ASN1_INTEGER to ASN1_STRING - c

Question

I am using openssl to get data about x509 certificate.
Is there a way to convert ASN1_INTEGER to ASN1_STRING, which can than easily be transformed to char array? Is there a way to convert it to any other human readable format?
EDIT: I'm using openssl compiled for iOS, as I am having the iOS project. Here is the code I am using to extract the serial number from the certificate:
ASN1_INTEGER *serial = X509_get_serialNumber(certificateX509);
long value = ASN1_INTEGER_get(serial);
NSLog(#"Serial %ld", value);
certificateX509 is a valid X509 object and I have managed to get some other fields from it (issuer name, expiry date and so on)
EDIT 2:
I finally came to a solution, which may not be the most straightforward one:
ASN1_INTEGER *serial = X509_get_serialNumber(certificateX509);
BIGNUM *bnser = ASN1_INTEGER_to_BN(serial, NULL);
int n = BN_num_bytes(bnser);
unsigned char outbuf[n];
int bin = BN_bn2bin(bnser, outbuf);
char *hexbuf = (char*) outbuf;
hexBuf then contains characters whose value needs to be read as hex integer in order to retrieve logical values.
I use NSMutableString to create a human readable string:
NSMutableString *str = [[NSMutableString alloc] init];
for (int i=0; i<n; i++) {
NSString *temp = [NSString stringWithFormat:#"%.6x", hexbuf[i]];
[str appendString:[NSString stringWithFormat:#"%# ", temp]];
}
If there is a simpler way, I would really like to know it.

Answer 1

The ascii hex conversion be done more simply using the built in BN_bn2hex(BIGNUM *) function
ASN1_INTEGER *serial = X509_get_serialNumber(certificateX509);
BIGNUM *bnser = ASN1_INTEGER_to_BN(serial, NULL);
char *asciiHex = BN_bn2hex(bnser);

Answer 2

One possibility is that you can extract the value of the ASN1_INTEGER as a normal C integer:
#include <openssl/asn1.h>
#include <stdio.h>
int main(int argc, char** argv) {
long value;
ASN1_INTEGER asn1int = {0};
ASN1_INTEGER_set(&asn1int, 42);
value = ASN1_INTEGER_get(&asn1int);
printf("The value is %ld.\n", value);
return 0;
}
Compiled like this:
gcc -Wall -o sploots sploots.c -lcrypto
this produces the output:
The value is 42.
To have the value as a string in an array of char, use snprintf.
I suspect there are also possibilities for using the BIO printing routines to dump the value to a BIO of some sort (perhaps a memory BIO). However, this approach seems simpler.
The way I arrived at this answer is that I looked through the OpenSSL headers for ASN1_INTEGER. After looking around for suitable APIs for a BIO-based solution, I noticed the ASN1_INTEGER_get function.
Looking around in OpenSSL header files is typically the way I learn how to use OpenSSL, since so much of the API is undocumented or incorrectly or incompletely documented.

Answer 3

I finally came to a solution, which may not be the most straightforward one:
ASN1_INTEGER *serial = X509_get_serialNumber(certificateX509);
BIGNUM *bnser = ASN1_INTEGER_to_BN(serial, NULL);
int n = BN_num_bytes(bnser);
unsigned char outbuf[n];
int bin = BN_bn2bin(bnser, outbuf);
char *hexBuf = (char*) outbuf;
hexBuf then contains characters whose value needs to be read as hex integer in order to retrieve logical values.
I use NSMutableString to create a human readable string:
NSMutableString *str = [[NSMutableString alloc] init];
for (int i=0; i<n; i++) {
NSString *temp = [NSString stringWithFormat:#"%.6x", hexbuf[i]];
[str appendString:[NSString stringWithFormat:#"%# ", temp]];
}

Answer 4

If you just want a readable NSString, BN_bn2dec is more consize than BN_bn2hex or BN_bn2bin.
No need to mess with hex encoding.
Here's my way, in iOS/ObjC, using pod 'OpenSSL-Universal', '1.0.2.10' :
ASN1_INTEGER *serialAsn1 = X509_get_serialNumber(certX509);
BIGNUM *serialBigNumber = ASN1_INTEGER_to_BN(serialAsn1, NULL);
char *serialChar = BN_bn2dec(serialBigNumber);
NSString *serialString = [NSString stringWithCString:(const char *) serialChar
encoding:NSUTF8StringEncoding];
Cheers.

Answer 5

This solution worked for me to get the serial number in hex:
ASN1_INTEGER* serial = X509_get_serialNumber(X509_certificate_ptr);
BIGNUM* bn = ASN1_INTEGER_to_BN(serial, NULL);
if (!bn) {
// log here "Unable to convert ASN1INTEGER to BN";
return "";
}
char* hex = BN_bn2hex(bn);
if (!hex) {
// Log here "Unable to convert BN to hex string";
return "";
}
cout << hex; // this is your serial number. Can be converted using std::to_string()
BN_free(bn);
ASN1_INTEGER_free(serial);
OPENSSL_free(hex);
return serial_number;
Without error checking and memory management the code is basically:
ASN1_INTEGER* serial = X509_get_serialNumber(X509_certificate_ptr);
BIGNUM* bn = ASN1_INTEGER_to_BN(serial, NULL);
char* hex = BN_bn2hex(bn);