INTERNET-DRAFT Andy Heffernan <draft-heffernan-tcp-md5-02.txt> cisco Systems September 26, 1996 TCP MD5 Signature Option Status of this Memo This document is an Internet Draft. 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." Please check the I-D abstract listing contained in each Internet Draft directory to learn the current status of this or any Internet Draft. Abstract This memo describes a TCP extension to enhance security for selected TCP applications. It defines a new TCP option for carrying an MD5 [1] digest in a TCP segment. This digest acts like a signature for that segment, incorporating information known only to the connection end points. Using this option in the way described in this paper significantly reduces the danger from security attacks on critical TCP applications on the Internet. This document specifies an experimental protocol for use in the Internet. 1.0 Introduction The primary motivation for this option is to allow applications using TCP as a transport to protect themselves against the introduction of spoofed segments into the connection stream. Of particular concern are resets. To spoof a connection using the scheme described in this paper, an Heffernan Expires March 26, 1997 [Page 1]
INTERNET-DRAFT TCP MD5 Signature Option September 26, 1996 attacker would not only have to guess sequence numbers, but would also have had to obtain the password included in the MD5 digest. This password never appears in the connection stream, and the actual form of the password is up to the application. It could even change during the lifetime of a particular connection so long as this change was synchronized on both ends (although retransmission can become problematical in some TCP implementations with changing passwords). Finally, there is no negotiation for the use of this option in a connection, rather it is purely a matter of site and/or application policy whether or not its connections use the option. 2.0 Proposal Every segment sent on a connection to be protected against spoofing will contain the 16-byte MD5 digest produced by applying the MD5 algorithm to the following items: -- the TCP pseudo-header -- the TCP header, excluding options, and assuming a checksum of zero -- the TCP segment data (if any) -- an independently-specified key or password, known to both TCPs and presumably connection-specific The header and pseudo-header are in network byte order. The nature of the key is deliberately left unspecified, but it must be known by both ends of the connection. A particular TCP implementation will determine what the application may specify as the key. Upon receiving a signed segment, the receiver must validate it by calculating its own digest from the same data (using its own key) and comparing the two digest. A failing comparison must result in the segment being dropped and must not produce any response back to the sender. Logging the failure is probably advisable. Unlike other proposed TCP extensions, the absence of the option in the SYN,ACK segment must not cause the sender to disable its sending of signatures. This negotiation is typically done to prevent some TCP implementations from misbehaving upon receiving options in non- SYN segments. This is not a problem, since the SYN,ACK will not be signed and will thus be ignored. The connection will never be made, and non-SYN segments with options will never be sent. More importantly, the sending of signatures must be under the complete control of the application, not at the mercy of the remote host not understanding the option. Heffernan Expires March 26, 1997 [Page 2]
INTERNET-DRAFT TCP MD5 Signature Option September 26, 1996 3.0 Syntax The proposed option has the following format: +---------+---------+-------------------+ | Kind=19 |Length=18| MD5 digest... | +---------+---------+-------------------+ | | +---------------------------------------+ | | +---------------------------------------+ | | +-------------------+-------------------+ | | +-------------------+ The MD5 digest is always 16 bytes in length, and the option would appear in every segment of a connection. 4.0 Some Implications 4.1 Connectionless Resets Connectionless resets will be ignored by the receiver of the resets, since the originator of the resets does not know the key, and so cannot generate the proper signatures for the segments. This means, for example, that connection attempts by a TCP which is generating signatures to a port with no listener will time out instead of being refused. Similarly, resets generated by a TCP in response to segments sent on a stale connection will also be ignored. 4.2 Performance The performance hit in calculating digests may inhibit the use of this option. Some measurements of a sample implementation showed that on a 25 MHz 68040, generating a signature for simple ACK segment took an average of 0.1448 ms, while generating a signature for a data segment carrying 4096 bytes of data took 4.688 ms on average. These times would be applied to both the input and output paths, with the input path also bearing the cost of a 16-byte compare. 4.3 TCP Header Size As with other options that are added to every segment, the size of the MD5 option must be factored into the MSS offered to the other side during connection negotiation. Specifically, the size of the Heffernan Expires March 26, 1997 [Page 3]
INTERNET-DRAFT TCP MD5 Signature Option September 26, 1996 header to subtract from the MTU (whether it is the MTU of the outgoing interface or IP's minimal MTU of 576 bytes) is now at least 18 bytes larger. The total header size is also an issue. The TCP header specifies where segment data starts with a 4-bit field which gives the total size of the header (including options) in 32-byte words. This means that the total size of the header plus option must be less than or equal to 60 bytes -- this leaves 40 bytes for options. As a concrete example, 4.4BSD defaults to sending window-scaling and timestamp information for connections it initiates. The most loaded segment will be the initial SYN packet to start the connection. With MD5 signatures, the SYN packet will contain the following: -- 4 bytes MSS option -- 4 bytes window scale option (3 bytes padded to 4 in 4.4BSD) -- 12 bytes for timestamp (4.4BSD pads the option as recommended in RFC 1323 Appendix A) -- 18 bytes for MD5 digest -- 2 bytes for end-of-option-list, to pad to a 32-bit boundary. This sums to 40 bytes, which just makes it. 5.0 References [1] Rivest, R, "The MD5 Message-Digest Algorithm," RFC 1321, MIT Laboratory for Computer Science, April 1992. Author's Address Andy Heffernan cisco Systems 170 West Tasman Drive San Jose, CA 95134 USA Phone: +1 408 526 8115 Email: ahh@cisco.com Heffernan Expires March 26, 1997 [Page 4]