openssl initialize RSA public key

Question

I have RSA base64 encoded public key that I need to use to verify digital signature. I don't understand how to initialize RSA with public key.

My code looks something like:

unsigned char *signature = ""; //signature string
char *original = ""; // my original string
unsigned char sha2HashDigest[SHA256_DIGEST_LENGTH];
SHA256(original, strlen(original), sha2HashDigest);

 char *key = "base64encodedKey";

 RSA *r = RSA_new();
//SET RSA public key?! how?

int result = RSA_verify(NID_sha256, sha2HashDigest, SHA256_DIGEST_LENGTH,
           signature, strlen(signatrue), r);
if (result != 1) // handle error

Note: I am doing this in iOS application, but I think it is irrelevant for the question.

UPDATE: I ended up using EVP as suggested by vond. Public key is PEM formatted file. This is my code:

   FILE *fp = fopen([keyFilePath UTF8String], "r");
    if (!fp) return NO;


    EVP_PKEY *pubKey = PEM_read_PUBKEY(fp,NULL,NULL,NULL);
    EVP_MD_CTX     md_ctx;
    EVP_MD_CTX_init(&md_ctx);

    EVP_VerifyInit(&md_ctx, EVP_sha256());
    EVP_VerifyUpdate (&md_ctx, (unsigned char*)[msgData bytes], [msgData length]);
    int  err = EVP_VerifyFinal (&md_ctx, (unsigned char*) sigData, (unsigned int)[sigData length], pubKey);
    EVP_PKEY_free (pubKey);

Answer 1

You might try the following:

const char *pub_key_pem = ...;

BIO *bio = BIO_new_mem_buf((void*)pub_key_pem, strlen(pub_key_pem));
RSA *rsa_pub = PEM_read_bio_RSAPublicKey(bio, NULL, NULL, NULL);

Note: PEM_read_bio_RSAPublicKey() expects PKCS#1 PEM format (with "BEGIN/END RSA PUBLIC KEY" dash lines in the first/last lines); if you have PEM with "BEGIN/END PUBLIC KEY", you should try PEM_read_bio_RSA_PUBKEY() instead. See explanation of the difference here and here.

If you don't have any of these dash lines in your base64 string, you may find it easier to decode the base64 string into a binary buffer, then use one of the d2i_RSAPublicKey() or d2i_RSA_PUBKEY() to get the RSA* public key from it.