Add signing and verifying arbitrary bytes (#41)

crypto/crypto.go

@@ -17,6 +17,9 @@ var txidPrefix = []byte("TX")
 // bidPrefix is prepended to a bid when signing it
 var bidPrefix = []byte("aB")
 
+// bytesPrefix is prepended to bytes when signing
+var bytesPrefix = []byte("NF")
+
 // RandomBytes fills the passed slice with randomness, and panics if it is
 // unable to do so
 func RandomBytes(s []byte) {
@@ -87,6 +90,23 @@ func rawSignTransaction(sk ed25519.PrivateKey, tx types.Transaction) (s types.Si
 	return
 }
 
+// SignBytes signs the bytes and returns the signature
+func SignBytes(sk ed25519.PrivateKey, bytesToSign []byte) (signature []byte, err error) {
+	// prepend the prefix for signing bytes
+	toBeSigned := bytes.Join([][]byte{bytesPrefix, bytesToSign}, nil)
+
+	// sign the bytes
+	signature = ed25519.Sign(sk, toBeSigned)
+	return
+}
+
+//VerifyBytes verifies that the signature is valid
+func VerifyBytes(pk ed25519.PublicKey, message, signature []byte) bool {
+	msgParts := [][]byte{bytesPrefix, message}
+	toBeVerified := bytes.Join(msgParts, nil)
+	return ed25519.Verify(pk, toBeVerified, signature)
+}
+
 // SignBid accepts a private key and a bid, and returns the signature of the
 // bid under that key
 func SignBid(sk ed25519.PrivateKey, bid types.Bid) (signedBid []byte, err error) {

crypto/crypto_test.go

@@ -4,6 +4,7 @@ import (
     "github.com/algorand/go-algorand-sdk/encoding/msgpack"
     "github.com/algorand/go-algorand-sdk/mnemonic"
     "testing"
+    "math/rand"
 
     "github.com/stretchr/testify/require"
     "golang.org/x/crypto/ed25519"
@@ -116,3 +117,18 @@ func TestMergeMultisigTransactions(t *testing.T) {
     expectedTxid := "2U2QKCYSYA3DUJNVP5T2KWBLWVUMX4PPIGN43IPPVGVZKTNFKJFQ"
     require.Equal(t, expectedTxid, txid)
 }
+
+func TestSignBytes(t *testing.T) {
+    account := GenerateAccount()
+    message := make([]byte, 15)
+    rand.Read(message)
+    signature, err := SignBytes(account.PrivateKey, message)
+    require.NoError(t, err)
+    require.True(t, VerifyBytes(account.PublicKey, message, signature))
+    if message[0] == 255 {
+        message[0] = 0
+    } else {
+        message[0] = message[0]+1
+    }
+    require.False(t, VerifyBytes(account.PublicKey, message, signature))
+}