ASN1_TIME_print functionality without BIO? - c

Question

As described in this question: Openssl C++ get expiry date, there is the possibility to write an ASN1 time into a BIO buffer and then read it back into a custom buffer buf:
BIO *bio;
int write = 0;
bio = BIO_new(BIO_s_mem());
if (bio) {
if (ASN1_TIME_print(bio, tm))
write = BIO_read(bio, buf, len-1);
BIO_free(bio);
}
buf[write]='\0';
return write;
How could this be achieved without using BIO at all? The ASN1_TIME_print function is only present when OPENSSL_NO_BIO is not defined. Is there a way to write the time directly into a given buffer?

Answer 1

You can try the sample code below. It doesn't use BIO, but should give you the same output as the OP's example. If you don't trust the ASN1_TIME string, you'll want to add some error checking for:
notBefore->data is > 10 chars
each char value is between '0' and '9'
values for year, month, day, hour, minute, second
type
You should test for the type (i.e. UTC), if you expect multiple types.
You should also test whether or not the date/time is GMT and add that to the string if you want the output to match exactly as if using BIOs. see:
openssl/crypto/asn1/t_x509.c - ASN1_UTCTIME_print or ASN1_GENERALIZEDTIME_print
ASN1_TIME* notBefore = NULL;
int len = 32;
char buf[len];
struct tm tm_time;
notBefore = X509_get_notBefore(x509_cert);
// Format ASN1_TIME with type UTC into a tm struct
if(notBefore->type == V_ASN1_UTCTIME){
strptime((const char*)notBefore->data, "%y%m%d%H%M%SZ" , &tm_time);
strftime(buf, sizeof(char) * len, "%h %d %H:%M:%S %Y", &tm_time);
}
// Format ASN1_TIME with type "Generalized" into a tm struct
if(notBefore->type == V_ASN1_GENERALIZEDTIME){
// I didn't look this format up, but it shouldn't be too difficult
}

Answer 2

I think this should be possible, at least in terms of writing the time directly into a given buffer -- but you'll still need to use BIOs.
Ideally, BIO_new_mem_buf would suit, given that it creates an in-memory BIO using a given buffer as the source. Unfortunately, that function treats the given buffer as read-only, which is not what we want. However, we can create our own function (let's call it BIO_new_mem_buf2), based on the BIO_new_mem_buf source code:
BIO *BIO_new_mem_buf2(void *buf, int len)
{
BIO *ret;
BUF_MEM *b;
size_t sz;
if (!buf) {
BIOerr(BIO_F_BIO_NEW_MEM_BUF, BIO_R_NULL_PARAMETER);
return NULL;
}
sz = (size_t)len;
if (!(ret = BIO_new(BIO_s_mem())))
return NULL;
b = (BUF_MEM *)ret->ptr;
b->data = buf;
b->length = sz;
b->max = sz;
return ret;
}
This is just like BIO_new_mem_buf, except that a) the len argument must indicate the size of the given buffer, and b) the BIO is not marked "readonly".
With the above, you should now be able to call:
ASN1_TIME_print(bio, tm)
and have the time appear in your given buffer.
Note that I have not tested the above code, so YMMV. Hope this helps!