"Unable to negotiate" and "no matching cipher found" errors when trying to connect with sftp.
This document (000020706) is provided subject to the disclaimer at the end of this document.
Environment
SUSE Linux Enterprise Server 15 SP4
SUSE Linux Enterprise Server for SAP Applications 15 SP4
SUSE Linux Enterprise Desktop 15 SP4
SUSE Linux Enterprise Server 15 SP3
SUSE Linux Enterprise Server for SAP Applications 15 SP3
SUSE Linux Enterprise Desktop 15 SP3
SUSE Linux Enterprise Server for SAP Applications 15 SP4
SUSE Linux Enterprise Desktop 15 SP4
SUSE Linux Enterprise Server 15 SP3
SUSE Linux Enterprise Server for SAP Applications 15 SP3
SUSE Linux Enterprise Desktop 15 SP3
Situation
sftp is unable to connect to a SFTP server and the following error message is displayed:
[Unable to negotiate with X.X.X.X port 22: no matching cipher found. Their offer: blowfish-cbc, aes256-cbc]
Resolution
The SFTP server likely supports only legacy CBC (Cipher Block Chain) ciphers such as blowfish-cbc, aes256-cbc, and the SFTP client doesn't accept those ciphers.
There are two possible solutions:
1) Upgrade and harden the SFTP server so that it supports more secure ciphers (best solution);
2) As a workaround, force the SFTP client to select the legacy cipher supported by the SFTP server for encrypting the data transfers, using the -c parameter of the sftp command;
Example:
From sftp man page:
There are two possible solutions:
1) Upgrade and harden the SFTP server so that it supports more secure ciphers (best solution);
2) As a workaround, force the SFTP client to select the legacy cipher supported by the SFTP server for encrypting the data transfers, using the -c parameter of the sftp command;
Example:
sftp -c aes128-cbc sftpuser@X.X.X.X
where X.X.X.X if the SFTP server IP address.
From sftp man page:
-c cipher
Selects the cipher to use for encrypting the data
transfers. This option is directly passed to ssh(1).
The list of available ciphers supported by the SFTP client may also be obtained using the command "ssh -Q cipher".
Cause
The SFTP client is trying to use more secure ciphers for encrypting the data transfers but the SFTP server probably supports only legacy CBC (Cipher Block Chain) ciphers.
Additional Information
sftp man page: https://man7.org/linux/man-pages/man1/sftp.1.html
Block cipher mode of operation: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation
SSH CBC vulnerability: https://www.kb.cert.org/vuls/id/958563
Plaintext Recovery Attacks Against SSH: https://www.isg.rhul.ac.uk/~kp/SandPfinal.pdf
OpenSSH Legacy Options https://www.openssh.com/legacy.html
Block cipher mode of operation: https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation
SSH CBC vulnerability: https://www.kb.cert.org/vuls/id/958563
Plaintext Recovery Attacks Against SSH: https://www.isg.rhul.ac.uk/~kp/SandPfinal.pdf
OpenSSH Legacy Options https://www.openssh.com/legacy.html
Disclaimer
This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.
- Document ID:000020706
- Creation Date: 22-Jul-2022
- Modified Date:17-Oct-2022
-
- SUSE Linux Enterprise Desktop
- SUSE Linux Enterprise Server
- SUSE Linux Enterprise Server for SAP Applications
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