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How to change key size in OpenSSH? I would like to change it from 1024 key size length to 512.

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I am pretty sure this is what you are after. You need to use ssh-keygen.

ssh-keygen -t rsa -b 512

this should create a 512 rsa crypto key

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There's also a ServerKeyBits option in sshd_config, can set that to a minimum of 512.

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That's only used for legacy SSH-1 connections, and is not used with SSH-2. – IQAndreas May 30 '15 at 23:52

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