Add --ssh-host-key-md option

Set checksum for SSH host public key.  Use same syntax with --checksum
option.  TYPE is hash type.  The supported hash type is sha-1 or
md5. DIGEST is hex digest.  For example:
sha-1=b030503d4de4539dc7885e6f0f5e256704edf4c3.  This option can be
used to validate server's public key when SFTP is used.  If this
option is not set, which is default, no validation takes place.

1 files changed, 15 insertions, 1 deletions

diff --git a/src/SocketCore.cc b/src/SocketCore.cc
index 320f04a6..701fc33e 100644
--- a/src/SocketCore.cc
+++ b/src/SocketCore.cc
@@ -989,7 +989,8 @@ bool SocketCore::tlsHandshake(TLSContext* tlsctx, const std::string& hostname)
 
 #ifdef HAVE_LIBSSH2
 
-bool SocketCore::sshHandshake()
+bool SocketCore::sshHandshake(const std::string& hashType,
+                              const std::string& digest)
 {
   wantRead_ = false;
   wantWrite_ = false;
@@ -1009,6 +1010,19 @@ bool SocketCore::sshHandshake()
     throw DL_ABORT_EX(fmt("SSH handshake failure: %s",
                           sshSession_->getLastErrorString().c_str()));
   }
+  if (!hashType.empty()) {
+    auto actualDigest = sshSession_->hostkeyMessageDigest(hashType);
+    if (actualDigest.empty()) {
+      throw DL_ABORT_EX(fmt("Empty host key fingerprint from SSH layer: "
+                            "perhaps hash type %s is not supported?",
+                            hashType.c_str()));
+    }
+    if (digest != actualDigest) {
+      throw DL_ABORT_EX(fmt("Unexpected SSH host key: expected %s, actual %s",
+                            util::toHex(digest).c_str(),
+                            util::toHex(actualDigest).c_str()));
+    }
+  }
   return true;
 }