libssh: libssh

libssh 0.12.1 The SSH library

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libssh

This is the online reference for developing with the libssh library. It documents the libssh C API and the C++ wrapper.

# Linking

We created a small howto how to link libssh against your application, read The Linking HowTo.

# Tutorial

You should start by reading The Tutorial, then reading the documentation of the interesting functions as you go.

# Features

The libssh library provides:

- Key Exchange Methods: sntrup761x25519-sha512, sntru.nosp@m.p761.nosp@m.x2551.nosp@m.9-sh.nosp@m.a512@.nosp@m.open.nosp@m.ssh.c.nosp@m.om, mlkem768x25519-sha256, mlkem768nistp256-sha256, mlkem1024nistp384-sha384, curve25519-sha256, curve.nosp@m.2551.nosp@m.9-sha.nosp@m.256@.nosp@m.libss.nosp@m.h.or.nosp@m.g, ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521, diffie-hellman-group1-sha1, diffie-hellman-group14-sha1
- GSSAPI Key Exchange Methods: gss-group14-sha256-*, gss-group16-sha512-*, gss-nistp256-sha256-*, gss-curve25519-sha256-*
- Public Key Algorithms: ssh-ed25519, ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521, ssh-rsa, rsa-sha2-512, rsa-sha2-256
- Ciphers: aes256-ctr, aes192-ctr, aes128-ctr, aes256-cbc (rijnd.nosp@m.ael-.nosp@m.cbc@l.nosp@m.ysat.nosp@m.or.li.nosp@m.u.se), aes192-cbc, aes128-cbc, 3des-cbc, blowfish-cbc
- Compression Schemes: zlib, zlib@.nosp@m.open.nosp@m.ssh.c.nosp@m.om, none
- MAC hashes: hmac-sha1, hmac-sha2-256, hmac-sha2-512, hmac-md5
- Authentication: none, password, public-key, keyboard-interactive, gssapi-with-mic, gssapi-keyex
- Channels: shell, exec (incl. SCP wrapper), direct-tcpip, subsystem, auth-.nosp@m.agen.nosp@m.t-req.nosp@m.@ope.nosp@m.nssh..nosp@m.com
- Global Requests: tcpip-forward, forwarded-tcpip
- Channel Requests: x11, pty, exit-status, signal, exit-signal, keepa.nosp@m.live.nosp@m.@open.nosp@m.ssh..nosp@m.com, auth-.nosp@m.agen.nosp@m.t-req.nosp@m.@ope.nosp@m.nssh..nosp@m.com
- Subsystems: sftp(version 3), OpenSSH Extensions
- SFTP: statv.nosp@m.fs@o.nosp@m.penss.nosp@m.h.co.nosp@m.m, fstat.nosp@m.vfs@.nosp@m.opens.nosp@m.sh.c.nosp@m.om
- Thread-safe: Just don't share sessions
- Non-blocking: it can be used both blocking and non-blocking
- Your sockets: the app hands over the socket, or uses libssh sockets
- OpenSSL, MBedTLS or gcrypt (deprecated): builds with either

# Additional Features

- Client and server support
- SSHv2 protocol support
- Supports Linux, UNIX, BSD, Solaris, OS/2 and Windows
- Automated test cases with nightly tests
- Event model based on poll(2), or a poll(2)-emulation.

# Copyright Policy

libssh is a project with distributed copyright ownership, which means we prefer the copyright on parts of libssh to be held by individuals rather than corporations if possible. There are historical legal reasons for this, but one of the best ways to explain it is that it’s much easier to work with individuals who have ownership than corporate legal departments if we ever need to make reasonable compromises with people using and working with libssh.

We track the ownership of every part of libssh via git, our source code control system, so we know the provenance of every piece of code that is committed to libssh.

So if possible, if you’re doing libssh changes on behalf of a company who normally owns all the work you do please get them to assign personal copyright ownership of your changes to you as an individual, that makes things very easy for us to work with and avoids bringing corporate legal departments into the picture.

If you can’t do this we can still accept patches from you owned by your employer under a standard employment contract with corporate copyright ownership. It just requires a simple set-up process first.

We use a process very similar to the way things are done in the Linux Kernel community, so it should be very easy to get a sign off from your corporate legal department. The only changes we’ve made are to accommodate the license we use, which is LGPLv2 (or later) whereas the Linux kernel uses GPLv2.

The process is called signing.

## How to sign your work

Once you have permission to contribute to libssh from your employer, simply email a copy of the following text from your corporate email address to:

contr.nosp@m.ibut.nosp@m.ing@l.nosp@m.ibss.nosp@m.h.org

    libssh Developer's Certificate of Origin. Version 1.0

    By making a contribution to this project, I certify that:

    (a) The contribution was created in whole or in part by me and I
        have the right to submit it under the appropriate
        version of the GNU General Public License; or

    (b) The contribution is based upon previous work that, to the best of
        my knowledge, is covered under an appropriate open source license
        and I have the right under that license to submit that work with
        modifications, whether created in whole or in part by me, under
        the GNU General Public License, in the appropriate version; or

    (c) The contribution was provided directly to me by some other
        person who certified (a) or (b) and I have not modified it.

    (d) I understand and agree that this project and the contribution are
        public and that a record of the contribution (including all
        metadata and personal information I submit with it, including my
        sign-off) is maintained indefinitely and may be redistributed
        consistent with the libssh Team's policies and the requirements of
        the GNU GPL where they are relevant.

    (e) I am granting this work to this project under the terms of the
        GNU Lesser General Public License as published by the
        Free Software Foundation; either version 2.1 of
        the License, or (at the option of the project) any later version.

    https://www.gnu.org/licenses/lgpl-2.1.html

We will maintain a copy of that email as a record that you have the rights to contribute code to libssh under the required licenses whilst working for the company where the email came from.

Then when sending in a patch via the normal mechanisms described above, add a line that states:

       Signed-off-by: Random J Developer <random@developer.example.org>

using your real name and the email address you sent the original email you used to send the libssh Developer’s Certificate of Origin to us (sorry, no pseudonyms or anonymous contributions.)

That’s it! Such code can then quite happily contain changes that have copyright messages such as:

       (c) Example Corporation.

and can be merged into the libssh codebase in the same way as patches from any other individual. You don’t need to send in a copy of the libssh Developer’s Certificate of Origin for each patch, or inside each patch. Just the sign-off message is all that is required once we’ve received the initial email.

Have fun and happy libssh hacking!

The libssh Team

# Internet standard

## Secure Shell (SSH)

The following RFC documents described SSH-2 protocol as an Internet standard.

- RFC 4250, The Secure Shell (SSH) Protocol Assigned Numbers
- RFC 4251, The Secure Shell (SSH) Protocol Architecture
- RFC 4252, The Secure Shell (SSH) Authentication Protocol
- RFC 4253, The Secure Shell (SSH) Transport Layer Protocol
- RFC 4254, The Secure Shell (SSH) Connection Protocol
- RFC 4255, Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints (not implemented in libssh)
- RFC 4256, Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
- RFC 4335, The Secure Shell (SSH) Session Channel Break Extension
- RFC 4344, The Secure Shell (SSH) Transport Layer Encryption Modes

It was later modified and expanded by the following RFCs.

- RFC 4419, Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
- RFC 4462, Generic Security Service Application Program Interface (GSS-API) Authentication and Key Exchange for the Secure Shell (SSH) Protocol
- RFC 4716, The Secure Shell (SSH) Public Key File Format (not implemented in libssh)
- RFC 5647, AES Galois Counter Mode for the Secure Shell Transport Layer Protocol (the algorithm negotiation implemented according to openssh.com)
- RFC 5656, Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
- RFC 6594, Use of the SHA-256 Algorithm with RSA, DSA, and ECDSA in SSHFP Resource Records (not implemented in libssh)
- RFC 6668, SHA-2 Data Integrity Verification for the Secure Shell (SSH) Transport Layer Protocol
- RFC 7479, Using Ed25519 in SSHFP Resource Records (not implemented in libssh)
- RFC 8160, IUTF8 Terminal Mode in Secure Shell (SSH)
- RFC 8270, Increase the Secure Shell Minimum Recommended Diffie-Hellman Modulus Size to 2048 Bits
- RFC 8308, Extension Negotiation in the Secure Shell (SSH) Protocol (only the "server-sig-algs" extension implemented)
- RFC 8332, Use of RSA Keys with SHA-256 and SHA-512 in the Secure Shell (SSH) Protocol
- RFC 8709, Ed25519 and Ed448 Public Key Algorithms for the Secure Shell (SSH) Protocol
- RFC 8731, Secure Shell (SSH) Key Exchange Method Using Curve25519 and Curve448
- RFC 9142, Key Exchange (KEX) Method Updates and Recommendations for Secure Shell (SSH)

There are also drafts that are being currently developed and followed.

- draft-miller-ssh-agent-08 SSH Agent Protocol
- draft-ietf-sshm-mlkem-hybrid-kex-09 PQ/T Hybrid Key Exchange with ML-KEM in SSH
- draft-ietf-sshm-ntruprime-ssh-06 Secure Shell (SSH) Key Exchange Method Using Hybrid Streamlined NTRU Prime sntrup761 and X25519 with SHA-512: sntrup761x25519-sha512
- draft-ietf-sshm-chacha20-poly1305-02 Secure Shell (SSH) authenticated encryption cipher: chacha20-poly1305
- draft-ietf-sshm-strict-kex-01 SSH Strict KEX extension

Interesting cryptography documents:

- PKCS #11, PKCS #11 reference documents, describing interface with smartcards.

## Secure Shell File Transfer Protocol (SFTP)

The protocol is not an Internet standard but it is still widely implemented. OpenSSH and most other implementation implement Version 3 of the protocol. We do the same in libssh.

- draft-ietf-secsh-filexfer-02.txt, SSH File Transfer Protocol

## Secure Shell Extensions

The OpenSSH project has defined some extensions to the protocol. We support some of them like the statvfs calls in SFTP or the ssh-agent.

- OpenSSH's deviations and extensions
- OpenSSH's pubkey certificate authentication
- OpenSSH private key format (openssh-key-v1)