+++ /dev/null
-This is the security documentation for tinc, a Virtual Private Network daemon.
-
- Copyright 2000,2001 Guus Sliepen <guus@sliepen.warande.net>,
- 2000,2001 Ivo Timmmermans <itimmermans@bigfoot.com>
-
- Permission is granted to make and distribute verbatim copies of
- this documentation provided the copyright notice and this
- permission notice are preserved on all copies.
-
- Permission is granted to copy and distribute modified versions of
- this documentation under the conditions for verbatim copying,
- provided that the entire resulting derived work is distributed
- under the terms of a permission notice identical to this one.
-
- $Id: SECURITY,v 1.1.2.4 2001/01/07 17:08:03 guus Exp $
-
-
-1. Authentication
-------------------
-
-The authentication protocol (see protocol.c for the up-to-date version) is:
-
- Client Server
- send_id(u)
- send_challenge(R)
- send_chal_reply(H)
- send_id(u)
- send_challenge(R)
- send_chal_reply(H)
- send_metakey(R)
- send_metakey(R)
- send_ack(u)
- send_ack(u)
- ---------------------------------------
- Other requests(E)...
-
- (u) Unencrypted,
- (R) RSA,
- (H) SHA1,
- (E) Encrypted with symmetric cipher.
-
-See section 4 for a detailed example version of the authentication.
-
-Authentication in tinc will be done in a way that is very similar to the way
-the SSH (Secure SHell) authentication protocol works. It is based on public
-key cryptography.
-
-Every tinc host has its own public/private key pair. Suppose there are two
-tinc hosts, A and B. If A and B trust each other, they store a copy of
-eachothers public key (in the same way passphrases were stored in versions
-of tinc <= 1.0pre2). They know these public keys beforehand, and the origin
-of the public keys has to be known for sure.
-
-To make sure that when a connection is made from A to B that B knows A is
-really who he claims to be, B encrypts a totally random string of bytes with
-A's public key. B also calculates the hash value from the unencrypted random
-string. B then sends the encrypted string to A. A then has to decrypt the
-string, calculate the hash value from that string and send it back to B. Since
-only he who possesses A's private key can decrypt this string, only he can send
-back the correct hash value. So, if B receives the same hash value he
-calculated himself, he knows for sure A is A.
-
-Both SSH and tinc use RSA for the public key cryptography. SSH uses MD5 as a
-secure hash algorithm, tinc uses SHA1. The reason for our choice of SHA1 is
-the fact that SHA1 is 160 bits instead of 128 (MD5), which makes brute force
-attacks harder. Also, the OpenSSL documentation recommends SHA1.
-
-2. Key exchange
-----------------
-
-The rest of the meta connection in tinc will be encrypted with a symmetric
-block cipher, since RSA is not really suited for this. When a connection is
-made, both sides have to agree on a key for this block cipher. To make sure
-that this key exchange is also done securely, and no man-in-the-middle attack
-is possible, RSA would be the best choice for exchanging keys.
-
-3. Symmetric cipher
---------------------
-
-Since the generalized encryption functions of OpenSSL are used, any symmetric
-cipher that is available in OpenSSL could possibly be used. The default however
-will be Blowfish. Blowfish is widely in use and still has not been cracked
-today (as far as we know). It also is one of the faster ciphers available.
-
-4. Detailed "example" of communication
----------------------------------------
-
-Tinc uses a peer-to-peer protocol, but during the authentication phase we will
-make a distinction between a server (a tinc daemon listening for incoming
-connections) and a client (a tinc daemon that is trying to connect to the tinc
-daemon playing server).
-
-The message strings here are kept short for clarity. The real length of the
-exchanged messages is indicated. The capital words ID, CHALLENGE, CHAL_REPLY,
-META_KEY and ACK are in reality replaced by the numbers 0, 1, 2, 3 and 4
-respectively.
-
-daemon message
---------------------------------------------------------------------------
-server <listening for connection>
-client <tries to connect>
-server <accepts connection>
-client ID client 8 0
- | | +-> options
- | +---> version
- +-------> name of tinc daemon
-server CHALLENGE 57fb4b2ccd70d6bb35a64c142f47e61d
- \______________________________/
- +-> KEYLENGTH bits totally random string, encrypted
- with client's public RSA key
-client CHAL_REPLY 191e23
- +-> 160 bits SHA1 value of the complete decrypted
- CHALLENGE sent by the server
-server ID server 8 0
- | | +-> options
- | +---> version
- +-------> name of tinc daemon
-client CHALLENGE da02add1817c1920989ba6ae2a49cecb
- \______________________________/
- +-> KEYLENGTH bits totally random string, encrypted
- with server's public RSA key
-server CHAL_REPLY 2bdeed
- +-> 160 bits SHA1 value of the complete decrypted
- CHALLENGE sent by the client
-client META_KEY 5f0823a93e35b69e7086ec7866ce582b
- \______________________________/
- +-> KEYLENGTH bits totally random string, encrypted
- with server's public RSA key
-server META_KEY 6ab9c1640388f8f045d1a07f8a672630
- \______________________________/
- +-> KEYLENGTH bits totally random string, encrypted
- with client's public RSA key
-client ACK
-server ACK
---------------------------------------------------------------------------
-
-When the server receives the ACK from the client, it should prepare itself
-for the fact that any subsequent data will be encrypted with the key the server
-sent itself in the META_KEY. Ofcourse, this key is taken from the decrypted
-version of that META_KEY, so that we will know for sure only the real client
-can send us messages. The same goes for the client when it receives an ACK.
-
-5. Encryption of VPN packets
------------------------------
-
-The VPN packets are also encrypted, but with a different key than the one used
-for the meta connection. The reason is that VPN packets can also come from
-other clients which do not have a meta connection with server. Each tinc daemon
-propagates (on request) a separate key for packets that it receives. This key
-is a random string, generated on the fly. Since it is exchanged using the meta
-connection, this key itself will be encrypted.
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