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Use OpenSSL RSA key with .Net
Problem:
The .Net framework throws an exception when attempting to use an RSA public key generated with OpenSSL due to a format mismatch. The key is in PKCS#1 format, while .Net expects a X.509 format.
Solution:
Conversion from PKCS#1 to X.509 Format:
To convert the RSA public key from PKCS#1 to X.509 format, use the PEM_write_bio_PUBKEY function instead of PEM_write_bio_RSAPublicKey. This will output the key in the SubjectPublicKeyInfo format with an OID and the public key.
Additionally, you will need to use EVP_PKEY_set1_RSA to convert the RSA key to an EVP_PKEY.
Example Code:
<code class="c++">// Include necessary headers #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 der1(BIO_new_file("rsa-public-1.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 SubjectPublicKeyInfo with OID and public key in ASN.1/DER rc = i2d_RSA_PUBKEY_bio(der1.get(), rsa.get()); ASSERT(rc == 1); // Write SubjectPublicKeyInfo with OID and public key in PEM rc = PEM_write_bio_PUBKEY(pem1.get(), pkey.get()); ASSERT(rc == 1); return 0; }</code>
This code generates an RSA key pair and writes the public key in both ASN.1/DER and PEM formats. You can then use the X.509 formatted public key with your .Net application.
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