Network Working Group P. Riikonen INTERNET-DRAFT draft-riikonen-silc-ke-auth-00.txt 28 June 2000 Expires: 28 Jan 2001 SILC Key Exchange and Authentication Protocols Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html The distribution of this memo is unlimited. Abstract This memo describes two protocols used in the Secure Internet Live Conferencing (SILC) protocol specified in the Secure Internet Live Conferencing, Protocol Specification internet-draft [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. SKE uses best parts of the SSH2 Key Exchange protocol, Station-To-Station (STS) protocol and the OAKLEY Key Determination protocol [OAKLEY]. The SILC Connection Authentication protocol provides user level authentication used when creating connections in SILC network. The protocol is transparent to the authentication data which means that it can be used to authenticate the user with, for example, passphrase (pre-shared- secret) or public key (and certificate). Riikonen [Page 1] INTERNET-DRAFT 28 June 2000 Table of Contents 1 Introduction .................................................. 2 2 SILC Key Exchange Protocol .................................... 3 2.1 Key Exchange Payloads ..................................... 3 2.1.1 Key Exchange Start Payload .......................... 4 2.1.2 Key Exchange 1 Payload .............................. 7 2.1.3 Key Exchange 2 Payload .............................. 9 2.2 Key Exchange Procedure .................................... 10 2.3 Processing the Key Material ............................... 12 2.4 SILC Key Exchange Groups .................................. 13 2.4.1 diffie-hellman-group1 ............................... 13 2.4.2 diffie-hellman-group2 ............................... 14 2.5 Key Exchange Status Types ................................. 14 3 SILC Connection Authentication Protocol ....................... 16 3.1 Connection Auth Payload ................................... 17 3.2 Connection Authentication Types ........................... 18 3.2.1 Passphrase Authentication ........................... 18 3.2.2 Public Key Authentication ........................... 18 3.3 Connection Authentication Status Types .................... 19 4 Security Considerations ....................................... 19 5 References .................................................... 19 6 Author's Address .............................................. 20 List of Figures Figure 1: Key Exchange Start Payload Figure 2: Key Exchange 1 Payload Figure 3: Key Exchange 2 Payload Figure 4: 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 internet-draft [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. SKE uses best parts of the SSH2 Key Exchange protocol, Station-To-Station (STS) protocol and the OAKLEY Key Determination protocol. The SILC Connection Authentication protocol provides user level authentication used when creating connections in SILC network. The protocol is transparent to the authentication data which means that it Riikonen [Page 2] INTERNET-DRAFT 28 June 2000 can be used to authenticate the user with, for example, passphrase (pre-shared- secret) or public key (and certificate). The basis of secure SILC session requires strong and secure key exchange protocol and authentication. The authentication protocol is entirely 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 refers constantly to other SILC protocol specification Internet Drafts that are a must read for those who wants to understand the function of these protocols. The most important references are the Secure Internet Live Conferencing, Protocol Specification [SILC1] and SILC Packet Protocol [SILC2] Internet Drafts. 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. 2 SILC Key Exchange Protocol SILC Key Exchange Protocol (SKE) is used to exchange shared secret between connecting entities. The result of this protocol is a key material used to secure the communication channel. The protocol uses Diffie-Hellman key exchange algorithm and its functionality is derived from several key exchange protocols. SKE uses best parts of the SSH2 Key Exchange protocol, Station-To-Station (STS) protocol and the OAKLEY Key Determination protocol. The protocol does not claim any conformance to any of these protocols, they were merely used as a reference when designing this protocol. 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 like commands, command replies and other communication between two entities. If connection is server to server connection, the keys are used to protect all traffic between those servers. In client connections usually all the packets are protected with this key except channel messages; channels has their own keys and they are not exchanged with this protocol. 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. Riikonen [Page 3] INTERNET-DRAFT 28 June 2000 There are several payloads used in the key exchange. As for all SILC packets, SILC Packet Header, described in [SILC2], is at the start of all packets, the same is done with these payloads as well. All fields in all payloads are always in MSB (most significant byte first) order. Following descriptions of these payloads. 2.1.1 Key Exchange Start Payload Key exchange between two entities always begins with a SILC_PACKET_KEY_EXCHANGE packet containing Key Exchange Start Payload. When performing key exchange between client and server, the client sends Key Exchange Start Payload to server filled with all security properties that the client supports. Server then checks if it supports the security properties. It then sends a Key Exchange Start Payload to client filled with security properties it selected from the payload client originally sent. The payload sent by server must include only one chosen property per list. When performing key exchange between server and server, the server who is contacting sends the Key Exchange Start Payload with security property list it supports to the other server. The contacted party then chooses the preferred properties same way as previously described. It then replies with the properties it wanted same way as previously described. The Key Exchange Start Payload is used to tell connecting entities what security properties and algorithms should be used in the communication. If perfect forward secrecy (PFS) is not desired (PFS is undefined by default) Key Exchange Start Payload is sent only once per session, thus, for example, re-keying will not cause sending of a new payload. If PFS is desired, re-keying will always cause new key exchange thus causes sending of a new Key Exchange Start Payload. When performing first key exchange this payload is never encrypted, as there are no existing keys to encrypt it with. If performing re-keying (PFS was selected) this payload is encrypted with the existing key and encryption algorithm. Cookie is also send in this payload. Cookie is used to uniform the payload so that none of the key exchange parties cannot determine this payload before hand. The cookie must be returned to the original sender 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 spaces (` '). Riikonen [Page 4] INTERNET-DRAFT 28 June 2000 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 + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Key Exchange Grp Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Key Exchange Groups ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PKCS Alg Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ PKCS Algorithms ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Encryption Alg Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Encryption Algorithms ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Hash Alg Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Hash Algorithms ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Compression Alg Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Compression Algorithms ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Key Exchange Start Payload Riikonen [Page 5] INTERNET-DRAFT 28 June 2000 o RESERVED (1 byte) - Reserved field. Sender fills this with zeroes (0). 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. Following flags are reserved for this field. No flags 0x00 In this case the field is ignored. No Reply 0x01 If set the receiver of the payload does not reply to the packet. 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. When PFS is used, re-keying and creating new keys for any particular purpose will cause new key exchange. Rest of the flags are reserved for the future and must not be set. o Payload Length (2 bytes) - Length of the entire Key Exchange Start payload. o Cookie (16 bytes) - Cookie that uniforms this payload so that each of the party cannot determine the payload before hand. o Key Exchange Grp Length (2 bytes) - The length of the key exchange group list, including this field as well. o Key Exchange Group (variable length) - The list of key exchange groups. See the section 2.1.2 SILC Key Exchange Groups for definitions of these groups. o PKCS Alg Length (2 bytes) - The length of the PKCS algorithms list, including this field as well. o PKCS Algorithms (variable length) - The list of PKCS algorithms. o Encryption Alg Length (2 bytes) - The length of the encryption Riikonen [Page 6] INTERNET-DRAFT 28 June 2000 algorithms list, including this field as well. o Encryption Algorithms (variable length) - The list of encryption algorithms. o Hash Alg Length (2 bytes) - The length of the Hash algorithms list, including this field as well. o Hash Algorithms (variable length) - The list of Hash algorithms. o Compression Alg Length (2 bytes) - The length of the compression algorithms list, including this field as well. o Compression Algorithms (variable length) - The list of compression algorithms. 2.1.2 Key Exchange 1 Payload Key Exchange 1 Payload is used to deliver computed public data from initiator to responder. This data is used to compute the shared secret, later by all parties. Key Exchange 1 Payload is only sent after the SILC_PACKET_KEY_EXCHANGE packet and the Key Exchange Start Payload has been processed by all the parties. This payload sends the initiator's public key to the responder. Responder may need the public key in which case it should be checked to be trusted by the responder. The payload may only be sent with SILC_PACKET_KEY_EXCHANGE_1 packet. It must not be sent in any other packet type. Following diagram represent the Key Exchange 1 Payload. Riikonen [Page 7] INTERNET-DRAFT 28 June 2000 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 Host (or certificate) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Public Data Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Public Data (e = g ^ x mod p) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Key Exchange 1 Payload o Public Key Length (2 bytes) - The length of the public key (or certificate), including this field and public key type field as well. 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 SSH 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. o Public Data Length (2 bytes) - The length of the public data computed by the responder, including this field as well. o Public Data (variable length) - The public data to be sent to the responder. See section 2.2 Key Exchange Riikonen [Page 8] INTERNET-DRAFT 28 June 2000 Procedure for detailed description how this field is computed. This value is binary encoded. 2.1.3 Key Exchange 2 Payload Key Exchange 2 Payload is used to deliver public key, computed public data and signature from responder to initiator. Initiator uses these public parts of the key exchange protocol to compute the shared secret. The payload may only be sent with SILC_PACKET_KEY_EXCHANGE_2 packet. It must not be sent in any other packet type. Following diagram represent the Key Exchange 2 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Public Key Length | Public Key Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Public Key of the Host (or certificate) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Public Data Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Public Data (f = g ^ y mod p) ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Signature Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Signature Data ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Key Exchange 2 Payload o Public Key Length (2 bytes) - The length of the public key (or certificate), including this field and public key type field as well. o Public Key Type (2 bytes) - The public key (or certificate) type. This field indicates the type of the public key in Riikonen [Page 9] INTERNET-DRAFT 28 June 2000 the packet. See previous sections for defined public key types. o Public Key of the host (variable length) - The public key of the sender (or its certificate). This is verified by the receiver of the packet. The type of this field is indicated by previous Public Key Type field. o Public Data Length (2 bytes) - The length of the public data computed by the responder, including this field as well. o Public Data (variable length) - The public data computed by the responder. See section 2.2 Key Exchange Procedure for detailed description how this field is computed. This value is binary encoded. o Signature Length (2 bytes) - The length of the signature, including the length of this field as well. o Signature Data (variable length) - The signature signed by the responder. The receiver of this signature must verify it. The verification is done using the public key received in this same payload. See section 2.2 Key Exchange Procedure for detailed description how to produce the signature. 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, and computes e = g ^ x mod p. The result e is then encoded into Key Exchange 1 Payload and sent to the responder. Riikonen [Page 10] INTERNET-DRAFT 28 June 2000 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(Key Exchange Start Payload data | Host 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 2 Payload and sends it to the initiator. 3. Initiator verifies that the public key provided in the payload is authentic, or if certificates are used it verifies the certificate. 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). 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 SILC_PACKET_FAILURE is sent to indicate that the key exchange protocol failed. Any other packets must not be sent or accepted during the key exchange except the SILC_PACKET_KEY_EXCHANGE_*, SILC_PACKET_DISCONNECT, SILC_PACKET_FAILURE and/or 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. 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 will start using the new keys. Riikonen [Page 11] INTERNET-DRAFT 28 June 2000 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. This is used in the key deriving process as a session identifier. Keys are derived from the key material as follows: Sending Initial Vector (IV) = hash(0 | KEY | HASH) Receiving Initial Vector (IV) = hash(1 | KEY | HASH) Sending Encryption Key = hash(2 | KEY | HASH) Receiving Encryption Key = hash(3 | KEY | HASH) HMAC Key = hash(4 | 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 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 is produced in following manner: K1 = hash(2 | KEY | HASH) K2 = hash(KEY | K1) K3 = hash(KEY | K1 | K2) ... Sending Encryption Key = K1 | K2 | K3 ... K1 = hash(3 | KEY | HASH) K2 = hash(KEY | K1) K3 = hash(KEY | 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 Riikonen [Page 12] INTERNET-DRAFT 28 June 2000 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 sending). The HMAC key is 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. 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. 2.4 SILC Key Exchange Groups Following groups may be used in the SILC Key Exchange protocol. The first group diffie-hellman-group1 is mandatory, other groups maybe 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 doesn't have to be the first on the list). 2.4.1 diffie-hellman-group1 The length of this group is 1024 bits. This is mandatory group. The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }. Its decimal value is 179769313486231590770839156793787453197860296048756011706444 423684197180216158519368947833795864925541502180565485980503 646440548199239100050792877003355816639229553136239076508735 759914822574862575007425302077447712589550957937778424442426 617334727629299387668709205606050270810842907692932019128194 467627007 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 Riikonen [Page 13] INTERNET-DRAFT 28 June 2000 The generator used with this prime is g = 2. The group order q is (p - 1) / 2. This group was taken from the OAKLEY specification. 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 decimal value is 241031242692103258855207602219756607485695054850245994265411 694195810883168261222889009385826134161467322714147790401219 650364895705058263194273070680500922306273474534107340669624 601458936165977404102716924945320037872943417032584377865919 814376319377685986952408894019557734611984354530154704374720 774996976375008430892633929555996888245787241299381012913029 459299994792636526405928464720973038494721168143446471443848 8520940127459844288859336526896320919633919 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 (p - 1) / 2. This group was taken from the OAKLEY specification. 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. Following status types are defined: Riikonen [Page 14] INTERNET-DRAFT 28 June 2000 0 SILC_SKE_STATUS_OK Protocol were exeucted succesfully. 1 SILC_SKE_STATUS_ERROR Unknown error occured. 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_PUBLIC_KEY Provided public key type is not supported. 8 SILC_SKE_STATUS_INCORRECT_SIGNATURE Provided signature was incorrect. Riikonen [Page 15] INTERNET-DRAFT 28 June 2000 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 server as well. Its other purpose is to provide information for the server about which type of connection this is. The type defines whether this is client, server or router connection. Server uses this information to create the ID for the connection. After the authentication protocol has been successfully completed SILC_PACKET_NEW_ID must be sent to the connecting party by the server. See section New ID Payload in [SILC2] for detailed description for this packet's payload. 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 everything checks out fine 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 is started by the connecting party by sending SILC_PACKET_CONNECTION_AUTH packet with Connection Auth Payload, described in the next section. This payload must include the authentication data. 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 must 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 at all. However, in this case the protocol still must be executed; the authentication data just is empty indicating no authentication is required. If authentication method is passphrase the authentication data is plaintext passphrase. As the payload is entirely encrypted it is safe to have plaintext passphrase. 3.2.1 Passphrase Authentication for more information. Riikonen [Page 16] INTERNET-DRAFT 28 June 2000 If authentication method is public key authentication the authentication data is signature of the hash value HASH plus Key Exchange Start Payload, established by the SILC Key Exchange protocol. This signature must then be verified by the server. See section 3.2.2 Public Key Authentication for more information. The connecting party of this protocol must wait after successful execution of this protocol for the SILC_PACKET_NEW_ID packet where it will receive the ID it will be using in the SILC network. Connecting party cannot start normal SILC session (sending messages or commands) until it has received its ID. The ID's are always created by the server except for server to server connection where servers create their own ID's. 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. 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 4: Connection Auth Payload o Payload Length (2 bytes) - Length of the entire Connection 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. Riikonen [Page 17] INTERNET-DRAFT 28 June 2000 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 or public key based authentication. 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 base 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. 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. 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 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. 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. The server must verify the data, thus it must keep the HASH and the Key Exchange Start Payload saved during SKE and authentication protocols. 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 Riikonen [Page 18] INTERNET-DRAFT 28 June 2000 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. Following status types are defined: 0 SILC_AUTH_OK Protocol was executed succesfully. 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. 5 References [SILC1] Riikonen, P., "Secure Internet Live Conferencing (SILC), Protocol Specification", Internet Draft, June 2000. [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft, June 2000. [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol", RFC 1459, May 1993. [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 19] INTERNET-DRAFT 28 June 2000 [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 Exhange (IKE)", RFC 2409, November 1998. [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. 6 Author's Address Pekka Riikonen Kasarmikatu 11 A4 70110 Kuopio Finland EMail: priikone@poseidon.pspt.fi This Internet-Draft expires 28 Jan 2001 Riikonen [Page 20]