RSA Public Key Conversion with just Modulus - c

Question

I received an RSA-2048 bit public key (256 bytes) as a file which contains just those 256 bytes. Which function in SSL enables me to load this key as an RSA structure so I can convert it to another format? This is in C code using openssl source.
I think it's the DER format, but I'm not 100% certain.

Answer 1

I just put this together and it seems to work correctly:
https://github.com/JonathonReinhart/rawrsa
#include <stdio.h>
#include <stdlib.h>
#include <libgen.h>
#include <openssl/bn.h>
#include <openssl/rsa.h>
#include <openssl/pem.h>
static const char* appname;
static void print_bn(const char *what, const BIGNUM *bn)
{
#ifdef DEBUG
char *str = BN_bn2hex(bn);
printf("%s (hex): %s\n", what, str);
OPENSSL_free(str);
#endif
}
static void usage(void)
{
fprintf(stderr, "Usage: %s modulus-file exponent\n", appname);
}
#define err(fmt, ...) \
fprintf(stderr, "%s: " fmt, appname, ##__VA_ARGS__)
int main(int argc, char *argv[])
{
appname = basename(argv[0]);
if (argc < 3) {
usage();
exit(1);
}
const char *modfile = argv[1];
const char *expstr = argv[2];
/* Read modulus */
FILE *mf = fopen(modfile, "rb");
if (!mf) {
err("Failed to open \"%s\": %m\n", modfile);
return 1;
}
unsigned char buf[256];
if (fread(buf, sizeof(buf), 1, mf) != 1) {
err("Failed to read %zu bytes of modulus\n", sizeof(buf));
return 1;
}
fclose(mf);
BIGNUM *mod = BN_bin2bn(buf, sizeof(buf), NULL);
if (!mod) {
err("BN_bin2bn() failed\n");
return 1;
}
print_bn("Modulus", mod);
/* Parse exponent */
BIGNUM *exp = NULL;
if (BN_dec2bn(&exp, expstr) == 0) {
err("BN_dec2bn() failed\n");
return 1;
}
print_bn("Exponent", exp);
/* Create RSA key */
RSA *rsa = RSA_new();
if (!rsa) {
err("RSA_new() failed\n");
return 1;
}
rsa->e = exp;
rsa->n = mod;
/* Write PEM-encoded RSA public key to stdout */
if (!PEM_write_RSAPublicKey(stdout, rsa)) {
err("PEM_write_RSAPublicKey() failed\n");
return 1;
}
return 0;
}
I use BN_bin2bn to create an OpenSSL bignum from raw binary data from a file. This is where we load your 256-byte modulus.
Then, I use BN_dec2bn to create a bignum from the exponent provided on the command line.
Next, I create an RSA object with RSA_new, and set the public exponent (rsa->e) and modulus (rsa->n).
Finally, I write the RSA public key to a PEM file with PEM_write_RSAPublicKey.
Example:
$ scons -Q
gcc -o main.o -c -Wall -Werror -g main.c
gcc -o rawrsa main.o -lcrypto
$ ./rawrsa key.bin 65537
-----BEGIN RSA PUBLIC KEY-----
MIIBCgKCAQEA9cFHSTQ6h1Ls/vx7B+V/84XVlLxUU1dU1mEr9ROAqWrZtfasvx2E
21lbva+AdJ/B4u6fGVhCEMgekXsRB65CqZfwL3DFL6tqam6GvrOyvZgAlQKrA54w
DaKMT8Kfg2I2K9W/HCkCOHczhuHhjFmeiV9BuQgpmcPcNz6UXBwU05d3g6oM/X4m
lEhEsaH4bqo1qsMX6jp6WnsR13GEfsYoYVmHgEbnKJyGpsoRVW6HQXLHvef9XLEJ
v9n7nLdHToya75svxJ3v9JugD3n6PiC48085/FWb9980o4hmG9iW5rehm4Dlui8c
TDnHkQSrvi9WLlZ+S8hdtwDRN/pVVTjgPAIDAQAB
-----END RSA PUBLIC KEY-----