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//
// Copyright 2021 The Sigstore Authors.
//
// 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 signature
import (
"crypto"
crand "crypto/rand"
"errors"
"fmt"
"io"
)
func isSupportedAlg(alg crypto.Hash, supportedAlgs []crypto.Hash) bool {
if supportedAlgs == nil {
return true
}
for _, supportedAlg := range supportedAlgs {
if alg == supportedAlg {
return true
}
}
return false
}
// ComputeDigestForSigning calculates the digest value for the specified message using a hash function selected by the following process:
//
// - if a digest value is already specified in a SignOption and the length of the digest matches that of the selected hash function, the
// digest value will be returned without any further computation
// - if a hash function is given using WithCryptoSignerOpts(opts) as a SignOption, it will be used (if it is in the supported list)
// - otherwise defaultHashFunc will be used (if it is in the supported list)
func ComputeDigestForSigning(rawMessage io.Reader, defaultHashFunc crypto.Hash, supportedHashFuncs []crypto.Hash, opts ...SignOption) (digest []byte, hashedWith crypto.Hash, err error) {
var cryptoSignerOpts crypto.SignerOpts = defaultHashFunc
for _, opt := range opts {
opt.ApplyDigest(&digest)
opt.ApplyCryptoSignerOpts(&cryptoSignerOpts)
}
hashedWith = cryptoSignerOpts.HashFunc()
if !isSupportedAlg(hashedWith, supportedHashFuncs) {
return nil, crypto.Hash(0), fmt.Errorf("unsupported hash algorithm: %q not in %v", hashedWith.String(), supportedHashFuncs)
}
if len(digest) > 0 {
if hashedWith != crypto.Hash(0) && len(digest) != hashedWith.Size() {
err = errors.New("unexpected length of digest for hash function specified")
}
return
}
digest, err = hashMessage(rawMessage, hashedWith)
return
}
// ComputeDigestForVerifying calculates the digest value for the specified message using a hash function selected by the following process:
//
// - if a digest value is already specified in a SignOption and the length of the digest matches that of the selected hash function, the
// digest value will be returned without any further computation
// - if a hash function is given using WithCryptoSignerOpts(opts) as a SignOption, it will be used (if it is in the supported list)
// - otherwise defaultHashFunc will be used (if it is in the supported list)
func ComputeDigestForVerifying(rawMessage io.Reader, defaultHashFunc crypto.Hash, supportedHashFuncs []crypto.Hash, opts ...VerifyOption) (digest []byte, hashedWith crypto.Hash, err error) {
var cryptoSignerOpts crypto.SignerOpts = defaultHashFunc
for _, opt := range opts {
opt.ApplyDigest(&digest)
opt.ApplyCryptoSignerOpts(&cryptoSignerOpts)
}
hashedWith = cryptoSignerOpts.HashFunc()
if !isSupportedAlg(hashedWith, supportedHashFuncs) {
return nil, crypto.Hash(0), fmt.Errorf("unsupported hash algorithm: %q not in %v", hashedWith.String(), supportedHashFuncs)
}
if len(digest) > 0 {
if hashedWith != crypto.Hash(0) && len(digest) != hashedWith.Size() {
err = errors.New("unexpected length of digest for hash function specified")
}
return
}
digest, err = hashMessage(rawMessage, hashedWith)
return
}
func hashMessage(rawMessage io.Reader, hashFunc crypto.Hash) ([]byte, error) {
if rawMessage == nil {
return nil, errors.New("message cannot be nil")
}
if hashFunc == crypto.Hash(0) {
return io.ReadAll(rawMessage)
}
hasher := hashFunc.New()
// avoids reading entire message into memory
if _, err := io.Copy(hasher, rawMessage); err != nil {
return nil, fmt.Errorf("hashing message: %w", err)
}
return hasher.Sum(nil), nil
}
func selectRandFromOpts(opts ...SignOption) io.Reader {
rand := crand.Reader
for _, opt := range opts {
opt.ApplyRand(&rand)
}
return rand
}
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