it is again important for latency to be very low. A third use case deals with Alarming data. Industrial control sensing equipment can be configured to send alarm information when it meets certain conditions. Often times this data is used to detect certain out-of-tolerance conditions, allowing an operator or automated system to take corrective action. Once again, in many systems the reading of this data doesn't grant the attacker information that can be exploited, it is generally just information regarding the physical state of the system. At the same time, being able to modify this data would allow an attacker to either trigger alarms falsely or to cover up evidence of an attack that might allow for detection of their malicious activity. Furthermore, sensors are often low powered devices that might struggle to process encrypted and authenticated data. Sending data that is just authenticated significantly eases the burden placed on these devices, yet still allows the data to be protected against any tampering threats. A fourth use case considers the protection of commands in the railway industry. In railway control systems, no confidentiality requirements are applied for the command exchange between an interlocking controller and a railway equipment controller (for instance, a railway point controller of a tram track where the position of the controlled point is publicly available). However, protecting integrity of those commands is vital, otherwise, an adversary could change the target position of the point by modifying the commands, which consequently could lead to the derailment of a passing train. Furthermore, requirements for providing blackbox recording of the safety related network traffic can only be fulfilled through using integrity only ciphers, to be able to provide the safety related commands to a third party, which is responsible for the analysis after an accident. The above use cases describe the relaxed requirements to provide confidentiality, and as these devices come with a small runtime memory footprint and reduced processing power, the need to minimize the number of cryptographic algorithms used is prioritized.
This document defines the following cipher suites for use in TLS 1.3: {0xC0, 0xB4} {0xC0, 0xB5} These cipher suites allow the use of SHA-256 or SHA-384 as the HMACs for data integrity protection as well as its use for HKDF. The authentication mechanisms remain unchanged with the intent to only update the cipher suites to relax the need for confidentiality.
The record payload protection as defined in can be retained when integrity only cipher suites are used. This section describes the mapping of record payload structures when integrity only cipher suites are employed. As integrity is provided with protection over the full record, the encrypted_record in the TLSCiphertext along with the additional_data input to AEADEncrypted as defined in Section 5.2 remains the same. The TLSCiphertext.length for the integrity cipher suites will be: TLSPlaintext.length + 32 TLSPlaintext.length + 48 The resulting encrypted_record is the concatenation of the TLSPlaintext with the resulting HMAC. With this mapping, the decrypt order as defined in Section 5.2 of remains the same. That is, the HMAC operation is of the form: AEAD-Encrypt-HMAC(write_key, nonce, additional_data, plaintext) = plaintext || HMAC(write_key, nonce || additional_data || plaintext) The encrypt and decrypt operations provide the integrity protection using HMAC SHA-256 or SHA-384 as described in .
IANA has granted registration the following specifically for this document: TLS_SHA256_SHA256 {0xC0, 0xB4} cipher suite and TLS_SHA384_SHA384 {0xC0, 0xB5} cipher suite. Note that both of these cipher suites are registered with the DTLS-OK column set to Y and the Recommneded column set to N
In general, with the exception of confidentiality and privacy, the security considerations detailed in and in apply to this document. Furthermore, as the cipher suites described in this document do not provide any confidentiality, it is important that they only be used in cases where there are no confidentiality or privacy requirements and concerns; and the runtime memory requirements can accommodate support for more cryptographic constructs. With the lack of data encryption specified in this draft, no confidentiality or privacy is provided for the data transported via the TLS session. To highlight the loss of privacy, the information carried in both the Server and Client certificates, while integrity protected, will be sent unencrypted. Similarly, other TLS extensions that may be carried in the Server's EncryptedExtensions message will only be integrity protected without provisions for confidentiality. Given the lack of confidentiality, it is of the utmost importance that these cipher suites never be enabled by default. As these cipher suites are meant to serve the IoT market, it is important that any IoT endpoint that uses them be explicitly configured with a policy of non-confidential communications.
The authors would like to acknowledge the work done by Industrial Communications Standards Groups (such as ODVA) as the motivation for this document. We would also like to thank Steffen Fries for providing a fourth use case. In addition, we are grateful for the advice and feedback from Joe Salowey, Blake Anderson and David McGrew.
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