SHA-1 (Secure hash algorithm) is a 160-bit hash algorithm that is at the heart of many web security protocols such as Secure Sockets Layer (SSL) and Transport Layer Security (TLS) since shortly after it was developed by the NSA (National Security Agency) in 1995. In 2005, a professor in China demonstrated an attack that could be successfully launched against the SHA-1 function, suggesting that the algorithm might not be secure enough for ongoing use. Because of this, NIST immediately recommended federal agencies begin moving away from SHA-1 toward stronger algorithms. In 2011, NIST mandated that many applications in federal agencies
SSH2 subsystems are a useful convenience feature to predefine remote commands for SSH clients to invoke easily. Subsystems provide a layer of abstraction for defining and invoking the remote commands. A subsystem need not be a separate program; it can invoke a function built into the SSH server itself. SFTP is the most common SSH subsystem that you are going to encounter. For example on Linux distributions, the default /etc/ssh/sshd_config file defines one subsystem, This is the configuration line on Fedora 20: # override default of no subsystems Subsystem sftp /usr/libexec/openssh/sftp-server Do not remove or comment out the above line.
Perfect Forward Secrecy (PFS) is a property of public-key encryption systems which generate random public keys per session for the purposes of key agreement which are not based on any sort of deterministic algorithm. A compromise of one message cannot lead to the compromise of another message or multiple messages. Twitter, Apache mod_ssh, SSL, TLS, and IPSec all support forward secrecy. According to the referenced Wikipedia article: Forward secrecy is designed to prevent the compromise of a long-term secret key from affecting the confidentiality of past conversations. However, forward secrecy (including perfect forward secrecy) cannot defend against a successful cryptanalysis