8. Network Layer Contents

Transcription

1 Contents 1 / 43 * Earlier Work * IETF IP sec Working Group * IP Security Protocol * Security Associations * Authentication Header * Encapsulation Security Payload * Internet Key Management Protocol * Modular Key Management Protocol * Internet Security Association Key Management Protocol * Discussion Earlier Work 2 / 43 Security Protocol 3 (SP3) * Network layer security protocol developed by NSA. * Uses cryptographic techniques to provide security services for connectionless version of the OSI network layer protocol. * Transport layer messages are processed by SP3 before being passed to lower network sublayers. 1

2 3 / 43 Network Layer Security Protocol (NLSP) * Developed by ISO. * To secure the connectionless network layer protocol (CLNP). * It is an incompatible descendent of SP3. Integrated NLSP (I-NLSP) * To provide security services for IPv4 and CLNP. * Security functions roughly similar to those of SP3. * I_NLSP provides additional functionality, e.g. security label processing. 4 / 43 * The protocols listed above are similar. * They all use encapsulation. * Outgoing plaintext packets are authenticated and/or encrypted and encapsulated in outer network layer headers. * Incoming packets are decapsulated, outer network layer headers are stripped off and the inner packets are authenticated and/or decrypted, and forwarded to the destination. * IP encapsulation requires no changes to existing Internet routing infrastructure. * Authenticated and/or encrypted IP packets have unencrypted, normal-looking outer IP header, can processed as usual. Outer IP header of 'swipe' Authenticated and/or encrypted inner IP packet (complete packet, including header) Authentication data+padding IP encapsulation 2

3 IETF IP sec Working Group 5 / 43 * IP would like to have sufficiently strong cryptographic security mechanisms. * The security mechanisms should be algorithm-independent, so that the cryptographic algorithm can be altered without effecting other parts of the implementation. * Wide variety of security policies should be supported. * The design should avoid adverse impacts on Internet user who do not want security functions. --> The result of this effort was the specification of a security architecture for IPv6 that comprises authentication and encryption mechanisms. * In 1992 the IETF IPsec WG standardized the IP Security Protocol (IPSP) and the Internet Key Management Protocol (IKMP). * The same security architecture that had been designed for IPv6 could be adapted for IPv4 as well. * The security mechanisms specified have to be retrofitted into IPv4 implementations and must be present in IPv6. IP Security Protocol (IPSP) 6 / 43 * IPv6 security architecture comprises data authentication and encryption mechanism, these mechanisms constitute the IPSP. The two security mechanisms of IPSP: * Authentication Header (AH): provides data origin authentication and connectionless data integrity services. It allows the recipient of an IP packet to verify that the originator is authentic and that the packet has not been altered during transmission. * Encapsulation Security Payload (ESP): provides connectionless data confidentiality services. It ensures that only the legitimate recipients of IP packet are able to read it. * Both mechanism are based on the concept of a security association (SA) and may be used together or separately. * Full protection against traffic analysis is not provided. 3

4 Security Associations (SA) 7 / 43 * An SA is an agreement between two or more parties on the security services that they want to use and how they are going to provide it. * This agreement is established through a common set of security related parameters like - authentication algorithm, mode and keys for the AH mechanism, - encryption algorithm, mode, and keys for the ESP mechanism, - the size and presence or absence of the encryption algorithm synchronisation or initialisation vector (IV), - the lifetime of the keys and the SA as a whole, - the source address, including network and subnet address, - the security level of the secured data, such as confidential, secret or unclassified. This is required if multilevel security (MLS) is provided 8 / 43 * When an IP packet is received, it can only be authenticated and/or decrypted if the receiver can link it with an appropriate SA. Hence the IP packet must convey a reference that points to the SA on the receiver's side. * In IPsec this reference is called a security parameter index (SPI). * Each SA is uniquely identified by an SPI value and a destination address. The SPI is a 32 bit value that is negotiated during a key management protocol execution. 4

5 * Depending on the granularity, three keying approaches for setting up SA, distinguished. a) host-oriented keying: all users on a host share the same session key, b) user-oriented keying: each user on a host has a unique sessions for another host, c) session-unique keying: a single session key assigned to a given IP (address, upper-layer protocol, and port number) e.g an FTP session uses another key than the same user's Telnet session. Host A Host B Host A Host B Host A K1 Host B K1 9 / 43 K K2 K2 K3 a) b) c) 10 / 43 * User-oriented and session-unique keying are superior to host-oriented keying. A single computer system will have more than one user, which do not necessarily trust each other. * When host-oriented keying is used and different users exist, it is possible for a user to determine the key via well-know methods, such as a plaintext attack. * The use and handling of SAs and SPI values are different for an individual receiver and for a group of receivers. * When packets are sent to a individual receiver the SPI is usually chosen by the receiver. It typically is the index of a local table of security contexts maintained by the receiver. * If packets are sent to a group of receivers the SPI must be common to all members of the group. Each member should be able to correlate the combination of group address and SPI with the parameters used for secure group communications. 5

6 Authentication Header (AH) 11 / 43 * The IPSP AH mechanism provides data origin authentication and connectionless data integrity services for IP packets. * The AH mechanism has also been extended to support reply protection. * The IPSP AH mechanism provides security by adding authentication data to IP packets. * The authentication data is computed by using a cryptographic authentication algorithm and a corresponding key. * The sender computes the authentication data prior to sending the packets. * The receiver verifies the data upon receipt. * Problem: some fields of the IP packet header may change in transit. * e.g., in the IPv6 header, the Hop Count field value is decremented at each hop, the IPv6 Destination and next address are swapped at every relay of the source route. * e.g., in the IPv4 header, the TTL field value may changed in transit as well. 12 / 43 That means: the sender must prepare a temporary version of the IP packet before computing the authentication data. * This temporary version must be independent of any modifications in transit. * For example the sender must perform for an IPv6 packet the following steps: - 'Hop Count' field set to zero. - If routing header is used, the IP Destination field must be set to the final destination. - Options whose C-bit is set must not be taken in account when computing the authentication data. 6

7 13 / 43 * The authentication data must be computed using a cryptographic algorithm. * Conventional checksum algorithm, such as the 16-bit checksum used by IP, or the 16- or 32-bit polynomial checksum used by Ethernet, should not be used, because they are too simple to break. * The idea of using a keyed one-way hash function for message authentication is to concatenated the message with a secret key and to use the one-way hash function to compute a corresponding message authentication code (MAC) from the concatenate message. * The one-way hash function should be collision-resistant, a feature of MD5, SHA-1, RIPEM. * Keyed MD5, as proposed for IPSP, operates by combining the message with a secret authentication key and then computing an MD5 hash value from the result. 14 / 43 * The key is prepended and appended to the message to prevent certain types of attacks. * The sequence of operations : - temporary version of the message is prepared, - MD5 operates on blocks of 16 bytes, the message padded with zero bytes. - the authentication data is computed as an MD5 hash value of a message obtained by concatenating the key, the message and the key again. * The result of this general procedure is a hash value. With MD5 or RIPEM the result is 128-bit hash value. * In general, the authentication algorithm is negotiated as part of the SA. * Keyed MD5 is the default algorithm in RFC

8 * It is also possible to use public key cryptography for AH computation and verification. * In this case the sender would digitally sign a message and the receiver would verify the digital signature accordingly. * The use of public key cryptography not only requires more computation power on both side, but also larger authentication data fields. * A typical size of a digital signature is bits, which is eight times the length of a keyed MD5 value. * The advantage is to provide non-repudiation of origin services. 15 / / 43 * The format of the IPSP authentication header: Next Length Reserved Header Security Parameter Index (SPI) Authentication Data (n*32 bit) 32 bit IPSP format * 8-bit Next Header field: type of the next payload header (of the IP Protocol) after the AH. * 8-bit length field: the length of the authentication data in 32 bit words. *16-bit Reserved field: for future use; currently set to zero. 32-bit SPI field: identifies the SA for the IP packet on the receiver side. 8

9 17 / 43 * The fixed 64-bit header is followed by the actual authentication data, encoded as a variable number n of 32-bit words. * Typical values for n are 4 in the case of MD4/5;if public keys are used n could be 32. * Protocol number 51 is assigned for the IPSP AH. * Note: presence or absence of an AH does not change IP's behaviour. * TCP/IP protocol implementations may be instructed to reject unauthenticated IP packets. 18 / 43 * To prevent most of the IP address against spoofing and session hijacking attacks, a revised format of the IPSP AH has been proposed. Next Length Reserved Header Security Parameter Index (SPI) Replay Prevention Authentication Data (n*32 bit) 32 bit IPSP format 1) An optional 32-bit Replay Prevention is introduced to provide protection against replay attacks and to assign every packet an unique identifier. 2) The proposal to truncate keyed one-way hash values to 96 bits has been adopted. 9

10 19 / 43 summary advantage: - Provides stronger securityfor the Internet. - AH mechanism does not effect the exportability, nor significantly increase implementation costs. disadvantage: - AH increases IP processing costs and communication latency in participating systems. - Increased latency is primarily due to the calculation of the authentication data by the sender and the receiver of an IP packet. Encapsulating Security Payload 20 / 43 * IPSP AH does not transform the payload data of an IP packet. * another security mechanism must be deployed for data confidentiality services. * This security mechanism is called the encapsulating security payload (ESP). * ESP provides data confidentiality services by encrypting and encapsulating either the payload of an IP packet (transport mode) or the entire packet (tunnel mode). 10

11 IPSP ESP format Security Parameter Index (SPI) * 32-bit SPI: security parameter index on the Initialization Vector (IV) receiver's side. Payload Data * Initialisation Vector (IV): variable number of 32-bit words, where the precise number is defined Padding as a parameter of the SA. * Payload Length: variable-length that includes the 32 bit data that is encrypted with the algorithm specified in the current SA (depends on whether the ESP mechanism is used in transport or tunnel mode). * Padding field: variable length, preferably filled with random bits, resulting length of the Payload data and Padding field is 6 modulo 8. * Pad Length: 8-bit, indicates the total length of the Padding field. * Payload Type: 8-bit, the protocol number for the payload data. Pad Length Paload Type IPSP ESP format 21 / 43 * Only SPI and IV, if present, are not encrypted. * Padding, Pad Length and Payload Type are encrypted together with the Payload Data. * The default algorithm is DES in CBC (cipher block chaining) mode. * ESP protocol number 50 is assigned for the IPSP. 22 / 43 11

12 23 / 43 Transport Mode: * ESP mechanism in transport mode is used to encrypt and encapsulate the upper-layer protocol data, such as ICMP,UDP or TCP. * No additional bandwidth required (no additional encrypted IP headers). * The sending host identifies the senders upper layer protocol and encapsulates the packet in ESP format. * Using the SA user identification and destination IP address applies the cryptographic transformation. * Resulting ESP forms the payload of the IP packet. * IP header is set to 50. * The receiving host processes the IP header and plaintext part of the ESP to obtain the SPI value. * This value is then used as an index for a local SPI table to find the negotiated SA parameters and cryptographic keys. * The encrypted part of the ESP is decrypted using the specified SA 24 / 43 Tunnel Mode: * Entire network layer protocol data units, such as IP or IPX packets, can be encrypted and encapsulated in new IP packets. * Tunnel mode ESP is primarily used by security gateways for packets that are not originating from that gateway but must be securely transmitted. * It can also be used to provide partial traffic flow confidentiality. * Also, a secure tunnel tunnel between security gateways can be created. 12

13 * Sending host obtains the SA using the user identification and destination IP address, and applies the corresponding cryptographic transformation to encrypt the entire IP packet. * That packet is encapsulated in an ESP, which is included as payload in a new IP packet. * IP protocol is set to 50 and the ESP type field is set to 4. * The receiving host processes the unencrypted IP header and plaintext part of the ESP to obtain the SPI value. * This value is used as an index for a local SPI table to find the negotiated SA parameters and cryptographic keys. * The encrypted part of the ESP is decrypted using the encryption algorithm and the key specified in the SA. * The receiver is able to extract the IP packet, that has been tunnelled in ESP through the Internet. 25 / / 43 advantages: * ESP provides stronger security within the Internet. * It will not affect or significantly increase the implementation costs. disadvantages: * Hosts that implement the ESP mechanism experience some performance impact. * Additional processing required for handling the ESP protocol at sending and receiving hosts. * Sender and receiver also need extra expend processor time (proportional to the size of of the packets) to perform actual encryption and decryption. Increased total processing time and latency. 13

14 27 / 43 * AH and ESP mechanism have been designed independently and can be applied separately or together. * If data confidentiality is not required only AH may be used. * Otherwise ESP can be used as well. Internet key management protocol (IKMP) 28 / 43 * Establishment of SA requires shared keys that are only know to the legitimate members of a group. * These keys can be manually configured or distributed and a small number of group is affected. * Efficient key management protocols are required for larger numbers of groups. 14

15 Modular Key Managment Protocol (MKMP) 29 / 43 * Prior to the standardization of IPSP and MKMP a security architecture for the internet was designed. The heart of this architecture are an IP secure tunnel protocol (IPST) and a corresponding modular key management protocol (MKMP) on top of UDP. * IPST is an encapsulation protocol. The packets may be authenticated and/or encrypted similar to IPSP. * MKMP refers to a set of protocols that has been designed for the management of cryptographic keys. Derivation and periodic refreshing of session keys are available. * IPST and MKMP have been prototyped on AIX3.2.5 for IBM's NetSp Secure Network Gateway firewall products. 30 / 43 * A typical key management scheme consist of two modules. -a master key module - a master key may be exchanged or agreed upon between the communicating parties, - a session key module - master key may be used for the derivation, agreement, and/or refreshing of session keys. Manual Key Center-based Certificate-based Movement Key Management Key Management Master Key Module Session Key Module MKMP approach 15

16 * An important design goal of MKMP was to minimize the numbers of exchanged messages and the computational overhead, and to guarantee a basic security principle for session keys (if a session key is derive future session keys and the master key are not compromised). 31 / 43 * Assume A and B already share a master key K from the master key module, and a nonce N b from a previous MKMP session. The following handshake can be used to generate a new session key SK. step1: A-->B: N a, <N a, N b >K step2: B-->A:N b ', <N b ', N a >K A provides B with a nonce N a and a MAC for the concatenation of N a and N b, computed using master key K. B verifies the authenticity of the MAC and returns another N b ' and a MAC (in case of a positive verification). A and B generate session key SK=f k (N b ',N a ), with f K as a pseudo-random function keyed with K. The session key is never transmitted between A and B, there is no need to authenticate and/or encrypt the key. 32 / 43 *note: MKMP is of limited practical use, because it does not provide a solution for the distribution of master key. 16

17 Internet Security Association Key Management Protocol (ISAKMP) * Was designed by NSA. * It can be implemented over any Internet or transport layer protocol. * UDP port 500 is reserved for ISAKMP. * Not bound to any fix mechanism or algorithm, but rather describes a general concept. * ISAKMP can be used to establish an SA between negotiating parties: - During first phase, a basic set of security attributes may be agreed. This basic set provides protection for ISAKMP exchange. It indicates the authentication and key exchange methods. The result is an ISAKMP SA. - In the second phase, the ISAKMP SA is used to negotiate the security services that will be contained in the SA for the other security protocol or application 33 / 43 * The two-phase negotiation causes some overhead, but it also provides additional benefits. * Once negotiated, the ISAKMP SA can be used to negotiate SAs for other entities. The ISAKMP SA can provide additional security during the second negotiation. * ISAKMP supports the creation of five exchanges for the establishment of SAs and related keying material. 1) Base Exchange: allows authentication and key exchange information to be transmitted together. Reduces the number of round trips at the expense of not providing identity protection, because identities are exchanged before a shared secret has been established. 2) Identity Protection Exchange: designed to separate authentication and key exchange information. Provides protection of the entities at the expense of additional messages. The key information is exchanged first, then the encrypted authentication information is sent. 34 / 43 17

18 3) Authentication-Only Exchange: Only allows authentication information to be transmitted. Only performs authentication, without computing keys and shared secrets 4) Aggressive Exchange: Establishes all security relevant information in a single exchange and reduces the overall round trip. This exchange is the most one, but it does not protect the secrecy of the identities. 5) Informational Exchange is designed as a one way transmittal of information that can be used for SA management. 35 / 43 conclusion: * ISAKMP provides a flexible and extensible framework for establishing and managing SAs and cryptographic keys. * The framework consists of header and payload definitions, exchange types for guiding message and payload exchange and some general processing guidelines. * ISAKMP does not define the mechanism that will be used to establishe and manage SAs and cryptographic keys in an authenticated and confidential way. 36 / 43 18

19 OAKLEY 37 / 43 * Can also be used to establish a shared secret with an assigned identifier and associated authenticated identities for the involved parties. * The key determination protocol is based on an authenticated Diffie-Hellman key exchange to achieve PFS for the shared secrets. * The two parties are allowed to select mutually agreeable supporting algorithms for encryption, key exchange, and authentication. * For example the protocol explicitly defines how the two parties can select mathematical structures for performing the Diffie-Hellman key exchange; they can use standard groups or define their own. * Three distinct group representations can be used: 1) Modular exponentiation groups (MODP). 2) Elliptic curve groups over the field GF(2 n ) (EC2N). 3) Elliptic curve groups over GF(p) (ECP). 38 / 43 * The OAKLEY key determination protocol was designed to be a compatible component of ISAKMP for managing SAs. Both run over UDP using a well-known port. * The goal of the OAKLEY protocol is the secure establishment of a common keying information state for the two parties involved. * These are a key name, secret keying material, the identification of the two parties, and three algorithms for use during authentication. * Three algorithms are used for encryption, hashing and authentication. * The OAKLAY main mode exchange has 5 optional features: 1) Stateless cookie exchange 2) PFS for the keying material 3) Secrecy for the identities 4) PFS for identity secrecy 5) Use of signatures (for non-repudiation) 19

20 * The parties can use all or none of these features. * They exchange messages until their requirements are fully met. * The three components of the OAKLEY key determination protocol are 1) Cookie exchange 2) Diffie-Hellman key exchange 3) Authentication 39 / 43 * Cookie exchange can optionally be stateless. * Diffie-Hellman key exchange is optional, but essential for PFS. * Authentication can be performed with privacy for identifiers, privacy for identifiers with PFS, as well as non-repudiation * Most key management protocols are based on UDP. * Since UDP is unreliable, but a key management protocol must be reliable, the reliability must be built into the key management protocols. * Another issue of UDP-based key management protocols is the effect of firewalls, which filter out UDP datagrams. 40 / 43 20

21 DISCUSSION * IPSP and IKMP are both mandatory for IPv6 implementations. * Most current implementation are based on various flavours of UNIX. * If Internet layer security protocols are to be widely deployed, they must be available for MS-DOS and Windows. * Problem: the most widely deployed TCP/IP implementation is not public. * To overcome this problem, a socket-based key management application programming interface (API) was proposed. * The idea is to come up with an environment that allows IPSP and IKMP to exist outside the operation system kernel, to enable implementation, debugging, and updating in a safe environment. 41 / 43 PF_KEY API. * User-level key management programs and the operating system kernel communicate through a socket using the protocol family PF_KEY. * It provides a set of messages that the kernel can use to indicate the need for a new or updated SA. Manual Key Key Management Network applications Maintenance Protocol Maintenance 42 / 43 PF_KEY User Space Network Sockets Interface Kernel Space Socket Interface Kernel Security Associations Table and Engine Network Security Policy Network Protocol Implementation Addition for the IP Security architecture to be implemented Traditional networking implementation Socket based key management API 21

22 * IPSP could be used to established a secure tunnel between two distant firewalls.the IP packet exchanges between two units would be encapsulate into IPSP packets transmitted from one firewall to the other through the Internet. Using AH if authentication is required or ESP if confidentially is needed. 43 / 43 *Use of IPSP and IKMP enables a company to set up a (secure) virtual private network (VPN). A VPN consists of a collection of hosts which could communicate over the public Internet. *An interesting open issue is compression in IPSP.there are two main reasons for putting compression in IPSP: -Properly encrypted data is no longer compressible.if compression and encryption are required, compression must always be performed first. -Internet layer compression can reduce AH and ESP processing costs, as well as the likelihood of IP packet fragmentation 22