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TUCoPS :: Unix :: General :: unix4985.htm

Tinc VPN Crypto Analysis white paper



10th Jan 2002 [SBWID-4985]
COMMAND

	Tinc VPN Crypto Analysis white paper

SYSTEMS AFFECTED

	All versions

PROBLEM

	Jerome Etienne [http://www.off.net/~jme] wrote :
	

	1  Security description
	

	   This section describes how tinc secures forwarded packets. The outgoing

	   packet begins with an \'salt\' of 2 bytes containing a cryptographically

	   strong random value. It plays the role of an IV according to the manual \"2

	   bytes of salt (random data) are added in front of the actual VPN packet,

	   so that two VPN packets with (almost) the same content do not seem to be

	   the same for eavesdroppers.\" The forwarded packet is appended. The couple

	   salt and forwarded is padded to be 64bit aligned (blowfish\'s block size).

	   The whole (salt, forwarded packet and padding) is encrypted with blowfish

	   in CBC.

	

	2  Vulnerabilities
	

	   This section explains how an attacker can modify packets (see section 2.1)

	   , replay them (see section 2.3), learn pattern of the plain text (see

	   section 2.2).

	

	  2.1  No packet authentication

	

	   The aim of encryption is to make the data unreadable for anybody who

	   doesn\'t know the key. It doesn\'t prevent an attacker from modifying the

	   data. People assume that an attacker won\'t do it because the attacker

	   wouldn\'t be able to choose the resulting clear text. But this section

	   shows that the attacker can choose the resulting clear text to some

	   extends and that modifying the cypher text data may be interesting even if

	   the attacker ignores the result.

	

	    2.1.1  To insert random data

	

	   If the attacker modifies the cipher text without choosing the resulting

	   clear text, it will likely produce random data. The legitimate user won\'t

	   detect the modification and will use them as if they were valid. As they

	   likely appears random, it will result of a Denial of Service (aka DoS).

	

	    2.1.2  To insert chosen data

	

	   The encryption mode is CBC[oST81,sec 5.3]. CBC allows cut/past attacks

	   i.e. the attacker can cut encrypted data from one part of a packet and

	   paste them in another location. As both data sections have been encrypted

	   by the same key, the clear text won\'t be completely random data.

	

	   This lack of authentication isn\'t a CBC flaw. Authentication isn\'t

	   considered a aim of the encryption mode, so most modes (e.g. ECB, CFB,

	   OFB) doesn\'t authenticate the data. To use another mode would be flawed in

	   the same way except if they explicitly protect against forgery. Recently

	   some modes including authentication popped up to speed up the encryption /

	   authentication couple but as far as i know they are all patented.

	

	   In very short, encrypting with CBC is Cn=Enc(Cn-1 xor Pn) where Enc(x) is

	   encrypting x, Pn is the nth block of plain text and Cn the nth block of

	   cipher text. For the first block, Cn-1 is an Initial vector (aka IV) which

	   may be public and must be unique for a given key. The decryption is Pn =

	   Dec(Cn) xor Cn-1. See [oST81,sec 5.3] for a longer description of CBC.

	

	   If the attacker copies s blocks from the location m to n (aka

	   [Cn,...,Cn+s-1] == [Cm,...,Cm+s-1]), Pn+1 up to Pn+s-1 will the same as

	   Pm+1 to Pm+s-1 and Pn will likely appears random. Cn (i.e. Cm) will be

	   decrypted as Pn = Dec(Cm) xor Cn-1 but Cm-1 and Cn-1 are different so Pn

	   will likely appears random. Nevertheless Pn+1 = Dec(Cn+1) xor Cn =

	   Dec(Cm+1) xor Cm = Pm+1, so Pn+1=Pm+1. So if the attacker has an idea of

	   the content of a group of blocks in a packet, he can copy them to the Nth

	   block, thus it can choose the content of it without being detected.

	

	   As usual packets aren\'t designed to appears random, its content may be

	   predictable to some extents (e.g. IP header) The attacker may use such

	   informations to guess the contents and do a knowledgeable cut/past.

	

	  2.2  Insecure IV

	

	   The aim of an IV is to hide the repetitive patterns inside the the

	   encrypted plain text, so it must be unique for a given key. Tinc\'s IV,

	   called salt in the source, are random so they aren\'t guaranteed to be

	   unique and are vulnerable to the birthday paradox. Moreover the IV is only

	   16bit long, so as a rule of thumb if tinc forwards 255 packets, there is a

	   probability of 50% to have at least 2 packets with the same IV.

	

	  2.3  No anti-replay protection

	

	   Tinc doesn\'t include any protection against packet\'s replay, so an

	   attacker who eavesdrops the encrypted packets can successfully replay them

	   later and the destination will consider them as legitimate.

	

	   The manual section 6.3.2 claims \"There is no extra provision against

	   replay attacks or alteration of packets. However, the VPN packets,

	   normally UDP or TCP packets themselves, contain checksums and sequence

	   numbers. Since those checksums and sequence numbers are encrypted, they

	   automatically become cryptographically secure. The kernel will handle any

	   checksum errors and duplicate packets.\"

	

	   We believe it is risky to base the security on assumption on the forwarded

	   packets. Moreover in this case, the assumptions are incorrect. UDP doesn\'t

	   have any sequence number. TCP do have sequence numbers but, for example,

	   an attacker can replay a TCP syn packet to perform a SYN flood attack on a

	   server behind the tinc peer.

	

	3  Conclusion
	

	   This text focuses on packet forwarding because i didn\'t found any hole in

	   the peer\'s authentication. WORK: to write

	

	References
	

	   [oST81]

	           National Institute of Standards and Technology. implementing and

	           using the nbs data encryption standard. Federal information

	           processing standards fips74, April 1981.

SOLUTION

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