| Internet-Draft | Operator Privacy | October 2021 |
| Moskowitz, et al. | Expires 10 April 2022 | [Page] |
This document describes a method of providing privacy for UAS Operator/Pilot 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.¶
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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 UAS Operator/Pilot information. Encrypting in place means that the ciphertext is exactly the same length as the cleartext, and directly replaces it.¶
An example of and an initial application of this mechanism is the 8 bytes of UAS Operator/Pilot (hereafter called simply Operator) longitude and latitude location in the ASTM System Message (Msg Type 0x4). This meets the Drip Requirements [drip-requirements], Priv-01.¶
It is assumed that the Operator, via the UAS, registers an operation with its USS. During this operation registration, the UAS and USS exchange public keys to use in the hybrid ECIES. The USS key may be long lived, but the UAS key SHOULD be unique to a specific operation. This provides protection if the ECIES secret is exposed from prior operations.¶
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 (AEAD tag). There is rarely any data in the messages that can be used as an IV. The AES-CFB16 mode of operation proposed here can encrypt a multiple of 2 bytes.¶
The System Message is not a simple, one-time, encrypt the PII with the ECIES derived key. The Operator may move during a operation 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 and not be at risk of attack by the content of other messages.¶
Another candidate message is the optional ASTM Operator ID Message (Msg Type 0x5) with its 20 character Operator ID field. The Operator ID does not change during an operation, so this is a one-time encryption operation for the operation. The same cipher SHOULD be used for all messages from the UAS and this will influence the cipher selection.¶
Future applications of this mechanism may be provided. The content of the System Message may change to meet CAA requirements, requiring encrypting a different amount of data. At that time, they will be added to this document.¶
Editor note: The Rules for allowing encryption need to be updated to handle the UA operating in Broadcast Remote ID only mode. That is conditions where the USS cannot notify the UAS to stop encrypting.¶
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.¶
See Drip Requirements [drip-requirements] for common DRIP terms.¶
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 operations to their USS. This operation reporting provides a mechanism for the USS and operator to establish an operation security context. Here it will be used to exchange public keys for use in ECIES.¶
The operator's ECIES public key SHOULD be unique for each operation. The USS ECIES public key may be unique for each operator and operation, but not required. For best post-compromise security (PCS), the USS ECIES 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 [NIST.SP.800-185].¶
The KMAC function provides a new, more efficient, key derivation function over HKDF [RFC5869]. This will be referred to as KKDF.¶
HKDF needs a minimum of 4 hash functions (e.g. SHA256). KKDF does an equivalent shared secret generation in a single Keccak Sponge operation.¶
When the USS - UAS Operation Security Context is established, the UAS provides its UAS ID (null padded to 20 characters per [F3411-19]) and a 256 bit random nonce to the USS. These are inputs, along with the ECDH keys to produce the shared secret as follows.¶
A 64 bit UNIX timestamp for the operation time is also included in the Operation Security Context. This will be used in the IV construction.¶
Per [NIST.SP.800-56Cr1], Section 4.1, Option 3:¶
Shared Secret = KMAC128(salt, IKM, L, S)
¶
L is the derived key bit length. Since only a single key is needed, L=128.¶
S is the byte string 01001011 || 01000100 || 01000110, which represents the sequence of characters "K", "D", and "F" in 8-bit ASCII.¶
salt = Nonce-USS | Nonce-UAS
¶
There are special security considerations for IKM per [RFC7748]. The IKM as follows:¶
IKM = Diffie-Hellman secret | USS-ID | RID
¶
The System Message contains 8 bytes of Operator specific information: Longitude and Latitude of the Remote Operator (Pilot in the field description) of the UA. The GCS MAY encrypt these as follows.¶
Editors Note: The next version of [F3411-19], currently in ballot, is adding a 2 byte Operator Altitude field, thus increasing the Operator specific information to 10 bytes. This change will be delineated via Protocol Version field. It is this future shift from a multiple of 4 bytes to a multiple of 2 bytes that is the reason to change from CFB32 in earlier drafts to CFB16 used now.¶
The 8 bytes of Operator information are encrypted, using the ECIES derived 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 operation registration. AES-CFB16 is the recommended default cipher.¶
ASTM Remote ID and Tracking messages [F3411-19] SHOULD be updated to allow Bit 5 of the Flags byte in the System Message set to "1" to indicate the Operator information is encrypted.¶
The USS similarly decrypts these 8 bytes and provides the information to authorized entities.¶
If the Operator location is encrypted the encrypted bit flag MUST be set to 1.¶
The Operator MAY be notified by the USS that the operation has entered a location or time where privacy of Operator location is not allowed. In this case the Operator MUST disable this privacy feature and send the location unencrypted or land the UA or route around the restricted area.¶
If the UAS looses connectivity to the USS, the privacy feature SHOULD be disabled or land the UA.¶
If the operation is in an area or time with no Internet Connectivity, the privacy feature MUST NOT be used.¶
An Observer receives a System Message with the encrypt bit set to 1. The Observer sends a query to its USS Display Provider containing the UA's ID and the encrypted fields.¶
The USS Display Provider MAY deny the request if the Observer does not have the proper authorization.¶
The USS Display Provider MAY reply to the request with the decrypted fields if the Observer has the proper authorization.¶
The USS Display Provider MAY reply to the request with the decrypting key if the Observer has the proper authorization.¶
The Observer MAY notify the USS through its USS Display Provider that content privacy for a UAS in this location/time is not allowed. If the Observer has the proper authorization for this action, the USS notifies the Operator to disable this privacy feature.¶
The Operator ID Message contains the 20 byte Operator ID. The GCS MAY encrypt these as follows.¶
The 20 bytes Operator ID is encrypted, using the ECIES derived 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 operation registration. AES-CFB16 is the recommended default cipher.¶
ASTM Remote ID and Tracking messages [F3411-19] SHOULD be updated to allow Operator ID Type in the Operator ID Message set to "1" to indicate the Operator ID is encrypted.¶
The USS similarly decrypts these 20 bytes and provides the information to authorized entities.¶
If the Operator ID is encrypted the Operator ID Type field MUST be set to 1.¶
The Operator MAY be notified by the USS that the operation has entered a location or time where privacy of Operator ID is not allowed. In this case the Operator MUST disable this privacy feature and send the ID unencrypted or land the UA or route around the restricted area.¶
If the UAS looses connectivity to the USS, the privacy feature SHOULD be disabled or land the UA.¶
If the operation is in an area or time with no Internet Connectivity, the privacy feature MUST NOT be used.¶
An Observer receives a Operator ID Message with the Operator ID Type field set to 1. The Observer sends a query to its USS Display Provider containing the UA's ID and the encrypted fields.¶
The USS Display Provider MAY deny the request if the Observer does not have the proper authorization.¶
The USS Display Provider MAY reply to the request with the decrypted fields if the Observer has the proper authorization.¶
The USS Display Provider MAY reply to the request with the decrypting key if the Observer has the proper authorization.¶
The Observer MAY notify the USS through its USS Display Provider that content privacy for a UAS in this location/time is not allowed. If the Observer has the proper authorization for this action, the USS notifies the Operator to disable this privacy feature.¶
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 operation time, along with its operation 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|Message_Type, 128). Inclusion of the ASTM Message_Type ensures a unique IV for each Message type that contains PII to encrypt.¶
AES-CFB16 would then be used to encrypt the Operator information.¶
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 (e.g. credit card numbers). The Feistal scheme is explained in Appendix A.¶
If 2 bytes of the 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 operation time, along with its operation 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|Message_Type, 112). Inclusion of the ASTM Message_Type ensures a unique IV for each Message type that contains PII to encrypt.¶
AES-CTR would then be used to encrypt the Operator information.¶
This document provides solution to PRIV-1 for PII in the ASTM System Message.¶
ASTM will need to make the following changes to the "Flags" in the System Message (Msg Type 0x4):¶
ASTM will need to make the following changes to the "Operator ID Type" in the Operator ID Message (Msg Type 0x5):¶
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.¶
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. The risk is mitigated due to the short-term value of the data. Further analysis is need to properly place risk.¶
The ASTM Remote ID Messages do 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/UAS. 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.¶
[RFC7748] warns about using Curve25519 and Curve448 in Diffie-Hellman for key derivation:¶
Designers using these curves should be aware that for each public key, there are several publicly computable public keys that are equivalent to it, i.e., they produce the same shared secrets. Thus using a public key as an identifier and knowledge of a shared secret as proof of ownership (without including the public keys in the key derivation) might lead to subtle vulnerabilities.¶
This applies here, but may have broader consequences. Thus two endpoint IDs are included with the Diffie-Hellman secret.¶
Section 4.1 of NIST SP 800-185 [NIST.SP.800-185] states:¶
"The KECCAK Message Authentication Code (KMAC) algorithm is a PRF and keyed hash function based on KECCAK . It provides variable-length output"¶
That is, the output of KMAC is indistinguishable from a random string, regardless of the length of the output. As such, the output of KMAC can be divided into multiple substrings, each with the strength of the function (KMAC128 or KMAC256) and provided that a long enough key is used, as discussed in Sec. 8.4.1 of SP 800-185.¶
For example KMAC128(K, X, 512, S), where K is at least 128 bits, can produce 4 128 bit keys each with a strength of 128 bits. That is a single sponge operation is replacing perhaps 5 HMAC-SHA256 operations (each 2 SHA256 operations) in HKDF.¶
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.
¶
The recommended ciphers come from discussions on the IRTF CFRG mailing list.¶