Distinguishing start of digitally signed message digest - digital-signature

Question

Since I just created this crude test using the functions from IETF RFC 4634, I don't know for certain whether I've used them correctly for HMAC-SHA-384-192, so I'll start with that code here:
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include "sha.h"
int main(int argc, char *argv[]) {
HMACContext hmac;
const unsigned char *keyarr = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef";
int err = hmacReset(&hmac, SHA384, keyarr, 48);
if (err != shaSuccess) {
printf("err 1\n");
exit(1);
}
const uint8_t testarray[65] = {'I',' ','a','m',' ','n','o','t',' ','a',' ','c','r','o','o','k','!'};
const unsigned char *prfkey = "abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789";
memcpy((void *)testarray + 17, (void *)prfkey, 48);
const int testlen = 65;
err = hmacInput(&hmac, testarray, testlen);
if (err != shaSuccess) {
printf("err 2\n");
exit(1);
}
uint8_t Message_Digest[USHAMaxHashSize];
err = hmacResult(&hmac, Message_Digest);
if (err != shaSuccess) {
printf("err 3\n");
exit(1);
}
int i;
for(i = 0; i < 24; i++) printf(" %02X", Message_Digest[i]);
putchar('\n');
}
If I've done everything right (other than selecting good keys) so far, I would ordinarily have a 24-byte (i.e., 192-bit) digest, but if I digitally sign the digest prior to appending it, my experience is that the signature block isn't a predictable length. I'm sure I could come up with any number of ways to identify the end of the message portion, but I don't want to make this a hack. What is the accepted way of doing this? (The signature will use ECDSA.)
I should also mention that this will be a multicast message using UDP inside ESP, since that puts constraints on message economy. (That's also the main reason for the problem--keeping it binary. The other is the practice of appending, rather than prefixing it with a byte count in front of it.)