Update crypto.md

doc/api/crypto.md

@@ -581,6 +581,33 @@ const bobSecret = bob.computeSecret(aliceKey);
 assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
 ```
 
+## Class: DiffieHellmanGroup
+<!-- YAML
+added: v0.7.5
+-->
+
+Usage:
+
+```js
+const name = 'modp1';
+const dh = crypto.createDiffieHellmanGroup(name);
+```
+It takes a well-known modp group as its argument but
+ otherwise works the same as `DiffieHellman`.
+
+`name` is taken from [RFC 2412][] (modp1 and 2) and [RFC 3526][]:
+```bash
+$ perl -ne 'print "$1\n" if /"(modp\d+)"/' src/node_crypto_groups.h
+modp1  #  768 bits
+modp2  # 1024 bits
+modp5  # 1536 bits
+modp14 # 2048 bits
+modp15 # etc.
+modp16
+modp17
+modp18
+```
+
 ### diffieHellman.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])
 <!-- YAML
 added: v0.5.0
@@ -1733,6 +1760,13 @@ Creates a `DiffieHellman` key exchange object and generates a prime of
 `primeLength` bits using an optional specific numeric `generator`.
 If `generator` is not specified, the value `2` is used.
 
+### crypto.createDiffieHellmanGroup()
+<!-- YAML
+added: v0.9.3
+-->
+
+An alias for [`crypto.getDiffieHellman()`][]
+
 ### crypto.createECDH(curveName)
 <!-- YAML
 added: v0.11.14
@@ -3226,6 +3260,7 @@ the `crypto`, `tls`, and `https` modules and are generally specific to OpenSSL.
 [`crypto.createSign()`]: #crypto_crypto_createsign_algorithm_options
 [`crypto.createVerify()`]: #crypto_crypto_createverify_algorithm_options
 [`crypto.getCurves()`]: #crypto_crypto_getcurves
+[`crypto.getDiffieHellman()`]: #crypto_crypto_getdiffiehellman
 [`crypto.getHashes()`]: #crypto_crypto_gethashes
 [`crypto.privateDecrypt()`]: #crypto_crypto_privatedecrypt_privatekey_buffer
 [`crypto.privateEncrypt()`]: #crypto_crypto_privateencrypt_privatekey_buffer