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