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/*
* ZLint Copyright 2024 Regents of the University of Michigan
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not
* use this file except in compliance with the License. You may obtain a copy
* of the License at http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
* implied. See the License for the specific language governing
* permissions and limitations under the License.
*/
package rfc
import (
"fmt"
"math/big"
"strings"
"github.com/zmap/zcrypto/x509"
"github.com/zmap/zlint/v3/lint"
"github.com/zmap/zlint/v3/util"
)
func init() {
lint.RegisterCertificateLint(&lint.CertificateLint{
LintMetadata: lint.LintMetadata{
Name: "e_incorrect_ku_encoding",
Description: "RFC 5280 Section 4.2.1.3 describes the value of a KeyUsage to be a DER encoded BitString, which itself defines that all trailing 0 bits be counted as being \"unused\".",
Citation: "Where ITU-T Rec. X.680 | ISO/IEC 8824-1, 21.7, applies, the bitstring shall have all trailing 0 bits removed before it is encoded.",
Source: lint.RFC5280,
EffectiveDate: util.ZeroDate,
},
Lint: func() lint.LintInterface { return &incorrectKuEncoding{} },
})
}
type incorrectKuEncoding struct{}
func NewIncorrectKuEncoding() lint.LintInterface {
return &incorrectKuEncoding{}
}
func (l *incorrectKuEncoding) CheckApplies(c *x509.Certificate) bool {
ku := util.GetExtFromCert(c, util.KeyUsageOID)
return ku != nil && len(ku.Value) > 0
}
func (l *incorrectKuEncoding) Execute(c *x509.Certificate) *lint.LintResult {
ku := util.GetExtFromCert(c, util.KeyUsageOID).Value
if len(ku) < 4 {
return &lint.LintResult{
Status: lint.Fatal,
Details: fmt.Sprintf("KeyUsage encodings must be at least four bytes long. Got %d bytes", len(ku)),
}
}
// Byte 0: Tag
// Byte 1: Length
// Byte 2: Unused bits
// Bytes 3..n: KeyUsage
declaredUnused := uint(ku[2])
actualUnused := big.NewInt(0).SetBytes(ku[3:]).TrailingZeroBits()
if declaredUnused == actualUnused {
return &lint.LintResult{Status: lint.Pass}
}
// Just a bit of formatting to a visualized binary form so
// it's easier for users to see what the exact binary that
// we're referring to so that they can debug their own certs.
binary := make([]string, len(ku))
for i, b := range ku {
binary[i] = fmt.Sprintf("%08b", b)
}
return &lint.LintResult{
Status: lint.Error,
Details: fmt.Sprintf(
"KeyUsage contains an inefficient encoding wherein the number of 'unused bits' is declared to be "+
"%d, but it should be %d. Raw Bytes: %v, Raw Binary: [%s]",
declaredUnused, actualUnused, ku, strings.Join(binary, " "),
)}
}
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