SFTPPlus Documentation

Start Page 8. User’s Guides 8.1. SSL / TLS / SSH protocols overview
server-side client-side security configuration

8.1. SSL / TLS / SSH protocols overview

8.1.1. Introduction

FTPS / HTTPS and Local Manager protocols use the SSL v3 or TLS v1 cryptographic protocols. SSL v3 is provided for backward compatibility with older endpoints. TLS v1 should be used whenever possible. TLS v1 is actually SSL v4, renamed before release.

SFTP and SCP protocols use the SSH v2 cryptographic protocol.

The SSH protocol is similar to the SSL/TLS protocols. At lower levels they all employ the same cryptographic algorithms, yet the protocols are incompatible.

The server supports all FIPS-140 compliant cryptographic algorithms, as well as other algorithms required by older or non FIPS-140 compliant endpoints.

What are colloquially known as SSL certificates should be referred to as X.509 certificates. The term SSL certificate became common due to the adoption of the X.509 certificate format by Netscape when it designed the original version of the SSL (Secure Socket Layer) protocol. SSL is now deprecated (being replaced by TLS) but the term SSL certificate is still common, and will likely persist for the near future.

8.1.2. Establishing trust

Trust is established by using Public Key Encryption. In a Public Key Encryption system, each party operates with pairs of keys. Those keys may be private or public.

Possession of a private key will prove the identity of the party. For this reason, a private key should never be shared.

In an SSL/TLS connection, trust is established using an external party, the Certificate Authority.

In an SSH connection, trust is established directly by the client accepting the server’s public key. SSL/TLS host and server validation

When the FTPS or HTTPS protocol is used, the configured certificate must be issued with the Subject Alternative Name (subjectAltName) or the Common Name (CN) containing the address used by the HTTPS or FTPS clients to connect to the server.

Multiple services can share the same set of keys and certificates.

If you have server-side certificates issued with different Common Names (CN), such as CN=acme-test.com and CN=acme1-test.com, ensure that there is a corresponding client certificate for each each of the CNs involved or that the name of the account is the same as the configured CN.

The port number is not included in SSL/TLS certificate validation.

When validating and troubleshooting SSL/TLS host and server, check both server-side and client-side error logs as the issue may be on the client or server-side certificates. If you only have access to the client-side or the server-side logs, such as working with external partners, please get in touch with them to obtain logs from the other side.

When there is an error code and message, the log information corresponds with the diagnostic information provided by OpenSSL.

8.1.3. Key Management

Default server installation will generate a self-signed SSL certificate, and pairs of 2048-bit RSA and 1024-bit DSA private/public keys.

To change the certificates used by the SSL/TLS service, you need to generate a new key and an associated certificate signing request. This can be done with external tools such as the IIS Manager, OpenSSL tools, or the Java KeyTool.

SSL certificates are stored in PEM format.

SSH keys are stored in OpenSSH format.

In the case that the SSH keys are breached, or you need to use keys with a different size, you can generate new keys using the Local Manager or the command line tool distributed with the server.

To manage public keys for SSH clients, please consult the documentation dedicated to SSH Key Authentication

8.1.4. Protecting private keys, public keys and certificates

The server handles private/public keys and certificates as normal files in the local file system.

The operating system’s file system security features should be used to protect private keys from being read by other parties. Public keys and certificates can be readable by all operating system users, as this does not pose any risk.

In general, they are stored as separate files, but you can have an SSL public key stored together with the private key in a single file.

When transferred or stored over an unsecured medium, the private keys should be encrypted with a password.

If private keys are breached, they can be used to impersonate the identity of the endpoint associated to them.

If the server’s private key is breached, it can be used to create a rogue server instance, which will look identical to the original instance for any remote endpoint.

Besides getting the private key, the attacker will also need to change the TCP/IP routing or the DNS server in use for the endpoint, so that the rogue server is accessible using the same IP or DNS name for the remote client.

If the client’s private key is breached, it can be used to impersonate the identity of the remote endpoint associated with that private key.

8.1.5. Private Key, Public Key, Certificate Backup / Recovery

Private keys, public keys and certificates can be backed up using regular file backup systems.

Backup systems require read access to absolutely all of the files, so private keys should be protected with a password.

For SSH keys, in the case that you lose the public key, you can recover it using the private key.

For SSL/TLS keys, in the case that you lose the certificate, you can ask your CA to send a copy of the certificate. In a worst-case scenario, you can generate a new CSR (Certificate Signing Request) for the same private key.

There is no method to recover private keys other than from backups.

8.1.6. Certificate Chaining

The X.509 standard used by SSL and TLS includes a model for setting up a hierarchy of certificate authorities.

At their core, SSL and TLS protocols use X.509 certificates to validate connection endpoints. To allow verification of the authenticity of these certificates, they are digitally signed by a certificate authority (a CA).

In a similar manner, a certificate authority’s certificate is signed by a higher level CA which delegates its trust to what is called an intermediate certificate authority.

This chain of trust can be traced back to a certificate signed by a CA known as a “Root Certificate Authority”. A Root CA is an authority whose certificate is considered ultimately trusted.

This whole process is called certificate chaining.

Unlike operating systems and browsers, SFTPPlus is not distributed with a single list of root CAs.

SFTPPlus contains a couple of narrow down lists of Root CAs, but each are targeted to a specific usage. For example, with the SharePoint Online AA list you will only set the component to accept from peer certificates signed by Microsoft IT SSL SHA2 and nothing else.

Administrators are required to explicitly configure the list of trusted root CAs, together with their intermediate CAs and the associated CRLs.

8.1.7. Performance considerations

Symmetric encryption algorithms are fast on modern computers. Private / public key asymmetric encryption and decryption are slow and require many more resources to compute compared to conventional single-key algorithms.

SSL/TLS and SSH actually use symmetric encryption to encrypt exchanged data. Asymmetric encryption is only used to exchange the (short-lived) symmetric keys.

SFTPPlus can handle hundreds of parallel active connections over a Gigabit Ethernet connection. With thousands of connections, our performance tests indicate that the bottleneck is not the CPU performing TLS/SSL/SSH cryptographic operations, but rather the transfer speed of the disk or the network bandwidth.

8.1.8. Weak ciphers and protocols

TLS/SSL and SSH protocols were introduced a long time ago, SSL 3.0 being introduced in 1996. Over time, some of the protocols or cryptographic algorithms proved to have design weaknesses or to be less secure.

The following cryptographic algorithms and protocols are not considered secure:

  • SSL at any version
  • RC4
  • MD5 message-digest algorithm
  • DES Data Encryption Standard symmetric-key algorithm
  • Export grade algorithms.

SSL version 2.0 is not supported as it contains a number of security flaws which ultimately led to the design of SSL version 3.0.

SSL version 3.0 is supported, but its usage is highly discouraged. As of 2014, the 3.0 version of SSL is considered insecure.

RC4 in SSL and TLS was at one time considered secure but as of March 2013, using RC4 in SSL and TLS is considered insecure.

The MD5 message-digest algorithm is a widely used cryptographic hash function, but with modern computers the security of the MD5 hash function is severely compromised. The algorithm is not included in the list of approved FIPS 140-2 hash functions.

The DES symmetric-key algorithm is vulnerable to brute force attack, and is not considered secure.

While the 3DES algorithm is approved by FIPS 140-2, it is no longer considered secure due to the vulnerability associated with the SWEET32 attack

U.S. cryptography export regulations define a set of algorithms with the intention of allowing them to be broken easily by the NSA, but not by other organizations with fewer computing resources. Nowadays, NSA capabilities from the 1990s can be matched by any personal computer, making those algorithms insecure.

While for some, FIPS 140-2 compliance is the gold standard for security, it was released in December 2002. With the fast pace at which the computer security landscape is evolving, a standard defined in 2002 should not be considered up to date.

FIPS 140-3 update was not yet released due to disagreement in the US government and the updated document is not yet ready for consumption as of this writing. Meanwhile, use the guidance from PCI and ISO/IEC 24759:2017 standards.

8.1.9. NULL ciphers

TLS/SSL can be used in non-authentication or non-encryption modes. These modes are disabled by default as they provide degraded security.

Non-encryption mode (eNULL) can be used in special cases when the remote peer is required to be authenticated, but the transmitted data is already encrypted using another method like a PGP encrypted file.


The OpenSSL version distributed with SUSE Linux Enterprise Linux version 12 does not allow using the eNULL ciphers.

Non-authentication mode (aNULL) is vulnerable to a “man in the middle” attack and its use is highly discouraged. In this mode, the connection will not validate the remote peer. Data sent in this mode is encrypted.

8.1.10. Certificate Revocation List Loading

When the CRL fails to be loaded for the first time, it is considered a critical failure and the component using the CRL is stopped.

This is done to help detecting configuration errors.

If the CRL was successfully loaded at least once, but then fails to be reloaded at the scheduled date and time, the loading will be retried with a delay of 4 hours.

The current loaded CRL is still considered valid, as long as the Next Update date and time is not reached.

If reloading the CRL still fails after the Next Update date and time is reached the current cached CRL is no longer valid and a new CRL reloading is scheduled in 4 hours.

In some special cases the current loaded CRL is considered invalid, even if the Next Update is not reached. The error messages will indicate whether the CRL is no longer valid.

When a SSL/TLS based service is started and CRL or CDP configuration is defined it will try to pre-cache the CRL by loading the CRL, even if no client has yet made a connection. In this way, when a client will later initiate a connection, the connection is not delayed while waiting for the CRL to be loaded.

A cached CRL is considered valid as long as the date and time advertised in the Next Update is not reached.

Redirection is not supported for the CRL URLs in order to mitigate redirection attacks or miss-configurations. Administrator need to always configure the final location of a CRL.