OpenSSH: how to disallow weak (<2048 bits) RSA keys

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Are there any configs for OpenSSH server to disallow weak (e.g. <2048 bits) RSA keys? I'm aware of PubkeyAcceptedKeyTypes which can disallow specific key types, incl. ssh-rsa, as a whole.




ssh security configuration openssh




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    – Pedro
    Mar 1 at 21:37














up vote
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Are there any configs for OpenSSH server to disallow weak (e.g. <2048 bits) RSA keys? I'm aware of PubkeyAcceptedKeyTypes which can disallow specific key types, incl. ssh-rsa, as a whole.




ssh security configuration openssh




share|improve this question




















  • This is an interesting question.
    – Pedro
    Mar 1 at 21:37












up vote
1
down vote

favorite









up vote
1
down vote

favorite











Are there any configs for OpenSSH server to disallow weak (e.g. <2048 bits) RSA keys? I'm aware of PubkeyAcceptedKeyTypes which can disallow specific key types, incl. ssh-rsa, as a whole.




ssh security configuration openssh




share|improve this question












Are there any configs for OpenSSH server to disallow weak (e.g. <2048 bits) RSA keys? I'm aware of PubkeyAcceptedKeyTypes which can disallow specific key types, incl. ssh-rsa, as a whole.





ssh security configuration openssh





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asked Feb 25 at 14:30









nodakai

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  • This is an interesting question.
    – Pedro
    Mar 1 at 21:37
















  • This is an interesting question.
    – Pedro
    Mar 1 at 21:37















This is an interesting question.
– Pedro
Mar 1 at 21:37




This is an interesting question.
– Pedro
Mar 1 at 21:37










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I can't see it would be run-time configurable, but you could change the compile-time definition of SSH_RSA_MINIMUM_MODULUS_SIZE in sshkey.h and recompile:



#define SSH_RSA_MINIMUM_MODULUS_SIZE 1024


to



#define SSH_RSA_MINIMUM_MODULUS_SIZE 2048


Either that, or just don't install keys that are shorter. It might be possible to scan the users' authorized_keys files for keys that are shorter than that, at least if we accept the length of the encoded key as an indicator of the length of the actual key.



The encoding for a 2048 bit RSA key seems to be 372 characters long, so something like this might match shorter keys:



grep -E 'ssh-rsa [a-zA-Z0-9+/=],371( |$)' file...





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    I can't see it would be run-time configurable, but you could change the compile-time definition of SSH_RSA_MINIMUM_MODULUS_SIZE in sshkey.h and recompile:



    #define SSH_RSA_MINIMUM_MODULUS_SIZE 1024


    to



    #define SSH_RSA_MINIMUM_MODULUS_SIZE 2048


    Either that, or just don't install keys that are shorter. It might be possible to scan the users' authorized_keys files for keys that are shorter than that, at least if we accept the length of the encoded key as an indicator of the length of the actual key.



    The encoding for a 2048 bit RSA key seems to be 372 characters long, so something like this might match shorter keys:



    grep -E 'ssh-rsa [a-zA-Z0-9+/=],371( |$)' file...





    share|improve this answer
























      up vote
      2
      down vote













      I can't see it would be run-time configurable, but you could change the compile-time definition of SSH_RSA_MINIMUM_MODULUS_SIZE in sshkey.h and recompile:



      #define SSH_RSA_MINIMUM_MODULUS_SIZE 1024


      to



      #define SSH_RSA_MINIMUM_MODULUS_SIZE 2048


      Either that, or just don't install keys that are shorter. It might be possible to scan the users' authorized_keys files for keys that are shorter than that, at least if we accept the length of the encoded key as an indicator of the length of the actual key.



      The encoding for a 2048 bit RSA key seems to be 372 characters long, so something like this might match shorter keys:



      grep -E 'ssh-rsa [a-zA-Z0-9+/=],371( |$)' file...





      share|improve this answer






















        up vote
        2
        down vote










        up vote
        2
        down vote









        I can't see it would be run-time configurable, but you could change the compile-time definition of SSH_RSA_MINIMUM_MODULUS_SIZE in sshkey.h and recompile:



        #define SSH_RSA_MINIMUM_MODULUS_SIZE 1024


        to



        #define SSH_RSA_MINIMUM_MODULUS_SIZE 2048


        Either that, or just don't install keys that are shorter. It might be possible to scan the users' authorized_keys files for keys that are shorter than that, at least if we accept the length of the encoded key as an indicator of the length of the actual key.



        The encoding for a 2048 bit RSA key seems to be 372 characters long, so something like this might match shorter keys:



        grep -E 'ssh-rsa [a-zA-Z0-9+/=],371( |$)' file...





        share|improve this answer












        I can't see it would be run-time configurable, but you could change the compile-time definition of SSH_RSA_MINIMUM_MODULUS_SIZE in sshkey.h and recompile:



        #define SSH_RSA_MINIMUM_MODULUS_SIZE 1024


        to



        #define SSH_RSA_MINIMUM_MODULUS_SIZE 2048


        Either that, or just don't install keys that are shorter. It might be possible to scan the users' authorized_keys files for keys that are shorter than that, at least if we accept the length of the encoded key as an indicator of the length of the actual key.



        The encoding for a 2048 bit RSA key seems to be 372 characters long, so something like this might match shorter keys:



        grep -E 'ssh-rsa [a-zA-Z0-9+/=],371( |$)' file...






        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Feb 25 at 14:51









        ilkkachu

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