> ## Documentation Index > Fetch the complete documentation index at: https://docs.aitriumos.com/llms.txt > Use this file to discover all available pages before exploring further. # SFTP Server Compatibility > SSH algorithms, authentication methods and connection behavior of AitriumOS as an SFTP client, for IT teams operating a trial's SFTP server. export const Audience = ({who}) =>
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; This page is for the IT team that operates a trial's SFTP server. It lists what AitriumOS supports as an SFTP client and the traffic your server will see from participating sites. Site IT teams should start with [Upload connection troubleshooting](/aitrium-os/connection-troubleshooting). **Version covered: AitriumOS 2.2.7.** AitriumOS uses the libssh2 1.11 SSH library. On Windows it uses the Windows cryptography API (CNG); on macOS it uses OpenSSL. The two platforms support different algorithm sets, so check both columns below. ## How sites connect * AitriumOS is an SSH-2 SFTP client. Each site's workstation connects **directly** to the host and port configured in the trial's integration. Traffic does not pass through Aitrium, and it cannot go through a web (HTTP) proxy. * Connections come from each **site's public egress IP address**. There is no fixed Aitrium address to allowlist. If your server restricts source addresses, each site must send you its egress addresses. * If your host name resolves to several addresses, such as IPv6 and IPv4, AitriumOS 2.2.7 tries them one after another, in the order the site's resolver returns them. ## Supported algorithms SSH uses the first algorithm in the client's list that the server also supports. AitriumOS offers the algorithms below in this order of preference. | Category | Windows | macOS | | - | - | - | | **Key exchange** | `diffie-hellman-group-exchange-sha256`, `diffie-hellman-group16-sha512`, `diffie-hellman-group18-sha512`, `diffie-hellman-group14-sha256` | `curve25519-sha256`, `curve25519-sha256@libssh.org`, `ecdh-sha2-nistp256`, `ecdh-sha2-nistp384`, `ecdh-sha2-nistp521`, then the same Diffie-Hellman methods as Windows | | **Host key** | `rsa-sha2-512`, `rsa-sha2-256`, then legacy `ssh-rsa` | `ecdsa-sha2-nistp256`, `ecdsa-sha2-nistp384`, `ecdsa-sha2-nistp521`, `ssh-ed25519`, `rsa-sha2-512`, `rsa-sha2-256`, then legacy `ssh-rsa` | | **Cipher** | `chacha20-poly1305@openssh.com`, `aes256-ctr`, `aes128-ctr` | `chacha20-poly1305@openssh.com`, `aes256-gcm@openssh.com`, `aes256-ctr`, `aes128-gcm@openssh.com`, `aes128-ctr` | | **MAC** | `hmac-sha2-256`, `hmac-sha2-256-etm@openssh.com`, `hmac-sha2-512`, `hmac-sha2-512-etm@openssh.com` | Same as Windows | * **Integrated ciphers.** `chacha20-poly1305@openssh.com` and the AES-GCM ciphers include their own integrity protection, so no separate MAC is negotiated with them. * **Strict key exchange.** AitriumOS 2.2.7 supports the OpenSSH strict key exchange extension (`kex-strict-c-v00@openssh.com`), which mitigates the Terrapin attack. * **Not supported:** post-quantum hybrid key exchange (such as `sntrup761x25519-sha512` or `mlkem768x25519-sha256`) and DSA host keys. Servers that offer post-quantum methods alongside the classic ones above are unaffected. * **Older algorithms**, such as SHA-1-based key exchange and MACs and CBC-mode ciphers, are still offered at the lowest preference so older servers keep working. A server that disables them is unaffected. ### Recommended server settings **Servers that work with AitriumOS today need no changes for 2.2.7.** AitriumOS 2.2.7 only adds algorithms and reorders its preferences; everything earlier versions could negotiate, including legacy `ssh-rsa` host keys and SHA-1 key exchange and MACs, is still accepted. The settings below are guidance for **new servers or when hardening** an existing one. When hardening, keep at least one algorithm from each row enabled so that **both Windows and macOS** workstations can connect: | Category | Keep at least one of | | - | - | | Key exchange | `diffie-hellman-group-exchange-sha256`, `diffie-hellman-group16-sha512`, `diffie-hellman-group18-sha512`, `diffie-hellman-group14-sha256` | | Host key | An **RSA host key**, preferably offered as `rsa-sha2-256` or `rsa-sha2-512` (legacy `ssh-rsa` is still accepted) | | Cipher | `chacha20-poly1305@openssh.com`, `aes256-ctr` or `aes128-ctr` | | MAC | `hmac-sha2-256` or `hmac-sha2-512`, with or without `-etm@openssh.com` (not needed if only `chacha20-poly1305@openssh.com` is offered) | **Windows workstations need an RSA host key and an AES-CTR or ChaCha20 cipher.** A server with only an Ed25519 or ECDSA host key, only elliptic-curve key exchange, or only AES-GCM ciphers accepts macOS workstations but not Windows workstations. Windows workstations then report *The SFTP server's security settings aren't supported by this version of AitriumOS*. ### Changes in AitriumOS 2.2.7 | | AitriumOS 2.2.6 | AitriumOS 2.2.7 | | - | - | - | | Encrypt-then-MAC (`hmac-sha2-256-etm@openssh.com`, `hmac-sha2-512-etm@openssh.com`) | Not supported | Supported | | `chacha20-poly1305@openssh.com` | Not supported | Supported | | AES-GCM | Not supported | Supported on macOS | | Preferred cipher | `aes256-ctr`, then `aes128-ctr` | `chacha20-poly1305@openssh.com`, `aes256-gcm@openssh.com`, `aes256-ctr`, `aes128-gcm@openssh.com`, `aes128-ctr`, where supported | | No algorithm in common | Generic connection error | *The SFTP server's security settings aren't supported by this version of AitriumOS* | A server that accepts **only** encrypt-then-MAC MACs requires AitriumOS 2.2.7 or later. ## Authentication | Method | Support | | - | - | | **Password** | Supported (SSH `password` method). | | **Private key** | Supported (SSH `publickey` method) with an **RSA** key, optionally protected by a passphrase. If both a key and a password are configured, the key is used. | | **Keyboard-interactive** | Not supported. A server that offers only `keyboard-interactive`, as some PAM-based setups do, rejects AitriumOS. Enable `password` or `publickey` for the trial's account. | | **Multi-factor sign-in** | Not supported. The trial's account must be able to sign in with a single password or key. | **RSA key signatures.** AitriumOS signs RSA key logins with `rsa-sha2-512` or `rsa-sha2-256`. A server that accepts **only** the legacy `ssh-rsa` signature for key logins works if it advertises that restriction to clients (OpenSSH 9.7 or later does); on older servers, also accept `rsa-sha2-256` or `rsa-sha2-512` in `PubkeyAcceptedAlgorithms`. **Private key formats.** PEM (`-----BEGIN RSA PRIVATE KEY-----`) works on every platform and is the recommended format. OpenSSH format (`-----BEGIN OPENSSH PRIVATE KEY-----`) is also accepted. On Windows, AitriumOS converts an OpenSSH-format key with the `ssh-keygen` tool from Windows' **OpenSSH Client** feature, so each site's workstation needs that feature installed. PuTTY `.ppk` keys are not accepted; export them in OpenSSH format with PuTTYgen first. Credentials are configured by the trial organization and delivered encrypted to authorized site users. Sites do not enter or hold them. See [Transfer credentials](/security/aitrium-os/data-flows#transfer-credentials). ## Traffic your server will see | Traffic | What happens | How often | | - | - | - | | **Upload** | Sign-in, then SFTP transfer of the prepared, de-identified dataset. | Each submission. On a network failure, AitriumOS tries to connect up to three times, 2 and then 4 seconds apart. A rejected sign-in is not retried. | | **Delivery check** | After each upload, AitriumOS reads the uploaded file's size on the server to confirm delivery. A full read-back with a SHA-256 comparison can be enabled per integration on request. | Each upload. | | **Reachability check** (from 2.2.6) | Connection, SSH identification and key exchange only, then disconnect with the reason `AitriumOS reachability check`. **No sign-in, no SFTP session, no data.** | At most once a day per workstation and destination, at a randomized time, and skipped after a successful upload or connection test in the last 24 hours. Also before a workstation's first submission to your server (at most once an hour), and when a site user selects **Check now**. | | **Connection test** | Sign-in with the trial's credentials, without transferring data. | Automatically when a site user opens the trial, at most 4 times per 24 hours per workstation and destination. A success is reused for 24 hours. After a rejected sign-in there are no automatic retries. Site users can also start a test. | Many workstations at one hospital can share a single public IP address. The limits above apply per workstation, so intrusion-prevention or ban rules (for example fail2ban) should allow for several workstations connecting from one address. The delivery check needs the trial's account to be able to read the attributes of files it has uploaded. The full read-back also needs read access to those files. ## Troubleshooting from the server side * **Connection closes after the key exchange, before sign-in, with the reason `AitriumOS reachability check`:** this is the handshake-only check working as designed. * **"No matching" key exchange, host key, cipher or MAC in your server log:** compare your server's enabled algorithms with [Recommended server settings](#recommended-server-settings). Tell sites to update to AitriumOS 2.2.7 if your server accepts only encrypt-then-MAC MACs or only ChaCha20. * **No connection attempt in your log while a site reports "No response" or "refused the connection":** the traffic is blocked before it reaches your server, by the site's outbound firewall or your own address restrictions. For compatibility questions, contact [support@aitriumos.com](mailto:support@aitriumos.com).