Use OpenSSL RSA key with .Net

I am using openssl 0.9.6g and I have created public/private keypair
using RSA_generate_key(). It gives me keys like:
-----BEGIN RSA PUBLIC KEY-----
...
-----END RSA PUBLIC KEY-----
I think what I am looking for is “how to convert rsa public key from pkcs#1 to x509 format.

Yeah, .Net can consume some ASN.1/DER encoded keys, and some PEM encoded keys. The difference is PKCS encoding versus Traditional encoding (OpenSSL calls it “Traditional”). The traditional encoding is the SubjectPublicKeyInfo and it includes the OID and the public key.

So you are looking for either an ASN.1/DER encoding or a PEM encoding that writes SubjectPublicKeyInfo, and not just the public key.

I have another module in .NET which throws an exception when passed in
this key due to its format. It takes format like:
-----BEGIN PUBLIC KEY-----
...
-----END PUBLIC KEY-----

In this case, use PEM_write_bio_PUBKEY rather than PEM_write_bio_RSAPublicKey.

PEM_write_bio_PUBKEY writes the SubjectPublicKeyInfo; while PEM_write_bio_RSAPublicKey writes only the public key.

You will need an EVP_PKEY, so use EVP_PKEY_set1_RSA to convert it.

This is a PKCS key in OpenSSL. Its just the public key. You would use PEM_write_RSAPublicKey to write it:

-----BEGIN RSA PUBLIC KEY-----

And this is a Traditional key in OpenSSL. Its the SubjectPublicKeyInfo, and it includes an OID for the algorithm (rsaEncryption) and the public key. You would use PEM_write_bio_PUBKEY to write it:

-----BEGIN PUBLIC KEY-----

Instead of saving the key with PEM_write_RSAPublicKey, you should write out the SubjectPublicKeyInfo structure in ASN.1/DER format with i2d_RSA_PUBKEY_bio; or write it out in PEM format with PEM_write_bio_PUBKEY.

The program below creates a RSA key pair, and then writes out the public key in all the formats. Be sure to save the private key, too.

(And I’m glad you have the C++ tag. unique_ptr makes this exercise so much easier).

#include <memory>
using std::unique_ptr;

#include <openssl/bn.h>
#include <openssl/rsa.h>
#include <openssl/pem.h>
#include <openssl/bio.h>
#include <openssl/x509.h>

#include <cassert>
#define ASSERT assert

using BN_ptr = std::unique_ptr<BIGNUM, decltype(&::BN_free)>;
using RSA_ptr = std::unique_ptr<RSA, decltype(&::RSA_free)>;
using EVP_KEY_ptr = std::unique_ptr<EVP_PKEY, decltype(&::EVP_PKEY_free)>;
using BIO_FILE_ptr = std::unique_ptr<BIO, decltype(&::BIO_free)>;

int main(int argc, char* argv[])
{
    int rc;

    RSA_ptr rsa(RSA_new(), ::RSA_free);
    BN_ptr bn(BN_new(), ::BN_free);

    BIO_FILE_ptr pem1(BIO_new_file("rsa-public-1.pem", "w"), ::BIO_free);
    BIO_FILE_ptr pem2(BIO_new_file("rsa-public-2.pem", "w"), ::BIO_free);
    BIO_FILE_ptr der1(BIO_new_file("rsa-public-1.der", "w"), ::BIO_free);
    BIO_FILE_ptr der2(BIO_new_file("rsa-public-2.der", "w"), ::BIO_free);

    rc = BN_set_word(bn.get(), RSA_F4);
    ASSERT(rc == 1);

    // Generate key
    rc = RSA_generate_key_ex(rsa.get(), 2048, bn.get(), NULL);
    ASSERT(rc == 1);

    // Convert RSA key to PKEY
    EVP_KEY_ptr pkey(EVP_PKEY_new(), ::EVP_PKEY_free);
    rc = EVP_PKEY_set1_RSA(pkey.get(), rsa.get());
    ASSERT(rc == 1);

    //////////

    // Write just the public key in ASN.1/DER
    // Load with d2i_RSAPublicKey_bio
    rc = i2d_RSAPublicKey_bio(der1.get(), rsa.get());
    ASSERT(rc == 1);

    // Write just the public key in PEM
    // Load with PEM_read_bio_RSAPublicKey
    rc = PEM_write_bio_RSAPublicKey(pem1.get(), rsa.get());
    ASSERT(rc == 1);

    // Write SubjectPublicKeyInfo with OID and public key in ASN.1/DER
    // Load with d2i_RSA_PUBKEY_bio
    rc = i2d_RSA_PUBKEY_bio(der2.get(), rsa.get());
    ASSERT(rc == 1);

    // Write SubjectPublicKeyInfo with OID and public key in PEM
    // Load with PEM_read_bio_PUBKEY
    rc = PEM_write_bio_PUBKEY(pem2.get(), pkey.get());
    ASSERT(rc == 1);

    return 0;
}

The set1 in EVP_PKEY_set1_RSA bumps the reference count, so you don’t get a segfault on a double free.

After executing the program, you get the expected PEM and ASN.1/DER:

$ cat rsa-public-1.pem 
-----BEGIN RSA PUBLIC KEY-----
MIIBCgKCAQEA0cgFv6wEcqoOhPtHdVmX4YFlCwodnSqooeCxFF1XadTS4sZkVJTC
kszHmRqXiXL2NmqnuDQsq6nLd+sNoU5yJJ+W1hwo7UToCyJ/81tS4n6mXvF8oilP
8YudD5QnBdW9LhqttBIN4Gk+Cxun+HG1rSJLGP9yiPPFd7DPiFz0Gd+juyWznWnP
gapDIWEKqANKma3j6b9eopBDWB0XAgU0HQ71MSNbcsPvDd23Ftx0re/7jG53V7Bn
eBy7fQsPmxcn4c74Lz4CvhOr7VdQpeBzNeG2CtkefKWyTk7Vu4FZnAgNd/202XAr
c6GmEQqD2M2zXH/nVZg5oLznECDVQ1x/pwIDAQAB
-----END RSA PUBLIC KEY-----

$ cat rsa-public-2.pem 
-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA0cgFv6wEcqoOhPtHdVmX
4YFlCwodnSqooeCxFF1XadTS4sZkVJTCkszHmRqXiXL2NmqnuDQsq6nLd+sNoU5y
JJ+W1hwo7UToCyJ/81tS4n6mXvF8oilP8YudD5QnBdW9LhqttBIN4Gk+Cxun+HG1
rSJLGP9yiPPFd7DPiFz0Gd+juyWznWnPgapDIWEKqANKma3j6b9eopBDWB0XAgU0
HQ71MSNbcsPvDd23Ftx0re/7jG53V7BneBy7fQsPmxcn4c74Lz4CvhOr7VdQpeBz
NeG2CtkefKWyTk7Vu4FZnAgNd/202XArc6GmEQqD2M2zXH/nVZg5oLznECDVQ1x/
pwIDAQAB
-----END PUBLIC KEY-----

$ dumpasn1 rsa-public-1.der 
  0 266: SEQUENCE {
  4 257:   INTEGER
       :     00 D1 C8 05 BF AC 04 72 AA 0E 84 FB 47 75 59 97
       :     E1 81 65 0B 0A 1D 9D 2A A8 A1 E0 B1 14 5D 57 69
       :     D4 D2 E2 C6 64 54 94 C2 92 CC C7 99 1A 97 89 72
       :     F6 36 6A A7 B8 34 2C AB A9 CB 77 EB 0D A1 4E 72
       :     24 9F 96 D6 1C 28 ED 44 E8 0B 22 7F F3 5B 52 E2
       :     7E A6 5E F1 7C A2 29 4F F1 8B 9D 0F 94 27 05 D5
       :     BD 2E 1A AD B4 12 0D E0 69 3E 0B 1B A7 F8 71 B5
       :     AD 22 4B 18 FF 72 88 F3 C5 77 B0 CF 88 5C F4 19
       :             [ Another 129 bytes skipped ]
265   3:   INTEGER 65537
       :   }

0 warnings, 0 errors.

$ dumpasn1 rsa-public-2.der 
  0 290: SEQUENCE {
  4  13:   SEQUENCE {
  6   9:     OBJECT IDENTIFIER rsaEncryption (1 2 840 113549 1 1 1)
 17   0:     NULL
       :     }
 19 271:   BIT STRING, encapsulates {
 24 266:     SEQUENCE {
 28 257:       INTEGER
       :         00 D1 C8 05 BF AC 04 72 AA 0E 84 FB 47 75 59 97
       :         E1 81 65 0B 0A 1D 9D 2A A8 A1 E0 B1 14 5D 57 69
       :         D4 D2 E2 C6 64 54 94 C2 92 CC C7 99 1A 97 89 72
       :         F6 36 6A A7 B8 34 2C AB A9 CB 77 EB 0D A1 4E 72
       :         24 9F 96 D6 1C 28 ED 44 E8 0B 22 7F F3 5B 52 E2
       :         7E A6 5E F1 7C A2 29 4F F1 8B 9D 0F 94 27 05 D5
       :         BD 2E 1A AD B4 12 0D E0 69 3E 0B 1B A7 F8 71 B5
       :         AD 22 4B 18 FF 72 88 F3 C5 77 B0 CF 88 5C F4 19
       :                 [ Another 129 bytes skipped ]
289   3:       INTEGER 65537
       :       }
       :     }
       :   }

0 warnings, 0 errors.

Related, see How to generate RSA private key using openssl?. It shows you how to write a RSA public and private key in a number of formats.

More Related Contents:

Leave a Comment Cancel reply