Security Working Group IPsec Working Group INTERNET-DRAFT J. Hughes, Editor June 1996 Expires in Six months Combined DES-CBC, HMAC and Replay Prevention Security Transform Status of this Memo This document is a submission to the IETF Internet Protocol Security (IPSEC) Working Group. Comments are solicited and should be addressed to the working group mailing list (ipsec@tis.com) or to the editor. 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 draft documents are valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Distribution of this memo is unlimited. Abstract This draft describes a combination of privacy, authentication, integrity and replay prevention into a single packet format. This document is the result of significant work by several major contributors and the IPsec working group as a whole. These contributors, cited later in this document, provided many of the key technical details summarized in this document. Hughes [Page 1] RFC DRAFT June 1996 Discussion This draft allows a combination of MD5 and DES-CBC. In addition to privacy, the goal of this transform is to ensure that the packet is authentic, can not be modified in transit, or replayed. For the purpose of this RFC, the terms conformance and compliance are synonymous. The claims of privacy, integrity, authentication, and replay prevention are made in this draft. A good general text describing the methods and algorithm are in [Schneier95]. Privacy is provided by DES-CBC [FIPS-46] [FIPS-46-1] [FIPS-74] [FIPS-81]. Integrity is provided by HMAC [Krawczyk96]. Authentication is provided since only the source and destination know the HMAC key. If the HMAC is correct, it proves that it must have been added by the source. Replay prevention is provided by the combination of a constantly increasing count, the SPI and the HMAC key. The integrity of the replay field is provided by the HMAC. Hughes [Page 2] RFC DRAFT June 1996 Packet Format +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--- | Security Parameters Index (SPI) | ^ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | --- | Replay Prevention Field (count) | | ^ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | | ~ Payload Data ~ | | | |HMAC | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DES | | Padding (0-7 bytes) | | CBC +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | Pad Length | Payload Type | v | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--- | | | | ~ HMAC digest ~ | | | v +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --- Security Parameters Index This field is negotiated at key setup and shall not be 0 [RFC-1825] Replay Prevention Replay Prevention is an unsigned 32 bit incrementing counter starting at a value of 1. The key (K, as described in a later section) must be changed frequently enough so that the counter is not allowed to wrap; in other words, the key must be changed before (2^32)-2 packets are transmitted using this key. For a given SPI, counter wrapping shall be considered to be a replay attack. (While a wrap is a replay attack, there is always the possibility that a packet can get duplicated, so the presence of a single or small number of duplicate packets is not an absolute indication of a replay attack.) The receiver must verify that for a given SPI the packets received have non-repeating (non-duplicate) counter values. This can be implemented as a simple increasing count test or the receiver may choose to accept out-of-order packets as long as it is guaranteed that packets can be received only once. For example, an implementation can use a sliding receive window, the size of which is an implementation detail. If such a receive window is supported, the receiver must ensure that it will accept packets within the current Hughes [Page 3] RFC DRAFT June 1996 window only once, and reject any packets it receives with a value that is less than the lower bound of the window. An example may allow the most recent 32 packets to be allowed to arrive out of order. That is, these 32 packets can arrive in any sequence relative to each other except that these packets are guaranteed to arrive only once. Appendix A has actual code that implement a 32 packet replay window and a test routine. The purpose of this routine is to show how it could be implemented. Payload The payload contains data that is described by the payload type field. This field is an integral number of bytes in length; the following padding and pad length fields will help provide alignment to a double word boundary. Padding The padding (pad bytes and pad length field) is used to align the following "payload type" field to end on a double word boundary (when counting from the start of the replay field). Padding bytes may be initialized with random data. At a minimum, the number of pad bytes added must be enough to align the payload type field on the next appropriate boundary. However, the sender may choose to include additional padding, provided that the alignment is maintained. In total, the sender can add 0-255 bytes of padding. Pad Length The pad length field indicates the number of pad bytes immediately preceding it. The range of valid values is 0-255, where a value of zero indicates that the byte immediately preceding the pad length field is the last byte of the payload. Hughes [Page 4] RFC DRAFT June 1996 Payload Type Describes what the payload is. The values are described in: ftp://ftp.isi.edu/in-notes/iana/assignments/protocol-numbers HMAC Digest The HMAC digest is a 128 bit residue described in [Krawczyk96]. This covers the SPI, replay, payload, padding, pad length, payload type. HMAC is a keyed algorithm and shall use the HMAC key as described in the section on keys. Encryption Transform Procedure CBC chaining with an IV_key is used. IV_key count|x1 x2 x3 | | | | |-------(+) --------(+) --------(+) | | | | | ------- | ------- | -------- k--| DES | | k--| DES | | k--| DES | ------- | ------- | -------- | | | | | |-----| |-----| |----... | | | y1 y2 y3 Where count is the Replay counter. x1, x2, x3 are the plaintext (x1 is 32 bits, all others are 64 bits). y1, y2, y3 are the ciphertext. The process of this transformation is comprised of the following 3 steps. 1. Taking the data and encapsulating it with the SPI, count, pad, pad length, and payload type. 2. Calculating the HMAC using the HMAC_key and creating the digest from the SPI, count, data, pad, pad length, and payload type and placing the result into the HMAC digest field. 3. Encrypting the count, data, pad, pad length, payload type, and HMAC digest using DES and the appropriate DES_key_ and IV_key. Hughes [Page 5] RFC DRAFT June 1996 (Note that the first DES block is a combination of the count and the first word of plaintext.) Decryption Transform Procedure CBC chaining with an IV_key is used. IV_key y1 y2 y3 | | | | | |------ |------ |----... | | | | | | | ------- | ------- | -------- | k--| DES | | k--| DES | | k--| DES | | ------- | ------- | -------- | | | | | | |-------(+) |-------(+) |-------(+) | | | (count|x1) x2 x3 The process of this transformation is comprised of the following 4 steps. 1. (Optional step) Decrypt the first bock of data using the appropriate DES_key and IV_key and then do a quick "sanity check" of the count. If the count has decreased below the window or has increased by more than 65k, then it is safe to discard this packet as either a replay, non-authentic or too old. If the count is within 65K, then the probability that the packet is authentic is 65535/65536. (The following replay check and HMAC check are both still required). 2. Decrypt the count (if not already done), data, pad, pad length, and payload type using DES and the appropriate DES_key_ and IV_key. 3. Calculate the HMAC using the HMAC_key and create the digest from the SPI, count, data, pad, pad length, and payload type and checking the result at digest at the end of the packet. If the digest is incorrect, discard the packet. 4. Check the count using the window algorithm discussed above. If the packet is duplicate or too old, discard the packet. Hughes [Page 6] RFC DRAFT June 1996 Key Material The key K is provided by the key management layer. This key is used to derive the symmetric keys, they are: DES_Key_I is the DES key for traffic from the initiator -> responder. DES_Key_R is the DES key for traffic from the responder -> initiator. HMAC_Key is the key for the HMAC Algorithm (This is the same for both directions, but since these are encrypted by different DES keys, then this is not a problem.) IV_key is used to stop code book attacks on the first block. The vertical bar symbol "|" is used to denote concatenation of bit strings. MD5(x|y) denotes the result of applying the MD5 function to the concatenated bit strings x and y. Truncate(x,n) denotes the result of truncating x to its first n bits. DES_Key_I = Truncate(MD5( D_Pad_I | K ),64) DES_Key_R = Truncate(MD5( D_Pad_R | K ),64) IV_Key = Truncate(MD5( I_Pad | K ),64) HMAC_Key = MD5( H_Pad | K ) where each _Pad_is 512 bit string. D_Pad_I = 0x5C repeated 64 times. D_Pad_R = 0x3A repeated 64 times. I_Pad = 0xAC repeated 64 times. H_Pad = 0x53 repeated 64 times. (Implementation note, The 16 byte intermediate residuals can be precalculated from these constants and stored to reduce processing overhead). Hughes [Page 7] RFC DRAFT June 1996 Security Considerations The ESP-DES-HMAC-RP transform described in this draft is immune to the [Bellovin96] attacks. (AH [RFC-1826], in some modes, can also provide immunity to these attack.) The implications of the size of K can be found in [Blaze96]. References [Bellovin96] Bellovin, S., "Problem Areas for the IP Security Protocols", AT&T Research, ftp://ftp.research.att.com/dist/smb/badesp.ps, July, 1996. [FIPS-46] US National Bureau of Standards, "Data Encryption Standard", Federal Information Processing Standard (FIPS) Publication 46, January 1977. [FIPS-46-1] US National Bureau of Standards, "Data Encryption Standard", Federal Information Processing Standard (FIPS) Publication 46-1, January 1988. [FIPS-74] US National Bureau of Standards, "Guidelines for Implementing and Using the Data Encryption Standard", Federal Information Processing Standard (FIPS) Publication 74, April 1981. [FIPS-81] US National Bureau of Standards, "DES Modes of Operation" Federal Information Processing Standard (FIPS) Publication 81, December 1980. [Krawczyk96] Krawczyk, H., Bellare, M., Canetti, R., "HMAC-MD5: Keyed-MD5 for Message Authentication", work-in-progress, http://info.internet.isi.edu:80/in-drafts/files/draft-ietf-ipsec- hmac-md5-00.txt, March, 1996 [Maughan96] Maughan, D., Schertler, M. Internet Security Association and Key Management Protocol (ISAKMP), work-in-progress, http://info.internet.isi.edu:80/in-drafts/files/draft-ietf-ipsec- isakmp-04.txt, February, 1996 [Orman96] Orman, H., "The Oakley Key Determination Protocol", work- in-progress, http://info.internet.isi.edu:80/in-drafts/files/draft- ietf-ipsec-oakley-00.txt, February, 1996. Hughes [Page 8] RFC DRAFT June 1996 [RFC-1825] Atkinson, R, "Security Architecture for the Internet Protocol", ftp://ds.internic.net/rfc/rfc1825.txt, August 1995. [RFC-1826] Atkinson, R, "IP Authentication Header", ftp://ds.internic.net/rfc/rfc1826.txt, August 1995. [Schneier95] Schneier, B., "Applied Cryptography Second Edition", John Wiley & Sons, New York, NY, 1995. ISBN 0-471-12845-7 [Blaze96] Blaze M., Diffie, W., Rivest, R., Schneier, B., Shimomura, T., Thompson, E., Wiener, M., "Minimal Key Lengths for Symmetric Ciphers to Provide Adequate Commercial Security", http://theory.lcs.mit.edu/~rivest/bsa-final-report.ascii, January, 1996 Acknowledgements This document is the result of significant work by several major contributors. They include (in alphabetical order): Robert W. Baldwin RSA Labs. Kevin Kingdon RSA Labs Hugo Krawczyk IBM Corporation Perry Metzger Piermont Information Services Phil Rogaway University of California at Davis Bill Simpson Computer Systems Consulting Services Hughes [Page 9] RFC DRAFT June 1996 David A Wagner University of California at Berkeley In addition, the contributions of the entire IPSEC Working Group are acknowledged. The IPsec working group can be contacted through the chairs: Ran Atkinson Cisco Systems Paul Lambert Oracle Corporation Editor's Address James P. Hughes Network Systems Corporation Hughes [Page 10] RFC DRAFT June 1996 Appendix A This is a routine that implements a 32 packet window. This is intended on being an implementation sample. #include #include typedef unsigned long u_long; enum { ReplayWindowSize = 32 }; u_long bitmap = 0; /* session state - must be 32 bits */ u_long lastSeq = 0; /* session state */ /* Returns 0 if packet disallowed, 1 if packet permitted */ int ChkReplayWindow(u_long seq); int ChkReplayWindow(u_long seq) { u_long diff; if (seq == 0) return 0; /* first == 0 or wrapped */ if (seq > lastSeq) { /* new larger sequence number */ diff = seq - lastSeq; if (diff < ReplayWindowSize) { /* In window */ bitmap = (bitmap << diff) | 1; /* set bit for this packet */ } else bitmap = 1; /* This packet has a "way larger" */ lastSeq = seq; return 1; /* larger is good */ } diff = lastSeq - seq; if (diff >= ReplayWindowSize) return 0; /* too old or wrapped */ if (bitmap & (1 << diff)) return 0; /* this packet already seen */ bitmap |= (1 << diff); /* mark as seen */ return 1; /* out of order but good */ } Hughes [Page 11] RFC DRAFT June 1996 char string_buffer[512]; #define STRING_BUFFER_SIZE sizeof(string_buffer) int main() { int result; u_long last, current, bits; printf("Input initial state (bits in hex, last msgnum):\n"); if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) exit(0); sscanf(string_buffer, "%lx %lu", &bits, &last); if (last != 0) bits |= 1; bitmap = bits; lastSeq = last; printf("bits:%08lx last:%lu\n", bitmap, lastSeq); printf("Input value to test (current):\n"); while (1) { if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) break; sscanf(string_buffer, "%lu", ¤t); result = ChkReplayWindow(current); printf("%-3s", result ? "OK" : "BAD"); printf(" bits:%08lx last:%lu\n", bitmap, lastSeq); } return 0; } Hughes [Page 12]