svn commit: r360137 - head/sys/netipsec

[John Baldwin]

Author: jhb
Date: Mon Apr 20 22:57:15 2020
New Revision: 360137
URL: https://svnweb.freebsd.org/changeset/base/360137

Log:
  Update comments about IVs used in IPsec ESP.
  
  Add some prose and a diagram describing the layout of the cipher IV
  for AES-CTR and AES-GCM and how it relates to the ESP IV stored in the
  packet after the ESP header.  Also, remove an XXX comment about the
  initial block counter value used for AES-CTR in esp_output as the
  current code matches the RFC (and the equivalent code in esp_input
  didn't have the XXX comment).
  
  Discussed with:	cem

Modified:
  head/sys/netipsec/xform_esp.c

Modified: head/sys/netipsec/xform_esp.c
==============================================================================
--- head/sys/netipsec/xform_esp.c	Mon Apr 20 22:24:49 2020	(r360136)
+++ head/sys/netipsec/xform_esp.c	Mon Apr 20 22:57:15 2020	(r360137)
@@ -406,22 +406,38 @@ esp_input(struct mbuf *m, struct secasvar *sav, int sk
 	crp->crp_payload_start = skip + hlen;
 	crp->crp_payload_length = m->m_pkthdr.len - (skip + hlen + alen);
 
+	/* Generate or read cipher IV. */
 	if (SAV_ISCTRORGCM(sav)) {
 		ivp = &crp->crp_iv[0];
 
-		/* GCM IV Format: RFC4106 4 */
-		/* CTR IV Format: RFC3686 4 */
-		/* Salt is last four bytes of key, RFC4106 8.1 */
-		/* Nonce is last four bytes of key, RFC3686 5.1 */
+		/*
+		 * AES-GCM and AES-CTR use similar cipher IV formats
+		 * defined in RFC 4106 section 4 and RFC 3686 section
+		 * 4, respectively.
+		 *
+		 * The first 4 bytes of the cipher IV contain an
+		 * implicit salt, or nonce, obtained from the last 4
+		 * bytes of the encryption key.  The next 8 bytes hold
+		 * an explicit IV unique to each packet.  This
+		 * explicit IV is used as the ESP IV for the packet.
+		 * The last 4 bytes hold a big-endian block counter
+		 * incremented for each block.  For AES-GCM, the block
+		 * counter's initial value is defined as part of the
+		 * algorithm.  For AES-CTR, the block counter's
+		 * initial value for each packet is defined as 1 by
+		 * RFC 3686.
+		 *
+		 * ------------------------------------------
+		 * | Salt | Explicit ESP IV | Block Counter |
+		 * ------------------------------------------
+		 *  4 bytes     8 bytes          4 bytes
+		 */
 		memcpy(ivp, sav->key_enc->key_data +
 		    _KEYLEN(sav->key_enc) - 4, 4);
-
+		m_copydata(m, skip + hlen - sav->ivlen, sav->ivlen, &ivp[4]);
 		if (SAV_ISCTR(sav)) {
-			/* Initial block counter is 1, RFC3686 4 */
 			be32enc(&ivp[sav->ivlen + 4], 1);
 		}
-
-		m_copydata(m, skip + hlen - sav->ivlen, sav->ivlen, &ivp[4]);
 		crp->crp_flags |= CRYPTO_F_IV_SEPARATE;
 	} else if (sav->ivlen != 0)
 		crp->crp_iv_start = skip + hlen - sav->ivlen;
@@ -813,22 +829,20 @@ esp_output(struct mbuf *m, struct secpolicy *sp, struc
 	crp->crp_payload_length = m->m_pkthdr.len - (skip + hlen + alen);
 	crp->crp_op = CRYPTO_OP_ENCRYPT;
 
-	/* Generate IV / nonce. */
+	/* Generate cipher and ESP IVs. */
 	ivp = &crp->crp_iv[0];
 	if (SAV_ISCTRORGCM(sav)) {
-		/* GCM IV Format: RFC4106 4 */
-		/* CTR IV Format: RFC3686 4 */
-		/* Salt is last four bytes of key, RFC4106 8.1 */
-		/* Nonce is last four bytes of key, RFC3686 5.1 */
+		/*
+		 * See comment in esp_input() for details on the
+		 * cipher IV.  A simple per-SA counter stored in
+		 * 'cntr' is used as the explicit ESP IV.
+		 */
 		memcpy(ivp, sav->key_enc->key_data +
 		    _KEYLEN(sav->key_enc) - 4, 4);
 		be64enc(&ivp[4], cntr);
 		if (SAV_ISCTR(sav)) {
-			/* Initial block counter is 1, RFC3686 4 */
-			/* XXXAE: should we use this only for first packet? */
 			be32enc(&ivp[sav->ivlen + 4], 1);
 		}
-
 		m_copyback(m, skip + hlen - sav->ivlen, sav->ivlen, &ivp[4]);
 		crp->crp_flags |= CRYPTO_F_IV_SEPARATE;
 	} else if (sav->ivlen != 0) {