Using Crypto++ generated RSA keys on OpenSSL

Question

Is there a way to use the RSA keys I've generated with the Crypto++ API in OpenSSL? What I am looking for is a way to store the keys in a format that both Crypto++ and OpenSSL can easily open them.

I'm writing a licensing scheme and would want to verify signatures and decrypt files using the Crypto++ API, but to generate the license files I would want to use a web interface (probably using PHP, which only supports OpenSSL) to generate and encrypt/sign the licenses.

I would write both applications using Crypto++ and call it from the PHP, but since the private key will be stored in a encrypted form, a password must be passed to the application and passing it on the command line doesn't seems to be a good idea to me.

Answer 1

Both Crypto++ and OpenSSL can handle PKCS#8 encoded keys. In crypto++, you can generate keys and convert to PKCS#8 buffer like this,

AutoSeededRandomPool rng;
RSAES_OAEP_SHA_Decryptor priv(rng, 2048);
string der;
StringSink der_sink(der);
priv.DEREncode(der_sink);
der_sink.MessageEnd();

// der.data() is the bytes you need

Now you just need to pass the bytes to PHP. You can save it in a file, send in a message.

The only gotcha is that PHP's OpenSSL interface only accepts PEM encoded PKCS#8. You can easily convert DER-encoded buffer into PEM like this in PHP,

<?php
function pkcs8_to_pem($der) {

    static $BEGIN_MARKER = "-----BEGIN PRIVATE KEY-----";
    static $END_MARKER = "-----END PRIVATE KEY-----";

    $value = base64_encode($der);

    $pem = $BEGIN_MARKER . "\n";
    $pem .= chunk_split($value, 64, "\n");
    $pem .= $END_MARKER . "\n";

    return $pem;
}
?>

You can also convert PKCS#8 to PEM in C++ if you prefer. The algorithm is very simple as you can see from the PHP code.

OpenSSL is so prevalent nowadays. I don't see any reason to use Crypto++ for common crypto applications like this.

Answer 2

Is there a way to use the RSA keys I've generated with the Crypto++ API in OpenSSL? What I am looking for is a way to store the keys in a format that both Crypto++ and OpenSSL can easily open them.

Yes. In addition to X.509 and PKCS #8 encoded keys (ZZ Coder's answer), you can also use PEM encoded keys including encrypted keys. Support for PEM encoded keys was donated to the project in July, 2014 for OpenSSL interop.

To use the PEM encoded keys, you need to fetch the Crypto++ PEM Pack and recompile the library. The PEM Pack is not part of the Crypto++ library as provided by Wei Dai at the Crypto++ website.

Once you install and recompile, its as simple as:

// Load a RSA public key
FileSource fs1("rsa-pub.pem", true);
RSA::PublicKey k1;
PEM_Load(fs1, k1);

// Load a encrypted RSA private key
FileSource fs2("rsa-enc-priv.pem", true);
RSA::PrivateKey k2;
PEM_Load(fs2, k2, "test", 4);

// Save an EC public key
DL_PublicKey_EC<ECP> k16 = ...;
FileSink fs16("ec-pub-xxx.pem", true);
PEM_Save(fs16, k16);

// Save an encrypted EC private key
DL_PrivateKey_EC<ECP> k18 = ...;
FileSink fs18("ec-enc-priv-xxx.pem", true);
PEM_Save(fs18, k18, "AES-128-CBC", "test", 4);

The keys look like so on-disk:

$ cat rsa-pub.pem
-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCg7ovcljEjZCFOdLWENBKE6FSk
Nke6OP79SMJABJw+JoEBpNddK6/v99IvA1qU76V0V4k8qLvhkVUtk9FArhhRsxeF
1fd8UVqgsT8j0YCVFcJ/ZA372ogpXyvc5aK9mZEiKE5TIF8qnDFFZiMWPrad1buk
hg+eFdo78QRLA5plEQIDAQAB
-----END PUBLIC KEY-----
$  
$ cat rsa-enc-priv.pem
-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: AES-128-CBC,E1A759E11CA515CE34B6E8CE5278C919
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-----END RSA PRIVATE KEY-----
$  
$ cat ec-pub.pem
-----BEGIN PUBLIC KEY-----
MFYwEAYHKoZIzj0CAQYFK4EEAAoDQgAEVwXjdIb2yy25QbIO0XiIHpySXwSpIAcz
v0Wdyq+fZ6BdJjs2jKvbs9pcRJn8yxlASWoz2R4NoHTZ2YokKsDfEg==
-----END PUBLIC KEY-----
$  
$ cat ec-enc-priv.pem
-----BEGIN EC PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: AES-128-CBC,F1DBC73E26DCD310888932C2762B3512

nikex48SFvtNOIrOEDipwmxaghjn4jtrvwI3d1H/VNq9yp26WqFZxBJCUPFBFLjH
auA+AHeUo3BVkNQPs0VO4FD5xR50mtc2tCJizzhyTTTypLc3lRkxmD1MpeZnWRy2
70foVtNSvLL/QLJqNJGm/G9kl0xPN4zAfOq7Txoscnk=
-----END EC PRIVATE KEY-----

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Related: for other useful Crypto++ patches, see the Category:Patch page on the Crypto++ wiki.