yk_challenge_response() returns 22 bytes response instead of expected 20

Hi,

I have YubiKey 4. Slot 1 is configured with HMAC.

ykpersonalize -1 -ochal-resp -ochal-hmac -ohmac-lt64

It seems yk_challenge_response() returns 22 bytes response instead of expected 20 bytes. By definition, the yk_challenge_response() function should fill ony 20 bytes in response but it seems 22 bytes are returned instead. They are not zeros and they changes with the input so they seems to be part of the hash.

Maybe, those 2 bytes are suppose to be undefined by yk_challenge_response() definition and so the behavior is correct and I should ignore them, but it is strange.

The example code calculates HMAC from the string and prints response. First 22 bytes are not zeros (well, they are zeros in 1/256 cases ;-)). Isn't it strange?

#include <stdio.h>
#include <unistd.h>
#include <time.h>
#include <string.h>

#include <yubikey.h>
#include <ykdef.h>
#include <ykcore.h>
#include <ykstatus.h>

int main(int argc, char **argv) {
    YK_KEY *yk = 0;

    if (!yk_init()) {
        fprintf(stderr, "yk_init failed\n");
        exit(1);
    }
    if (!(yk = yk_open_key(0))) {
        fprintf(stderr, "yk_open_key failed\n");
        exit(1);
    }

    int i;
    unsigned char *challenge = "xxsx";
    unsigned int len = strlen(challenge);
    unsigned char response[SHA1_MAX_BLOCK_SIZE];
    memset(response, 0, sizeof(response));
    int yk_cmd = SLOT_CHAL_HMAC1;
    if(! yk_challenge_response(yk, yk_cmd, true, len,
                challenge, sizeof(response), response)) {
        fprintf(stderr, "yk_challenge_response failed\n");
        exit(1);
    }
    for (i = 0; i < sizeof(response); ++i) {
        printf("%d\n", (int)response[i]);
    }

    return 0;
}

Yubico / yubikey-personalization

yk_challenge_response() returns 22 bytes response instead of expected 20 #124