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crypto/x509: avoid crypto/rand.Int to generate serial number 

It's probabyl safe enough, but just reading bytes from rand and then
using SetBytes is simpler, and doesn't require allowing calls from
crypto into math/big's Lsh, Sub, and Cmp.

Change-Id: I6a6a4656761f7073f9e149f288c48e97048ab13c
Reviewed-on: https://go-review.googlesource.com/c/go/+/643278
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>
This commit is contained in:
Filippo Valsorda 2025-01-22 13:42:33 +01:00 committed by Gopher Robot
parent 555974734f
commit 6e8d7a113c
1 changed files with 12 additions and 19 deletions
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31
src/crypto/x509/x509.go
View File

@ -27,7 +27,6 @@ import (
"crypto/ecdsa"
"crypto/ed25519"
"crypto/elliptic"
cryptorand "crypto/rand"
"crypto/rsa"
"crypto/sha1"
"crypto/x509/pkix"
@ -1673,25 +1672,19 @@ func CreateCertificate(rand io.Reader, template, parent *Certificate, pub, priv
serialNumber := template.SerialNumber
if serialNumber == nil {
// Generate a serial number following RFC 5280 Section 4.1.2.2 if one is not provided.
// Requirements:
// - serial number must be positive
// - at most 20 octets when encoded
maxSerial := big.NewInt(1).Lsh(big.NewInt(1), 20*8)
for {
var err error
serialNumber, err = cryptorand.Int(rand, maxSerial)
if err != nil {
return nil, err
}
// If the serial is exactly 20 octets, check if the high bit of the first byte is set.
// If so, generate a new serial, since it will be padded with a leading 0 byte during
// encoding so that the serial is not interpreted as a negative integer, making it
// 21 octets.
if serialBytes := serialNumber.Bytes(); len(serialBytes) > 0 && (len(serialBytes) < 20 || serialBytes[0]&0x80 == 0) {
break
}
// Generate a serial number following RFC 5280, Section 4.1.2.2 if one
// is not provided. The serial number must be positive and at most 20
// octets *when encoded*.
serialBytes := make([]byte, 20)
if _, err := io.ReadFull(rand, serialBytes); err != nil {
return nil, err
}
// If the top bit is set, the serial will be padded with a leading zero
// byte during encoding, so that it's not interpreted as a negative
// integer. This padding would make the serial 21 octets so we clear the
// top bit to ensure the correct length in all cases.
serialBytes[0] &= 0b0111_1111
serialNumber = new(big.Int).SetBytes(serialBytes)
}
// RFC 5280 Section 4.1.2.2: serial number must be positive