crypto/rand: Currently using deprecated API for random number generation on Windows

[randomvariable]

What version of Go are you using (go version)?

N/A

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

Any release of Windows supported by Microsoft

What did you do?

Reviewing cryptographic protocols for a downstream project

What did you expect to see?

Use of a modern Windows API to retrieve entropy, namely BCryptGenRandom from CryptoNG, which is compliant with CTR_DRBG from NIST SP800-90.

What did you see instead?

Go calls CryptGenRandom from a deprecated API. The algorithm is not fully documented and is only known in part.

[bcmills]

CC @FiloSottile @agl

[jtn7]

I noticed, recently, that crypto/rand used CryptGenRandom instead of (BCryptGenRandom). So I decided to call it manually.

In case anyone is interested:

var (
	bcrypt, libErr     = syscall.LoadLibrary("bcrypt.dll")
	genRandom, procErr = syscall.GetProcAddress(bcrypt, "BCryptGenRandom")
)

const BCRYPT_USE_SYSTEM_PREFERRED_RNG = 0x00000002

func bcryptGenRandom(buf []byte) (ret uintptr, callErr error) {
	const nargs = 4
	ret, _, callErr = syscall.Syscall6(
		genRandom,
		nargs,
		0,
		uintptr(unsafe.Pointer(&buf[0])),
		uintptr(len(buf)),
		BCRYPT_USE_SYSTEM_PREFERRED_RNG,
		0,
		0,
	)
	return
}

[networkimprov]

cc @alexbrainman @mattn @zx2c4 @iwdgo

[zx2c4]

The RNG function that we should be using on Windows is actually RtlGenRandom. I'll submit a patch to use that. We already have a wrapper around it in getRandomData.

[gopherbot]

Change https://golang.org/cl/210057 mentions this issue: crypto/rand: generate random numbers using RtlGenRandom on Windows