diff --git a/cipher-chachapoly.c b/cipher-chachapoly.c
new file mode 100644
index 0000000..20628ab
--- /dev/null
+++ b/cipher-chachapoly.c
@@ -0,0 +1,114 @@
+/*
+ * Copyright (c) 2013 Damien Miller <djm@mindrot.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/* $OpenBSD: cipher-chachapoly.c,v 1.2 2013/11/21 02:50:00 djm Exp $ */
+
+#include "includes.h"
+
+#include <sys/types.h>
+#include <stdarg.h> /* needed for log.h */
+#include <string.h>
+#include <stdio.h>  /* needed for misc.h */
+
+#include "log.h"
+#include "misc.h"
+#include "cipher-chachapoly.h"
+
+void chachapoly_init(struct chachapoly_ctx *ctx,
+    const u_char *key, u_int keylen)
+{
+	if (keylen != (32 + 32)) /* 2 x 256 bit keys */
+		fatal("%s: invalid keylen %u", __func__, keylen);
+	chacha_keysetup(&ctx->main_ctx, key, 256);
+	chacha_keysetup(&ctx->header_ctx, key + 32, 256);
+}
+
+/*
+ * chachapoly_crypt() operates as following:
+ * Copy 'aadlen' bytes (without en/decryption) from 'src' to 'dest'.
+ * Theses bytes are treated as additional authenticated data.
+ * En/Decrypt 'len' bytes at offset 'aadlen' from 'src' to 'dest'.
+ * Use POLY1305_TAGLEN bytes at offset 'len'+'aadlen' as the
+ * authentication tag.
+ * This tag is written on encryption and verified on decryption.
+ * Both 'aadlen' and 'authlen' can be set to 0.
+ */
+int
+chachapoly_crypt(struct chachapoly_ctx *ctx, u_int seqnr, u_char *dest,
+    const u_char *src, u_int len, u_int aadlen, u_int authlen, int do_encrypt)
+{
+	u_char seqbuf[8];
+	u_char one[8] = { 1, 0, 0, 0, 0, 0, 0, 0 }; /* NB. little-endian */
+	u_char expected_tag[POLY1305_TAGLEN], poly_key[POLY1305_KEYLEN];
+	int r = -1;
+
+	/*
+	 * Run ChaCha20 once to generate the Poly1305 key. The IV is the
+	 * packet sequence number.
+	 */
+	bzero(poly_key, sizeof(poly_key));
+	put_u64(seqbuf, seqnr);
+	chacha_ivsetup(&ctx->main_ctx, seqbuf, NULL);
+	chacha_encrypt_bytes(&ctx->main_ctx,
+	    poly_key, poly_key, sizeof(poly_key));
+	/* Set Chacha's block counter to 1 */
+	chacha_ivsetup(&ctx->main_ctx, seqbuf, one);
+
+	/* If decrypting, check tag before anything else */
+	if (!do_encrypt) {
+		const u_char *tag = src + aadlen + len;
+
+		poly1305_auth(expected_tag, src, aadlen + len, poly_key);
+		if (timingsafe_bcmp(expected_tag, tag, POLY1305_TAGLEN) != 0)
+			goto out;
+	}
+	/* Crypt additional data */
+        if (aadlen) {
+		chacha_ivsetup(&ctx->header_ctx, seqbuf, NULL);
+		chacha_encrypt_bytes(&ctx->header_ctx, src, dest, aadlen);
+	}
+	chacha_encrypt_bytes(&ctx->main_ctx, src + aadlen,
+	    dest + aadlen, len);
+
+	/* If encrypting, calculate and append tag */
+	if (do_encrypt) {
+		poly1305_auth(dest + aadlen + len, dest, aadlen + len,
+		    poly_key);
+	}
+	r = 0;
+
+ out:
+	bzero(expected_tag, sizeof(expected_tag));
+	bzero(seqbuf, sizeof(seqbuf));
+	bzero(poly_key, sizeof(poly_key));
+	return r;
+}
+
+int
+chachapoly_get_length(struct chachapoly_ctx *ctx,
+    u_int *plenp, u_int seqnr, const u_char *cp, u_int len)
+{
+	u_char buf[4], seqbuf[8];
+
+	if (len < 4)
+		return -1; /* Insufficient length */
+	put_u64(seqbuf, seqnr);
+	chacha_ivsetup(&ctx->header_ctx, seqbuf, NULL);
+	chacha_encrypt_bytes(&ctx->header_ctx, cp, buf, 4);
+	*plenp = get_u32(buf);
+	return 0;
+}
+