Security NTP Protocol
David L. Mills University of Delaware http://www.eecis.udel.edu/~mills mailto:[e-mail]
Sir John Tenniel; Alice's Adventures in Wonderland,Lewis Carroll
Apr 20, 2009
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Security protocol requirements
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It must interoperate with the existing NTP architecture model and protocol design. In particular, it must support the symmetric key scheme described in RFC-1305. It must provide for the independent collection of cryptographic values and time values. A NTP packet is accepted for processing only when the required cryptographic values have been obtained and verified and the NTP header has passed all sanity checks. It must not significantly degrade the potential accuracy of the NTP synchronization algorithms. In particular, it must not make unreasonable demands on the network or host processor and memory resources. It must be resistant to cryptographic attacks, specifically those identified in the security model above. In particular, it must be tolerant of operational or implementation variances, such as packet loss or misorder, or suboptimal configurations.
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Apr 20, 2009
Security protocol requirements (continued)
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It must build on a widely available suite of cryptographic algorithms, yet be independent of the particular choice. In particular, it must not require data encryption other than incidental to signature and cookie encryption operations. It must function in all the modes supported by NTP, including server, symmetric and broadcast modes. It must not require intricate per-client or per-server configuration other than the availability of the required cryptographic keys and certificates. The reference implementation must contain provisions to generate cryptographic key files specific to each client and server.
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Apr 20, 2009
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Autokey security protocol
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NTP and Autokey protocols work independently for each client, with tentative outcomes confirmed only after both succeed. Public keys and certificates are obtained and verified relatively infrequently using X.509 certificates and certificate trails. Session keys are derived from public keys using fast algorithms. Each NTP message is individually authenticated using session key and message digest (keyed MD5). A proventic trail is a sequence of NTP servers each synchronized and cryptographically verified to the next lower stratum server and ending on one or more trusted servers. Proventic trails are constructed from each server to the trusted servers at decreasing stratum levels. When server time and at least one proventic trail are verified, the peer is admitted to the population used to synchronize the system clock.
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Apr 20, 2009
Session keys
Source Address
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Dest Key ID Address NTPv4 Session Key
Cookie
Hash
NTPv4 session keys have four 32-bit words (16 octets total). The session key value is the 16-octet MD5 message digest of the session key. Key IDs have pseudo-random values and are used only once. A special key ID value of zero is used as a NAK reply. In multicast mode and in any message including an extension field, the cookie has a public value (zero). In client/server modes the cookie is a hash of the addresses and a private value. In symmetric modes the cookie is a random roll; in case both peers generate cookies, the agreed cookie is the EXOR of the two values.
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Apr 20, 2009
Computing the cookie
Client Address Server Address Key ID (0) Private Value Cookie
Compute Hash
Compute Signature Signature and Timestamp
Cookie
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The server generates a cookie unique to the client and server addresses and its own private value. It returns the cookie, signature and timestamp to the client in an extension field. The cookie is transmitted from server to client encrypted by the clien public key. The server uses the cookie to validate requests and construct replies. The client uses the cookie to validate the reply and checks that the request key ID matches the reply key ID.
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Apr 20, 2009
Generating the session key list
Source Address Dest Address Cookie Key ID Session Key ID List Final Index Final Key ID
Compute Hash Index n Next Key ID Index n + 1
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Compute Signature
Signature
The server rolls a random 32-bit seed as the initial key ID and selects the cookie. Messages with a zero cookie contain only public values. The initial session key is constructed using the given addresses, cookie and initial key ID. The session key value is stored in the key cache. The next session key is constructed using the first four octets of the session key value as the new key ID. The server continues to generate the full list. The final index number and last key ID are provided in an extension field with signature and timestamp.
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Apr 20, 2009
Sending messages
NTP Header and Extension Fields Compute Hash Key ID Session Key ID List
Message Authenticator Code (MAC)
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The message authenticator code (MAC) consists of the MD5 message digest of the NTP header and extension fields using the session key ID and value stored in the key cache. The server uses the session key ID list in reverse order and discards each key value after use. An extension field containing the last index number and key ID is included in the first packet transmitted (last on the list). This extension field can be provided upon request at any time. When all entries in the key list are used, a new one is generated.
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Apr 20, 2009
Receiving messages
NTP Header and Extension Fields Compute Hash Message Authenticator Code (MAC) Key ID Message Digest
Message Digest
Compare
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The intent is not to hide the message contents, just verify where it came from and that it has not been modified in transit. The MAC message digest is compared with the computed digest of the NTP header and extension fields using the session key ID in the MAC and the key value computed from the addresses, key ID and cookie. If the cookie is zero, the message contains public values. Anybody can validate the message or make a valid message containing any values. If the cookie has been determined by secret means, nobody except the parties to the secret can validate a message or make a valid message.
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Apr 20, 2009
NTP protocol header and timestamp formats
NTP Protocol Header Format (32 bits) LI VN Mode Strat Poll Root Delay Root Dispersion Reference Identifier Prec LI VN Strat Poll Prec leap warning indicator version number (4) stratum (0-15) poll interval (log2) precision (log2)
Reference Timestamp (64) Originate Timestamp (64) Cryptosum Receive Timestamp (64) Transmit Timestamp (64) Extension Field 1 (optional) Extension Field 2... (optional) Authenticator (Optional) Key/Algorithm Identifier Message Digest (128)
NTP Timestamp Format (64 bits) Seconds (32) Fraction (32) Value is in seconds and fraction since 0h 1 January 1900 NTP v4 Extension Field Field Type Length Extension Field (padded to 32-bit boundary) Last field padded to 64-bit boundary NTP v3 and v4 NTP v4 only authentication only
Authenticator uses MD5 cryptosum of NTP header plus extension fields (NTPv4) Apr 20, 2009 10
NTPv4 extension fields
Field Type Length Association ID Timestamp Filestamp Value Length Value Signature Length Signature Padding (as needed) Value Fields (optional) NTP Extension Field
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New extension fields format defined for NTP Version 4
Fields are processed in order. Requests may be transmitted with or without value fields. Last field padded to 64-bit boundary; all others padded to 32-bit boundary. Field length covers all payload and padding.
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Apr 20, 2009
Host status word
0 Digest/Signature NID 16 Client 24 Ident 28 Host
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Host status word is constructed at initialization time.
Client and server exchange status words with offered identity schemes Both client and server agree on the same scheme
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Digest/Signature NID
16 bits for Network ID of the message digest/signature encryption scheme
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Client
8 bits available for client Autokey protocol operations
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Host
8 bits available for host Autokey operations and offered identity scheme
Apr 20, 2009
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Autokey protocol exchanges
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Parameter Exchange (ASSOC message - not signed)
Exchange host names; agree on digest/signature and schemes. identity Optional: verify host name/address using reverse-DNS.
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Certificate Exchange (CERT message)
Obtain and verify certificates on the trail to a trusted root certificate.
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Identity Exchange (IFF, GQ and MV messages)
Verify server identity using agreed identity scheme (TC, IFF, GQ, MV).
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Values Exchange (COOKIE and AUTO messages)
Obtain and verify the cookie, autokey values and leapseconds table, depending on the association mode (client-server, broadcast, symmetric).
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Signature Exchange (SIGN message)
Request the server to sign and return a client certificate. The exchange is valid only when the client has synchronized to a proventic source and the server identity has been confirmed.
Apr 20, 2009
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Parameter and certificate exchanges
Client Send host name and status word (unsigned) Agree digest NID and ID scheme Assoc Request Assoc Response Certificate Request Verify signature Certificate Response Send X.509 certificate and certificate and signature Server Agree digest NID And ID scheme Send host name and status word (unsigned)
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Initial exchange of host status words defines server message digest and signature encryption algorithm and identity scheme. The Certificate Request/Response cycle repeats as needed.
Primary (stratum 1) certificate is explicitly trusted and self-signed. Secondary certificates are signed by the next lower stratum server and validated with its public key.
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Apr 20, 2009
Identification exchange
Client Compute nonce1 and send Server Compute nonce2 and response Challenge Request
Verify response Challenge Response Send response and signature and signature
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This is a challenge-response scheme
Client Alice and server Bob share a common set of parameters and a private group key b. Alice rolls random nonce r and sends to Bob. Bob rolls random nonce k, computes a one-way function f(r, k, b) and sends to Alice. Alice computes some function g(f, b) to verify that Bob knows b.
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The signature prevents message modification and binds the response to Bob's private key. An interceptor can see the challenge and response, but cannot determine k or b or how to construct a response acceptable to Alice.
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Apr 20, 2009
Cookie exchange
Client Active Peer Server Passive Peer
Send public key and signature Verify signature; decrypt cookie
Cookie Request Cookie Response
Verify signature; encrypt cookie Send encrypted cookie and signature
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Client sends public key to server without signature when not synchronized. Symmetric active peer sends public key and signature to passive peer when synchronized. Server cookie is encrypted from the hash of source/destination addresses, zero key ID and server private value. Symmetric passive cookie is a random value for every exchange. Server private value is refreshed and protocol restarted once per day.
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Apr 20, 2009
Autokey exchange
Client Active Peer Server Passive Peer
Send request and signature Verify signature; install values
Autokey Request Autokey Response
Verify signature Send autokey values and signature
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Server generates key list and signature calculated to last about one hour. Client sends request to server without signature when not synchronized. Server replies with the last index number and key ID on the list. Broadcast server uses AUTO response for the first message after regenerating the key and ASSOC response for all other messages.
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Sign certificate exchange
Client Active Peer Server Passive Peer
Send certificate and signature Verify signature; install certificate;
Sign Request Sign Response
Verify signature; sign certificate Send certificate and signature.
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This is used to authenticate client to server, with server acting as de facto certificate authority using encrypted credential scheme TBD. Client sends certificate to server with or without signature. Server extracts request data and signs with server private key. Client verifies certificate and signature. Subsequently, client supplies this certificate rather than self-signed certificate, so clients can verify with server public key.
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Apr 20, 2009
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Broadcast/multicast mode
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The broadcast server sends messages at fixed intervals.
The first message sent after regenerating the key list includes the autokey values and signature. Other messages include the server association ID, but no signature. This is used as a handle for clients to request the autokey values if necessary. These messages are considered public values, so the cookie value is zero.
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When a multicast client receives the first message, it temporarily switches to client/server mode in order to calibrate the network propagation delay and authenticate the server. The client first obtains the parameters and verifies the certificate, identity and signature as in client/server mode, then obtains the autokey values and signature. When the propagation delay is ...