Send a SSH_MSG_EXT_INFO with server-sig-algs when the client indicates they support extensions by sending ext-info-c. This allows modern clients that don't do ssh-rsa anymore because of sha1 to still use RSA keys with sha2.

Robin_Jadoul

Apr 12, 2023, 1:05 PM

EUHO3DAZ4D3LMHYJDLVLXBHIFAUKXMNFNDLE5ZHG4T7SVJMG55EAC

Dependencies

• [2] 7FRJYUI6 Reboot because of a bad change

Change contents

• edit in thrussh/src/server/session.rs at line 119
  [2.191449]

  [2.191449]
  + pub(crate) fn send_server_sig_algs(&mut self) {

  + // This should be only ever called when this is valid

  + if let Some(Kex::NewKeys(newkeys)) = &self.common.kex {

  + let mut buf = CryptoVec::new();

  + buf.push(msg::EXT_INFO);

  + buf.push_u32_be(1); // nr-extensions

  + buf.extend_ssh_string(b"server-sig-algs"); // extension-name

  + let algs = self

  + .common

  + .config

  + .preferred

  + .key

  + .iter()

  + .map(|n| n.0)

  + .collect::<Vec<_>>()

  + .join(",");

  + buf.extend_ssh_string(algs.as_bytes()); // extension-value

  + newkeys.cipher.write(&buf, &mut self.common.write_buffer);

  + } else {

  + unreachable!(

  + "Trying to send server-sig-algs SSH_MSG_EXT_INFO when we didn't just send NEWKEYS"

  + );

  + }

  + }

  +
• edit in thrussh/src/server/mod.rs at line 662
  [2.224638]

  [2.224638]
  + let client_supports_ext = kexdh.names.client_supports_ext;
• edit in thrussh/src/server/mod.rs at line 669
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  [2.224889]
  + if client_supports_ext {

  + // RFC 8308 (2.4): Send SSH_MSG_EXT_INFO as the next packet following the server's first SSH_MSG_NEWKEYS.

  + // Since we have no session established, this should have been the first time

  + session.send_server_sig_algs();

  + }
• edit in thrussh/src/negotiation.rs at line 34
  [2.269535]

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  + pub client_supports_ext: bool,

  + pub server_supports_ext: bool,
• edit in thrussh/src/negotiation.rs at line 141
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  + let client_supports_ext = Self::select(&[kex::CLIENT_SUPPORTS_EXT], kex_string).is_some();

  + let server_supports_ext = Self::select(&[kex::SERVER_SUPPORTS_EXT], kex_string).is_some();

  +
• edit in thrussh/src/negotiation.rs at line 204
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  [2.274587]
  + client_supports_ext,

  + server_supports_ext,
• edit in thrussh/src/msg.rs at line 60
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  +

  + // https://www.rfc-editor.org/rfc/rfc8308#section-2.3

  + pub const EXT_INFO: u8 = 7;
• edit in thrussh/src/kex.rs at line 48
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  [2.308346]
  + pub const CLIENT_SUPPORTS_EXT: Name = Name("ext-info-c");

  + pub const SERVER_SUPPORTS_EXT: Name = Name("ext-info-s");