Improve the comment explaining why the previous change matches OpenSSL's

behavior.

ok jsing

1 files changed, 15 insertions, 8 deletions

diff --git a/src/lib/libcrypto/evp/e_chacha.c b/src/lib/libcrypto/evp/e_chacha.c
index bc496241e6..198eaef09f 100644
--- a/src/lib/libcrypto/evp/e_chacha.c
+++ b/src/lib/libcrypto/evp/e_chacha.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: e_chacha.c,v 1.6 2020/01/26 02:39:58 tb Exp $ */
+/* $OpenBSD: e_chacha.c,v 1.7 2020/01/26 07:34:05 tb Exp $ */
 /*
  * Copyright (c) 2014 Joel Sing <jsing@openbsd.org>
  *
@@ -34,7 +34,15 @@ static const EVP_CIPHER chacha20_cipher = {
         .nid = NID_chacha20,
         .block_size = 1,
         .key_len = 32,
-        .iv_len = 16, /* OpenSSL has 8 byte counter followed by 8 byte iv */
+        /* 
+         * The 128 bit EVP IV is split for ChaCha into four 32 bit pieces:
+         *                      counter[0]      counter[1]      iv[0]   iv[1]
+         * OpenSSL exposes these as;
+         *      openssl_iv =    counter[0]      iv[0]           iv[1]   iv[2]
+         * Due to the cipher internal state's symmetry, these are functionally
+         * equivalent.
+         */
+        .iv_len = 16,
         .flags = EVP_CIPH_STREAM_CIPHER | EVP_CIPH_ALWAYS_CALL_INIT |
             EVP_CIPH_CUSTOM_IV,
         .init = chacha_init,
@@ -50,17 +58,16 @@ EVP_chacha20(void)
 
 static int
 chacha_init(EVP_CIPHER_CTX *ctx, const unsigned char *key,
-    const unsigned char *iv, int enc)
+    const unsigned char *openssl_iv, int enc)
 {
         if (key != NULL)
                 ChaCha_set_key((ChaCha_ctx *)ctx->cipher_data, key,
                     EVP_CIPHER_CTX_key_length(ctx) * 8);
-        if (iv != NULL) {
-                const unsigned char *openssl_iv = iv + 8;
-                const unsigned char *counter = iv;
+        if (openssl_iv != NULL) {
+                const unsigned char *iv = openssl_iv + 8;
+                const unsigned char *counter = openssl_iv;
 
-                ChaCha_set_iv((ChaCha_ctx *)ctx->cipher_data, openssl_iv,
-                    counter);
+                ChaCha_set_iv((ChaCha_ctx *)ctx->cipher_data, iv, counter);
         }
         return 1;
 }