试试这个：

char* mKey="-----BEGIN RSA PRIVATE KEY-----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\n1WJAxZeCFHWL6+/84zoDWwzXN0xQFzO0ZspxxQNFqCI=\n-----END RSA PRIVATE KEY-----";

BIO* bo = BIO_new( BIO_s_mem() );
BIO_write( bo, mKey,strlen(mKey));

EVP_PKEY* pkey = 0;
PEM_read_bio_PrivateKey( bo, &pkey, 0, 0 );

BIO_free(bo);

RSA* rsa = EVP_PKEY_get1_RSA( pkey );

下面是在C++中使用openssl库从字符串获取公钥的代码。

#include <openssl/bio.h>
#include <openssl/rsa.h>

const std::string RsaPublicKey = "-----BEGIN PUBLIC KEY---";
RSA* public_key_;
BIO* bo = BIO_new(BIO_s_mem());
BIO_write(bo, RsaPublicKey.c_str(), RsaPublicKey.length());
PEM_read_bio_RSA_PUBKEY(bo, &public_key_, nullptr, nullptr);
BIO_free(bo);
RSA_free(public_key_);