Load RSA private key from a PEM encoded private key - c++

Question

I'm trying to load an RSA private key from a std::string that contains the private key in PEM format, like this:
-----BEGIN RSA PRIVATE KEY-----
MIIBOgIBAAJBAK8Q+ToR4tWGshaKYRHKJ3ZmMUF6jjwCS/u1A8v1tFbQiVpBlxYB
paNcT2ENEXBGdmWqr8VwSl0NBIKyq4p0rhsCAQMCQHS1+3wL7I5ZzA8G62Exb6RE
INZRtCgBh/0jV91OeDnfQUc07SE6vs31J8m7qw/rxeB3E9h6oGi9IVRebVO+9zsC
IQDWb//KAzrSOo0P0yktnY57UF9Q3Y26rulWI6LqpsxZDwIhAND/cmlg7rUz34Pf
SmM61lJEmMEjKp8RB/xgghzmCeI1AiEAjvVVMVd8jCcItTdwyRO0UjWU4JOz0cnw
5BfB8cSIO18CIQCLVPbw60nOIpUClNxCJzmMLbsrbMcUtgVS6wFomVvsIwIhAK+A
YqT6WwsMW2On5l9di+RPzhDT1QdGyTI5eFNS+GxY
-----END RSA PRIVATE KEY-----
And I wonder if anyone can help me to use this key instead of generating a random with the following statement.
CryptoPP::RSA::PrivateKey rsaPrivate;
rsaPrivate.GenerateRandomWithKeySize (rnd, 512);

Answer 1

The key is PEM encoded. You need to strip the PEM header and footer, then convert from Base64 back to DER/BER, and finally use Crypto++'s BERDecodePrivateKey.
There's some reading on the subject at the Crypto++ wiki under Keys and Formats. Below is the code to perform the conversion (I don't believe Stack Overflow has a working example of it in Crypto++).
string RSA_PRIV_KEY =
"-----BEGIN RSA PRIVATE KEY-----\n"
"MIIBOgIBAAJBAK8Q+ToR4tWGshaKYRHKJ3ZmMUF6jjwCS/u1A8v1tFbQiVpBlxYB\n"
"paNcT2ENEXBGdmWqr8VwSl0NBIKyq4p0rhsCAQMCQHS1+3wL7I5ZzA8G62Exb6RE\n"
"INZRtCgBh/0jV91OeDnfQUc07SE6vs31J8m7qw/rxeB3E9h6oGi9IVRebVO+9zsC\n"
"IQDWb//KAzrSOo0P0yktnY57UF9Q3Y26rulWI6LqpsxZDwIhAND/cmlg7rUz34Pf\n"
"SmM61lJEmMEjKp8RB/xgghzmCeI1AiEAjvVVMVd8jCcItTdwyRO0UjWU4JOz0cnw\n"
"5BfB8cSIO18CIQCLVPbw60nOIpUClNxCJzmMLbsrbMcUtgVS6wFomVvsIwIhAK+A\n"
"YqT6WwsMW2On5l9di+RPzhDT1QdGyTI5eFNS+GxY\n"
"-----END RSA PRIVATE KEY-----";
static string HEADER = "-----BEGIN RSA PRIVATE KEY-----";
static string FOOTER = "-----END RSA PRIVATE KEY-----";
size_t pos1, pos2;
pos1 = RSA_PRIV_KEY.find(HEADER);
if(pos1 == string::npos)
throw runtime_error("PEM header not found");
pos2 = RSA_PRIV_KEY.find(FOOTER, pos1+1);
if(pos2 == string::npos)
throw runtime_error("PEM footer not found");
// Start position and length
pos1 = pos1 + HEADER.length();
pos2 = pos2 - pos1;
string keystr = RSA_PRIV_KEY.substr(pos1, pos2);
// Base64 decode, place in a ByteQueue
ByteQueue queue;
Base64Decoder decoder;
decoder.Attach(new Redirector(queue));
decoder.Put((const byte*)keystr.data(), keystr.length());
decoder.MessageEnd();
// Write to file for inspection
FileSink fs("decoded-key.der");
queue.CopyTo(fs);
fs.MessageEnd();
try
{
CryptoPP::RSA::PrivateKey rsaPrivate;
rsaPrivate.BERDecodePrivateKey(queue, false /*paramsPresent*/, queue.MaxRetrievable());
// BERDecodePrivateKey is a void function. Here's the only check
// we have regarding the DER bytes consumed.
ASSERT(queue.IsEmpty());
}
catch (const Exception& ex)
{
cerr << ex.what() << endl;
exit (1);
}
After loading the key, you can validate it with:
AutoSeededRandomPool prng;
bool valid = rsaPrivate.Validate(prng, 3);
if(!valid)
cerr << "RSA private key is not valid" << endl;
And print it with:
cout << "N: " << rsaPrivate.GetModulus() << endl << endl;
cout << "E: " << rsaPrivate.GetPublicExponent() << endl << endl;
cout << "D: " << rsaPrivate.GetPrivateExponent() << endl << endl;
If the key is password protected, then Crypto++ cannot decode it. The library lacks the support to perform the decryption. In this case, you can convert it to BER/DER using the following OpenSSL command. Then you can use the key material with Crypto++.
openssl pkcs8 -nocrypt -in rsa-key.pem -inform PEM -topk8 -outform DER -out rsa-key.der
The sample program wrote the key to file with this:
FileSink fs("decoded-key.der");
queue.CopyTo(fs);
fs.MessageEnd();
The CopyTo leaves the bytes in the queue for use later. You can dump the file with an ASN.1 tool, like Gutmann's dumpasn1:
$ dumpasn1 decoded-key.der
0 314: SEQUENCE {
4 1: INTEGER 0
7 65: INTEGER
: 00 AF 10 F9 3A 11 E2 D5 86 B2 16 8A 61 11 CA 27
: 76 66 31 41 7A 8E 3C 02 4B FB B5 03 CB F5 B4 56
: D0 89 5A 41 97 16 01 A5 A3 5C 4F 61 0D 11 70 46
: 76 65 AA AF C5 70 4A 5D 0D 04 82 B2 AB 8A 74 AE
: 1B
74 1: INTEGER 3
77 64: INTEGER
: 74 B5 FB 7C 0B EC 8E 59 CC 0F 06 EB 61 31 6F A4
: 44 20 D6 51 B4 28 01 87 FD 23 57 DD 4E 78 39 DF
: 41 47 34 ED 21 3A BE CD F5 27 C9 BB AB 0F EB C5
: E0 77 13 D8 7A A0 68 BD 21 54 5E 6D 53 BE F7 3B
143 33: INTEGER
: 00 D6 6F FF CA 03 3A D2 3A 8D 0F D3 29 2D 9D 8E
: 7B 50 5F 50 DD 8D BA AE E9 56 23 A2 EA A6 CC 59
: 0F
178 33: INTEGER
: 00 D0 FF 72 69 60 EE B5 33 DF 83 DF 4A 63 3A D6
: 52 44 98 C1 23 2A 9F 11 07 FC 60 82 1C E6 09 E2
: 35
213 33: INTEGER
: 00 8E F5 55 31 57 7C 8C 27 08 B5 37 70 C9 13 B4
: 52 35 94 E0 93 B3 D1 C9 F0 E4 17 C1 F1 C4 88 3B
: 5F
248 33: INTEGER
: 00 8B 54 F6 F0 EB 49 CE 22 95 02 94 DC 42 27 39
: 8C 2D BB 2B 6C C7 14 B6 05 52 EB 01 68 99 5B EC
: 23
283 33: INTEGER
: 00 AF 80 62 A4 FA 5B 0B 0C 5B 63 A7 E6 5F 5D 8B
: E4 4F CE 10 D3 D5 07 46 C9 32 39 78 53 52 F8 6C
: 58
: }
0 warnings, 0 errors.