无法将NSString / NSData转换为SecKeyRef

Question

无法将NSString/NSData转换为SecKeyRef。  我用Google搜索了，但找不到合适的答案。

我需要使用公钥进行RSA加密。我从服务器获取公钥作为NSData。需要将转换作为参数传递给下面的代码。

- (NSString *)encryptRSA:(NSString *)plainTextString key:(SecKeyRef)publicKey {
    size_t cipherBufferSize = SecKeyGetBlockSize(publicKey);
    uint8_t *cipherBuffer = malloc(cipherBufferSize);
    uint8_t *nonce = (uint8_t *)[plainTextString UTF8String];
    SecKeyEncrypt(publicKey,
                  kSecPaddingOAEP,
                  nonce,
                  strlen( (char*)nonce ),
                  &cipherBuffer[0],
                  &cipherBufferSize);
    NSData *encryptedData = [NSData dataWithBytes:cipherBuffer length:cipherBufferSize];
    //return [encryptedData base64EncodedStringWithOptions:NSDataBase64Encoding64CharacterLineLength];
    return [DJUtility base64forData:encryptedData];
}