SRS uses base64 encoding for the checksum, however while this is ok when we

only have MTA in the loop, some implementations like Dovecot's LMTP dislike
finding '/' in an e-mail address. Since checksum is meant to be verified at
the MX that generated the SRS encoding, use alternate rfc354 base64 encode,
swapping '/' with '_' and '+' with '-'.

ok eric@ millert@

3 files changed, 32 insertions, 10 deletions

diff --git a/usr.sbin/smtpd/smtpd.h b/usr.sbin/smtpd/smtpd.h
index 7ba6a472a19..ceefa269279 100644
--- a/usr.sbin/smtpd/smtpd.h
+++ b/usr.sbin/smtpd/smtpd.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: smtpd.h,v 1.639 2019/09/20 17:46:05 gilles Exp $	*/
+/*	$OpenBSD: smtpd.h,v 1.640 2019/09/29 10:03:49 gilles Exp $	*/
 
 /*
  * Copyright (c) 2008 Gilles Chehade <gilles@poolp.org>
@@ -1708,6 +1708,8 @@ int session_socket_error(int);
 int getmailname(char *, size_t);
 int base64_encode(unsigned char const *, size_t, char *, size_t);
 int base64_decode(char const *, unsigned char *, size_t);
+int base64_encode_rfc3548(unsigned char const *, size_t,
+		      char *, size_t);
 
 void log_trace_verbose(int);
 void log_trace(int, const char *, ...)
diff --git a/usr.sbin/smtpd/srs.c b/usr.sbin/smtpd/srs.c
index dc34ae486b9..05737d8d9c9 100644
--- a/usr.sbin/smtpd/srs.c
+++ b/usr.sbin/smtpd/srs.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: srs.c,v 1.2 2019/09/21 06:40:48 semarie Exp $	*/
+/*	$OpenBSD: srs.c,v 1.3 2019/09/29 10:03:49 gilles Exp $	*/
 
 /*
  * Copyright (c) 2019 Gilles Chehade <gilles@poolp.org>
@@ -125,7 +125,7 @@ srs0_encode(const char *sender, const char *rcpt_domain)
 		return sender;
 
 	/* compute HHHH */
-	base64_encode(srs_hash(env->sc_srs_key, tmp), SHA_DIGEST_LENGTH,
+	base64_encode_rfc3548(srs_hash(env->sc_srs_key, tmp), SHA_DIGEST_LENGTH,
 	    md, sizeof md);
 
 	/* prepend SRS0=HHHH= prefix */
@@ -157,7 +157,7 @@ srs1_encode_srs0(const char *sender, const char *rcpt_domain)
 		return sender;
 
 	/* compute HHHH */
-	base64_encode(srs_hash(env->sc_srs_key, tmp), SHA_DIGEST_LENGTH,
+	base64_encode_rfc3548(srs_hash(env->sc_srs_key, tmp), SHA_DIGEST_LENGTH,
 		md, sizeof md);
 
 	/* prepend SRS1=HHHH= prefix */
@@ -196,7 +196,7 @@ srs1_encode_srs1(const char *sender, const char *rcpt_domain)
 		return sender;
 
 	/* compute HHHH */
-	base64_encode(srs_hash(env->sc_srs_key, tmp + 5), SHA_DIGEST_LENGTH,
+	base64_encode_rfc3548(srs_hash(env->sc_srs_key, tmp + 5), SHA_DIGEST_LENGTH,
 		md, sizeof md);
 
 	/* prepend SRS1=HHHH= prefix skipping previous hops' HHHH */
@@ -234,14 +234,14 @@ srs0_decode(const char *rcpt)
 		return NULL;
 
 	/* compute checksum */
-	base64_encode(srs_hash(env->sc_srs_key, rcpt+5), SHA_DIGEST_LENGTH,
+	base64_encode_rfc3548(srs_hash(env->sc_srs_key, rcpt+5), SHA_DIGEST_LENGTH,
 	    md, sizeof md);
 
 	/* compare prefix checksum with computed checksum */
 	if (strncmp(md, rcpt, 4) != 0) {
 		if (env->sc_srs_key_backup == NULL)
 			return NULL;
-		base64_encode(srs_hash(env->sc_srs_key_backup, rcpt+5),
+		base64_encode_rfc3548(srs_hash(env->sc_srs_key_backup, rcpt+5),
 		    SHA_DIGEST_LENGTH, md, sizeof md);
 		if (strncmp(md, rcpt, 4) != 0)
 			return NULL;
@@ -302,14 +302,14 @@ srs1_decode(const char *rcpt)
 		return NULL;
 
 	/* compute checksum */
-	base64_encode(srs_hash(env->sc_srs_key, rcpt+5), SHA_DIGEST_LENGTH,
+	base64_encode_rfc3548(srs_hash(env->sc_srs_key, rcpt+5), SHA_DIGEST_LENGTH,
 	    md, sizeof md);
 
 	/* compare prefix checksum with computed checksum */
 	if (strncmp(md, rcpt, 4) != 0) {
 		if (env->sc_srs_key_backup == NULL)
 			return NULL;
-		base64_encode(srs_hash(env->sc_srs_key_backup, rcpt+5),
+		base64_encode_rfc3548(srs_hash(env->sc_srs_key_backup, rcpt+5),
 		    SHA_DIGEST_LENGTH, md, sizeof md);
 		if (strncmp(md, rcpt, 4) != 0)
 			return NULL;
diff --git a/usr.sbin/smtpd/util.c b/usr.sbin/smtpd/util.c
index fff0f774aee..2138d1a480f 100644
--- a/usr.sbin/smtpd/util.c
+++ b/usr.sbin/smtpd/util.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: util.c,v 1.147 2019/08/28 19:46:20 eric Exp $	*/
+/*	$OpenBSD: util.c,v 1.148 2019/09/29 10:03:49 gilles Exp $	*/
 
 /*
  * Copyright (c) 2000,2001 Markus Friedl.  All rights reserved.
@@ -860,6 +860,26 @@ base64_decode(char const *src, unsigned char *dest, size_t destsize)
 	return __b64_pton(src, dest, destsize);
 }
 
+int
+base64_encode_rfc3548(unsigned char const *src, size_t srclen,
+	      char *dest, size_t destsize)
+{
+	size_t i;
+	int ret;
+
+	if ((ret = base64_encode(src, srclen, dest, destsize)) == -1)
+		return -1;
+
+	for (i = 0; i < destsize; ++i) {
+		if (dest[i] == '/')
+			dest[i] = '_';
+		else if (dest[i] == '+')
+			dest[i] = '-';
+	}
+
+	return ret;
+}
+
 void
 log_trace(int mask, const char *emsg, ...)
 {