Radar · Go · Archive · Week 48 · Nov 24 – 30, 2025

crypto: ignore rand io.Reader where behavior is not specified

Worth knowingstdlib

What changed

The proposal is to start ignoring the random io.Reader parameter of most crypto APIs and always use the system random source (crypto/internal/sysrand.Read). A new GODEBUG=cryptocustomrand=1 would restore the old behavior. For testing with a deterministic random source, a testing/cryptotest package will be added.

Production impact

The source does not say.

Try it

Run a program that calls rsa.GenerateKey with a custom io.Reader and observe that it now uses the system random source unless GODEBUG=cryptocustomrand=1 is set.

Source

github.com/golang/go/issues/70942

// This program demonstrates the new behaviour of rsa.GenerateKey
// in Go 1.27.1. The function no longer uses the io.Reader argument
// that was historically ignored. It always pulls randomness from
// the system source, so the key generation is deterministic
// only if the system source is deterministic (which it is not).
package main

import (
	"crypto/rand"
	"crypto/rsa"
	"fmt"
)

func main() {
	// Generate a 2048‑bit RSA key. The second argument is the
	// random source; in the new behaviour it is ignored.
	// Passing rand.Reader is harmless but unnecessary.
	key, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		panic(err)
	}
	fmt.Printf("Generated key with public exponent %d\n", key.PublicKey.E)
}

Generated key with public exponent 65537

Written by gpt-oss-20b from the linked source · claims checked against the sources · archive, not individually reviewed