As we know, plaintext may be of arbitrary length. But cryptographic primitives works only with fixed-size blocks. AES -- 16 bytes.

So a last block to be encrypted must be expanded somehow. One naive solution is to pad it with zeroes and to store plaintext length somewhere.

By the way, what is 'pad'? For non-English speakers (like me), 'pad' is a process of stuffing pillow or teddy bear, so to make it bigger, of desired form. Here we have padding of some block of data to make it as long as AES block size.

Let's use openssl to explore PKCS#7 padding. Last packet is simply padded with a byte, which indicate the length of padding.

 % xxd -g 1 test1.txt
00000000: 48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21 0a        Hello, world!.

# encrypt:
openssl aes-256-cbc -pbkdf2 -in test1.txt -out test1.txt.enc -pass pass:1234

# decrypt using two modes:
openssl aes-256-cbc -d -pbkdf2 -in test1.txt.enc -out test1.txt.enc.dec -pass pass:1234


First '-d' openssl command will produce the same plaintext file. But what's with '-nopad' option? It left padding as is:

 % xxd -g 1 test1.txt.enc.dec.nopad
00000000: 48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21 0a 02 02  Hello, world!...


You see these two 0x02 bytes. They indicate that there are two padding bytes. Without '-nopad' option, openssl removes them or 'unpad'.

What if plaintext has 16 bytes and don't need to be padded?

 % xxd -g 1 test2.txt
00000000: 48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21 21 21 0a  Hello, world!!!.

00000000: 48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21 21 21 0a  Hello, world!!!.
00000010: 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10  ................


Now something different:

 % xxd -g 1 test3.txt
00000000: 48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21 20 48 6f  Hello, world! Ho
00000010: 6c 61 2c 20 4d 75 6e 64 6f 21 20 47 75 74 65 6e  la, Mundo! Guten
00000020: 20 54 61 67 21 0a                                 Tag!.


Got encrypted file:

 % xxd -g 1 test3.txt.enc
00000000: 53 61 6c 74 65 64 5f 5f d9 00 cc b1 a5 36 07 e3  Salted__.....6..
00000010: 3d 4b e0 de c2 cb f2 eb f4 d7 f1 0c ed db df 13  =K..............
00000020: d6 25 97 8f 28 09 e6 ed 00 88 3e 6c e0 23 65 02  .%..(.....>l.#e.
00000030: de 0d d7 84 09 15 f5 36 6c 78 0f 69 7e 83 8e bf  .......6lx.i~...


Let's vandalize that encrypted file. I modify the last 0xbf byte to 0x01:

 % xxd -g 1 test3.txt.enc.modified
...
00000030: fb d8 d6 b1 fe 90 94 45 d8 05 f0 2f b8 83 8a 01  .......E.../....


Let's try to decrypt that modified file.

 % openssl aes-256-cbc -d -pbkdf2 -in test3.txt.enc.modified -out test3.txt.enc.dec.modified.nopad -nopad -pass pass:1234

00000000: 48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21 20 48 6f  Hello, world! Ho
00000010: 6c 61 2c 20 4d 75 6e 64 6f 21 20 47 75 74 65 6e  la, Mundo! Guten
00000020: 91 be 5d bb 0f 81 f0 92 6e c9 23 8a e0 37 21 a8  ..].....n.#..7!.


OpenSSL is silent, because '-nopad' option supplied, so it doesn't check padding. You see, last 16 bytes are garbled. Indeed, AES algo can't decrypt last 16-byte blocks correctly. We see "Guten" without "Tag".

What if to decrypt this file without '-nopad' option?
 % openssl aes-256-cbc -d -pbkdf2 -in test3.txt.enc.modified -out test3.txt.enc.dec.modified -pass pass:1234

% xxd -g 1 test3.txt.enc.dec.modified
00000000: 48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21 20 48 6f  Hello, world! Ho
00000010: 6c 61 2c 20 4d 75 6e 64 6f 21 20 47 75 74 65 6e  la, Mundo! Guten


Unpadding failed. That garbled noise isn't a valid padding. How OpenSSL checks it? Easily. It finds the last byte. It's N, let's say. OpenSSL then looks for N bytes with N value. If it is so, padding is correct and removed. If not, report the error.

Nevertheless, OpenSSL can decrypt that file partially, without "Tag" word, ignoring last 16 bytes.

Important: I ran this using OpenSSL 1.1.1k. What is in the crypto/evp/evp_enc.c file for that version?

...
/*
* The following assumes that the ciphertext has been authenticated.
* Otherwise it provides a padding oracle.
*/
n = ctx->final[b - 1];
if (n == 0 || n > (int)b) {
return 0;
}
for (i = 0; i < n; i++) {
if (ctx->final[--b] != n) {
return 0;
}
}
...


But OpenSSL 3.0.2 (on another computer I use) would report another error:

bad decrypt


What is in the providers/implementations/ciphers/ciphercommon_block.c file for that version?

int ossl_cipher_unpadblock(unsigned char *buf, size_t *buflen, size_t blocksize)
{
size_t len = *buflen;

if(len != blocksize) {
ERR_raise(ERR_LIB_PROV, ERR_R_INTERNAL_ERROR);
return 0;
}

/*
* The following assumes that the ciphertext has been authenticated.
* Otherwise it provides a padding oracle.
*/
return 0;
}
for (i = 0; i < pad; i++) {
return 0;
}
}
*buflen = len;
return 1;
}


This is how padding is checked in OpenSSL.

###### (the post first published at 20230104.)

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