diff --git a/lib/sync.js b/lib/sync.js
index a0c322a..811151f 100644
--- a/lib/sync.js
+++ b/lib/sync.js
@@ -13,6 +13,26 @@ var sizes = {
 	ripemd160: 20
 };
 
+var blockSizes = {
+	__proto__: null,
+	blake2b512: 128, // Should use the built-in key support instead of HMAC.
+	blake2s256: 64,  // (ie PBKDF2-BLAKE2b vs PBKDF2-HMAC-BLAKE2b)
+	md5: 64,
+	ripemd160: 64,
+	rmd160: 64,
+	sha1: 64,
+	sha224: 64,
+	sha256: 64,
+	sha384: 128,
+	sha512: 128,
+	'sha512-224': 128,
+	'sha512-256': 128,
+	'sha3-224': 144,
+	'sha3-256': 136,
+	'sha3-384': 104,
+	'sha3-512': 72
+};
+
 var mapping = {
 	__proto__: null,
 	'sha-1': 'sha1',
@@ -24,6 +44,7 @@ var mapping = {
 };
 
 var createHmac = require('create-hmac');
+var createHash = require('create-hash');
 var Buffer = require('safe-buffer').Buffer;
 
 var checkParameters = require('./precondition');
@@ -38,9 +59,13 @@ function pbkdf2(password, salt, iterations, keylen, digest) {
 	var lowerDigest = (digest || 'sha1').toLowerCase();
 	var mappedDigest = mapping[lowerDigest] || lowerDigest;
 	var size = sizes[mappedDigest];
-	if (typeof size !== 'number' || !size) {
+	var blockSize = blockSizes[mappedDigest];
+	if (typeof size !== 'number' || !size || typeof blockSize !== 'number' || !blockSize) {
 		throw new TypeError('Digest algorithm not supported: ' + digest);
 	}
+	if (password.length > blockSize) {
+		password = createHash(mappedDigest).update(password).digest();
+	}
 
 	var DK = Buffer.allocUnsafe(keylen);
 	var block1 = Buffer.allocUnsafe(salt.length + 4);