Internet-Draft Operator Privacy April 2020
Moskowitz, et al. Expires 4 October 2020 [Page]
Intended Status:
Standards Track
R. Moskowitz
HTT Consulting
S. Card
AX Enterprize
A. Wiethuechter
AX Enterprize

Operator Privacy for RemoteID Messages


This document describes a method of providing privacy for Operator information specified in the ASTM UAS Remote ID and Tracking messages. This is achieved by encrypting, in place, those fields containing Operator sensitive data using a hybrid ECIES.

Status of This Memo

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This Internet-Draft will expire on 4 October 2020.

Table of Contents

1. Introduction

This document defines a mechanism to provide privacy in the ASTM Remote ID and Tracking messages [F3411-19] by encrypting, in place, those fields that contain sensitive Operator information. An example of such, and the initial application of this mechanism is the 8 bytes of Operator longitude and latitude location in the System Message.

It is assumed that the Operator registers a mission with a USS. During this mission registration, the Operator and USS exchange public keys to use in the hybrid ECIES. The USS key may be long lived, but the Operator key SHOULD be unique to a specific mission. This provides protection if the ECIES secret is exposed from prior missions.

The actual Tracking message field encryption MUST be an "encrypt in place" cipher. There is rarely any room in the tracking messages for a cipher IV or encryption MAC. There is rarely any data in the messages that can be used as an IV. A number of ciphers are proposed here that can encrypt exactly 64 bits. It is not a simple, encrypt these 64 bits with these ECIES derived key. The Operator may move during a mission and these fields change, correspondingly. Further, not all messages will be received by the USS, so each message's encryption must stand on its own, but not be at risk of attack by the content of other messages.

Future applications of this mechanism may be provided. The content of the System Message may change, requiring encrypting a different amount of data. At that time, they will be added to this document.

2. Terms and Definitions

2.1. Requirements Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2.2. Definitions

Broadcast Remote ID. A method of sending RID messages as 1-way transmissions from the UA to any Observers within radio range.
Civil Aeronautics Administration. An example is the Federal Aviation Administration (FAA) in the United States of America.
Elliptic Curve Integrated Encryption Scheme. A hybrid encryption scheme which provides semantic security against an adversary who is allowed to use chosen-plaintext and chosen-ciphertext attacks.
Ground Control Station. The part of the UAS that the remote pilot uses to exercise C2 over the UA, whether by remotely exercising UA flight controls to fly the UA, by setting GPS waypoints, or otherwise directing its flight.
Referred to in other UAS documents as a "user", but there are also other classes of RID users, so we prefer "observer" to denote an individual who has observed an UA and wishes to know something about it, starting with its RID.
Network Remote ID. A method of sending RID messages via the Internet connection of the UAS directly to the UTM.
Remote ID. A unique identifier found on all UA to be used in communication and in regulation of UA operation.
Unmanned Aircraft. In this document UA's are typically though of as drones of commercial or military variety. This is a very strict definition which can be relaxed to include any and all aircraft that are unmanned.
Unmanned Aircraft System. Composed of Unmanned Aircraft and all required on-board subsystems, payload, control station, other required off-board subsystems, any required launch and recovery equipment, all required crew members, and C2 links between UA and the control station.
UAS Service Supplier. Provide UTM services to support the UAS community, to connect Operators and other entities to enable information flow across the USS network, and to promote shared situational awareness among UTM participants. (From FAA UTM ConOps V1, May 2018).
UAS Traffic Management. A "traffic management" ecosystem for uncontrolled operations that is separate from, but complementary to, the FAA's Air Traffic Management (ATM) system.

3. The Operator - USS Security Relationship

All CAAs have rules defining which UAS must be registered to operate in their National Airspace. This includes UAS and Operator registration in a USS. Further, operator's are expected to report flight missions to their USS. This mission reporting provides a mechanism for the USS and operator to establish a mission security context. Here it will be used to exchange public keys for use in ECIES.

The operator's public key SHOULD be unique for each mission. The USS public key may be unique for each operator and mission, but not required. For best post-compromise security (PCS), even the USS public key should be changed over some operational window.

The public key algorithm should be Curve25519 [RFC7748]. Correspondingly, the ECIES 128 bit shared secret should be generated using KMAC as specified in sec 5 of [new-crypto].

4. System Message Privacy

The System Message contains 8 bytes of Operator specific information: Longitude and Latitude of the Remote Pilot of the UA. The GCS can encrypt these as follows.

The 8 bytes of Operator information are encrypted, using the ECIES 128 bit shared secret with one of the cipher's specified below. The choice of cipher is based on USS policy and is agreed to as part of the mission registration. AES-CFB16 is the recommended default cipher.

Bit 2 of the Flags byte is set to "1" to indicate the Operator information is encrypted.

The USS similarly decrypts these 8 bytes and provides the information to authorized entities.

4.1. Using AES-CFB16 in the System Message

CFB16 is defined in [NIST.SP.800-38A], Section 6.3. This is the Cipher Feedback (CFB) mode operating on 16 bits at a time. This variant of CFB can be used to encrypt any multiple of 2 bytes of cleartext.

The Operator includes a 64 bit UNIX timestamp for the mission time, along with its mission pubic key. The Operator also includes the UA MAC address (or multiple addresses if flying multiple UA).

The 128 bit IV for AES-CFB16 is constructed by the Operator and USS as: SHAKE128(MAC|UTCTime, 128).

AES-CFB16 would then be used to encrypt the Operator information.

4.2. Using Speck in the System Message

Speck [ISO ...., Reference needed] is a 64 bit block cipher and can be applied directly to the 8 bytes Operator information, using the 128 bit Operator/USS shared secret.

4.3. Using a Feistel scheme in the System Message

If the encryption speed doesn't matter, we can use the following approach based on the Feistel scheme. This approach is already being used in format-preserving encryption. The Feistal scheme is explained in Appendix A.

4.4. Using AES-CTR in the System Message

If 2 bytes of the System Message can be set aside to contain a counter that is incremented each time the Operator information changes, AES-CTR can be used as follows.

The Operator includes a 64 bit UNIX timestamp for the mission time, along with its mission pubic key. The Operator also includes the UA MAC address (or multiple addresses if flying multiple UA).

The high order bits of an AES-CTR counter is constructed by the Operator and USS as: SHAKE128(MAC|UTCTime, 112).

AES-CTR would then be used to encrypt the Operator information.

5. IANA Considerations


6. Security Considerations

An attacker has no known text after decrypting to determine a successful attack. An attacker can make assumptions about the high order byte values for Operator Longitude and Latitude that may substitute for known cleartext. There is no knowledge of where the operator is in relation to the UA. Only if changing location values "make sense" might an attacker assume to have revealed the operator's location.

6.1. CFB16 Risks

Using the same IV for different Operator information values with CFB16 presents a cyptoanalysis risk. Typically only the low order bits would change as the Operators position changes. Thus the first 2 encrypted bytes would not change, and only subsequent bytes would. The risk is mitigated due to the short-term value of the data. Further analysis is need to properly place risk.

6.2. Speck Risks

The use of Speck for the block cipher has risks. Speck has been extensively analyzed and generally rejected as an AES alternative. But here it is being used as a 64 bit block cipher which AES is not. The risk is mitigated as the key is used to protect a limited number of blocks. In a 4 hour mission with a System Message every 10 seconds, there are only 1,440 applications of the Speck cipher, provided that the operator reported to the UA a new location within those 10 second windows.

6.3. Crypto Agility

The Remote ID System Message does not provide any space for a crypto suite indicator or any other method to manage crypto agility.

All crypto agility is left to the USS policy and the relation between the USS and operator. The selection of the ECIES public key algorithm, the shared secret key derivation function, and the actual symmetric cipher used for on the System Message are set by the USS which informs the operator what to do.

7. Acknowledgments

The recommendation to use Speck for the block cipher comes after discussions on the IRTF CFRG mailing list. Better known ciphers will not work for this situation without changes to the System Message content.

8. Normative References

Barker, E., Chen, L., and R. Davis, "Recommendation for key-derivation methods in key-establishment schemes", National Institute of Standards and Technology report, DOI 10.6028/nist.sp.800-56cr1, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

9. Informative References

ASTM International, "Standard Specification for Remote ID and Tracking", , <>.
Moskowitz, R., Card, S., and A. Wiethuechter, "New Cryptographic Algorithms for HIP", Work in Progress, Internet-Draft, draft-moskowitz-hip-new-crypto-04, , <>.
Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, , <>.

Appendix A. Feistel Scheme

This approach is already being used in format-preserving encryption.

According to the theory, to provide CCA security guarantees (CCA = Chosen Ciphertext Attacks) for m-bit encryption X |-> Y, we should choose d >= 6. It seems very ineffective that when shortening the block length, we have to use 6 times more block encryptions. On the other hand, we preserve both the block cipher interface and security guarantees in a simple way.

How to encrypt an m-bit plaintext X using an n-bit block cipher
    E = {E_K} for n > m?

    Enc(X, K):
      1. Y <- X.
      2. Split Y into 2 equal parts: Y = Y1 || Y2
      (let us assume for simplicity that m is even).
      3. For i = 1, 2, ..., d do:
        Y <- Y2 || (Y1 ^ first_m/2_bits(E_K(Y2 || Ci)),
      where Ci is a (n - m/2)-bit round constant.
      4. Y <- Y2 || Y1.
      5. Return Y.

    Dec(Y, K):
      1. X <- Y.
      2. Split X into 2 equal parts: X = X1 || X2.
      3. For i = d, ..., 2, 1 do:
        X <- X2 || (X1 ^ first_m/2_bits(E_K(X2 || Ci)).
      4. X <- X2 || X1.
      5. Return X.

Authors' Addresses

Robert Moskowitz
HTT Consulting
Oak Park, MI 48237
United States of America
Stuart W. Card
AX Enterprize
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Adam Wiethuechter
AX Enterprize
4947 Commercial Drive
Yorkville, NY 13495
United States of America