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RSA/SHA1 signature type disabled by default in OpenSSH

Contributed by rueda on 2021-08-30 from the Really Senile Algorthms
dept.

In a message to tech@ Damien Miller (djm@) explained the consequences
of his recent commit:

    [...]
    RSA/SHA1, a.k.a the "ssh-rsa" signature type is now disabled by default
    in OpenSSH.

    While The SSH protocol confusingly uses overlapping names for key and
    signature algorithms, this does not stop the use of RSA keys and there
    is no need to regenerate "ssh-rsa" keys - most servers released in the
    last five years will automatically negotiate the use of RSA/SHA-256/512
    signatures.

    This has been coming for a long time, but I do expect it will be
    distruptive for some people as there are likely to be some devices
    out there that cannot be upgraded to support the safer algorithms.

    In these cases, it is possible to selectively re-enable RSA/SHA1
    support by specifying PubkeyAcceptedAlgorithms=+ssh-rsa in the
    ssh_config(5) or sshd_config(5) for the endpoint.

    Please report any problems here, to bugs@ or to openssh@
    [...]

TL;DR:

  * The "ssh-rsa" signature type is now disabled by default.
  * "ssh-rsa" signatures can be selectively re-enabled if necessary.
  * RSA ("ssh-rsa") keys are not affected by this change and remain
    valid.

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