One of my professors mentioned in class that there is a way of using PKCS#7 padding to have the padding persistent after decryption.

For example, if I encrypt a 20-byte file using

openssl enc -aes-128-ecb -in input.txt -out encrypted.txt -K 0123456789 -v

I obviously get the padded difference of:

bytes read   :       20
bytes written:       32

So 12 bytes of padding was added.

After decryption of the file, I want the padding to remain.

The only method I could think of was making the bytes in the original input.txt file the same character that PKCS#7 uses to pad, so that when it's time to unpad, it doesn't know what to unpad. I haven't been able to find exactly which character it uses, or if it's not a character. I've read some about the standard here and here, but with no real luck.

I'm still reading on it, but I haven't had any eureka moments yet.

An example of what I'm looking for would be if I gave it a 4-byte file such as:


And it encrypted it, but added 12 bytes of C at the end, I'd want the decrypted file to be:


1 Answer 1


In OpenSSL there is an -nopad option. If you don't want the OpenSSL removing the padding bytes, add the -nopad option.

openssl enc -d  -nopad -aes-128-ecb -in  encrypted.txt -K 0123456789 -v -out decrypted.txt

Note that you cannot see as C because the OpenSSL doesn't print in hex.

To see in hex you can use xxd command

 xxd -r decrypted.txt

00000000: 3132 3334 3534 3334 3837 3433 3733 3838  1234543487437388
00000010: 3431 3431 3433 3436 410a 0606 0606 0606  41414346A.......

See, the 6 06 and the plaintext was


Note: Note: In PKCS#7 padding with 128-bit block cipher we don't see Cs, but we have 0C, 1C,2C,3C,4C,5C,6C,7C. What you mention is PKCS#5. See the difference in What is the difference between PKCS#5 padding and PKCS#7 padding

  • 1
    $\begingroup$ Thanks, this was very helpful. After decrypting with the -nopad option it did retain the padding. The padding is showing as 12 ^L's which makes sense. $\endgroup$ Sep 30, 2019 at 16:15
  • $\begingroup$ For some reason xxd can't read the decrypted text. The error message makes me think it's because the decrypted text isn't in hex anymore. $\endgroup$ Sep 30, 2019 at 16:16
  • $\begingroup$ I know that ^M is a newline character from windows, but I'm not familiar with ^L $\endgroup$ Sep 30, 2019 at 16:24
  • 1
    $\begingroup$ Can you try 'xxd -b' $\endgroup$
    – kelalaka
    Sep 30, 2019 at 16:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.