improve TLS1PRF comments

tls1prf.go

@@ -16,7 +16,7 @@ func SupportsTLS1PRF() bool {
 		(vMajor >= 1 && vMinor >= 1)
 }
 
-// TLS1PRF implements the TLS 1.0/1.1 pseudo-random function if h is nil or crypto.MD5SHA1,
+// TLS1PRF implements the TLS 1.0/1.1 pseudo-random function if h is nil,
 // else it implements the TLS 1.2 pseudo-random function.
 // The pseudo-random number will be written to result and will be of length len(result).
 func TLS1PRF(result, secret, label, seed []byte, h func() hash.Hash) error {
@@ -93,8 +93,10 @@ func TLS1PRF(result, secret, label, seed []byte, h func() hash.Hash) error {
 	if C.go_openssl_EVP_PKEY_derive(ctx, base(result), &outLen) != 1 {
 		return newOpenSSLError("EVP_PKEY_derive")
 	}
+	// The Go standard library expects TLS1PRF to return the requested number of bytes,
+	// fail if it doesn't.
 	if outLen != C.size_t(len(result)) {
-		return errors.New("tls1-prf: entropy limit reached")
+		return errors.New("tls1-prf: derived less bytes than requested")
 	}
 	return nil
 }