service: implement a function for signing data

service/sign.go

@@ -0,0 +1,58 @@
+package service
+
+import (
+	"crypto/ecdsa"
+
+	crypto "github.com/nspcc-dev/neofs-crypto"
+)
+
+// Returns data from DataSignatureAccumulator for signature creation/verification.
+//
+// If passed DataSignatureAccumulator provides a SignedDataReader interface, data for signature is obtained
+// using this interface for optimization. In this case, it is understood that reading into the slice D
+// that the method DataForSignature returns does not change D.
+func dataForSignature(src SignedDataSource) ([]byte, error) {
+	r, ok := src.(SignedDataReader)
+	if !ok {
+		return src.SignedData()
+	}
+
+	buf := bytesPool.Get().([]byte)
+	defer func() {
+		bytesPool.Put(buf)
+	}()
+
+	if size := r.SignedDataSize(); size <= cap(buf) {
+		buf = buf[:size]
+	} else {
+		buf = make([]byte, size)
+	}
+
+	n, err := r.ReadSignedData(buf)
+	if err != nil {
+		return nil, err
+	}
+
+	return buf[:n], nil
+
+}
+
+// DataSignature returns the signature of data obtained using the private key.
+//
+// If passed data container is nil, ErrNilSignedDataSource returns.
+// If passed private key is nil, crypto.ErrEmptyPrivateKey returns.
+// If the data container or the signature function returns an error, it is returned directly.
+func DataSignature(src SignedDataSource, key *ecdsa.PrivateKey) ([]byte, error) {
+	if src == nil {
+		return nil, ErrNilSignedDataSource
+	} else if key == nil {
+		return nil, crypto.ErrEmptyPrivateKey
+	}
+
+	data, err := dataForSignature(src)
+	if err != nil {
+		return nil, err
+	}
+
+	return crypto.Sign(key, data)
+}