diff --git a/auth.go b/auth.go
index a9c6a66..f7c27d5 100644
--- a/auth.go
+++ b/auth.go
@@ -21,6 +21,9 @@ import (
 // verifyHandshakeSignature verifies a signature against unhashed handshake contents.
 func verifyHandshakeSignature(sigType uint8, pubkey crypto.PublicKey, hashFunc crypto.Hash, signed, sig []byte) error {
 	if hashFunc != directSigning {
+		if !hashFunc.Available() {
+			return fmt.Errorf("hash function unavailable: %v", hashFunc)
+		}
 		h := hashFunc.New()
 		h.Write(signed)
 		signed = h.Sum(nil)
diff --git a/cipher_suites.go b/cipher_suites.go
index 3cb9fa3..bc508fe 100644
--- a/cipher_suites.go
+++ b/cipher_suites.go
@@ -13,6 +13,7 @@ import (
 	"crypto/rc4"
 	"crypto/sha1"
 	"crypto/sha256"
+	_ "crypto/sha512" // for crypto.SHA384
 	"fmt"
 	"hash"
 	"runtime"