From: Loup Vaillant <loup@loup-vaillant.fr>
Date: Thu, 19 Oct 2017 21:27:00 +0000 (+0200)
Subject: Added crypto_wipe() (Erases buffers)
X-Git-Url: https://git.codecow.com/?a=commitdiff_plain;h=c113eb3444eabdb3f3e22f462005bed1803df357;p=Monocypher.git

Added crypto_wipe() (Erases buffers)

I've been convinced that wiping secrets might be useful to mitigate
some side channel attacks where the attacker might read your memory
after you're done processing those secrets.

This probably wasn't the only copy, though.
---

diff --git a/src/monocypher.c b/src/monocypher.c
index 7c00955..d6dccf1 100644
--- a/src/monocypher.c
+++ b/src/monocypher.c
@@ -89,6 +89,14 @@ static int zerocmp32(const u8 p[32])
     return crypto_verify32(p, zero);
 }
 
+void crypto_wipe(void *secret, size_t size)
+{
+    volatile u8 *v_secret = (u8*)secret;
+    FOR (i, 0, size) {
+        v_secret[i] = 0;
+    }
+}
+
 /////////////////
 /// Chacha 20 ///
 /////////////////
@@ -409,6 +417,8 @@ void crypto_poly1305_final(crypto_poly1305_ctx *ctx, u8 mac[16])
     u += (i64)(ctx->h[1]) + ctx->pad[1];  store32_le(mac +  4, u);  u >>= 32;
     u += (i64)(ctx->h[2]) + ctx->pad[2];  store32_le(mac +  8, u);  u >>= 32;
     u += (i64)(ctx->h[3]) + ctx->pad[3];  store32_le(mac + 12, u);
+
+    crypto_wipe(ctx, sizeof(*ctx));
 }
 
 void crypto_poly1305_auth(u8     mac[16],  const u8 *message,
@@ -600,6 +610,7 @@ void crypto_blake2b_final(crypto_blake2b_ctx *ctx, u8 *hash)
     FOR (i, nb_words * 8, ctx->hash_size) {
         hash[i] = (ctx->hash[i / 8] >> (8 * (i % 8))) & 0xff;
     }
+    crypto_wipe(ctx, sizeof(*ctx));
 }
 
 void crypto_blake2b_general(u8       *hash   , size_t hash_size,
@@ -900,6 +911,9 @@ void crypto_argon2i(u8       *hash,      u32 hash_size,
         extended_hash(hash_area, 1024, initial_hash, 72);
         load_block(&tmp_block, hash_area);
         copy_block(blocks + 1, &tmp_block);
+
+        crypto_wipe(initial_hash,   72);
+        crypto_wipe(hash_area   , 1024);
     }
 
     // Actual number of blocks
@@ -937,6 +951,11 @@ void crypto_argon2i(u8       *hash,      u32 hash_size,
     u8 final_block[1024];
     store_block(final_block, blocks + (nb_blocks - 1));
     extended_hash(hash, hash_size, final_block, 1024);
+
+    crypto_wipe(final_block , 1024);
+    // Note: the work area is *not* wiped by default.  It would erase
+    // the final hash if the user wanted to use it to store the final
+    // hash, for instance to conserve stack space.
 }
 
 ////////////////////////////////////
@@ -1537,6 +1556,8 @@ void crypto_sign_final(crypto_sign_ctx *ctx, u8 signature[64])
         signature[i] = half_sig[i];
     }
     modL(signature + 32, s);  // second half of the signature = s
+
+    crypto_wipe(ctx, sizeof(*ctx));
 }
 
 void crypto_sign(u8        signature[64],
@@ -1646,7 +1667,7 @@ void crypto_lock_update(crypto_lock_ctx *ctx, u8 *cipher_text,
 
 void crypto_lock_final(crypto_lock_ctx *ctx, u8 mac[16])
 {
-    crypto_poly1305_final (&(ctx->poly), mac);
+    crypto_poly1305_final(&(ctx->poly), mac);
 }
 
 void crypto_unlock_update(crypto_lock_ctx *ctx, u8 *plain_text,
@@ -1676,6 +1697,7 @@ void crypto_aead_lock(u8        mac[16],
     crypto_lock_auth   (&ctx, ad, ad_size);
     crypto_lock_update (&ctx, cipher_text, plain_text, text_size);
     crypto_lock_final  (&ctx, mac);
+    crypto_wipe(&(ctx.chacha), sizeof(ctx.chacha));
 }
 
 int crypto_aead_unlock(u8       *plain_text,
@@ -1693,6 +1715,7 @@ int crypto_aead_unlock(u8       *plain_text,
         return -1; // reject forgeries before wasting our time decrypting
     }
     crypto_lock_encrypt(&ctx, plain_text, cipher_text, text_size);
+    crypto_wipe(&(ctx.chacha), sizeof(ctx.chacha));
     return 0;
 }
 
diff --git a/src/monocypher.h b/src/monocypher.h
index a8ad38a..de439bf 100644
--- a/src/monocypher.h
+++ b/src/monocypher.h
@@ -10,6 +10,10 @@ int crypto_verify16(const uint8_t a[16], const uint8_t b[16]);
 int crypto_verify32(const uint8_t a[32], const uint8_t b[32]);
 int crypto_verify64(const uint8_t a[64], const uint8_t b[64]);
 
+// Erase a buffer from existence.
+// It probably wasn't the only copy...
+void crypto_wipe(void *secret, size_t size);
+
 ////////////////
 /// Chacha20 ///
 ////////////////
@@ -134,6 +138,8 @@ typedef crypto_sha512_ctx crypto_hash_ctx;
 typedef crypto_blake2b_ctx crypto_hash_ctx;
 #endif
 
+// EdDSA signatures context.  Do not rely on its contents or its size,
+// they may change without notice.
 typedef struct {
     crypto_hash_ctx hash;
     uint8_t buf[96];
@@ -158,6 +164,8 @@ void crypto_sign(uint8_t        signature [64],
 
 int crypto_check_public_key(const uint8_t public_key[32]);
 
+// EdDSA check context.  Do not rely on its contents or its size, they
+// may change without notice.
 typedef struct {
     crypto_hash_ctx hash;
     uint8_t sig[64];
@@ -187,6 +195,9 @@ int crypto_key_exchange(uint8_t       shared_key      [32],
 ////////////////////////////////
 /// Authenticated encryption ///
 ////////////////////////////////
+
+// Authenticated encryption context.  Do not rely on its contents or
+// its size, they may change without notice.
 typedef struct {
     crypto_chacha_ctx   chacha;
     crypto_poly1305_ctx poly;