15.1.1.1. Default secure SSL/SLS configuration¶

When using the secure value for the ssl_cipher_list, the following algorithms are enabled:

HIGH:!PSK:!RSP:!eNULL:!aNULL:!RC4:!MD5:!DES:!3DES:!aDH:!kDH:!DSS

This list provides maximum compatibility with existing deployments while avoiding deprecated ciphers.

SFTPPlus uses the OpenSSL library provided by the Python cryptography module. To benefit from upstream security updates for the bundled OpenSSL library, keep your SFTPPlus deployments always updated. This OpenSSL version might not provide all the ciphers which are required by older SSL/TLS versions of the standard. This is valid especially for cryptographic methods which in recent years were discovered to no longer be secured. For example, SSLv3 is no longer provided at all. While 3DES was considered secure at the beginning of 2016, in August 2016 it was discovered that it is vulnerable to the SWEET32 attack. Therefore, 3DES support is no longer included with latest updates for most operating systems.

To verify the list of ciphers available for your operating system use:

openssl ciphers -V

15.1.1.2. SSL/TLS versions¶

• SSL v3 (considered not secure)

• TLS v1.0

• TLS v1.1

• TLS v1.2

• TLS v1.3

15.1.1.3. File formats¶

TLS / X.509 certificates and keys can be stored and read by SFTPPlus in the following formats:

• PKCS #8 / PEM

• PKCS #12 / PFX

15.1.1.4. Public-key cryptographic systems¶

• DSS/DSA

• RSA

15.1.1.5. Key agreement algorithms¶

• DHE, EDH, DH - ephemeral prime factorization Diffie-Hellman (DH) key agreement

• EECDH, ECDHE, ECDH - ephemeral elliptic curve Diffie-Hellman (ECDH) key agreement

For the DH key agreement, SFTPPlus uses a DH parameter for the 2 generator with a size of 2048 bits. Contact us if you require a different DH parameter for your configuration.

15.1.1.6. Hash functions¶

• MD5

• SHA-1 (FIPS 140-2 compatible)

• SHA-2 (for OpenSSL 0.9.8 or newer) (FIPS 140-2 compatible)

15.1.1.7. Encryption algorithms¶

• 3DES (FIPS 140-2 compatible, vulnerable to SWEET32 attacks)

• AES 128 and AES 256 (FIPS 140-2 compatible)

• RC4

• Blowfish

15.1.2.1. Default SFTP/SCP secure configuration¶

When using the secure value for the ssh_cipher_list, the following algorithms are enabled. These are listed below according to preference:

# Host keys
ssh-ed25519'
ecdsa-sha2-nistp256
ecdsa-sha2-nistp384
ecdsa-sha2-nistp521
rsa-sha2-512
rsa-sha2-256
ssh-rsa

# Ciphers
aes256-ctr
aes192-ctr
aes128-ctr

# MACs
# SHA1 and MD5 might look weak, but the way they are used in SSH
# does not allow for the possibility of a collision attack.
hmac-sha2-256
hmac-sha2-512

# Key Exchanges
# See RFC for current recommendation (check updates).
# This is based on:
# https://tools.ietf.org/id/draft-ietf-curdle-ssh-kex-sha2-09.html
curve25519-sha256 (with OpneSSL 1.1.1 or newer)
curve25519-sha256@libssh.org (with OpneSSL 1.1.1 or newer)
ecdh-sha2-nistp521
ecdh-sha2-nistp384
ecdh-sha2-nistp256
diffie-hellman-group-exchange-sha256
diffie-hellman-group18-sha512
diffie-hellman-group17-sha512
diffie-hellman-group16-sha512
diffie-hellman-group15-sha512
diffie-hellman-group14-sha256

This list provides maximum compatibility with existing deployments while avoiding deprecated ciphers.

15.1.2.2. Public-key cryptographic systems¶

Here is the list of supported public-key cryptographic systems ordered by SFTPPlus' preference during the negotiation phase:

• Ed25519 (ssh-ed25519)

• ECDSA (ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521)

• RSA (rsa-sha2-512, rsa-sha2-256, ssh-rsa)

• DSS/DSA (ssh-dss)

15.1.2.3. SSH Key Exchange¶

Here is the list of supported SSH key exchanges, ordered on the preference of SFTPPlus during the negotiation phase:

• curve25519-sha256

• curve25519-sha256@libssh.org

• ecdh-sha2-nistp521

• ecdh-sha2-nistp384

• ecdh-sha2-nistp256

• diffie-hellman-group-exchange-sha256 (FIPS 140-2 compatible)

• diffie-hellman-group-exchange-sha1 (FIPS 140-2 compatible)

• diffie-hellman-group14-sha1 (FIPS 140-2 compatible)

• diffie-hellman-group1-sha1 (FIPS 140-2 compatible, but no longer considered secure to modern standards)

• diffie-hellman-group14-sha256 (RFC 8268 for transition to newer group sizes)

• diffie-hellman-group15-sha512 (RFC8268)

• diffie-hellman-group16-sha512 (RFC8268)

• diffie-hellman-group17-sha512 (RFC8268)

• diffie-hellman-group18-sha512 (RFC8268)

The fixed group prime numbers are the one specified in RFC3526.

15.1.2.4. Keyed-hash message authentication code (HMAC)¶

Here is the list of supported HMAC, ordered on the preference of SFTPPlus during the negotiation phase:

• hmac-sha2-512 (FIPS 140-2 compatible)

• hmac-sha2-256 (FIPS 140-2 compatible)

• hmac-sha1 (FIPS 140-2 compatible)

• hmac-md5

15.1.2.5. Symmetric encryption algorithms¶

Here is the list of supported symmetric encryption algorithms, ordered on the preference of SFTPPlus during the negotiation phase:

• aes256-ctr, aes256-cbc, aes192-ctr, aes192-cbc, aes128-ctr, aes128-cbc (FIPS 140-2 compatible)

• cast128-ctr, cast128-cbc

• blowfish-ctr, blowfish-cbc

• 3des-ctr, 3des-cbc (FIPS 140-2 compatible, vulnerable to SWEET32 attacks)

15.1.4.1. Public-key cryptographic systems¶

• DSS/DSA

• RSA (RSA-E, RSA-S)

• ELG-E

15.1.4.2. Hash functions¶

• MD5

• SHA1

• RIPEMD160

• SHA256

• SHA384

• SHA512

• SHA224

15.1.4.3. Symmetric encryption algorithms¶

• IDEA

• 3DES

• CAST5

• BLOWFISH

• AES (AES128)

• AES192

• AES256

• TWOFISH

• CAMELLIA128

• CAMELLIA192

• CAMELLIA256

15.1.4.4. Compression¶

• Uncompressed

• ZIP

• ZLIB

15.1.5.1. TOTP - Time-Based One-Time Passwords¶

The Time-Based One-Time Password (TOTP) authentication method adds an extra layer of security on top of the usual username/password credentials.

A unique code valid for a limited number of seconds is used for validation.

The code is generated using helper applications like Google Authenticator or FreeOTP.

To use a unique password per session, this unique code has to be added at the end of the regular password. By appending the unique code to the regular password, the new method of authentication is still compatible with the traditional username and password authentication system. No extra changes are required for the file transfer client.

This implements a two factor authentication method (2FA) in which both the password and the unique code are used to authenticate the connection.

SFTPPlus supports the TOTP algorithm as defined in RFC 6238

The following parameters are supported: * 6 digits * 30 seconds interval * SHA1

Two-factor authentication will succeed as long as the received token is within one time step of 30 seconds (+/- 30s).

Authenticating twice with the same multi-factor authentication token will fail. This prevents replay attacks.