lib/oracle.go: add OracleAESRandomEncrypt

1 files changed, 68 insertions, 0 deletions

diff --git a/lib/oracle.go b/lib/oracle.go
new file mode 100644
index 0000000..a77793a
--- /dev/null
+++ b/lib/oracle.go
@@ -0,0 +1,68 @@
+// Copyright © 2020 rsiddharth <s@ricketyspace.net>
+// SPDX-License-Identifier: ISC
+
+package lib
+
+import (
+	"crypto/rand"
+	"math/big"
+)
+
+// Given an input `in`, this function AES encrypts `in` using a
+// randomly generate 16-byte key using ECB or CBC mode and returns the
+// cipher.
+func OracleAESRandomEncrypt(in []byte) []byte {
+	// Generate random key.
+	key := make([]byte, 16)
+	_, err := rand.Read(key)
+	if err != nil {
+		panic(err)
+	}
+	// Generate random initialization vector; needed for AES CBC.
+	iv := make([]byte, 16)
+	_, err = rand.Read(iv)
+	if err != nil {
+		panic(err)
+	}
+
+	// Add 5-10 bytes at the beginning and end of `in`
+	in = append(randomBytes(5, 10), in...)
+	in = append(in, randomBytes(5, 10)...)
+
+	// Randomly encrypt `in` with AES in ECB or CBC mode.
+	m, err := rand.Int(rand.Reader, big.NewInt(2))
+	if err != nil {
+		panic(err)
+	}
+	var out []byte
+	if m.Int64() == 0 {
+		// Encrypt with AES in ECB mode.
+		out = AESEncryptECB(in, key)
+	} else {
+		// Encrypt with AES in CBC mode.
+		out = AESEncryptCBC(in, key, iv)
+	}
+	return out
+}
+
+// Randomly generates `min` to `max` bytes.
+func randomBytes(min, max int64) []byte {
+	var rn *big.Int
+	var err error
+	for {
+		rn, err = rand.Int(rand.Reader, big.NewInt(max+1))
+		if err != nil {
+			panic(err)
+		}
+		if rn.Int64() >= min {
+			break
+		}
+	}
+
+	bs := make([]byte, rn.Int64())
+	_, err = rand.Read(bs)
+	if err != nil {
+		panic(err)
+	}
+	return bs
+}