how to authenticate with git-svn on a subversion repository

hello !

i would like to use git-svn to commit some modifications to a Subversion repository but i don't know where i can enter my username and password to commit to the repository ? Is there any special file to put them. For now i get an error:

# git-svn commit remotes/git-svn
Autorisation refusée: MKACTIVITY de 
'/svn/!svn/act/8115f5df-c0da-4a6d-91ef-135dcb76141c': Échec à 
l'autorisation (http://projet.archlinuxfr.org) at /usr/bin/git-svn line 553
512 at /usr/bin/git-svn line 574
	main::commit_lib('f45ba41060287fedfcedfb5fc4c29ecfe30a7466') called at 
/usr/bin/git-svn line 480
	main::commit('remotes/git-svn') called at /usr/bin/git-svn line 172

Thanks