Re: [libsrtp] AES CM implementation in SRTP

23 Jun 2020


      Hilman,
You might want to use srtp_octet_string_hex_string() to dump the 
contents of buffers, since the v128_hex_string() is intended for use 
with 16 octet strings.  While AES is a block cipher that operates on 16 
octet blocks, the output from ICM over the keystream can be as little as 
one octet.
The keystream is the length of the data to encrypt. And, yes, one takes 
that keystream and XORs it with the plaintext to produce the ciphertext 
as illustrated here: https://tools.ietf.org/html/rfc3711#page-20
The ciphertext is the same length as the original plaintext, which is 
also the same length as the keystream (in practice, since no padding is 
used).
Paul
------ Original Message ------
From: "Hilman Fitriana via libsrtp" libsrtp@lists.packetizer.com
To: libsrtp@lists.packetizer.com
Sent: 6/22/2020 3:14:08 PM
Subject: [libsrtp] AES CM implementation in SRTP
...
Hello. I want to ask about the implementation of the AES ICM method in 
the SRTP payload encryption process.
In my understanding, the AES ICM encryption process works by doing 
`bitwise XOR` between the `keystream buffer` and the `buffer itself 
(plaintext)`. Keystream buffers are generated through the AES process 
with the input value of iv and the master key.
Following is the implementation of the AES CM method in the srtp 
library.
static srtp_err_status_t srtp_aes_icm_encrypt(void *cv,unsigned 

char *buf,unsigned int *enc_len){
       srtp_aes_icm_ctx_t *c = (srtp_aes_icm_ctx_t *)cv;
       unsigned int bytes_to_encr = *enc_len;
       unsigned int i;
       uint32_t *b;
       printf("\nbuffer (plaintext)in hexa: %s 
\n",v128_hex_string(buf));
   //.............................................................
   //pass
   /* now loop over entire 16-byte blocks of cast-iron */
   for (i = 0; i < (bytes_to_encr / sizeof(v128_t)); i++) {
    /* fill buffer with new keystream */
       srtp_aes_icm_advance(c);

/*
* add keystream into the data buffer (this would be a lot faster
* if we could assume 32-bit alignment!)
*/

#if ALIGN_32


//.............................................................
           //pass
    #else
           if ((((uintptr_t)buf) & 0x03) != 0) {
           } else {
               printf("\nkeystream 
:%s\n",v128_hex_string(&c->keystream_buffer));
               b = (uint32_t *)buf;
               *b++ ^= c->keystream_buffer.v32[0];
               *b++ ^= c->keystream_buffer.v32[1];
               *b++ ^= c->keystream_buffer.v32[2];
               *b++ ^= c->keystream_buffer.v32[3];
               buf = (uint8_t *)b;
           }
    #endif /* #if ALIGN_32 */
       }
//.............................................................
           //pass
 printf("\nbuffer after encrypt 

(ciphertext):%s\n",v128_hex_string(buff));
    return srtp_err_status_ok;
    }
I then tried to display the plaintext stored in buf as well as the 
keystream used for bitwise xor operations. However, when displaying 
ciphertext, it does not show the results of bitwise xor `plaintext` and 
`keystream`. Can anyone help me understand the code?
Thank you

Re: [libsrtp] AES CM implementation in SRTP

Paul E. Jones