@seanmcken and @Tiran,

Neither rarcrack nor JtR crack RAR passwords. Both rarcrack and JtR are used just to enumerate a list of passwords to try and are sent, painfully one by one, to the rar command to see if it returns OK.

(the command is: unrar t -y -p$PASSWORD $FILE)

This is incredibly stupid^Winefficient. But I guess it's the easy (only?) way unless details of the RAR encryption are known.