[man]

BAD MSG:
     AUTHORIZED_KEYS FILE FORMAT
     [...]

     Each line of the file contains one key (empty lines and lines
     starting with a `#' are ignored as comments).  Each RSA public
     key consists of the following fields, separated by spaces:
     options, bits, exponent, modulus, comment.  Each protocol version
     2 public key consists of: options, key­ type, base64 encoded key,
     comment.  The options fields are optional; its presence is
     determined by whether the line starts with a number or not (the
     option field never starts with a number).  The bits, exponent,
     modu­ lus and comment fields give the RSA key for protocol
     version 1; the com­ ment field is not used for anything (but may
     be convenient for the user to identify the key).  For protocol
     version 2 the keytype is ``ssh-dss'' or ``ssh-rsa''.


This is a test to test the validity of the key, so that user cannot
put invalid stuff in ~/authorized_keys.

It was setup by the FSF sysadmins.

I wonder, do you think one could add "command=/bin/bash " in his key
in a site that doesn't use an authorized_keys prefix (ie Gna!)?


Is the problem, the fact you want to support SSH Protocol v1?

-- 
Sylvain