123 lines
5.6 KiB
Text
123 lines
5.6 KiB
Text
This is the security documentation for tinc, a Virtual Private Network daemon.
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Copyright 2000 Guus Sliepen <guus@sliepen.warande.net>,
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2000 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 1.1.2.2 2000/09/17 20:11:59 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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---------------------------------------
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Any negotations about the meta protocol
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encryption go here(u).
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---------------------------------------
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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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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 it's 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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Instead of doing RSA encryption again, tinc will use a part of the random
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string that was exchanged during the authentication phase as the key for the
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symmetric cipher. Some symmetric ciphers require a random initialisation vector
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for improved security. This vector can be taken from the random string as well.
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Is this secure? I (Guus Sliepen) think at this moment that it is:
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- Since the random string cannot be decrypted by anyone eavesdropping or
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playing man-in-the-middle, the symmetric key cannot be known by sniffing.
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- The unencrypted returned hash value is supposed to be cryptographically
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secure. Furthermore, it can only at most give a way 160 bits of information
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from the complete random string which is longer than the key for the
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symmetric cipher, so very few bits will actualy contain information about
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the symmetric cipher key alone, if any.
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- If the RSA encryption is cracked, the rest of the communications can be
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decrypted anyway.
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- If the symmetric cipher encryption is cracked without using the information
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from the encrypted random strings or the hash values, this still won't give
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the full plaintext for the random string, so it won't facilitate a known-
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plaintext attack on the RSA encryption.
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- RSA and symmetric ciphers are fundamentally different. It is very unlikely
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that the overlap of both will create any interference that will facilitate
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an easier-than-brute-force attack.
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Other options for key exchange could be:
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* A second exchange of RSA encrypted random strings.
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This is equal to the former scheme just without knowing the hash value of
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the unecrypted random string.
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* Diffie-Hellman with RSA signing.
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This should be very secure, but there are a lot of pitholes with using both
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encryption with public keys and private keys together with the same keypair.
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* Diffie-Hellman with passphrases.
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This is what tinc <= 1.0pre2 used to do. Passphrases are secret, exchanging
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them must be done with great care, nobody may eavesdrop. Exchanging public
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keys on the other hand is much safer, everybody may eavesdrop, just as long
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as you are sure that the public key itself belongs to the right owner.
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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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