Network Working Group April 1991 Internet-Draft SNMP Security Protocols James M. Galvin Trusted Information Systems, Inc. Keith McCloghrie Hughes LAN Systems, Inc. James R. Davin MIT Laboratory for Computer Science April 8, 1991 Contents 1 Status of this Memo 1 2 Acknowledgements 1 3 Introduction 1 3.1 Threats 3 3.2 Goals and Constraints 5 3.3 Security Services 6 3.4 Mechanisms 6 4 SNMP Party 7 5 SNMP MD4 Authentication Protocol 10 5.1 Generating a Message 13 5.2 Receiving a Message 14 6 SNMP DES Privacy Protocol 16 6.1 Generating a Message 17 6.2 Receiving a Message 18 7 Clock and Secret Distribution 19 7.1 Initial Configuration 21 7.2 Clock Distribution 24 7.3 Secret Distribution 25 7.4 Crash Recovery 28 INTERNET-DRAFT April 1991 8 Security Considerations 30 8.1 Conformance 31 8.2 Protocol Correctness 32 8.2.1 Clock Monotonicity Mechanism 33 8.2.2 Data Integrity Mechanism 33 8.2.3 Data Origin Authentication Mechanism 33 8.2.4 Restricted Administration Mechanism 34 8.2.5 Ordered Delivery Mechanism 35 8.2.6 Message Timeliness Mechanism 36 8.2.7 Confidentiality Mechanism 36 Galvin, McCloghrie, Davin [Page ii] INTERNET-DRAFT April 1991 1 Status of this Memo This draft document will be submitted to the RFC editor as a protocol specification. Distribution of this memo is unlimited. Please send comments to the authors: James M. Galvin , Keith McCloghrie , and James R. Davin . 2 Acknowledgements 3 Introduction The Simple Network Management Protocol (SNMP) specification [1] allows for the protection of network management operations by a variety of security protocols. The SNMP administrative model described in [2] provides a framework for securing SNMP network management. In the context of that framework, this memo defines protocols to support the following three security services: o data integrity, o data origin authentication, and o data confidentiality. In the model described in [2], each SNMP party is, by definition, associated with a single authentication protocol. The authentication protocol provides a mechanism by which SNMP management communications transmitted by the party may be reliably identified as having originated from that party. The authentication protocol defined in this memo also reliably determines that the message received is the message that was sent. Similarly, each SNMP party is, by definition, associated with a single privacy protocol. The privacy protocol provides a Galvin, McCloghrie, Davin [Page 1] INTERNET-DRAFT April 1991 mechanism by which SNMP management communications transmitted to said party are protected from disclosure. The privacy protocol in this memo specifies that only authenticated messages may be protected from disclosure. These protocols are secure alternatives to the so-called "trivial" protocol defined in [1]. USE OF THE TRIVIAL PROTOCOL ALONE DOES NOT CONSTITUTE SECURE NETWORK MANAGEMENT. THEREFORE, A NETWORK MANAGEMENT SYSTEM THAT IMPLEMENTS ONLY THE TRIVIAL PROTOCOL IS NOT CONFORMANT TO THIS SPECIFICATION. The SNMP MD4 authentication protocol is described in Section 5. It provides a data integrity service by having the originator compute a digest over an appropriate portion of a message and sending that digest to the recipient, with the message, for verification. The data origin authentication service is supported by prefixing the message with a secret value known only to the originator and recipient, prior to computing the digest. Thus, data integrity is supported explicitly while data origin authentication is supported implicitly in the verification of the digest. The SNMP DES privacy protocol is described in Section 6. This protocol protects messages from disclosure by encrypting their contents according to a secret cryptographic key known only to the originator and recipient. The additional functionality afforded by this protocol is assumed to justify its additional computational cost. The SNMP MD4 authentication protocol depends on the existence of loosely synchronized clocks between the originator and recipient of a message. Maintaining such clocks may be accomplished by a variety of strategies. Section 7.2 presents one such strategy that is particularly suited to the demands of SNMP network management. Galvin, McCloghrie, Davin [Page 2] INTERNET-DRAFT April 1991 Both protocols described here require the sharing of secret information between the originator of a message and its recipient. The protocol specifications assume the existence of the necessary secrets. The selection of such secrets and their secure distribution to appropriate parties may be accomplished by a variety of strategies. Section 7.3 presents one such strategy that is particularly suited to the demands of SNMP network management. 3.1 Threats Several of the classical threats to network protocols are applicable to the network management problem and therefore would be applicable to any SNMP security protocol. Other threats are not applicable to the network management problem. This section discusses principal threats, secondary threats and threats which are of lesser importance. The principal threats against which any SNMP security protocol should provide protection are: Modification of Information. The SNMP protocol provides the means for management stations to manipulate the value of objects in a managed agent. The modification threat is the danger that some party may alter in-transit messages generated by an authorized party in such a way as to effect unauthorized management operations. Masquerade. The SNMP administrative model includes a model of access control. Access control necessarily depends on knowledge of the origin of a message. The masquerade threat is the danger that management operations not authorized for some party may be attempted by that party by assuming the identity of another party that has the appropriate authorizations. Two secondary threats are also identified: Galvin, McCloghrie, Davin [Page 3] INTERNET-DRAFT April 1991 Message Stream Modification. The SNMP protocol is based upon connectionless transport services. The message stream modification threat is the danger that messages may be arbitrarily re-ordered, delayed or replayed to effect unauthorized management operations. This threat may arise either by the work of a malicious attacker or by the natural operation of a subnetwork service. All of these threats must be addressed by a security protocol. Disclosure. The disclosure threat is the danger of eavesdropping on the exchanges between managed agents and a management station. Protecting against this threat is mandatory when the SNMP is used to administer private parameters on which its security is based. Protecting against the disclosure threat may also be required as a matter of local policy. There are at least two threats that a SNMP security protocol need not protect against. No attempt was made to provision protection against: Denial of Service A SNMP security protocol need not attempt to address the broad range of attacks by which service to authorized parties is denied. Indeed, such denial-of-service attacks are in many cases indistinguishable from the type of network failures with which any viable network management protocol must cope as a matter of course. Traffic Analysis In addition, a SNMP security protocol need not attempt to address traffic analysis attacks. It is believed that many traffic patterns are predictable -- agents are probably managed on a regular basis by a relatively small number of management stations -- and therefore there is no significant advantage afforded by protecting against traffic analysis. Galvin, McCloghrie, Davin [Page 4] INTERNET-DRAFT April 1991 3.2 Goals and Constraints Based on the foregoing account of threats in the SNMP network management environment, the goals of a SNMP security protocol are enumerated below. 1. The protocol should provide for verification that each received SNMP message has not been modified during its transmission through the network. 2. The protocol should provide for verification of the identity of the originator of each received SNMP message. 3. The protocol should provide that the apparent time of generation for each received SNMP message is recent. 4. The protocol should provide that the apparent time of generation for each received SNMP message is subsequent to that for all previously delivered messages of similar origin. 5. The protocol should provide, when necessary, that the contents of each received SNMP message are protected from disclosure. In addition to the principal goal of supporting secure network management, the design of a SNMP security protocol is also influenced by the following constraints: 1. When requirements of effective management in times of network stress are inconsistent with those of security, the former are preferred. 2. Neither the SNMP nor its underlying security mechanisms should depend upon the ready availability of other network services (e.g., Network Time Protocol (NTP) or secret/key management protocols). 3. A security mechanism should entail no changes to the basic SNMP network management philosophy. Galvin, McCloghrie, Davin [Page 5] INTERNET-DRAFT April 1991 3.3 Security Services The security services necessary to support the goals of the SNMP security protocol are as follows. Data Integrity. The provision of the property that data and data sequences have not been altered or destroyed in an unauthorized manner. Data Origin Authentication. The corroboration that the claimed origin of received data is correct. Data Confidentiality. The provision of the property that information is not made available or disclosed to unauthorized individuals, entities or processes. The protocols specified in this memo require both data integrity and data origin authentication to be used at all times. For these protocols, it is not possible to realize data integrity without data origin authentication, nor is it possible to realize data origin authentication without data integrity. Further, there is no provision for data confidentiality without both data integrity and data origin authentication. 3.4 Mechanisms The mechanisms chosen to support each of the security services and goals of the SNMP security protocols are as follows. o In support of data integrity, the use of the MD4 [3] message digest algorithm is chosen. A 128-bit digest is calculated over an appropriate portion of a SNMP message and included as part of the message sent to the recipient. o In support of data origin authentication and data integrity, the portion of a SNMP message that is Galvin, McCloghrie, Davin [Page 6] INTERNET-DRAFT April 1991 digested is first prefixed with a secret value shared by the originator of that message and its intended recipient. o To protect against the threat of message reordering, a timestamp value is included in each message generated. A recipient evaluates the timestamp to determine if the message is recent, and it uses the timestamp to determine if the message is ordered relative to other messages it has received. In conjunction with other readily available information (e.g., the request-id), the timestamp also indicates whether or not the message is a replay of a previous message. o In support of data confidentiality, the use of the Data Encryption Standard (DES) [4] in the Cipher Block Chaining mode of operation [5] is chosen. An appropriate portion of the message is encrypted prior to being transmitted to its recipient. 4 SNMP Party Recall from [2], a SNMP party can be represented by an ASN.1 value with the following syntax. SnmpParty ::= SEQUENCE { partyIdentity OBJECT IDENTIFIER, partyTDomain OBJECT IDENTIFIER, partyTAddr OCTET STRING, partyProxyFor OBJECT IDENTIFIER, partyAuthProt OBJECT IDENTIFIER, partyAuthClock TimeTicks, Galvin, McCloghrie, Davin [Page 7] INTERNET-DRAFT April 1991 partyAuthLastMsg TimeTicks, partyAuthNonce INTEGER, partyAuthPrivate OCTET STRING, partyAuthPublic OCTET STRING, partyAuthLifetime INTEGER, partyPrivProt OBJECT IDENTIFIER, partyPrivPrivate OCTET STRING, partyPrivPublic OCTET STRING } For each SnmpParty value that supports the generation of messages using the SNMP MD4 authentication protocol or the receipt of messages via the SNMP DES privacy protocol, the significance of each of its components is as follows. o Its partyIdentity component is called the identity of the party and corresponds to the party identity. o Its partyTDomain component is called the transport domain and indicates the kind of transport service by which the party receives network management traffic. An example of a transport domain is rfc1157Domain (SNMP over UDP). o Its partyTAddr component is called the transport addressing information and represents a transport service address by which the party receives network management traffic. o Its partyProxyFor component is called the proxied party and represents the identity of a second SNMP Galvin, McCloghrie, Davin [Page 8] INTERNET-DRAFT April 1991 party or other management entity with which interaction may be necessary to satisfy received management requests. In this context, the value noProxy signifies that the party responds to received management requests by entirely local mechanisms. o Its partyAuthProt component is called the authentication protocol and identifies a protocol by which all messages generated by the party are authenticated as to integrity and origin. In this context, the value md4AuthProt, defined in [7], signifies that messages generated by the party are authenticated using the SNMP MD4 authentication protocol. o Its partyAuthClock component is called the authentication clock and represents a notion of the current time that is specific to the party. o Its partyAuthLastMsg component is called the ratchet and represents a notion of time associated with the most recent, authentic protocol message generated by the party. o Its partyAuthNonce component is called the nonce and represents a monotonically increasing integer associated with the most recent, authentic protocol message generated by the party. The nonce associated with a particular message distinguishes it among all others transmitted in the same unit TimeTicks interval. o Its partyAuthPrivate component is called the private authentication key and represents the secret value needed to support the authentication protocol. This component is a 16 octet value. o Its partyAuthPublic component is called the public authentication key and represents any public value that may be needed to support the authentication protocol. This component is not relevant. o Its partyAuthLifetime component is called the lifetime and represents an administrative upper bound Galvin, McCloghrie, Davin [Page 9] INTERNET-DRAFT April 1991 on acceptable delivery delay for protocol messages generated by the party. o Its partyPrivProt component is called the privacy protocol and identifies a protocol by which all protocol messages received by the party are protected from disclosure. In this context, the value desPrivProt, defined in [7], signifies that messages received by the party are protected from disclosure by the SNMP DES privacy protocol. o Its partyPrivPrivate component is called the private privacy key and represents the secret value needed to support the privacy protocol. This component is a 16 octet value, of which the first 8 octets are a DES key and the second 8 octets are a DES Initialization Vector. o Its partyPrivPublic component is called the public privacy key and represents any public value that may be needed to support the privacy protocol. This component is not relevant. 5 SNMP MD4 Authentication Protocol This section describes the SNMP MD4 authentication protocol. It provides both for verifying the integrity of a received message (i.e., the message received is the message sent) and for verifying the origin of a message (i.e., the reliable identification of the originator). The integrity of the message is protected by computing a digest over an appropriate portion of a message. The digest is computed by the originator of the message, transmitted with the message and verified by the recipient of the message. A secret value known only to the originator and recipient of the message is prefixed to the message prior to the digest computation. Thus, the origin of the message is known implicitly with the verification of the digest. Galvin, McCloghrie, Davin [Page 10] INTERNET-DRAFT April 1991 Recall from [2], a SNMP management communication is represented by an ASN.1 value with the following syntax. SnmpMgmtCom ::= [1] IMPLICIT SEQUENCE { dstParty OBJECT IDENTIFIER, srcParty OBJECT IDENTIFIER, pdu PDUs } For each SnmpMgmtCom value that represents a SNMP management communication, the following statements are true: o Its dstParty component is called the destination and identifies the SNMP party to which the communication is directed. o Its srcParty component is called the source and identifies the SNMP party from which the communication is originated. o Its pdu component has the form and significance attributed to it in [1] Recall from [2], a SNMP authenticated management communication is represented by an ASN.1 value with the following syntax. SnmpAuthMsg ::= [1] IMPLICIT SEQUENCE { authInfo ANY, - defined by authentication protocol authData SnmpMgmtCom } Galvin, McCloghrie, Davin [Page 11] INTERNET-DRAFT April 1991 For each SnmpAuthMsg value that represents a SNMP authenticated management communication, the following statements are true: o Its authInfo component is called the authentication information and represents information required in support of the authentication protocol used by the SNMP party originating the message. The detailed significance of the authentication information is specific to the authentication protocol in use; it has no effect on the application semantics of the communication other than its use by the authentication protocol in determining whether the communication is authentic or not. o Its authData component is called the authentication data and represents a SNMP management communication. In support of the SNMP MD4 authentication protocol, an authInfo component is of data type AuthInformation. It is represented by an ASN.1 value with the following syntax. AuthInformation ::= [1] IMPLICIT SEQUENCE { authTimestamp TimeTicks, authNonce INTEGER, authDigest OCTET STRING } For each AuthInformation value that represents authentication information, the following statements are true: o Its authTimestamp component is called the authentication timestamp and represents the time of the Galvin, McCloghrie, Davin [Page 12] INTERNET-DRAFT April 1991 generation of the message according to the partyAuthClock of the SNMP party that originated it. o Its authNonce component is called the authentication nonce and represents an integer value evaluated according to the authTimestamp value. In particular, the granularity of the authentication timestamp limits the total number of management communications to 100 per second. The authentication nonce is a monotonically increasing sequence number, that is reset for each new authentication timestamp value. o Its authDigest component is called the authentication digest and represents the digest computed over an appropriate portion of the message, where the message is temporarily prefixed with a secret value for the purposes of computing the digest. The SNMP MD4 authentication protocol is identified by the ASN.1 object identifier value md4AuthProt. 5.1 Generating a Message This section describes the behavior of a SNMP protocol entity when it acts as a SNMP party for which the authentication protocol is administratively specified as md4AuthProt. Insofar as the behavior of a SNMP protocol entity when transmitting protocol messages is defined generically in [2], only those aspects of that behavior that are specific to the SNMP MD4 authentication protocol are described below. In particular, this section describes the encapsulation of a SNMP management communication into a SNMP authenticated management communication. According to [2], a SnmpAuthMsg value is constructed during Step 3 of generic processing. In particular, it states the authInfo component is constructed according to the partyAuthProt value of the SNMP party originating the Galvin, McCloghrie, Davin [Page 13] INTERNET-DRAFT April 1991 message. When that value is md4AuthProt, the procedures executed by a SNMP protocol entity whenever either a management request or a trap notification is to be transmitted by a SNMP party are as follows. 1. The local database is consulted to determine the authentication clock, ratchet, nonce and private authentication key of the SNMP party originating the message. 2. The authTimestamp component is set to the retrieved authentication clock value. 3. If the ratchet is equal to the authentication clock, the nonce is incremented. Otherwise the nonce is set to zero. The authNonce component is set to the nonce value. In the local database, the originating SNMP party's partyAuthNonce is set to the nonce value and the originating SNMP party's partyAuthLastMsg is set to the authentication clock. 4. The authentication digest is temporarily set to the private authentication key. The SnmpAuthMsg is serialized according to the conventions of [6] and [1]. An MD4 digest is computed over this octet sequence according to [3]. The authDigest component is set to the computed digest value. As set forth in [2], the SnmpAuthMsg value is then encapsulated according to the appropriate privacy protocol into a SnmpPrivMsg value. This latter value is then serialized and transmitted to the receiving SNMP party. 5.2 Receiving a Message This section describes the behavior of a SNMP protocol entity upon receipt of a protocol message from a SNMP party for which the authentication protocol is administratively specified Galvin, McCloghrie, Davin [Page 14] INTERNET-DRAFT April 1991 as md4AuthProt. Insofar as the behavior of a SNMP protocol entity when receiving protocol messages is defined generically in [2], only those aspects of that behavior that are specific to the SNMP MD4 authentication protocol are described below. According to [2], a SnmpAuthMsg value is evaluated during Step 9 of generic processing. In particular, it states the SnmpAuthMsg is evaluated according to the partyAuthProt value of the SNMP party that originated the message. When that value is md4AuthProt, the procedures executed by a SNMP protocol entity whenever a management communication is received by a SNMP party are as follows. 1. If the authInfo component value is not an AuthInformation value, the message is evaluated as unauthentic. Otherwise, the value is extracted from the SnmpAuthMsg value. 2. The local database is consulted to determine the authentication clock, ratchet, nonce, private authentication key and lifetime of the SNMP party that originated the message. 3. If the authTimestamp component plus the lifetime is less than the authentication clock, the message is evaluated as unauthentic. 4. If the authTimestamp component is less than the ratchet, the message is evaluated as unauthentic. 5. If the authTimestamp component is equal to the ratchet and if the authNonce component is less than or equal to the nonce, the message is evaluated as unauthentic. 6. The authDigest component is extracted and temporarily recorded. 7. A new SnmpAuthMsg is constructed such that its authDigest component is set to the private Galvin, McCloghrie, Davin [Page 15] INTERNET-DRAFT April 1991 authentication key and its other components are set to the value of the corresponding components in the received SnmpAuthMsg. This new SnmpAuthMsg value is serialized according to the conventions of [6] and [1]. An MD4 digest is computed over this octet sequence according to [3]. 8. If the computed digest value is not equal to the previously recorded digest value, the message is evaluated as unauthentic. 9. If the authTimestamp component exceeds the ratchet, in the local database the originating SNMP party's partyAuthLastMsg value is set to the authTimestamp value and the originating SNMP party's partyAuthNonce value is set to zero. 10. If the authTimestamp component equals the ratchet, in the local database the originating SNMP party's partyAuthNonce value is set to the authNonce component. If the SnmpAuthMsg value is evaluated as unauthentic, an authentication failure is noted and the received message is discarded without further processing. Otherwise, processing of the received message continues as specified in [2]. 6 SNMP DES Privacy Protocol This section describes the SNMP DES privacy protocol. It provides for protection from disclosure of a received message. An appropriate portion of the message is encrypted according to a secret key known only to the originator and recipient of the message. The SNMP DES privacy protocol is identified by the ASN.1 object identifier value desPrivProt. Galvin, McCloghrie, Davin [Page 16] INTERNET-DRAFT April 1991 Recall from [2], a SNMP private management communication is represented by an ASN.1 value with the following syntax. SnmpPrivMsg ::= [1] IMPLICIT SEQUENCE { privDst OBJECT IDENTIFIER, privData [1] IMPLICIT OCTET STRING } For each SnmpPrivMsg value that represents a SNMP private management communication, the following statements are true: o Its privDst component is called the private destination and identifies the SNMP party to which the communication is directed. o Its privData component is called the private data and represents the (possibly encrypted) serialization (according to the conventions of [6] and [1]) of a SNMP authenticated management communication (according to the conventions of the SNMP MD4 authentication protocol). 6.1 Generating a Message This section describes the behavior of a SNMP protocol entity when it acts as a SNMP party for which the privacy protocol is administratively specified as desPrivProt. Insofar as the behavior of a SNMP protocol entity when transmitting a protocol message is defined generically in [2], only those aspects of that behavior that are specific to the SNMP DES privacy protocol are described below. In particular, this section describes the encapsulation of a SNMP authenticated management communication into a SNMP private management communication. Galvin, McCloghrie, Davin [Page 17] INTERNET-DRAFT April 1991 According to [2], a SnmpPrivMsg value is constructed during Step 5 of generic processing. In particular, it states the privdata component is constructed according to the partyPrivProt value of the SNMP party receiving the message. When that value is desPrivProt, the procedures executed by a SNMP protocol entity whenever either a management request or a trap notification is to be transmitted by a SNMP party are as follows. 1. If the SnmpAuthMsg is not authenticated according to the conventions of md4AuthProt, the generation of the SnmpPrivMsg fails according to a local procedure, without further processing. 2. The local database is consulted to determine the private privacy key of the SNMP party receiving the message. The encryption key and the initialization vector are extracted from the private privacy key. 3. The SnmpAuthMsg is serialized according to the conventions of [6] and [1]. 4. The serialized value is encrypted according to the DES [4] in the Cipher Block Chaining mode of operation [5], using the extracted encryption key and initialization vector. 5. The privData component is set to the encrypted value. As set forth in [2], the SnmpPrivMsg value is then serialized and transmitted to the receiving SNMP party. 6.2 Receiving a Message This section describes the behavior of a SNMP protocol entity when it acts as a SNMP party for which the privacy protocol is administratively specified as desPrivProt. Insofar as the behavior of a SNMP protocol entity when receiving a protocol Galvin, McCloghrie, Davin [Page 18] INTERNET-DRAFT April 1991 message is defined generically in [2], only those aspects of that behavior that are specific to the SNMP DES privacy protocol are described below. According to [2], a privData component of a received SnmpPrivMsg value is evaluated during Step 4 of generic processing. In particular, it states the privData component is evaluated according to the partyPrivProt value of the SNMP party receiving the message. When that value is desPrivProt, the procedures executed by a SNMP protocol entity whenever a management communication is received by a SNMP party are as follows. 1. The local database is consulted to determine the private privacy key of the SNMP party receiving the message. The encryption key and the initialization vector are extracted from the private privacy key. 2. The privData component is decrypted according to the DES [4] in the Cipher Block Chaining mode of operation [5], using the extracted encryption key and initialization vector. Processing of the received message continues as specified in [2]. 7 Clock and Secret Distribution The protocols described in Sections 5 and 6 assume the existence of loosely synchronized clocks and secret values. Although a variety of strategies may be employed, there are three requirements that apply to any strategy employed. o If the value of an authentication clock is decreased, the ratchet and private authentication key must be changed concurrently. When the value of an authentication clock is decreased, messages that have been sent with a timestamp value Galvin, McCloghrie, Davin [Page 19] INTERNET-DRAFT April 1991 between the value of the authentication clock and its new value may be replayed. Changing the private authentication key obviates this threat. However, changing the authentication clock and the private authentication key is not sufficient to ensure proper operation. If the ratchet is not reduced similarly to the authentication clock, no message will be considered authentic until the value of the authentication clock exceeds the value of the ratchet. o The private authentication key and private privacy key must be known only to the parties requiring knowledge of them. Protecting the secrets from disclosure is critical to the security of the protocols. In particular, if the secrets are distributed via a network, the secrets must be protected with a protocol that supports confidentiality, e.g., the SNMP DES privacy protocol. Further, knowledge of the secrets must be as restricted as possible within an implementation. In particular, although the secrets may be known to one or more persons during the initial configuration of a device, the secrets should be immediately changed such that their actual value is known only to the software. A management station has the additional responsibility of recovering the state of all parties whenever it boots. This may require recording the secrets on a long-term storage device. Access to information on this device must be as restricted as is practically possible. o There must exist at least one SNMP protocol entity that assumes the role of a responsible management station. This management station is responsible for ensuring that all authentication clocks are synchronized and for changing the secret values when necessary. Although more than one management station may participate in the responsibility, their coordination is essential to the secure management of the network. The mechanism by which multiple management stations ensure that no Galvin, McCloghrie, Davin [Page 20] INTERNET-DRAFT April 1991 more than one of them attempts to synchronize the clocks or update the secrets at any one time is a local implementation issue. This section describes one strategy for maintaining loosely synchronized clocks and distributing secret values among SNMP parties that support the SNMP MD4 authentication protocol and the SNMP DES privacy protocol. First, the procedures by which a SNMP protocol entity is initially configured are specified. Lastly, the procedures by which a SNMP protocol entity recovers from a "crash" are specified. 7.1 Initial Configuration This section describes the initial configuration of a SNMP protocol entity that supports the SNMP MD4 authentication protocol or both the SNMP MD4 authentication protocol and the SNMP DES privacy protocol. When a network device is first installed, its initial configuration must be done manually, i.e., a person must physically visit the device and enter the initial values for each SNMP party. This suggests the person will have knowledge of the initial secret values. This process may be repeated for each SNMP party that is added over time. In general, however, the security of a system is enhanced as the number of entities that know a secret is reduced. Requiring a person to physically visit a device every time a SNMP party is configured exposes the secrets unnecessarily. In particular, the initial authentication secret is 128 bits long, plus 64 bits each for the initial DES key and initialization vector. Clearly these values will need to be recorded on a medium in order to be transported between a responsible management station and a managed agent. Alternatively, a small set of SNMP parties may be configured for each SNMP protocol entity, one pair of which may be used to initially configure all other SNMP parties. Galvin, McCloghrie, Davin [Page 21] INTERNET-DRAFT April 1991 In fact, there is a minimal, useful set of SNMP parties that could be configured between each responsible management station and managed agent. This minimal set includes one of each of the following for both the responsible management station and the managed agent. o a SNMP party identity with its partyAuthProt and partyPrivProt set to noAuth and noPriv, respectively o a SNMP party identity with its partyAuthProt and partyPrivProt set to md4AuthProt and noPriv, respectively o a SNMP party identity with its partyAuthProt and partyPrivProt set to md4AuthProt and desPrivProt, respectively The last of these SNMP parties in both the responsible management station and the managed agent could be used to configure all other SNMP parties. It is the only suitable identity for this purpose, since it is the only identity that supports data confidentiality. Data confidentiality is necessary in order to protect the distributed secrets from disclosure to unauthorized entities. Configuring one pair of SNMP parties to be used to configure all other parties has the advantage of only exposing one pair of secrets, the secrets used to configure the minimal, useful set identified above. To limit this exposure, a management station should change these values as its first operation upon completion of the initial configuration. In this way, secrets are known only to the peers requiring knowledge of them in order to communicate. The Management Information Base (MIB) document supporting these security protocols specifies 6 initial party identities and initial values, which, by convention, are assigned to the parties and their associated parameters. [7] Galvin, McCloghrie, Davin [Page 22] INTERNET-DRAFT April 1991 All 6 parties should be configured in each new managed agent and its responsible management station. The responsible management station should be configured before the managed agent, since the management station can be used to generate the initial secrets and provide them to a person, on a suitable medium, for distribution to the managed agent. The following sequence of steps specifies how to initially configure a managed agent and its responsible management station upon its installation. 1. Determine the initial values for each of the components of the SNMP party to be configured. Some of these values may be computed by the responsible management station, some may be specified in the MIB document and some may be administratively determined. 2. Configure the parties in the responsible management station, according to the set of initial values. If the management station is computing some initial values to be entered into the agent, an appropriate medium must be present to record the values. 3. Configure the parties in the managed agent, according to the set of initial values. 4. The responsible management station must synchronize the authentication clock values for each party it shares with the managed agent. The following sequence of events specifies how this should be done. (a) The responsible management station must retrieve the authentication clocks for each party to be reconfigured. This must be an unauthenticated request, since the management station does not know if the clocks are synchronized. (b) The values received must be used to again retrieve the authentication clocks for each party to be reconfigured. This must be an authenticated request, so the actual values can be verified. Galvin, McCloghrie, Davin [Page 23] INTERNET-DRAFT April 1991 (c) The values received must be used to determine an appropriate value for each of the authentication clocks. These new values must be distributed to the appropriate set of parties in order to synchronize the clocks, using an authenticated message. 5. The responsible management station should change the secret values just configured to ensure the actual values are known only to the peers requiring knowledge of them in order to communicate. To do this, a management station generates new secrets for each party to be reconfigured and distributes those secrets using a protocol that protects them from disclosure, e.g., desPrivProt. (see Section 7.3) If the managed agent does not support a protocol that protects messages from disclosure, then automatic maintenance and configuration of parties is not possible, i.e., the last step above is not possible. The secrets can only be changed by a physical visit to the device. If there are other SNMP protocol entities requiring knowledge of the secrets, the responsible management station must distribute the information upon completion of the initial configuration. The mechanism used must protect the secrets from disclosure to unauthorized entities, e.g., the SNMP DES privacy protocol is an acceptable mechanism. 7.2 Clock Distribution A responsible management station must ensure the authentication clock value for a particular SNMP party is loosely synchronized among all the local databases in which it appears. The skew among the clock values must be accounted for in the lifetime value, in addition to the expected communication delivery delay. The detection of a skewed authentication clock may be identified by a number of strategies, including knowledge of Galvin, McCloghrie, Davin [Page 24] INTERNET-DRAFT April 1991 the accuracy of the system clock, unauthenticated queries of the local database and recognition of authentication failures originated by the party. The sequence of steps by which a responsible management station changes the authentication clock for a particular SNMP party is similar to that used to change a secret value. (see Section 7.3) However, since the clock value need not be protected from disclosure, it is not necessary for the destination to support a privacy protocol to distribute clock values. If the authentication clock for a particular SNMP party ever reaches the maximal TimeTicks value,(The value of interest may be TimeTicks less lifetime. When authenticating a message, its authentication timestamp is added to lifetime and compared to the authentication clock. A SNMP protocol entity must guarantee that the sum is never greater than the maximum TimeTicks value.) the clock must halt at that value and the lifetime value must be set to zero. In this state, the only authenticated request a managed agent should process for this party is one that alters the value of at least its authentication clock, private authentication key, ratchet, nonce and lifetime. In order to reset these values, the responsible management station must set the authentication timestamp in the message to the maximum TimeTicks value. The nonce value is used to distinguish multiple messages. If the value of the authentication clock for a particular SNMP party is ever altered such that its new value is less than its old value, its ratchet and private authentication key must be altered at the same time. 7.3 Secret Distribution This section describes one strategy by which a SNMP protocol entity that supports both the SNMP MD4 authentication protocol and the SNMP DES privacy protocol can change the Galvin, McCloghrie, Davin [Page 25] INTERNET-DRAFT April 1991 secrets for a particular SNMP party. The frequency with which the secrets of a SNMP party should be changed is a local administrative issue. However, the more frequently a secret is used, the more frequently it should be changed. At a minimum, the secrets must be changed at least every maximum TimeTicks value. The following sequence of steps specifies how a responsible management station changes a secret value for a particular SNMP party, i.e., how to change the private authentication key or the private privacy key. 1. The responsible management station generates a new secret value. 2. The responsible management station encapsulates a SNMP Set request in a SNMP private management communication with at least the following properties. o Its source supports the SNMP MD4 authentication protocol. o Its destination supports the SNMP DES privacy protocol. 3. The SNMP private management communication is transmitted to its destination. 4. Upon receiving the request, the recipient processes the message according to [1] and [2]. 5. The recipient encapsulates a SNMP Set response in a SNMP private management communication with at least the following properties. o Its source supports the SNMP MD4 authentication protocol. o Its destination supports the SNMP DES privacy protocol. 6. The SNMP private management communication is transmitted to its destination. Galvin, McCloghrie, Davin [Page 26] INTERNET-DRAFT April 1991 7. Upon receiving the response, the responsible management station updates its local database with the new value. If the responsible management station does not receive a response to its request, there are two possible causes. o The request may not have been delivered to the destination. o The response may not have been delivered to the originator of the request. In order to distinguish the two possible error conditions, a responsible management station should check the destination to see if the change has occurred. Unfortunately, since the secret values are unreadable, this is not possible. One alternative is to set the public value corresponding to the secret being changed to a recognizable, novel value, i.e., set partyAuthPublic when changing partyAuthPrivate, or set partyPrivPublic when changing partyPrivPrivate. In this way, the responsible management station may retrieve the public value when a response is not received, and verify if the change has taken place. One other scenario worthy of mention is using a SNMP party to change its own secrets. In this case, the destination will change its local database prior to generating a response. Thus, the response will be constructed according to the new value. However, the responsible management station will not update its local database until after the response is received. This suggests the responsible management station will actually receive a response which will be evaluated as unauthentic. The responsible management station may either account for this scenario as a special case, or use the mechanism described above to verify if the change has taken place, i.e., setting the public value to a recognizable, novel value when changing a secret value. Galvin, McCloghrie, Davin [Page 27] INTERNET-DRAFT April 1991 7.4 Crash Recovery The authentication clock of a SNMP party is a critical component of the overall security of the protocols. The inclusion of a reliable representation of a clock in a SNMP protocol entity enhances the overall security. A reliable clock representation continues to increase according to the passage of time, even when the local SNMP protocol entity -- due to power loss or other system failure -- may not be operating. An example of a reliable clock representation is that provided by battery-powered clock-calendar devices incorporated into some contemporary systems. If a managed agent crashes and does not reboot in time for its responsible management station to prevent its authentication clock from reaching its maximum value, upon reboot the clock should be halted at its maximal value. The procedures specified in Section 7.2 would then apply. If a managed network element supports a reliable clock representation,(It is assumed that management stations will always support reliable clock representations, where a person present during the recovery setting the clock is considered a reliable representation.) recovering from a crash requires no special actions. For each SNMP party in the local database for such a SNMP protocol entity, its identity, authentication clock, private authentication key and private privacy key must enjoy non-volatile, incorruptible representations. If possible, lifetime should also enjoy a non-volatile, incorruptible representation. Upon reboot, the remaining elements of each SNMP party are set as follows.(It is assumed the only security protocols are the two specified in this memo. Should there exist other security protocols, the authentication protocol and the privacy protocol would also require non-volatile, incorruptible representations.) o If the private authentication key is not noAuth, the authentication protocol is set to md4AuthProt. Galvin, McCloghrie, Davin [Page 28] INTERNET-DRAFT April 1991 o The ratchet is initialized to the value of the authentication clock. o The nonce is initialized to zero. o If the lifetime is not retained, it should be initialized to zero. o If the private privacy key is not noPriv, the privacy protocol is set to desPrivProt. Upon detecting that a managed agent has rebooted, a responsible management station must reset all other values, and the lifetime if it was not retained. In order to reset the lifetime, the responsible management station must set the authentication timestamp in the message to the sum of the authentication clock and lifetime. This is an artificial advancement of the authentication timestamp in order to guarantee the message will be authentic when received by the recipient. If, however, a managed network element does not support a reliable clock representation, the following set of procedures apply to all SNMP protocol entities residing at that element. For each SNMP party in the local database for a SNMP protocol entity, its identity, private authentication key and private privacy key must enjoy non-volatile, incorruptible representations. Upon reboot, the remaining elements of each SNMP party are set as follows. o If the private authentication key is not noAuth, the authentication protocol is set to md4AuthProt. o The authentication clock is initialized to the maximum TimeTicks value. o The ratchet is initialized to the maximum TimeTicks value. o The nonce is initialized to zero. Galvin, McCloghrie, Davin [Page 29] INTERNET-DRAFT April 1991 o The lifetime is initialized to zero. o If the private privacy key is not noPriv, the privacy protocol is set to desPrivProt. In this state, the only authenticated request a managed agent should process for this party is one that alters the value of at least its authentication clock, private authentication key, ratchet, nonce and lifetime. In order to reset these values, the responsible management station must set the authentication timestamp in the message to the maximum TimeTicks value. The nonce value is used to distinguish multiple messages. 8 Security Considerations To address the possibility of message duplication (malicious or otherwise), a management station should discard SNMP responses for which neither the request-id component nor the represented management information corresponds to any currently outstanding request. Owing to the possible loss or suppression of authentication failure traps, due to authentication clock skew or inconsistent notions of shared secrets, a management station should not interpret an agent's lack of response to an authenticated SNMP management communication as a conclusive indication of agent or network failure. In order either to facilitate administration of a SNMP party for which clock synchronization may be lapsed or to provide for continued management in times of network stress, a management station implementation may provide for arbitrary, artificial advancement of the timestamp on locally generated messages. The security of a SNMP party based upon the SNMP MD4 authentication protocol depends upon the magnitude of its lifetime: a large lifetime increases the vulnerability to replay attacks. Thus, the lifetime for a party must be chosen (by the local administration) to be as small as possible, given the Galvin, McCloghrie, Davin [Page 30] INTERNET-DRAFT April 1991 accuracy of clock devices available, relevant round-trip communications delays and the frequency with which a responsible management station will be able to verify all clock values. The implementation of a management station may, when explicitly authorized, provide for dynamic adjustment of the lifetime in order to accommodate changing network conditions. 8.1 Conformance A SNMP protocol entity implementation that claims conformance to this memo satisfies the following requirements: 1. For each SNMP party about which it maintains information in a local database, it must, to the maximal extent possible, prohibit read-access to the private authentication key and private encryption key under all circumstances except as required to generate and/or validate SNMP messages with respect to that party. This prohibition includes prevention of read-access by the agent's human operators. 2. In the event that it loses knowledge of a party's parameters, (in particular, the private authentication key, private encryption key, authentication clock, or ratchet) it must alter its record of these quantities to random values, so subsequent interaction with that party requires manual redistribution of new secrets and other parameters. 3. For each SNMP party about which it maintains information in a local database, it must prohibit alterations to its record of the authentication clock for that party independently of alterations to its record of the private authentication key (unless the clock alteration is an advancement). 4. If it selects new value(s) for a party's secret(s), it must avoid bad or obvious choices for said secret(s). Choices Galvin, McCloghrie, Davin [Page 31] INTERNET-DRAFT April 1991 to be avoided are boundary values (such as all-zeros), and predictable values (such as the same value as previously, or selecting from a predetermined set). 5. For each SNMP party about which it maintains information in a local database, it must never increase its record of the lifetime for that party except as may be explicitly authorized (via imperative command or securely represented configuration information) by the responsible network administrator. 6. For each SNMP party about which it maintains information in a local database, it must never allow its record of the authentication clock for that party to be incremented beyond the maximal TimeTicks value and so "roll-over" to zero. 8.2 Protocol Correctness The correctness of these SNMP security protocols with respect to the stated goals depends on the following assumptions: 1. The MD4 message digest algorithm satisfies its design criteria.(In particular, it must be computationally infeasible to discover two messages that share the same digest value.) 2. The DES cryptographic algorithm satisfies its design criteria.(In particular, it must be computationally infeasible to determine the cleartext message from the ciphertext message without knowledge of the DES key used in the transformation.) 3. Local notions of a party's authentication clock while it is associated with a specific key value are monotonically non-decreasing (i.e., they never run backwards) in the absence of administrative manipulations. 4. The secrets for a particular SNMP party are known only to authorized SNMP protocol entities. Galvin, McCloghrie, Davin [Page 32] INTERNET-DRAFT April 1991 5. Local notions of the authentication clock for a particular SNMP party are never altered (except for advancements) without also altering the private authentication key. For each mechanism of the protocol, an informal account of its contribution to the required goals is presented below. 8.2.1 Clock Monotonicity Mechanism By pairing each sequence of a clock's values with a unique key, the protocols partially realize goals 3 and 4, and the conjunction of this property with assumption 1 above is sufficient for the claim that all local notions of a party's authentication clock are, in general, non-decreasing with time. 8.2.2 Data Integrity Mechanism The protocols require computation of the MD4 message digest computed over the SNMP message prepended by the secret for the relevant party. By virtue of this mechanism and assumptions 1 and 4, the protocols realize goal 1. Normally, the inclusion of the message digest value with the digested message would not be sufficient to guarantee data integrity, since the digest value can be modified in addition to the message while it is enroute. However, since not all of the digested message is included in the transmission to the destination, it is not possible to substitute both a message and a digest value while enroute to a destination. 8.2.3 Data Origin Authentication Mechanism The data integrity mechanism requires the use of a secret value known only to communicating parties. By virtue of this mechanism and assumptions 1 and 4, the protocols explicitly prevent unauthorized modification of messages. Data origin Galvin, McCloghrie, Davin [Page 33] INTERNET-DRAFT April 1991 authentication is implicit if the message digest value can be verified. That is, the protocols realize goal 2. 8.2.4 Restricted Administration Mechanism This memo requires that implementations preclude administrative alterations of the authentication clock for a particular party independently from its private authentication key (unless that clock alteration is an advancement). An example of an efficient implementation of this restriction is provided in a pseudocode fragment below. This pseudocode fragment meets the requirements of assumption 5. Pseudocode Fragment. Observe that the requirement is not for simultaneous alteration but to preclude independent alteration. This latter requirement is fairly easily realized in a way that is consistent with the defined semantics of the SNMP Set operation. Although alteration in a single request is clearly more efficient -- and less complicated in the face of agent failures -- than alteration in multiple requests, security requirements are satisfied by either strategy so long as the logical dependency between the clock and the secret is assured. Void partySetKey (party, newKeyValue) - if (party->clockAltered) - party->clockAltered = FALSE; party->keyAltered = FALSE; party->keyInUse = newKeyValue; party->clockInUse = party->clockCache; } else - party->keyAltered = TRUE; party->keyCache = newKeyValue; } } Galvin, McCloghrie, Davin [Page 34] INTERNET-DRAFT April 1991 Void partySetClock (party, newClockValue) - if (party->keyAltered) - party->keyAltered = FALSE; party->clockAltered = FALSE; party->clockInUse = newClockValue; party->keyInUse = party->keyCache; } else - party->clockAltered = TRUE; party->clockCache = newClockValue; } } 8.2.5 Ordered Delivery Mechanism The definition of the SNMP security protocol requires that, if the timestamp value on a received message does not exceed the timestamp of the most recent validated message locally delivered from the originating party, then that message is not delivered. Otherwise, the record of the timestamp for the most recent locally delivered validated message is updated. if (msgIsValidated) - if (timestampOfReceivedMsg > party->timestampOfLastDeliveredMsg) - party->timestampOfLastDeliveredMsg = timestampOfReceivedMsg; } else - msgIsValidated = FALSE; } } Although not explicitly represented in the pseudocode above, in the SNMP MD4 authentication protocol, the ordered Galvin, McCloghrie, Davin [Page 35] INTERNET-DRAFT April 1991 delivery mechanism must ensure that, when the authentication timestamp of the received message is equal to the ratchet, received messages continue to be delivered as long as their nonce values are monotonically increasing. By virtue of this mechanism, the protocols realize goal 4. 8.2.6 Message Timeliness Mechanism The definition of the SNMP security protocols requires that, if the authentication timestamp value on a received message -- augmented by an administratively chosen lifetime value -- is less than the local notion of the clock for the originating SNMP party, the message is not delivered. if (timestampOfReceivedMsg + party->administrativeLifetime <= party->localNotionOfClock) - msgIsValidated = FALSE; } By virtue of this mechanism, the protocols realize goal 3. In cases in which the local notions of a particular SNMP party clock are moderately well-synchronized, the timeliness mechanism effectively limits the age of validly delivered messages. Thus, if an attacker diverts all validated messages for replay much later, the delay introduced by this attack is limited to a period that is proportional to the skew among local notions of the party clock. 8.2.7 Confidentiality Mechanism The protocols require the use of the DES when the data confidentiality service is required. By virtue of this mechanism and assumption 2, the protocols realize goal 5. Galvin, McCloghrie, Davin [Page 36] INTERNET-DRAFT April 1991 References [1] Jeffrey D. Case, Mark S. Fedor, Martin L. Schoffstall, and James R. Davin. A Simple Network Management Protocol (SNMP). RFC 1157, DDN Network Information Center, SRI International, May 1990. Obsoletes RFC1098. [2] SNMP administrative model. In preparation., March 1991. [3] Ronald L. Rivest. The md4 message digest algorithm. RFC 1186, DDN Network Information Center, SRI International, October 1990. [4] FIPS Publication 46-1. Data Encryption Standard. National Institute of Standards and Technology, U. S. Department of Commerce, Washington, D.C., January 1977. Federal Information Processing Standard (FIPS). [5] FIPS Publication 81. DES Modes of Operation. National Institute of Standards and Technology, U. S. Department of Commerce, Washington, D.C., December 1980. Federal Information Processing Standard (FIPS). [6] Information Processing -- Open Systems Interconnection -- Specification of Basic Encoding Rules for Abstract Syntax Notation One (ASN.1). International Organization for Standardization/International Electrotechnical Institute, 1987. International Standard 8825. [7] Keith McCloghrie, James R. Davin, and James M. Galvin. Experimental Definitions of Managed Objects for Administration of SNMP Parties. RFC DRAFT, DDN Network Information Center, SRI International, 1991. Working Draft. Galvin, McCloghrie, Davin [Page 37]