Network Working Group P. Riikonen Internet-Draft draft-riikonen-silc-ke-auth-09.txt 15 January 2007 Expires: 15 July 2007 SILC Key Exchange and Authentication Protocols Status of this Draft By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This memo describes two protocols used in the Secure Internet Live Conferencing (SILC) protocol, specified in the Secure Internet Live Conferencing, Protocol Specification [SILC1]. The SILC Key Exchange (SKE) protocol provides secure key exchange between two parties resulting into shared secret key material. The protocol is based on Diffie-Hellman key exchange algorithm and its functionality is derived from several key exchange protocols. The second protocol, SILC Connection Authentication protocol provides user level authentication used when creating connections in SILC network. The protocol supports passphrase (pre-shared secret) authentication and public key (and certificate) authentication based on digital signatures. Riikonen [Page 1] Internet-Draft 15 January 2007 Table of Contents 1 Introduction .................................................. 2 1.1 Requirements Terminology .................................. 3 2 SILC Key Exchange Protocol .................................... 3 2.1 Key Exchange Payloads ..................................... 4 2.1.1 Key Exchange Start Payload .......................... 4 2.1.2 Key Exchange Payload ................................ 9 2.2 Key Exchange Procedure .................................... 11 2.3 Processing the Key Material ............................... 13 2.4 SILC Key Exchange Groups .................................. 15 2.4.1 diffie-hellman-group1 ............................... 15 2.4.2 diffie-hellman-group2 ............................... 15 2.4.3 diffie-hellman-group3 ............................... 16 2.5 Key Exchange Status Types ................................. 16 3 SILC Connection Authentication Protocol ....................... 18 3.1 Connection Auth Payload ................................... 19 3.2 Connection Authentication Types ........................... 20 3.2.1 Passphrase Authentication ........................... 20 3.2.2 Public Key Authentication ........................... 21 3.3 Connection Authentication Status Types .................... 21 4 Security Considerations ....................................... 22 5 References .................................................... 22 6 Author's Address .............................................. 23 7 Full Copyright Statement ...................................... 24 List of Figures Figure 1: Key Exchange Start Payload Figure 2: Key Exchange Payload Figure 3: Connection Auth Payload 1 Introduction This memo describes two protocols used in the Secure Internet Live Conferencing (SILC) protocol specified in the Secure Internet Live Conferencing, Protocol Specification [SILC1]. The SILC Key Exchange (SKE) protocol provides secure key exchange between two parties resulting into shared secret key material. The protocol is based on Diffie-Hellman key exchange algorithm and its functionality is derived from several key exchange protocols, such as SSH2 Key Exchange protocol, Station-To-Station (STS) protocol and the OAKLEY Key Determination protocol [OAKLEY]. The second protocol, SILC Connection Authentication protocol provides user level authentication used when creating connections in SILC Riikonen [Page 2] Internet-Draft 15 January 2007 network. The protocol supports passphrase (pre-shared secret) authentication and public key (and certificate) authentication based on digital signatures. The basis of secure SILC session requires strong and secure key exchange protocol and authentication. The authentication protocol is secured and no authentication data is ever sent in the network without encrypting and authenticating it first. Thus, authentication protocol may be used only after the key exchange protocol has been successfully completed. This document constantly refers to other SILC protocol specifications that should be read to be able to fully understand the functionality and purpose of these protocols. The most important references are the Secure Internet Live Conferencing, Protocol Specification [SILC1] and the SILC Packet Protocol [SILC2]. The protocol is intended to be used with the SILC protocol thus it does not define own framework that could be used. The framework is provided by the SILC protocol. 1.1 Requirements Terminology The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC2119]. 2 SILC Key Exchange Protocol SILC Key Exchange Protocol (SKE) is used to exchange shared secret material used to secure the communication channel. The protocol use Diffie-Hellman key exchange algorithm and its functionality is derived from several key exchange protocols, such as SSH2 Key Exchange protocol, Station-To-Station (STS) protocol and the OAKLEY Key Determination protocol [OAKLEY]. The protocol does not claim any conformance to any of these protocols, they were only used as a reference when designing this protocol. The protocol can mutually authenticate the negotiating parties during the key exchange. The purpose of SILC Key Exchange protocol is to create session keys to be used in current SILC session. The keys are valid only for some period of time (usually an hour) or at most until the session ends. These keys are used to protect packets traveling between the two entities. Usually all traffic is secured with the key material derived from this protocol. The Diffie-Hellman implementation used in the SILC SHOULD be compliant Riikonen [Page 3] Internet-Draft 15 January 2007 to the PKCS #3. 2.1 Key Exchange Payloads During the key exchange procedure public data is sent between initiator and responder. This data is later used in the key exchange procedure. There are several payloads used in the key exchange. As for all SILC packets, SILC Packet Header, described in [SILC2], is at the beginning of all packets sent in during this protocol. All the fields in the following payloads are in MSB (most significant byte first) order. 2.1.1 Key Exchange Start Payload The key exchange between two entities MUST be started by sending the SILC_PACKET_KEY_EXCHANGE packet containing Key Exchange Start Payload. Initiator sends the Key Exchange Start Payload to the responder filled with all security properties it supports. The responder then checks whether it supports the security properties. It then sends a Key Exchange Start Payload to the initiator filled with security properties it selected from the original payload. The payload sent by responder MUST include only one chosen property per list. The character encoding for the security property values as defined in [SILC1] SHOULD be UTF-8 [RFC2279] in Key Exchange Start Payload. The Key Exchange Start Payload is used to tell connecting entities what security properties and algorithms should be used in the communication. The Key Exchange Start Payload is sent only once per session. Even if the PFS (Perfect Forward Secrecy) flag is set the Key Exchange Start Payload is not re-sent. When PFS is desired the Key Exchange Payloads are sent to negotiate new key material. The procedure is equivalent to the very first negotiation except that the Key Exchange Start Payload is not sent. As this payload is used only with the very first key exchange the payload is never encrypted, as there are no keys to encrypt it with. A cookie is also sent in this payload. A cookie is used to randomize the payload so that none of the key exchange parties can determine this payload before the key exchange procedure starts. The cookie MUST be returned to the original sender unmodified by the responder. Following diagram represents the Key Exchange Start Payload. The lists mentioned below are always comma (',') separated and the list MUST NOT include white spaces (' '). Riikonen [Page 4] Internet-Draft 15 January 2007 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RESERVED | Flags | Payload Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Cookie + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version String Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Version String ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Key Exchange Grp Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Key Exchange Groups ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PKCS Alg Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ PKCS Algorithms ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Encryption Alg Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Encryption Algorithms ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Hash Alg Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Hash Algorithms ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HMAC Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ HMACs ~ | | Riikonen [Page 5] Internet-Draft 15 January 2007 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Compression Alg Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Compression Algorithms ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Key Exchange Start Payload o RESERVED (1 byte) - Reserved field. Sender fills this with zero (0) value. o Flags (1 byte) - Indicates flags to be used in the key exchange. Several flags can be set at once by ORing the flags together. The following flags are reserved for this field: No flags 0x00 In this case the field is ignored. IV Included 0x01 This flag is used to indicate that Initialization Vector (IV) in encryption will be included in the ciphertext which the recipient must use in decryption. At the beginning of the SILC packet, before the SILC Packet header an 8-bit Security ID (SID) MUST be placed. After the SID, the IV MUST be placed. After the IV, a 32-bit MSB first ordered packet sequence number MUST be placed. The SID and IV MUST NOT be encrypted, but the sequence number MUST be included in encryption. The recipient MUST use the sequence number during MAC verification [SILC2]. All fields however are authenticated with MAC. The Security ID is set to value 0 when the key exchange is performed for the first time. It is monotonically increased after each re-key, wrapping eventually. The SID in combination with the current session can be used to identify which key has been used to encrypt an incoming packet. This is especially important after rekey when using UDP/IP protocol, where packets may be lost or reordered. A packet with unknown SID will result into discarding the packet as it cannot be decrypted. After rekey, implementation Riikonen [Page 6] Internet-Draft 15 January 2007 should understand that it may still receive packets with old SID and be prepared to decrypt them with the old key. With this flag it is possible to use SILC protocol on unreliable transport such as UDP/IP which may cause packet reordering and packet losses. By default, this flag is not set and thus IV is not included in the ciphertext. Setting this flag increases the packet length by one ciphertext block plus 1 byte for the Security ID and 32 bits for the sequence number. Responder MAY override this flag for the initiator, however without this flag UDP connection cannot be used. The flag MAY also be used in TCP connection. When using with UDP/IP implementations SHOULD use anti-replay methods where an anti-replay window defines what packets are replays. An example of anti-window protocol is in [RFC2406] Section 3.4.2 with example source code in [RFC2401] Appendix C. While [RFC2401] and [RFC2406] does not relate to SILC, the anti-replay method used is applicable in SILC. PFS 0x02 Perfect Forward Secrecy (PFS) to be used in the key exchange protocol. If not set, re-keying is performed using the old key. See the [SILC1] for more information on this issue. When PFS is used, re-keying and creating new keys for any particular purpose MUST cause new key exchange with new Diffie-Hellman exponent values. In this key exchange only the Key Exchange Payload is sent and the Key Exchange Start Payload MUST NOT be sent. When doing PFS the Key Exchange Payloads are encrypted with the old keys. Mutual Authentication 0x04 Both of the parties will perform authentication by providing signed data for the other party to verify. By default, only responder will provide the signature data. If this is set then the initiator must also provide it. Initiator MAY set this but also responder MAY set this even if initiator did not set it. Rest of the flags are reserved for the future and Riikonen [Page 7] Internet-Draft 15 January 2007 MUST NOT be set. o Payload Length (2 bytes) - Length of the entire Key Exchange Start payload, not including any other field. o Cookie (16 bytes) - Cookie that randomize this payload so that each of the party cannot determine the payload before hand. This field MUST be present. o Version String Length (2 bytes) - The length of the Version String field, not including any other field. o Version String (variable length) - Indicates the version of the sender of this payload. Initiator sets this when sending the payload and responder sets this when it replies by sending this payload. See [SILC1] for definition for the version string format. This field MUST be present and include valid version string. o Key Exchange Grp Length (2 bytes) - The length of the key exchange group list, not including any other field. o Key Exchange Group (variable length) - The list of key exchange groups. See the section 2.4 SILC Key Exchange Groups for definitions of these groups. This field MUST be present. o PKCS Alg Length (2 bytes) - The length of the PKCS algorithms list, not including any other field. o PKCS Algorithms (variable length) - The list of PKCS algorithms. This field MUST be present. o Encryption Alg Length (2 bytes) - The length of the encryption algorithms list, not including any other field. o Encryption Algorithms (variable length) - The list of encryption algorithms. This field MUST be present. o Hash Alg Length (2 bytes) - The length of the Hash algorithm list, not including any other field. o Hash Algorithms (variable length) - The list of Hash algorithms. The hash algorithms are mainly used in the SKE protocol. This field MUST be present. o HMAC Length (2 bytes) - The length of the HMAC list, not including any other field. Riikonen [Page 8] Internet-Draft 15 January 2007 o HMACs (variable length) - The list of HMACs. The HMAC's are used to compute the Message Authentication Code (MAC) of the SILC packets. This field MUST be present. o Compression Alg Length (2 bytes) - The length of the compression algorithms list, not including any other field. o Compression Algorithms (variable length) - The list of compression algorithms. This field MAY be omitted. 2.1.2 Key Exchange Payload Key Exchange payload is used to deliver the public key (or certificate), the computed Diffie-Hellman public value and possibly signature data from one party to the other. When initiator is using this payload and the Mutual Authentication flag is not set then the initiator MUST NOT provide the signature data. If the flag is set then the initiator MUST provide the signature data so that the responder can verify it. The Mutual Authentication flag is usually used when a separate authentication protocol will not be executed for the initiator of the protocol. This is case for example when the SKE is performed between two SILC clients. In normal case, where client is connecting to a server, or server is connecting to a router the Mutual Authentication flag MAY be omitted. However, if the connection authentication protocol for the connecting entity is not based on digital signatures (it is based on pre-shared key or there is no authentication) then the Mutual Authentication flag SHOULD be enabled. This way the connecting entity has to provide proof of possession of the private key for the public key it will provide in this protocol. When performing re-key with PFS selected this is the only payload that is sent in the SKE protocol. The Key Exchange Start Payload MUST NOT be sent at all. However, this payload does not have all the fields present. In the re-key with PFS the public key and a possible signature data SHOULD NOT be present. If they are present they MUST be ignored. The only field that is present is the Public Data that is used to create the new key material. In the re-key the Mutual Authentication flag, that may be set in the initial negotiation, MUST also be ignored. This payload is sent inside SILC_PACKET_KEY_EXCHANGE_1 and inside SILC_PACKET_KEY_EXCHANGE_2 packet types. The initiator uses the SILC_PACKET_KEY_EXCHANGE_1 and the responder the latter. The following diagram represent the Key Exchange Payload. Riikonen [Page 9] Internet-Draft 15 January 2007 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Public Key Length | Public Key Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Public Key of the party (or certificate) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Public Data Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Public Data ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Signature Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Signature Data ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Key Exchange Payload o Public Key Length (2 bytes) - The length of the Public Key (or certificate) field, not including any other field. o Public Key Type (2 bytes) - The public key (or certificate) type. This field indicates the type of the public key in the packet. Following types are defined: 1 SILC style public key (mandatory) 2 SSH2 style public key (optional) 3 X.509 Version 3 certificate (optional) 4 OpenPGP certificate (optional) 5 SPKI certificate (optional) The only required type to support is type number 1. See [SILC1] for the SILC public key specification. See SSH2 public key specification in [SSH-TRANS]. See X.509v3 certificate specification in [PKIX-Part1]. See OpenPGP certificate specification in [PGP]. See SPKI certificate specification in [SPKI]. If this field includes zero (0) or unsupported type number the protocol MUST be aborted sending SILC_PACKET_FAILURE message and the connection SHOULD be closed immediately. Riikonen [Page 10] Internet-Draft 15 January 2007 o Public Key (or certificate) (variable length) - The public key or certificate of the party. This public key may be used to verify the digital signature. The public key or certificate in this field is encoded in the manner as defined in their respective definitions; see previous field. o Public Data Length (2 bytes) - The length of the Public Data field, not including any other field. o Public Data (variable length) - The public data to be sent to the receiver (computed Diffie-Hellman public values). See section 2.2 Key Exchange Procedure for detailed description how this field is computed. This field is MP integer and is encoded as defined in [SILC1]. o Signature Length (2 bytes) - The length of the signature, not including any other field. o Signature Data (variable length) - The signature signed by the sender. The receiver of this signature MUST verify it. The verification is done using the sender's public key. See section 2.2 Key Exchange Procedure for detailed description how to produce the signature. If the Mutual Authentication flag is not set then initiator MUST NOT provide this field and the Signature Length field MUST be set to zero (0) value. If the flag is set then also the initiator MUST provide this field. The responder always MUST provide this field. The encoding for signature is defined in [SILC1]. 2.2 Key Exchange Procedure The key exchange begins by sending SILC_PACKET_KEY_EXCHANGE packet with Key Exchange Start Payload to select the security properties to be used in the key exchange and later in the communication. After Key Exchange Start Payload has been processed by both of the parties the protocol proceeds as follows: Setup: p is a large and public safe prime. This is one of the Diffie Hellman groups. q is order of subgroup (largest prime factor of p). g is a generator and is defined along with the Diffie Hellman group. 1. Initiator generates a random number x, where 1 < x < q, Riikonen [Page 11] Internet-Draft 15 January 2007 and computes e = g ^ x mod p. The result e is then encoded into Key Exchange Payload, with the public key (or certificate) and sent to the responder. If the Mutual Authentication flag is set then initiator MUST also produce signature data SIGN_i which the responder will verify. The initiator MUST compute a hash value HASH_i = hash(Initiator's Key Exchange Start Payload | public key (or certificate) | e). The '|' stands for concatenation. It then signs the HASH_i value with its private key resulting a signature SIGN_i. 2. Responder generates a random number y, where 1 < y < q, and computes f = g ^ y mod p. It then computes the shared secret KEY = e ^ y mod p, and, a hash value HASH = hash(Initiator's Key Exchange Start Payload | public key (or certificate) | Initiator's public key (or certificate) | e | f | KEY). It then signs the HASH value with its private key resulting a signature SIGN. It then encodes its public key (or certificate), f and SIGN into Key Exchange Payload and sends it to the initiator. If the Mutual Authentication flag is set then the responder SHOULD verify that the public key provided in the payload is authentic, or if certificates are used it verifies the certificate. The responder MAY accept the public key without verifying it, however, doing so may result to insecure key exchange (accepting the public key without verifying may be desirable for practical reasons on many environments. For long term use this is never desirable, in which case certificates would be the preferred method to use). It then computes the HASH_i value the same way initiator did in the phase 1. It then verifies the signature SIGN_i from the payload with the hash value HASH_i using the received public key. 3. Initiator verifies that the public key provided in the payload is authentic, or if certificates are used it verifies the certificate. The initiator MAY accept the public key without verifying it, however, doing so may result to insecure key exchange (accepting the public key without verifying may be desirable for practical reasons on many environments. For long term use this is never desirable, in which case certificates would be the preferred method to use). Riikonen [Page 12] Internet-Draft 15 January 2007 Initiator then computes the shared secret KEY = f ^ x mod p, and, a hash value HASH in the same way as responder did in phase 2. It then verifies the signature SIGN from the payload with the hash value HASH using the received public key. If any of these phases is to fail the SILC_PACKET_FAILURE MUST be sent to indicate that the key exchange protocol has failed, and the connection SHOULD be closed immediately. Any other packets MUST NOT be sent or accepted during the key exchange except the SILC_PACKET_KEY_EXCHANGE_*, SILC_PACKET_FAILURE and SILC_PACKET_SUCCESS packets. The result of this protocol is a shared secret key material KEY and a hash value HASH. The key material itself is not fit to be used as a key, it needs to be processed further to derive the actual keys to be used. The key material is also used to produce other security parameters later used in the communication. See section 2.3 Processing the Key Material for detailed description how to process the key material. If the Mutual Authentication flag was set the protocol produces also a hash value HASH_i. This value, however, must be discarded. After the keys are processed the protocol is ended by sending the SILC_PACKET_SUCCESS packet. Both entities send this packet to each other. After this both parties MUST start using the new keys. 2.3 Processing the Key Material Key Exchange protocol produces secret shared key material KEY. This key material is used to derive the actual keys used in the encryption of the communication channel. The key material is also used to derive other security parameters used in the communication. Key Exchange protocol produces a hash value HASH as well. The keys MUST be derived from the key material as follows: Sending Initial Vector (IV) = hash(0x0 | KEY | HASH) Receiving Initial Vector (IV) = hash(0x1 | KEY | HASH) Sending Encryption Key = hash(0x2 | KEY | HASH) Receiving Encryption Key = hash(0x3 | KEY | HASH) Sending HMAC Key = hash(0x4 | KEY | HASH) Receiving HMAC Key = hash(0x5 | KEY | HASH) The Initial Vector (IV) is used in the encryption when doing for example CBC mode. As many bytes as needed are taken from the start of Riikonen [Page 13] Internet-Draft 15 January 2007 the hash output for IV. Sending IV is for sending key and receiving IV is for receiving key. For receiving party, the receiving IV is actually sender's sending IV, and, the sending IV is actually sender's receiving IV. Initiator uses IV's as they are (sending IV for sending and receiving IV for receiving). The Encryption Keys are derived as well from the hash(). If the hash() output is too short for the encryption algorithm more key material MUST be produced in the following manner: K1 = hash(0x2 | KEY | HASH) K2 = hash(KEY | HASH | K1) K3 = hash(KEY | HASH | K1 | K2) ... Sending Encryption Key = K1 | K2 | K3 ... K1 = hash(0x3 | KEY | HASH) K2 = hash(KEY | HASH | K1) K3 = hash(KEY | HASH | K1 | K2) ... Receiving Encryption Key = K1 | K2 | K3 ... The key is distributed by hashing the previous hash with the original key material. The final key is a concatenation of the hash values. For Receiving Encryption Key the procedure is equivalent. Sending key is used only for encrypting data to be sent. The receiving key is used only to decrypt received data. For receiving party, the receive key is actually sender's sending key, and, the sending key is actually sender's receiving key. Initiator uses generated keys as they are (sending key for sending and receiving key for receiving). The HMAC keys are used to create MAC values to packets in the communication channel. As many bytes as needed are taken from the start of the hash output to generate the MAC keys. These procedures are performed by all parties of the key exchange protocol. This MUST be done before the protocol has been ended by sending the SILC_PACKET_SUCCESS packet, to assure that parties can successfully process the key material. This same key processing procedure MAY be used in the SILC in some other circumstances as well. Any changes to this procedure is defined separately when this procedure is needed. See the [SILC1] and the [SILC2] for these circumstances. Riikonen [Page 14] Internet-Draft 15 January 2007 2.4 SILC Key Exchange Groups The Following groups may be used in the SILC Key Exchange protocol. The first group diffie-hellman-group1 is REQUIRED, other groups MAY be negotiated to be used in the connection with Key Exchange Start Payload and SILC_PACKET_KEY_EXCHANGE packet. However, the first group MUST be proposed in the Key Exchange Start Payload regardless of any other requested group (however, it does not have to be the first in the list). 2.4.1 diffie-hellman-group1 The length of this group is 1024 bits. This is REQUIRED group. The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }. Its hexadecimal value is FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381 FFFFFFFF FFFFFFFF The generator used with this prime is g = 2. The group order q is (p - 1) / 2. This group was taken from RFC 2412. 2.4.2 diffie-hellman-group2 The length of this group is 1536 bits. This is OPTIONAL group. The prime is 2^1536 - 2^1472 - 1 + 2^64 * { [2^1406 pi] + 741804 }. Its hexadecimal value is FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F 83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D 670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF The generator used with this prime is g = 2. The group order q is Riikonen [Page 15] Internet-Draft 15 January 2007 (p - 1) / 2. This group was taken from RFC 3526. 2.4.3 diffie-hellman-group3 The length of this group is 2048 bits. This is OPTIONAL group. This prime is: 2^2048 - 2^1984 - 1 + 2^64 * { [2^1918 pi] + 124476 }. Its hexadecimal value is FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F 83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D 670C354E 4ABC9804 F1746C08 CA18217C 32905E46 2E36CE3B E39E772C 180E8603 9B2783A2 EC07A28F B5C55DF0 6F4C52C9 DE2BCBF6 95581718 3995497C EA956AE5 15D22618 98FA0510 15728E5A 8AACAA68 FFFFFFFF FFFFFFFF The generator used with this prime is g = 2. The group order q is (p - 1) / 2. This group was taken from RFC 3526. Additional larger groups are defined in RFC 3526 and may be used in SKE by defining name for them using the above name format. 2.5 Key Exchange Status Types This section defines all key exchange protocol status types that may be returned in the SILC_PACKET_SUCCESS or SILC_PACKET_FAILURE packets to indicate the status of the protocol. Implementations may map the status types to human readable error message. All types except the SILC_SKE_STATUS_OK type MUST be sent in SILC_PACKET_FAILURE packet. The length of status is 32 bits (4 bytes). The following status types are defined: 0 SILC_SKE_STATUS_OK Protocol were executed successfully. Riikonen [Page 16] Internet-Draft 15 January 2007 1 SILC_SKE_STATUS_ERROR Unknown error occurred. No specific error type is defined. 2 SILC_SKE_STATUS_BAD_PAYLOAD Provided KE payload were malformed or included bad fields. 3 SILC_SKE_STATUS_UNSUPPORTED_GROUP None of the provided groups were supported. 4 SILC_SKE_STATUS_UNSUPPORTED_CIPHER None of the provided ciphers were supported. 5 SILC_SKE_STATUS_UNSUPPORTED_PKCS None of the provided public key algorithms were supported. 6 SILC_SKE_STATUS_UNSUPPORTED_HASH_FUNCTION None of the provided hash functions were supported. 7 SILC_SKE_STATUS_UNSUPPORTED_HMAC None of the provided HMACs were supported. 8 SILC_SKE_STATUS_UNSUPPORTED_PUBLIC_KEY Provided public key type is not supported. 9 SILC_SKE_STATUS_INCORRECT_SIGNATURE Provided signature was incorrect. 10 SILC_SKE_STATUS_BAD_VERSION Provided version string was not acceptable. Riikonen [Page 17] Internet-Draft 15 January 2007 11 SILC_SKE_STATUS_INVALID_COOKIE The cookie in the Key Exchange Start Payload was malformed, because responder modified the cookie. 3 SILC Connection Authentication Protocol Purpose of Connection Authentication protocol is to authenticate the connecting party with server. Usually connecting party is client but server may connect to router server as well. Its other purpose is to provide information for the server about which type of entity the connection is. The type defines whether the connection is client, server or router connection. Server use this information to create the ID for the connection. Server MUST verify the authentication data received and if it is to fail the authentication MUST be failed by sending SILC_PACKET_FAILURE packet. If authentication is successful the protocol is ended by server by sending SILC_PACKET_SUCCESS packet. The protocol is executed after the SILC Key Exchange protocol. It MUST NOT be executed in any other time. As it is performed after key exchange protocol all traffic in the connection authentication protocol is encrypted with the exchanged keys. The protocol MUST be started by the connecting party by sending the SILC_PACKET_CONNECTION_AUTH packet with Connection Auth Payload, described in the next section. This payload MUST include the authentication data. The authentication data is set according authentication method that MUST be known by both parties. If connecting party does not know what is the mandatory authentication method it MAY request it from the server by sending SILC_PACKET_CONNECTION_AUTH_REQUEST packet. This packet is not part of this protocol and is described in section Connection Auth Request Payload in [SILC2]. However, if connecting party already knows the mandatory authentication method sending the request is not necessary. See [SILC1] and section Connection Auth Request Payload in [SILC2] also for the list of different authentication methods. Authentication method MAY also be NONE, in which case the server does not require authentication. However, in this case the protocol still MUST be executed; the authentication data is empty indicating no authentication is required. If authentication method is passphrase the authentication data is plaintext passphrase. As the payload is encrypted it is safe to have plaintext passphrase. It is also provided as plaintext passphrase Riikonen [Page 18] Internet-Draft 15 January 2007 because the receiver may need to pass the entire passphrase into a passphrase verifier, and a message digest of the passphrase would prevent this. See the section 3.2.1 Passphrase Authentication for more information. If authentication method is public key authentication the authentication data is a digital signature of the hash value of hash HASH and Key Exchange Start Payload, established by the SILC Key Exchange protocol. This signature MUST then be verified by the server. See the section 3.2.2 Public Key Authentication for more information. See the section 4 SILC Procedures in [SILC1] for more information about client creating connection to server, and server creating connection to router, and how to register the session in the SILC Network after successful Connection Authentication protocol. 3.1 Connection Auth Payload Client sends this payload to authenticate itself to the server. Server connecting to another server also sends this payload. Server receiving this payload MUST verify all the data in it and if something is to fail the authentication MUST be failed by sending SILC_PACKET_FAILURE packet. The payload may only be sent with SILC_PACKET_CONNECTION_AUTH packet. It MUST NOT be sent in any other packet type. The following diagram represent the Connection Auth Payload. 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Payload Length | Connection Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Authentication Data ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Connection Auth Payload o Payload Length (2 bytes) - Length of the entire Connection Riikonen [Page 19] Internet-Draft 15 January 2007 Auth Payload. o Connection Type (2 bytes) - Indicates the type of the connection. See section Connection Auth Request Payload in [SILC2] for the list of connection types. This field MUST include valid connection type or the packet MUST be discarded and authentication MUST be failed. o Authentication Data (variable length) - The actual authentication data. Contents of this depends on the authentication method known by both parties. If no authentication is required this field does not exist. 3.2 Connection Authentication Types SILC supports two authentication types to be used in the connection authentication protocol; passphrase authentication or public key authentication based on digital signatures. The following sections defines the authentication methods. See [SILC2] for defined numerical authentication method types. 3.2.1 Passphrase Authentication Passphrase authentication or pre-shared key based authentication is simply an authentication where the party that wants to authenticate itself to the other end sends the passphrase that is required by the other end, for example server. The plaintext passphrase is put to the payload, that is then encrypted. The plaintext passphrase MUST be in UTF-8 [RFC2279] encoding. If the passphrase is in the sender's system in some other encoding it MUST be UTF-8 encoded before transmitted. The receiver MAY change the encoding of the passphrase to its system's default character encoding before verifying the passphrase. If the passphrase matches with the one in the server's end the authentication is successful. Otherwise SILC_PACKET_FAILURE MUST be sent to the sender and the protocol execution fails. This is REQUIRED authentication method to be supported by all SILC implementations. When password authentication is used it is RECOMMENDED that maximum amount of padding is applied to the SILC packet. This way it is not possible to approximate the length of the password from the encrypted packet. Riikonen [Page 20] Internet-Draft 15 January 2007 3.2.2 Public Key Authentication Public key authentication may be used if passphrase based authentication is not desired. The public key authentication works by sending a digital signature as authentication data to the other end, say, server. The server MUST then verify the signature by the public key of the sender, which the server has received earlier in SKE protocol, or which the server has cached locally at some previous time. The signature is computed using the private key of the sender by signing the HASH value provided by the SKE protocol previously, and the Key Exchange Start Payload from SKE protocol that was sent to the server. These are concatenated and hash function is used to compute a hash value which is then signed. auth_hash = hash(HASH | Key Exchange Start Payload); signature = sign(auth_hash); The hash() function used to compute the value is the hash function negotiated in the SKE protocol. The server MUST verify the data, thus it must keep the HASH and the Key Exchange Start Payload saved during SKE and authentication protocols. These values can be discarded after Connection Authentication protocol is completed. If the verified signature matches the sent signature, the authentication were successful and SILC_PACKET_SUCCESS is sent. If it failed the protocol execution is stopped and SILC_PACKET_FAILURE is sent. This is REQUIRED authentication method to be supported by all SILC implementations. 3.3 Connection Authentication Status Types This section defines all connection authentication status types that may be returned in the SILC_PACKET_SUCCESS or SILC_PACKET_FAILURE packets to indicate the status of the protocol. Implementations may map the status types to human readable error message. All types except the SILC_AUTH_STATUS_OK type MUST be sent in SILC_PACKET_FAILURE packet. The length of status is 32 bits (4 bytes). The following status types are defined: 0 SILC_AUTH_OK Protocol was executed successfully. Riikonen [Page 21] Internet-Draft 15 January 2007 1 SILC_AUTH_FAILED Authentication failed. 4 Security Considerations Security is central to the design of this protocol, and these security considerations permeate the specification. Common security considerations such as keeping private keys truly private and using adequate lengths for symmetric and asymmetric keys must be followed in order to maintain the security of this protocol. 5 References [SILC1] Riikonen, P., "Secure Internet Live Conferencing (SILC), Protocol Specification", Internet Draft, January 2007. [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft, January 2007. [SILC4] Riikonen, P., "SILC Commands", Internet Draft, January 2007. [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol", RFC 1459, May 1993. [IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810, April 2000. [IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC 2811, April 2000. [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC 2812, April 2000. [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC 2813, April 2000. [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol", Internet Draft. [PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440, November 1998. [SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693, September 1999. Riikonen [Page 22] Internet-Draft 15 January 2007 [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key Infrastructure, Certificate and CRL Profile", RFC 2459, January 1999. [Schneier] Schneier, B., "Applied Cryptography Second Edition", John Wiley & Sons, New York, NY, 1996. [Menezes] Menezes, A., et al, "Handbook of Applied Cryptography", CRC Press 1997. [OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol", RFC 2412, November 1998. [ISAKMP] Maughan D., et al, "Internet Security Association and Key Management Protocol (ISAKMP)", RFC 2408, November 1998. [IKE] Harkins D., and Carrel D., "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography Specifications, Version 2.0", RFC 2437, October 1998. [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. [RFC2401] Kent, S., et al, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [RFC2406] Kent, S., et al, "Security Architecture for the Internet Protocol", RFC 2406, November 1998. 6 Author's Address Pekka Riikonen Helsinki Finland EMail: priikone@iki.fi Riikonen [Page 23] Internet-Draft 15 January 2007 7 Full Copyright Statement Copyright (C) The Internet Society (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Riikonen [Page 24]