Request for Comments: 4705
GigaBeam High-Speed Radio Link Encryption
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Copyright © The Internet Society (2006).
This document describes the encryption and key management used by GigaBeam as part of the WiFiber(tm) family of radio link products. The security solution is documented in the hope that other wireless product development efforts will include comparable capabilities.
The GigaBeam WiFiber(tm) product family provides a high-speed point- to-point radio link. Data rates exceed 1 gigabit/second at a distance of about a mile. The transmission beam width is less than one degree, which means that attempts to intercept the signal are most successful when the attacker is either between the transmitter and receiver or the attacker is directly behind the receiver. Since interception is possible, some customers require confidentiality and integrity protection for the data on the radio link. This document describes the security solution designed and deployed by GigaBeam to provide these security services.
The GigaBeam security solution employs:
- AES-GCM [GCM] with a custom security protocol specified in this document to provide confidentiality and integrity protection of subscriber traffic on the radio link;
o AES-CBC [CBC] and HMAC-SHA-1 [HMAC] with IPsec ESP [ESP] to provide confidentiality and integrity protection of management traffic between the radio control modules; o AES-CBC [CBC] and HMAC-SHA-1 [HMAC] with the IKE protocol [IKE] to provide confidentiality and integrity protection of key management traffic between the radio control modules; and
- OAKLEY key agreement [OAKLEY] and RSA digital signatures [PKCS1] are used with IKE to establish keying material and to provide authentication.
AES-GCM is used with the custom security protocol in a manner that is very similar to its use in ESP [ESP-GCM].
2. GigaBeam High-Speed Radio Link Overview
The GigaBeam high-speed radio link appears to be a fiber interface between two network devices. Figure 1 illustrates the connection of two devices that normally communicate using Gigabit Ethernet over a fiber optic cable.
+---------+ +----------+ +----------+ +---------+ | | | +----/ | | | | | Network | | GigaBeam | / | GigaBeam | | Network | | Device +=====+ Radio | /---- + Radio +=====+ Device | | | | | | | | | +---------+ ^ +----------+ ^ +----------+ ^ +---------+ | | | | | | Gigabit Ethernet | Gigabit Ethernet GigaBeam Radio Link
Figure 1. GigaBeam Radio Link Example.
Gigabit Ethernet traffic is encoded in 8B/10B format. The GigaBeam Radio Control Module (RCM) removes this coding to recover the 8-bit characters plus an indication of whether the character is a control code. The radio link frame is constructed from 224 10-bit input words, and a 4-way interleaved (56,50,10) Reed-Solomon Forward Error Correction (FEC) block is employed. Conversion of the Gigabit Ethernet data from 8B/10B format creates 224 bits of additional capacity in each frame, and another 196 bits is gained by recoding the 9-bit data using 64B/65B block codes. This additional 420 bits of capacity is used for the framing overhead required for FEC and link control.
2.1. GigaBeam Radio Link Frame Format
The GigaBeam radio link frame fields are summarized in Figure 2, which also provides the length of each field in bits.
Field Length Description ----- ------ ----------- SYNC 11 Frame Synchronization Pattern ('10110111000'b) KEYSEL 1 Indicates which AES key was used for this frame PN 40 AES-GCM Packet Number FLAGS 28 Control bits, one bit for each 64B/65B data block DCC 8 Data Communications Channel DATA 1792 Data (28 encrypted 64B/65B code blocks) TAG 96 Authentication Tag SPARE 24 Reserved for alternative FEC algorithms CHECK 240 Reed-Solomon Check Words for 4 10-bit symbol (56,50) code
Figure 2. GigaBeam Radio Link Frame Structure.
Each of the fields in the GigaBeam 2240-bit radio link frame is described below.
SYNC Synchronization field, an 11-bit Barker code. Always set to '10110111000'b. KEYSEL Key Selector -- select the appropriate key register for this frame. Two key registers are maintained to allow seamless rollover between encryption keys. PN Packet Number -- needed by AES-GCM; it carries the unique counter value for this frame. The value is incremented for each frame. FLAGS Control bits, one for each 64B/65B data block carried in the DATA field. If the bit is set, then the corresponding 64B/65B block in the DATA field contains a control code. This field is integrity protected by AES- GCM. DCC Data Communications Channel -- each frame carries one octet of the point-to-point data communications channel between the two radio control modules. See Section 2.2 for more information on the DCC. DATA Subscriber data carried as 28 64B/65B code blocks. This field is encrypted and integrity protected by AES-GCM. TAG The authentication tag generated by AES-GCM, truncated to 96 bits. SPARE 24 bits, set to zero. CHECK Forward error correction check value -- 24 check symbols are generated by a 4-way interleaved Reed-Solomon (56,50,10) algorithm. FEC is always active, but correction can be selectively enabled. For each frame, FEC processing also returns the number of bit errors, the number of symbols in error, and whether the FEC processing failed for the frame. This information allows an estimation of the bit error rate for the link.
2.2. Data Communications Channel
The Data Communications Channel (DCC) field reserves eight bits in each 2240-bit radio link frame for use in constructing a dedicated point-to-point link between the two RCMs. The DCC content is connected to a Universal Asynchronous Receiver/Transmitter (UART) controller that processes the DCC bit stream to provide an asynchronous serial channel that is visible to the RCM operating system. The Point-to-Point Protocol (PPP) [PPP] is used on the serial channel to create a simple two-node Internet Protocol (IP) network. Each IP datagram is spread over a large number of radio link frames. This two-node IP network carries management protocols between the GigaBeam RCMs.
IKE [IKE] runs on this two-node IP network to manage all cryptographic keying material. IPsec ESP [ESP] is used in the usual fashion to protect all non-IKE traffic on the data control channel. IPsec ESP employs AES-CBC as described in [ESP-CBC] and HMAC-SHA-1 as described in [ESP-HMAC].
3. Radio Link Processing
The fiber interface constantly provides a stream of data encoded in 8B/10B format. A radio link frame is constructed from 224 10-bit input words. Conversion of the data from 8B/10B format creates 224 bits of additional capacity in each frame, and then recoding using 64B/65B block codes creates another 196 bits of additional capacity. After encryption, the 64B/65B blocks are carried in the DATA field, and the control code indicator bits are carried in the FLAGS field. The additional capacity is used for the framing overhead.
Processing proceeds as follows:
- encryption and integrity protection as described in Section 3.1;
- forward error correction (FEC) processing as described in Section 3.2;
- scrambling as described in Section 3.3; and
- differential encoding as described in Section 3.4.
3.1. Encryption and Integrity Protection
The GigaBeam RCM contains two key registers. The single-bit KEYSEL field indicates which of the two registers was used for the frame.
AES-GCM [GCM] employs counter mode for encryption. Therefore, a unique value for each frame is needed to construct the counter. The counter includes a 32-bit salt value provided by IKE and a 40-bit packet number from the PN field in the radio link frame. The same counter value must not be used for more than one frame encrypted with the same key. The 128-bit counter block is constructed as shown in Figure 3. The first 96 bits of the AES counter block are called the Nonce in the AES-GCM algorithm description. Note that AES-GCM uses BLOCK values of zero and one for its own use. The values beginning with two are used for encrypting the radio link frame payload.
Field Length Description ----- ------ ----------- SALT 32 Salt value generated during the IKE exchange MBZ1 24 These bits must be zero PN 40 AES-GCM Packet Number carried in PN field MBZ2 28 These bits must be zero BLOCK 4 Block counter used in AES-GCM
Figure 3. AES Counter Block Construction.
AES-GCM is used to protect the FLAGS and DATA fields. The FLAGS field is treated as authenticated header data, and it is integrity protected, but it is not encrypted. The DATA field is encrypted and authenticated. The TAG field contains the authentication tag generated by AES-GCM, truncated to 96 bits.
Reception processing performs decryption and integrity checking. If the integrity checks fail, to maintain a continuous stream of traffic, the frame is replaced with K30.7 control characters. These control characters are normally used to mark errors in the data stream. Without encryption and integrity checking, these control characters usually indicate 8B/10B running disparity or code errors.
3.2. Forward Error Correction (FEC)
The 224 10-bit data symbols that make up each radio link frame are grouped into 4 subframes each consisting of 56 symbols. The subframes are formed by symbol interleaving. A Reed-Solomon Code, RS(56,50), designed for 10-bit symbols is applied to each subframe.
This Reed Solomon Code detects 6 errors and corrects 3 errors within each subframe. The FEC function is always active; however, it is possible to disable correction of the received data to support debugging.
The scrambler ensures that long series of one bits and long series of zero bits do not occur. When encryption is enabled, long series of common bit values is very unlikely; however, during the initial IKE exchange, the radio link frame payload is all zero bits.
The scrambling polynomial is (1 + x^14 + x^15). All words of a frame except the SYNC pattern are scrambled prior to transmission using this linear feedback shift register (LFSR). The LFSR is initialized to all ones at the start of each frame, prior to the first processed bit. Each processed input bit is added modulo 2 (i.e., an XOR) to the output of the x15 tap to form the output bit.
On reception, an identical process is performed after frame synchronization and prior to subsequent processing to recover the original bit pattern.
3.4. Differential Encoding
The data stream is differentially encoded to avoid symbol ambiguity at the receiver. Since the demodulator could produce true or inverted data depending on the details of the radio frequency (RF) and intermediate frequency (IF) processing chains, differential encoding is used to ensure proper reception of the intended bit value. A zero bit is encoded as no change from the previous output bit, and a one bit is encoded as a change from the previous output bit. Thus, an output bit is the result of XORing the unencoded bit with the previously transmitted encoded bit.
On reception, a complementary operation will be performed to produce the decoded datastream. The bitstream is formed by XORing the received encoded bit and the previously received encoded bit.
4. Key Management
The Internet Key Exchange (IKE) is used for key management [IKE]. IKE has two phases. In Phase 1, two Internet Security Association and Key Management Protocol (ISAKMP) peers establish a secure, authenticated channel with which to communicate. This is called the ISAKMP Security Association (SA). In the GigaBeam environment, the Phase 1 exchange is IKE Aggressive Mode with signatures and certificates. The RSA signature algorithm is used.
Phase 2 negotiates the Security Associations for the GigaBeam custom security protocol that protects subscriber traffic and IPsec ESP that protects management traffic between the GigaBeam RCMs. In the GigaBeam environment, the Phase 2 exchange is IKE Quick Mode, without perfect forward secrecy (PFS). The information exchanged along with Quick Mode is protected by the ISAKMP SA. That is, all payloads except the ISAKMP header are encrypted. A detailed description of Quick Mode can be found in Section 5.5 of [IKE].
When the Security Association is no longer needed, the ISAKMP Delete Payload is used to tell the peer GigaBeam device that it is being discarded.
Each GigaBeam device generates its own public/private key pair. This generation is performed at the factory, and the public key is certified by a Certification Authority (CA) in the factory. The certificate includes a name of the following format:
C=US O=GigaBeam Corporation OU=GigaBeam WiFiber(tm) SerialNumber=<device-model-identifier>/<device-serial-number>
The ISAKMP Certificate Payload is used to transport certificates, and in the GigaBeam environment, the "X.509 Certificate - Signature" certificate encoding type (indicated by a value of 4) is always used.
GigaBeam devices are always installed in pairs. At installation time, each one is configured with the device model identifier and device serial number of its peer. The device model identifier and device serial number of a backup device can also be provided. An access control check is performed when certificates are exchanged. The certificate subject name must match one of these configured values, and the certification path must validate to a configured trust anchor, such as the GigaBeam Root CA, using the validation rules in [PKIX1].
4.2. Oakley Groups
With IKE, several possible Diffie-Hellman groups are supported. These groups originated with the Oakley protocol and are therefore called "Oakley Groups".
GigaBeam devices use group 14, which is described in Section 3 of [MODP].
4.3. Security Protocol Identifier
The ISAKMP proposal syntax was specifically designed to allow for the simultaneous negotiation of multiple Phase 2 security protocol suites. The identifiers for the IPsec Domain of Interpretation (DOI) are given in [IPDOI].
The GigaBeam custom security protocol has been assigned the PROTO_GIGABEAM_RADIO protocol identifier, with a value of 5.
The PROTO_GIGABEAM_RADIO specifies the use of the GigaBeam radio link frame structure, which uses a single algorithm for both confidentiality and authentication. The following table indicates the algorithm values that are currently defined.
Transform ID Value ------------ ----- RESERVED 0 GIGABEAM_AES128_GCM 1
4.4. Keying Material
GIGABEAM_AES128_GCM requires 20 octets of keying material (called KEYMAT in [IKE]). The first 16 octets are the 128-bit AES key, and the remaining four octets are used as the salt value in the AES counter block.
Presently, AES with a 128-bit key is the only encryption algorithm that is supported. Other encryption algorithms could be registered in the future.
4.5. Identification Type Values
The following table lists the assigned values for the Identification Type field found in the ISAKMP Identification Payload.
ID Type Value ------- ----- RESERVED 0 ID_IPV4_ADDR 1 ID_FQDN 2 ID_USER_FQDN 3 ID_IPV4_ADDR_SUBNET 4 ID_IPV6_ADDR 5 ID_IPV6_ADDR_SUBNET 6 ID_IPV4_ADDR_RANGE 7 ID_IPV6_ADDR_RANGE 8 ID_DER_ASN1_DN 9 ID_DER_ASN1_GN 10 ID_KEY_ID 11
The ID_DER_ASN1_DN will be used when negotiating security associations for use with the GigaBeam custom security protocol. The provided distinguished name must match the peer's subject name provided in the X.509 certificate.
4.6. Security Parameter Index
The least significant bit of the Security Parameter Index (SPI) is used in the GigaBeam custom security protocol. When two GigaBeam custom security protocol security associations are active at the same time for communications in the same direction, the least significant bit of the SPI must be different to ensure that these active security associations can be distinguished by the single bit in the GigaBeam custom security protocol.
4.7. Key Management Latency
The IKE exchange over the DCC must complete before subscriber data can be exchanged in the GigaBeam radio link frame payload. Since each radio link frame carries a small portion of an IP datagram, many radio link frames carrying all zero bits must be sent and received to complete the IKE exchange.
Once the initial keying material is in place, the IKE exchanges to establish subsequent keying material can be performed concurrent with the transfer of subscriber data in the radio link frame payload. The KEYSEL field in the radio link frame is used to indicate which keying material is being used.
The PN field in radio link frame provides a continuous indication of the number of frames that have been encrypted with a particular key. Once a threshold is exceeded, the IKE exchanges begin to establish the replacement keying material. Currently, the exchanges begin when half of the packet numbers have been used, but any threshold can be employed, as long as the replacement keying material is available before the packet counters are exhausted.
5. Security Considerations
The security considerations in [IKE], [OAKLEY], [PKCS1], and [ESP] apply to the security system defined in this document.
Confidentiality and integrity are provided by the use of negotiated algorithms. AES-GCM [GCM] is used with the GigaBeam custom security protocol to provide confidentiality and integrity protection of subscriber traffic on the radio link. AES-CBC [CBC] and HMAC-SHA-1 [HMAC] are used with IPsec ESP [ESP] to provide confidentiality and integrity protection of management traffic between the radio control modules.
AES-GCM makes use of AES Counter mode to provide confidentiality. Unfortunately, it is very easy to misuse counter mode. If counter block values are ever used for more than one frame with the same key, then the same key stream will be used to encrypt both frames, and the confidentiality guarantees are voided. Using AES Counter mode with the same counter values to encrypt two plaintexts under the same key leaks the plaintext. The automated key management described here is intended to prevent this from ever happening.
Since AES has a 128-bit block size, regardless of the mode employed, the ciphertext generated by AES encryption becomes distinguishable from random values after 2^64 blocks are encrypted with a single key. Since the GigaBeam radio link frame allows for up to 2^40 fixed- length frames in a single security association, there is no possibility for more than 2^64 blocks to be encrypted with one key.
The lifetime of a particular AES key can be shorter than 2^40 frames. A smaller threshold can be used as a trigger to transition to the next key. This capability allows straightforward implementation of policies that require the key to be changed after a particular volume of traffic or a particular amount of time.
There are fairly generic precomputation attacks against all block cipher modes that allow a meet-in-the-middle attack against the key. These attacks require the creation and searching of huge tables of ciphertext associated with known plaintext and known keys. Assuming that the memory and processor resources are available for a precomputation attack, then the theoretical strength of AES Counter mode (and any other block cipher mode) is limited to 2^(n/2) bits, where n is the number of bits in the key. The use of long keys is the best countermeasure to precomputation attacks. The unpredictable nonce value in the counter block significantly increases the size of the table that the attacker must compute to mount a successful precomputation attack.
Rekeying with Quick Mode results in new keys to protect GigaBeam radio link frames; however, these keys are generated from the same Diffie-Hellman shared secret. In order to limit the amount of data that would be exposed by the disclosure of this Diffie-Hellman shared secret or the associated Diffie-Hellman private key, implementations should periodically rekey using a new Phase 1 exchange.
Diffie-Hellman exponents used in IKE Phase 1 should be erased from memory immediately after use. Likewise, AES and HMAC-SHA-1 keying material should be erased from memory when it is no longer needed.
This security solution makes use of IKEv1 [IKE]. IKEv1 was selected over IKEv2 [IKEv2] primarily due to the availability of an implementation for the processing environment. The use of IKEv2 would provide some useful capabilities, such as Diffie-Hellman group negotiation. These additional capabilities would not significantly improve the security of the overall key management solution that runs on the two-node IP network.
6. IANA Considerations
IANA has assigned one IPsec Security Protocol Identifier in http://www.iana.org/assignments/isakmp-registry for PROTO_GIGABEAM_RADIO. It was assigned the value 5.
7. Informative References
[CBC] Dworkin, M., "Recommendation for Block Cipher Modes of Operation: Methods and Techniques," NIST Special Publication 800-38A, December 2001. [ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005. [ESP-CBC] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher Algorithm and Its Use with IPsec", RFC 3602, September 2003. [ESP-GCM] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating Security Payload (ESP)", RFC 4106, June 2005. [ESP-HMAC] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within ESP and AH", RFC 2404, November 1998. [GCM] McGrew, D. and J. Viega, "The Galois/Counter Mode of Operation (GCM)", Submission to NIST. http://csrc.nist.gov/CryptoToolkit/modes/proposedmodes/ gcm/gcm-spec.pdf, January 2004. [Soon: NIST SP 800-38D.] [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, February 1997. [IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [IKEv2] Kaufman, C., "The Internet Key Exchange (IKEv2) Protocol", RFC 2306, December 2005. [IPDOI] Piper, D., "The Internet IP Security Domain of Interpretation for ISAKMP", RFC 2407, November 1998. [MODP] Kivinen, T. and M. Kojo. "More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)", RFC 3526, May 2003. [OAKLEY] Orman, H., "The Oakley Key Determination Protocol", RFC 2412, November 1998. [PKCS1] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC 2313, March 1998. [PKIX1] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002. [PPP] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994.
The authors thank Bob Sutherland and Dave Marcellas for their contributions and review.
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
470 Springpark Place, Suite 900
Herndon, VA 20170
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