crypto: move crypto buffer helpers

internal/crypto/buffer.go

@@ -0,0 +1,19 @@
+package crypto
+
+// NewBlobBuffer returns a buffer that is large enough to hold a blob of size
+// plaintext bytes, including the crypto overhead.
+func NewBlobBuffer(size int) []byte {
+	return make([]byte, size, size+Extension)
+}
+
+// PlaintextLength returns the plaintext length of a blob with ciphertextSize
+// bytes.
+func PlaintextLength(ciphertextSize int) int {
+	return ciphertextSize - Extension
+}
+
+// CiphertextLength returns the encrypted length of a blob with plaintextSize
+// bytes.
+func CiphertextLength(plaintextSize int) int {
+	return plaintextSize + Extension
+}