The issue is solved ! Dropbear didn't accept to open a SSH access because the pubkey saved in authorized_keys in right format !
My mistake is that until now, I used an old version of putty (<0.61) and the public keys created by PUTYYgen haven't a format identical with those generated by dropbear: Before Putty release 0.61 , Putty inserts one "=" between encode pubkey and comment field! Whilst, Dropbear inserts only one space. By confusion, I saved the dropbear pubkey in the old putty format. So, the key wasn't been found when an SSH access attempt by NAS1 in NAS2 !
