From 2865b20e43c865c9a06d084b3c26b500ddb48faf Mon Sep 17 00:00:00 2001
From: Fabian Peter Hammerle <fabian@hammerle.me>
Date: Fri, 27 Dec 2019 00:29:32 +0100
Subject: [PATCH 1/2] added option to verify server's cert fingerprint

https://github.com/corecode/dma/pull/71
https://github.com/corecode/dma/issues/10
---
 conf.c   | 17 ++++++++++++++++-
 crypto.c | 28 ++++++++++++++++++++++++++++
 dma.8    |  3 +++
 dma.c    |  1 +
 dma.conf |  4 ++++
 dma.h    |  1 +
 6 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/conf.c b/conf.c
index 3e3c7d3..c4eb247 100644
--- a/conf.c
+++ b/conf.c
@@ -218,7 +218,22 @@ parse_conf(const char *config_path)
 			config.masquerade_user = user;
 		} else if (strcmp(word, "STARTTLS") == 0 && data == NULL)
 			config.features |= STARTTLS;
-		else if (strcmp(word, "OPPORTUNISTIC_TLS") == 0 && data == NULL)
+		else if (strcmp(word, "FINGERPRINT") == 0) {
+			if (strlen(data) != SHA256_DIGEST_LENGTH * 2) {
+				errlogx(EX_CONFIG, "invalid sha256 fingerprint length");
+			}
+			unsigned char *fingerprint = malloc(SHA256_DIGEST_LENGTH);
+			if (fingerprint == NULL) {
+				errlogx(EX_CONFIG, "fingerprint allocation failed");
+			}
+			for (unsigned int i = 0; i < SHA256_DIGEST_LENGTH; i++) {
+				if(sscanf(data + 2 * i, "%02hhx", &fingerprint[i]) != 1) {
+					errlogx(EX_CONFIG, "failed to read fingerprint");
+				}
+			}
+			free(data);
+			config.fingerprint = fingerprint;
+		} else if (strcmp(word, "OPPORTUNISTIC_TLS") == 0 && data == NULL)
 			config.features |= TLS_OPP;
 		else if (strcmp(word, "SECURETRANSFER") == 0 && data == NULL)
 			config.features |= SECURETRANS;
diff --git a/crypto.c b/crypto.c
index f9bc360..c37345d 100644
--- a/crypto.c
+++ b/crypto.c
@@ -78,6 +78,29 @@ init_cert_file(SSL_CTX *ctx, const char *path)
 	return (0);
 }
 
+int
+verify_server_fingerprint(const X509 *cert)
+{
+	unsigned char fingerprint[EVP_MAX_MD_SIZE] = {0};
+	unsigned int fingerprint_len = 0;
+	if(!X509_digest(cert, EVP_sha256(), fingerprint, &fingerprint_len)) {
+		syslog(LOG_WARNING, "failed to load fingerprint of server's certicate: %s",
+			   ssl_errstr());
+		return (1);
+	}
+	if(fingerprint_len != SHA256_DIGEST_LENGTH) {
+		syslog(LOG_WARNING, "sha256 fingerprint has unexpected length of %d bytes",
+		       fingerprint_len);
+		return (1);
+	}
+	if(memcmp(fingerprint, config.fingerprint, SHA256_DIGEST_LENGTH) != 0) {
+		syslog(LOG_WARNING, "fingerprints do not match");
+		return (1);
+	}
+	syslog(LOG_DEBUG, "verified server's fingerprint");
+	return (0);
+}
+
 int
 smtp_init_crypto(int fd, int feature, struct smtp_features* features)
 {
@@ -173,6 +196,11 @@ smtp_init_crypto(int fd, int feature, struct smtp_features* features)
 	if (cert == NULL) {
 		syslog(LOG_WARNING, "remote delivery deferred: Peer did not provide certificate: %s",
 		       ssl_errstr());
+		return (1);
+	}
+	if(config.fingerprint != NULL && verify_server_fingerprint(cert)) {
+		X509_free(cert);
+		return (1);
 	}
 	X509_free(cert);
 
diff --git a/dma.8 b/dma.8
index d29c85b..ea18292 100644
--- a/dma.8
+++ b/dma.8
@@ -226,6 +226,9 @@ Uncomment if you want TLS/SSL secured transfer.
 Uncomment if you want to use STARTTLS.
 Only useful together with
 .Sq SECURETRANS .
+.It Ic FINGERPRINT Xo
+Pin the server certificate by specifying its SHA256 fingerprint.
+Only makes sense if you use a smarthost.
 .It Ic OPPORTUNISTIC_TLS Xo
 (boolean, default=commented)
 .Xc
diff --git a/dma.c b/dma.c
index 4006bce..d62bf2c 100644
--- a/dma.c
+++ b/dma.c
@@ -85,6 +85,7 @@ struct config config = {
 	.mailname	= NULL,
 	.masquerade_host = NULL,
 	.masquerade_user = NULL,
+	.fingerprint = NULL,
 };
 
 
diff --git a/dma.conf b/dma.conf
index 6d5a51c..fa95fc1 100644
--- a/dma.conf
+++ b/dma.conf
@@ -25,6 +25,10 @@
 # SECURETRANSFER)
 #STARTTLS
 
+# Pin the server certificate by specifying its SHA256 fingerprint.
+# Only makes sense if you use a smarthost.
+#FINGERPRINT 1234567890ABCDEF1234567890ABCDEF1234567890ABCDEF1234567890ABCDEF
+
 # Uncomment if you have specified STARTTLS above and it should be allowed
 # to fail ("opportunistic TLS", use an encrypted connection when available
 # but allow an unencrypted one to servers that do not support it)
diff --git a/dma.h b/dma.h
index ed0d0fc..5841362 100644
--- a/dma.h
+++ b/dma.h
@@ -138,6 +138,7 @@ struct config {
 	const char *mailname;
 	const char *masquerade_host;
 	const char *masquerade_user;
+	const unsigned char *fingerprint;
 
 	/* XXX does not belong into config */
 	SSL *ssl;

From ff520537486a32f1946b8076f17c7cdd1cd0b5d4 Mon Sep 17 00:00:00 2001
From: Simon Schubert <2@0x2c.org>
Date: Sun, 29 Dec 2019 14:44:11 +0100
Subject: [PATCH 2/2] rephrase messages without possessive

---
 crypto.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/crypto.c b/crypto.c
index c37345d..e2d621b 100644
--- a/crypto.c
+++ b/crypto.c
@@ -84,7 +84,7 @@ verify_server_fingerprint(const X509 *cert)
 	unsigned char fingerprint[EVP_MAX_MD_SIZE] = {0};
 	unsigned int fingerprint_len = 0;
 	if(!X509_digest(cert, EVP_sha256(), fingerprint, &fingerprint_len)) {
-		syslog(LOG_WARNING, "failed to load fingerprint of server's certicate: %s",
+		syslog(LOG_WARNING, "failed to load fingerprint of server certicate: %s",
 			   ssl_errstr());
 		return (1);
 	}
@@ -97,7 +97,7 @@ verify_server_fingerprint(const X509 *cert)
 		syslog(LOG_WARNING, "fingerprints do not match");
 		return (1);
 	}
-	syslog(LOG_DEBUG, "verified server's fingerprint");
+	syslog(LOG_DEBUG, "successfully verified server certificate fingerprint");
 	return (0);
 }