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I created a 5 MiB random file using openssl with head:

$ openssl enc -aes-256-ctr -pass pass:1o0SxTnYvbtjFtKLiuv3ccPebLOJiUU -nosalt < /dev/zero | head -c 5M > /mnt/pny1/file1

It created a file of the correct size, but also gave an error message:

error writing output file

I tried it with dd and got the same result:

$ openssl enc -aes-256-ctr -pass pass:1o0SxTnYvbtjFtKLiuv3ccPebLOJiUU -nosalt < /dev/zero | dd of=/mnt/pny1/file2 bs=1M count=5 iflag=fullblock
5+0 records in
5+0 records out
error writing output file
5242880 bytes (5.2 MB) copied, 0.0194212 s, 270 MB/s

I then tried overwriting a loop device using openssl with cat:

# openssl enc -aes-256-ctr -pass pass:1o0SxTnYvbtjFtKLiuv3ccPebLOJiUU -nosalt < /dev/zero | cat > /dev/loop0
cat: write error: No space left on device
error writing output file

Always error writing output file.

Why is this error occurring and how do I prevent it?

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3 Answers 3

Reset to default
6

It's openssls incorrect handling of SIGPIPE. head and also dd will close the pipe when they have recieved their 5 MB. This produces an EPIPE error on the write() system call when the openssl process tries to write to the pipe:

EPIPE    fd is connected to a pipe or socket whose reading end is closed. When this
         happens the writing process will also receive a SIGPIPE signal.

openssl then died due to a signal rather than exiting normally. You can check that in your example using:

$ openssl enc -aes-256-ctr -pass pass:password -nosalt < /dev/zero | head -c 5M >file
error writing output file
$ echo ${PIPESTATUS[@]}
1 0

The $PIPESTATUS array contains the exit values of the processes within the pipes. You see the openssl process exited with exit code 1.

However, to overcome that error, use head or dd before openssl in the pipe:

$ head -c 5M /dev/zero | openssl enc -aes-256-ctr -pass pass:password -nosalt >file
$ echo ${PIPESTATUS[@]}
0 0
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3
  • Does cat also close the pipe when it reaches uses up all the space on the block device? Commented Dec 10, 2015 at 3:53
  • I meant to say: does cat also close the pipe when it's finished overwriting all of the space on the block device? Commented Dec 10, 2015 at 4:02
  • @EmmaV When cat is no longer able to write to the device it stops, yes. Commented Dec 10, 2015 at 15:12
1

If the data for decrypting is too little, then you can run into this problem. I used a GNU parallel utility. Had this problem (error writing output file). Fixed it with adding min size for blocks using --blocksize. Also, openssl is kind of a weirdo, it returns this error on any problem I encountered (wrong password for decrypting, wrong data for decrypting). So be aware.

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0

If other answers don't fix your problem, try padding the message to a longer length.

Use the length of the key, or a multiple of that.

Nothing else answered the above question so I decided to check the message I was encoding. It was very short. I padded it with spaces, up to the length of the key, and that fixed it.

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