[cs631apue] Final Project Digest

[Jason Ajmo]

After speaking with Pat about this, he pointed something out that I didn't
previously consider.

Since the raw data is just bytes, it can have a null-terminator somewhere
in it, even though it isn't the end of ciphertext. Therefore, strlen()
should not be used when calculating lengths. The length should be the
number returned from read(2).

On Tue, Dec 5, 2017 at 12:19 PM Jason Ajmo <jajmo at stevens.edu> wrote:

> Thanks for the update.
>
> I'm having another weird bug. Decryption works sometimes, but this is due
> to the ciphertext length not always being a multiple of 16 (AES-256 block
> length).
>
> Directly after reading from stdin, I have the following two debug lines:
>         printf("read %lu bytes\n", nread);
>         printf("encrypted length: %lu\n", strlen(buff + 8 + SALT_LENGTH));
>
> It always reads 176 bytes when using the same constant input. However, the
> encrypted length is 160 when it's properly decrypted, and various other
> lengths when it fails (I've seen 95, 53, 43, etc.) `buff` is properly
> null-terminated. `nread` is the return value of read(2).
>
> The exact error is:
> 140187583043396:error:0606506D:digital envelope
> routines:EVP_DecryptFinal_ex:wrong final block
> length:/usr/src/crypto/external/bsd/openssl/dist/crypto/evp/evp_enc.c:505
>
> Any advice would be greatly appreciated. I've tried debugging myself and
> looking at various online resources, but haven't found anything too helpful.
>
> On Tue, Dec 5, 2017 at 12:03 PM Jan Schaumann <jschauma at stevens.edu>
> wrote:
>
>> Jason G Ajmo <jajmo at stevens.edu> wrote:
>>
>> > I began working on the code last night to decrypt an input passed to
>> > crysh using the professor's examples, but I'm only able to decrypt the
>> > input assuming it uses an MD5 digest instead of SHA1.
>>
>> You're right.  The examples provided in the manual page did not use the
>> right digest.  I've updated the man page.  The correct input generation
>> is:
>>
>> echo "date" | openssl enc -aes-256-cbc -md sha1 | crysh
>>
>> -Jan
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>>
> --
> Jason Ajmo
> Stevens Institute of Technology
> B.S. Cybersecurity '17
> M.S. Computer Science '18
> 0x56FA3123
>
-- 
Jason Ajmo
Stevens Institute of Technology
B.S. Cybersecurity '17
M.S. Computer Science '18
0x56FA3123
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