From e8ef3d40dd1bacd23cc5ba083e2c05aad7156db0 Mon Sep 17 00:00:00 2001
From: Jonathan Rudenberg <jonathan@titanous.com>
Date: Thu, 3 Jul 2014 21:54:44 -0700
Subject: [PATCH] Remove an alloc by pre-allocating the dest slice

---
 box/box.go | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/box/box.go b/box/box.go
index 6b09bf1..04e3938 100644
--- a/box/box.go
+++ b/box/box.go
@@ -90,6 +90,13 @@ func (c *Crypter) Encrypt(dst []byte, ephKey *Key, plaintext []byte, padLen int)
 		}
 		ephKey = &k
 	}
+	dstPrefixLen := len(dst)
+	// Allocate a new slice that can fit the full encrypted box if the current dst doesn't fit
+	if encLen := c.EncryptedLen(len(plaintext) + padLen); cap(dst)-len(dst) < encLen {
+		newDst := make([]byte, len(dst), len(dst)+encLen)
+		copy(newDst, dst)
+		dst = newDst
+	}
 
 	dh1 := c.Cipher.DH(ephKey.Private, c.ReceiverKey.Public)
 	dh2 := c.Cipher.DH(c.SenderKey.Private, c.ReceiverKey.Public)
@@ -100,8 +107,13 @@ func (c *Crypter) Encrypt(dst []byte, ephKey *Key, plaintext []byte, padLen int)
 	cc1 := c.Cipher.NewCipher(cv1)
 	cc2 := c.Cipher.NewCipher(c.ChainVar)
 
-	header := cc1.Encrypt(ephKey.Public, ephKey.Public, c.SenderKey.Public)
-	return noiseBody(cc2, header, padLen, plaintext, header), nil
+	dst = append(dst, ephKey.Public...)
+	dst = cc1.Encrypt(dst, ephKey.Public, c.SenderKey.Public)
+	return noiseBody(cc2, dst, padLen, plaintext, dst[dstPrefixLen:]), nil
+}
+
+func (c *Crypter) EncryptedLen(n int) int {
+	return n + (2 * c.Cipher.DHLen()) + (2 * c.Cipher.MACLen()) + 4
 }
 
 func (c *Crypter) Decrypt(ciphertext []byte) ([]byte, error) {