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

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. Even worse, it appears that versions of Java prior to 1.7 don't support Diffie-Hellman parameters longer than 1024 bits!

Validating Parameters

The Diffie-Hellman parameters should be validated after loading. To perform paramter validation, you call DH_check. DH_check returns 0 or a bitmask values of the following:

  • DH_CHECK_P_NOT_PRIME (0x01)
  • DH_CHECK_P_NOT_SAFE_PRIME (0x02)
  • DH_UNABLE_TO_CHECK_GENERATOR (0x04)
  • DH_NOT_SUITABLE_GENERATOR (0x08)

The validation code might look as follows (error checking omitted for clarity): 

BIO* bio = ...;
DH* dh = PEM_read_bio_DHparams(bio, NULL, NULL, NULL);

int rc, codes = 0;
rc = DH_check(dh, &codes);
assert(rc == 1);

if(BN_is_word(dh->g, DH_GENERATOR_2))
{
    long residue = BN_mod_word(dh->p, 24);
    if(residue == 11 || residue == 23) {
        codes &= ~DH_NOT_SUITABLE_GENERATOR;
    }
}

if (codes & DH_UNABLE_TO_CHECK_GENERATOR)
    printf("DH_check: failed to test generator\n");

if (codes & DH_NOT_SUITABLE_GENERATOR)
    printf("DH_check: g is not a suitable generator\n");

if (codes & DH_CHECK_P_NOT_PRIME)
    printf("DH_check: p is not prime\n");

if (codes & DH_CHECK_P_NOT_SAFE_PRIME)
    printf("DH_check: p is not a safe prime\n");

The additional call to BN_mod_word(dh->p, 24) (and unmasking of DH_NOT_SUITABLE_GENERATOR) is performed to ensure your program accepts IETF group parameters. OpenSSL checks the prime is congruent to 11 when g = 2; while the IETF's primes are congruent to 23 when g = 2. Without the test, the IETF parameters would fail validation. For details, see Diffie-Hellman Parameter Check (when g = 2, must p mod 24 == 11?).

Elliptic curve Diffie-Hellman

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)

For more information, see Elliptic Curve Diffie Hellman and Elliptic Curve Cryptography.

RFC 3526 PEM Encoded Groups

Below are three Diffie-Hellman MODP groups specified in RFC 3526, More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE) (the 1024-bit parameter is from RFC 2409). They can be used with PEM_read_bio_DHparams and a memory BIO. RFC 3526 also offers 1536-bit, 6144-bit and 8192-bit primes. 

static const char g_dh1024_sz[] =
    -----BEGIN DH PARAMETERS-----\n"
    MIGHAoGBAP//////////yQ/aoiFowjTExmKLgNwc0SkCTgiKZ8x0Agu+pjsTmyJR\n"
    Sgh5jjQE3e+VGbPNOkMbMCsKbfJfFDdP4TVtbVHCReSFtXZiXn7G9ExC6aY37WsL\n"
    /1y29Aa37e44a/taiZ+lrp8kEXxLH+ZJKGZR7OZTgf//////////AgEC\n"
    -----END DH PARAMETERS-----\n"

static const char g_dh2048_sz[] =
    "-----BEGIN DH PARAMETERS-----\n"
    "MIIBCAKCAQEA///////////JD9qiIWjCNMTGYouA3BzRKQJOCIpnzHQCC76mOxOb\n"
    "IlFKCHmONATd75UZs806QxswKwpt8l8UN0/hNW1tUcJF5IW1dmJefsb0TELppjft\n"
    "awv/XLb0Brft7jhr+1qJn6WunyQRfEsf5kkoZlHs5Fs9wgB8uKFjvwWY2kg2HFXT\n"
    "mmkWP6j9JM9fg2VdI9yjrZYcYvNWIIVSu57VKQdwlpZtZww1Tkq8mATxdGwIyhgh\n"
    "fDKQXkYuNs474553LBgOhgObJ4Oi7Aeij7XFXfBvTFLJ3ivL9pVYFxg5lUl86pVq\n"
    "5RXSJhiY+gUQFXKOWoqsqmj//////////wIBAg==\n"
    "-----END DH PARAMETERS-----";

static const char g_dh3072_sz[] =
    "-----BEGIN DH PARAMETERS-----\n"
    "MIIBiAKCAYEA///////////JD9qiIWjCNMTGYouA3BzRKQJOCIpnzHQCC76mOxOb\n"
    "IlFKCHmONATd75UZs806QxswKwpt8l8UN0/hNW1tUcJF5IW1dmJefsb0TELppjft\n"
    "awv/XLb0Brft7jhr+1qJn6WunyQRfEsf5kkoZlHs5Fs9wgB8uKFjvwWY2kg2HFXT\n"
    "mmkWP6j9JM9fg2VdI9yjrZYcYvNWIIVSu57VKQdwlpZtZww1Tkq8mATxdGwIyhgh\n"
    "fDKQXkYuNs474553LBgOhgObJ4Oi7Aeij7XFXfBvTFLJ3ivL9pVYFxg5lUl86pVq\n"
    "5RXSJhiY+gUQFXKOWoqqxC2tMxcNBFB6M6hVIavfHLpk7PuFBFjb7wqK6nFXXQYM\n"
    "fbOXD4Wm4eTHq/WujNsJM9cejJTgSiVhnc7j0iYa0u5r8S/6BtmKCGTYdgJzPshq\n"
    "ZFIfKxgXeyAMu+EXV3phXWx3CYjAutlG4gjiT6B05asxQ9tb/OD9EI5LgtEgqTrS\n"
    "yv//////////AgEC\n"
    "-----END DH PARAMETERS-----";

static const char g_dh4096_sz[] =
    "-----BEGIN DH PARAMETERS-----\n"
    "MIICCAKCAgEA///////////JD9qiIWjCNMTGYouA3BzRKQJOCIpnzHQCC76mOxOb\n"
    "IlFKCHmONATd75UZs806QxswKwpt8l8UN0/hNW1tUcJF5IW1dmJefsb0TELppjft\n"
    "awv/XLb0Brft7jhr+1qJn6WunyQRfEsf5kkoZlHs5Fs9wgB8uKFjvwWY2kg2HFXT\n"
    "mmkWP6j9JM9fg2VdI9yjrZYcYvNWIIVSu57VKQdwlpZtZww1Tkq8mATxdGwIyhgh\n"
    "fDKQXkYuNs474553LBgOhgObJ4Oi7Aeij7XFXfBvTFLJ3ivL9pVYFxg5lUl86pVq\n"
    "5RXSJhiY+gUQFXKOWoqqxC2tMxcNBFB6M6hVIavfHLpk7PuFBFjb7wqK6nFXXQYM\n"
    "fbOXD4Wm4eTHq/WujNsJM9cejJTgSiVhnc7j0iYa0u5r8S/6BtmKCGTYdgJzPshq\n"
    "ZFIfKxgXeyAMu+EXV3phXWx3CYjAutlG4gjiT6B05asxQ9tb/OD9EI5LgtEgqSEI\n"
    "ARpyPBKnh+bXiHGaEL26WyaZwycYavTiPBqUaDS2FQvaJYPpyirUTOjbu8LbBN6O\n"
    "+S6O/BQfvsqmKHxZR05rwF2ZspZPoJDDoiM7oYZRW+ftH2EpcM7i16+4G912IXBI\n"
    "HNAGkSfVsFqpk7TqmI2P3cGG/7fckKbAj030Nck0BjGZ//////////8CAQI=\n"
    "-----END DH PARAMETERS-----";
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