From edf695a35ea537189a9d6ebace32d71feec3b73d Mon Sep 17 00:00:00 2001
From: Neale Pickett <neale@woozle.org>
Date: Tue, 21 Sep 2010 21:59:49 -0600
Subject: [PATCH] Make arc4 more streamy

---
 src/arc4.c | 27 ++++++++++++++++-----------
 src/arc4.h |  7 ++-----
 2 files changed, 18 insertions(+), 16 deletions(-)

diff --git a/src/arc4.c b/src/arc4.c
index 2a2ec84..641d46d 100644
--- a/src/arc4.c
+++ b/src/arc4.c
@@ -4,6 +4,12 @@
 
 #define swap(a, b) do {int _swap=a; a=b, b=_swap;} while (0)
 
+struct arc4_ctx {
+  uint8_t S[256];
+  uint8_t i;
+  uint8_t j;
+};
+
 void
 arc4_init(struct arc4_ctx *ctx, uint8_t const *key, size_t keylen)
 {
@@ -22,25 +28,24 @@ arc4_init(struct arc4_ctx *ctx, uint8_t const *key, size_t keylen)
   ctx->j = 0;
 }
 
+uint8_t
+arc4_pad(struct arc4_ctx *ctx)
+{
+  ctx->i = (ctx->i + 1) % 256;
+  ctx->j = (ctx->j + ctx->S[ctx->i]) % 256;
+  swap(ctx->S[ctx->i], ctx->S[ctx->j]);
+  return ctx->S[(ctx->S[ctx->i] + ctx->S[ctx->j]) % 256];
+}
+
 void
 arc4_crypt(struct arc4_ctx *ctx,
            uint8_t *obuf, uint8_t const *ibuf, size_t buflen)
 {
-  int    i = ctx->i;
-  int    j = ctx->j;
   size_t k;
 
   for (k = 0; k < buflen; k += 1) {
-    uint8_t mask;
-
-    i = (i + 1) % 256;
-    j = (j + ctx->S[i]) % 256;
-    swap(ctx->S[i], ctx->S[j]);
-    mask = ctx->S[(ctx->S[i] + ctx->S[j]) % 256];
-    obuf[k] = ibuf[k] ^ mask;
+    obuf[k] = ibuf[k] ^ arc4_pad(ctx);
   }
-  ctx->i = i;
-  ctx->j = j;
 }
 
 void
diff --git a/src/arc4.h b/src/arc4.h
index b8f41fb..5ae3bd7 100644
--- a/src/arc4.h
+++ b/src/arc4.h
@@ -4,13 +4,10 @@
 #include <stdint.h>
 #include <stdlib.h>
 
-struct arc4_ctx {
-  uint8_t S[256];
-  uint8_t i;
-  uint8_t j;
-};
+struct arc4_ctx;
 
 void arc4_init(struct arc4_ctx *ctx, uint8_t const *key, size_t keylen);
+uint8_t arc4_pad(struct arc4_ctx *ctx);
 void arc4_crypt(struct arc4_ctx *ctx,
                 uint8_t *obuf, uint8_t const *ibuf, size_t buflen);
 void arc4_crypt_buffer(uint8_t const *key, size_t keylen,