[dm-crypt] some explanations would be welcome

[Pascal]

hi,

I've a large file (some Gb) that I want to encrypt without copying it, so I
do this :

# md5sum myfile
d2e539a1ff722e9f8f8f97605ca4ec1e -
# cryptsetup --type plain open myfile enc
Enter passphrase:
# dd bs=4M < myfile > /dev/mapper/enc
w+1 records in
w+1 records out
x bytes copied, y s, z MB/s
# md5sum < /dev/mapper/enc
d2e539a1ff722e9f8f8f97605ca4ec1e -

and all is ok for encryption.
now, I want to decrypt my encrypted file in the same way, so I do this :

# dd bs=4M < /dev/mapper/enc | md5sum
d2e539a1ff722e9f8f8f97605ca4ec1e -

it starts well but the result does not go as desired :

# dd bs=4M < /dev/mapper/enc > myfile
w+1 records in
w+1 records out
x bytes copied, y s, z MB/s
# md5sum myfile
120f0695aa0c2c0940f8a5a4c0bbd18f -

I tried increasing the bs (32M) but the result is the same :-(

what is my problem ?

regards.
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