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From: Marcel Logen <333200007110-0201@ybtra.de>
Newsgroups: sci.crypt
Subject: Re: What are the chances of this encrytion being broken?
Date: Wed, 26 Mar 2025 20:25:25 +0100
Organization: Bureau Logen
Message-ID: <20250326we192526@o15.ybtra.de>
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colin in sci.crypt:

>On 26/03/25 10:54, Marcel Logen wrote:
>>colin in sci.crypt:
>>>On 25/03/25 12:18, Marcel Logen wrote:

[...]

>>>>The Base64 decoded 'text' has 528 bytes.
>>[...]
>>
>>>Possibly 33 128 bit blocks ( aes has a block size 0f 128 bits )
>>
>>32, I think.
>>
>>512 bytes of plaintext become 528 bytes of ciphertext
>>with AES256 CBC (without salt).
>
>I can produce 528 bytes of ciphertext with 513 bytes of plaintext. ie an
>extra block is added.
>
>eg:
>$ cat 512bytes.txt | aespipe -e aes256 -P password.txt | wc -c
>512
>$ cat 513bytes.txt | aespipe -e aes256 -P password.txt | wc -c
>528

Ah, OK. I have found the cause: the padding.

| user15@o15:/tmp$ stat -c '%s' 512bytes.txt 
| 512
| user15@o15:/tmp$ openssl enc -aes-256-cbc -in 512bytes.txt   -salt -pass pass:1234 -pbkdf2 | wc -c
| 544
| user15@o15:/tmp$ openssl enc -aes-256-cbc -in 512bytes.txt -nosalt -pass pass:1234 -pbkdf2 | wc -c
| 528
| user15@o15:/tmp$ openssl enc -aes-256-cbc -in 512bytes.txt -nosalt -pass pass:1234 -pbkdf2 -nopad | wc -c
| 512

| user15@o15:/tmp$ stat -c '%s' 513bytes.txt 
| 513
| user15@o15:/tmp$ openssl enc -aes-256-cbc -in 513bytes.txt -nosalt -pass pass:1234 -pbkdf2 | wc -c
| 528
| user15@o15:/tmp$ openssl enc -aes-256-cbc -in 513bytes.txt -nosalt -pass pass:1234 -pbkdf2 -nopad | wc -c
| bad encrypt
| 40E7A9630B7F0000:error:1C80006B:Provider routines:ossl_cipher_generic_block_final:wrong final block length:../providers/implementations/ciphers/ciphercommon.c:420:
| 512

Marcel                                                       (Lines: 53)
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