1 files changed, 111 insertions, 0 deletions
diff --git a/pkg/signature/message.go b/pkg/signature/message.go
new file mode 100644
index 0000000..6f8449e
--- /dev/null
+++ b/pkg/signature/message.go
@@ -0,0 +1,111 @@
+//
+// 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
+}