doc: add caveats of algs and key size in crypto

Add description of user responsibility in the choice of cypto
algorithms and its key length. Some of recommendations for the safer
use are also described.

PR-URL: #3479
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/crypto.markdown

@@ -97,18 +97,18 @@ Creates and returns a hash object, a cryptographic hash with the given
 algorithm which can be used to generate hash digests.
 
 `algorithm` is dependent on the available algorithms supported by the
-version of OpenSSL on the platform. Examples are `'sha1'`, `'md5'`,
-`'sha256'`, `'sha512'`, etc.  On recent releases, `openssl
+version of OpenSSL on the platform. Examples are `'sha256'`,
+`'sha512'`, etc.  On recent releases, `openssl
 list-message-digest-algorithms` will display the available digest
 algorithms.
 
-Example: this program that takes the sha1 sum of a file
+Example: this program that takes the sha256 sum of a file
 
     var filename = process.argv[2];
     var crypto = require('crypto');
     var fs = require('fs');
 
-    var shasum = crypto.createHash('sha1');
+    var shasum = crypto.createHash('sha256');
 
     var s = fs.ReadStream(filename);
     s.on('data', function(d) {
@@ -511,21 +511,21 @@ expected.
 ## crypto.getDiffieHellman(group_name)
 
 Creates a predefined Diffie-Hellman key exchange object.  The
-supported groups are: `'modp1'`, `'modp2'`, `'modp5'` (defined in [RFC
-2412][]) and `'modp14'`, `'modp15'`, `'modp16'`, `'modp17'`,
-`'modp18'` (defined in [RFC 3526][]).  The returned object mimics the
-interface of objects created by [crypto.createDiffieHellman()][]
-above, but will not allow to change the keys (with
-[diffieHellman.setPublicKey()][] for example).  The advantage of using
-this routine is that the parties don't have to generate nor exchange
-group modulus beforehand, saving both processor and communication
-time.
+supported groups are: `'modp1'`, `'modp2'`, `'modp5'` (defined in
+[RFC 2412][], but see [Caveats](#crypto_caveats)) and `'modp14'`,
+`'modp15'`, `'modp16'`, `'modp17'`, `'modp18'` (defined in
+[RFC 3526][]).  The returned object mimics the interface of objects
+created by [crypto.createDiffieHellman()][] above, but will not allow
+changing the keys (with [diffieHellman.setPublicKey()][] for example).
+The advantage of using this routine is that the parties do not have to
+generate nor exchange group modulus beforehand, saving both processor
+and communication time.
 
 Example (obtaining a shared secret):
 
     var crypto = require('crypto');
-    var alice = crypto.getDiffieHellman('modp5');
-    var bob = crypto.getDiffieHellman('modp5');
+    var alice = crypto.getDiffieHellman('modp14');
+    var bob = crypto.getDiffieHellman('modp14');
 
     alice.generateKeys();
     bob.generateKeys();
@@ -768,6 +768,26 @@ default, set the `crypto.DEFAULT_ENCODING` field to 'binary'.  Note
 that new programs will probably expect buffers, so only use this as a
 temporary measure.
 
+## Caveats
+
+The crypto module still supports some algorithms which are already
+compromised. And the API also allows the use of ciphers and hashes
+with a small key size that are considered to be too weak for safe use.
+
+Users should take full responsibility for selecting the crypto
+algorithm and key size according to their security requirements.
+
+Based on the recommendations of [NIST SP 800-131A]:
+
+- MD5 and SHA-1 are no longer acceptable where collision resistance is
+  required such as digital signatures.
+- The key used with RSA, DSA and DH algorithms is recommended to have
+  at least 2048 bits and that of the curve of ECDSA and ECDH at least
+  224 bits, to be safe to use for several years.
+- The DH groups of `modp1`, `modp2` and `modp5` have a key size
+  smaller than 2048 bits and are not recommended.
+
+See the reference for other recommendations and details.
 
 [createCipher()]: #crypto_crypto_createcipher_algorithm_password
 [createCipheriv()]: #crypto_crypto_createcipheriv_algorithm_key_iv
@@ -779,3 +799,4 @@ temporary measure.
 [RFC 3526]: http://www.rfc-editor.org/rfc/rfc3526.txt
 [crypto.pbkdf2]: #crypto_crypto_pbkdf2_password_salt_iterations_keylen_digest_callback
 [EVP_BytesToKey]: https://www.openssl.org/docs/crypto/EVP_BytesToKey.html
+[NIST SP 800-131A]: http://csrc.nist.gov/publications/nistpubs/800-131A/sp800-131A.pdf