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