crypto/rsa: drop contradictory promise to keep PublicKey modulus secret

We claim to treat N as secret (and indeed bigmod is constant time in
relation to the modulus) but at the same time we warn that all inputs to
VerifyPKCS1v15 and Verify are public:

> The inputs are not considered confidential, and may leak through
> timing side channels, or if an attacker has control of part of the
> inputs.

See #67043 (which focuses on the inverse, recovering signatures by
controlling the public key input to Verify), and in particular
#67043 (comment).

Stopping the Verify adaptive attack would require significantly more
complexity, the kind that has caused vulnerabilities in the past (e.g.
CVE-2016-2107). On the other hand, assuming that a public key is
confidential is unlikely to work in practice, since it can be recovered
from just two valid (message, signature) pairs. See for example
https://keymaterial.net/2024/06/15/reconstructing-public-keys-from-signatures/.

This comment was introduced in CL 552935, not really due to a need to
specify that N was secret, but rather to clarify that E is not (so it
could be used in variable-time exponentiation).

Change-Id: I6a6a6964f3f8d2dc2fcc13ce938b271c9de9666b
Reviewed-on: https://go-review.googlesource.com/c/go/+/687616
Reviewed-by: Roland Shoemaker <[email protected]>
Auto-Submit: Filippo Valsorda <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: FiloSottile

src/crypto/rsa/rsa.go

@@ -63,9 +63,8 @@ var bigOne = big.NewInt(1)
 
 // A PublicKey represents the public part of an RSA key.
 //
-// The value of the modulus N is considered secret by this library and protected
-// from leaking through timing side-channels. However, neither the value of the
-// exponent E nor the precise bit size of N are similarly protected.
+// The values of N and E are not considered confidential, and may leak through
+// side channels, or could be mathematically derived from other public values.
 type PublicKey struct {
 	N *big.Int // modulus
 	E int      // public exponent