OpenSSL/Diffie-Hellman parameters

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To use Perfect Forward Secrecy cipher suites, you must set up Diffie-Hellman parameters (on the server side), or the PFS cipher suites will be silently ignored.

Diffie-Hellman[edit]

SSL_CTX_set_tmp_dh is used to set the Diffie-Hellman parameters for a context. One of the easiest ways to get Diffie-Hellman parameters to use with this function is to generate random Diffie-Hellman parameters with the dhparam command-line program with the -C option, and embed the resulting code fragment in your program. For example, openssl dhparam -C 2236 might result in:

#ifndef HEADER_DH_H
#include <openssl/dh.h>
#endif
DH *get_dh2236()
        {
        static unsigned char dh2236_p[]={
                0x0F,0x52,0xE5,0x24,0xF5,0xFA,0x9D,0xDC,0xC6,0xAB,0xE6,0x04,
                0xE4,0x20,0x89,0x8A,0xB4,0xBF,0x27,0xB5,0x4A,0x95,0x57,0xA1,
                0x06,0xE7,0x30,0x73,0x83,0x5E,0xC9,0x23,0x11,0xED,0x42,0x45,
                0xAC,0x49,0xD3,0xE3,0xF3,0x34,0x73,0xC5,0x7D,0x00,0x3C,0x86,
                0x63,0x74,0xE0,0x75,0x97,0x84,0x1D,0x0B,0x11,0xDA,0x04,0xD0,
                0xFE,0x4F,0xB0,0x37,0xDF,0x57,0x22,0x2E,0x96,0x42,0xE0,0x7C,
                0xD7,0x5E,0x46,0x29,0xAF,0xB1,0xF4,0x81,0xAF,0xFC,0x9A,0xEF,
                0xFA,0x89,0x9E,0x0A,0xFB,0x16,0xE3,0x8F,0x01,0xA2,0xC8,0xDD,
                0xB4,0x47,0x12,0xF8,0x29,0x09,0x13,0x6E,0x9D,0xA8,0xF9,0x5D,
                0x08,0x00,0x3A,0x8C,0xA7,0xFF,0x6C,0xCF,0xE3,0x7C,0x3B,0x6B,
                0xB4,0x26,0xCC,0xDA,0x89,0x93,0x01,0x73,0xA8,0x55,0x3E,0x5B,
                0x77,0x25,0x8F,0x27,0xA3,0xF1,0xBF,0x7A,0x73,0x1F,0x85,0x96,
                0x0C,0x45,0x14,0xC1,0x06,0xB7,0x1C,0x75,0xAA,0x10,0xBC,0x86,
                0x98,0x75,0x44,0x70,0xD1,0x0F,0x20,0xF4,0xAC,0x4C,0xB3,0x88,
                0x16,0x1C,0x7E,0xA3,0x27,0xE4,0xAD,0xE1,0xA1,0x85,0x4F,0x1A,
                0x22,0x0D,0x05,0x42,0x73,0x69,0x45,0xC9,0x2F,0xF7,0xC2,0x48,
                0xE3,0xCE,0x9D,0x74,0x58,0x53,0xE7,0xA7,0x82,0x18,0xD9,0x3D,
                0xAF,0xAB,0x40,0x9F,0xAA,0x4C,0x78,0x0A,0xC3,0x24,0x2D,0xDB,
                0x12,0xA9,0x54,0xE5,0x47,0x87,0xAC,0x52,0xFE,0xE8,0x3D,0x0B,
                0x56,0xED,0x9C,0x9F,0xFF,0x39,0xE5,0xE5,0xBF,0x62,0x32,0x42,
                0x08,0xAE,0x6A,0xED,0x88,0x0E,0xB3,0x1A,0x4C,0xD3,0x08,0xE4,
                0xC4,0xAA,0x2C,0xCC,0xB1,0x37,0xA5,0xC1,0xA9,0x64,0x7E,0xEB,
                0xF9,0xD3,0xF5,0x15,0x28,0xFE,0x2E,0xE2,0x7F,0xFE,0xD9,0xB9,
                0x38,0x42,0x57,0x03,
                };
        static unsigned char dh2236_g[]={
                0x02,
                };
        DH *dh;

        if ((dh=DH_new()) == NULL) return(NULL);
        dh->p=BN_bin2bn(dh2236_p,sizeof(dh2236_p),NULL);
        dh->g=BN_bin2bn(dh2236_g,sizeof(dh2236_g),NULL);
        if ((dh->p == NULL) || (dh->g == NULL))
                { DH_free(dh); return(NULL); }
        return(dh);
        }

which can then be used like this:

DH *dh = get_dh2236 ();
if (1 != SSL_CTX_set_tmp_dh (ctx, dh))
  error ();
DH_free (dh);

Be sure to choose a bit length appropriate to the security level you want to achieve, although keep in mind that Diffie-Hellman parameters longer than 2236 bits may be incompatible with older versions of NSS.

Elliptic curve Diffie-Hellman[edit]

For elliptic curve Diffie-Hellman, you can do something like this:

EC_KEY *ecdh = EC_KEY_new_by_curve_name (NID_X9_62_prime256v1);
if (! ecdh)
  error ();
if (1 != SSL_CTX_set_tmp_ecdh (ctx, ecdh))
  error ();
EC_KEY_free (ecdh);

Or, in OpenSSL 1.0.2 (not yet released, as of Feb 2013) and higher, you should be able to do:

SSL_CTX_set_ecdh_auto (ctx, 1)