there is no encrypted tftp.
try scp or https if you are concerned about encryption..to see if your equipment supports it, type following:
router#copy ?
/erase Erase destination file system.
/noverify Don't verify image signature before reload.
/verify Verify image signature before reload.
archive: Copy from archive: file system
cns: Copy from cns: file system
flash: Copy from flash: file system
ftp: Copy from ftp: file system
http: Copy from http: file system
https: Copy from https: file system
ips-sdf Copy from current IPS signature configuration
null: Copy from null: file system
nvram: Copy from nvram: file system
pram: Copy from pram: file system
rcp: Copy from rcp: file system
running-config Copy from current system configuration
scp: Copy from scp: file system
startup-config Copy from startup configuration
system: Copy from system: file system
tftp: Copy from tftp: file system
xmodem: Copy from xmodem: file system
ymodem: Copy from ymodem: file system