Don't store an HMAC key for longer than we need

The HMAC_CTX structure stores the original key in case the ctx is reused
without changing the key.

However, HMAC_Init_ex() checks its parameters such that the only code path
where the stored key is ever used is in the case where HMAC_Init_ex is
called with a NULL key and an explicit md is provided which is the same as
the md that was provided previously. But in that case we can actually reuse
the pre-digested key that we calculated last time, so we can refactor the
code not to use the stored key at all.

With that refactor done it is no longer necessary to store the key in the
ctx at all. This means that long running ctx's will not keep the key in
memory for any longer than required. Note though that the digested key
*is* still kept in memory for the duration of the life of the ctx.

Fixes #10743

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10763)

diff --git a/crypto/hmac/hmac.c b/crypto/hmac/hmac.c
index 51dd91237cf6cf7a88fa288c25b5941359707390..bc49b26ab1b49e17f00fb2553125edea519d064f 100644 (file)
--- a/crypto/hmac/hmac.c
+++ b/crypto/hmac/hmac.c
@@ -18,16 +18,17 @@
 int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int len,
                  const EVP_MD *md, ENGINE *impl)
 {
-    int rv = 0;
-    int i, j, reset = 0;
+    int rv = 0, reset = 0;
+    int i, j;
     unsigned char pad[HMAC_MAX_MD_CBLOCK_SIZE];
+    unsigned int keytmp_length;
+    unsigned char keytmp[HMAC_MAX_MD_CBLOCK_SIZE];
 
     /* If we are changing MD then we must have a key */
     if (md != NULL && md != ctx->md && (key == NULL || len < 0))
         return 0;
 
     if (md != NULL) {
-        reset = 1;
         ctx->md = md;
     } else if (ctx->md) {
         md = ctx->md;
@@ -44,35 +45,34 @@ int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int len,
 
     if (key != NULL) {
         reset = 1;
+
         j = EVP_MD_block_size(md);
-        if (!ossl_assert(j <= (int)sizeof(ctx->key)))
+        if (!ossl_assert(j <= (int)sizeof(keytmp)))
             return 0;
         if (j < len) {
             if (!EVP_DigestInit_ex(ctx->md_ctx, md, impl)
                     || !EVP_DigestUpdate(ctx->md_ctx, key, len)
-                    || !EVP_DigestFinal_ex(ctx->md_ctx, ctx->key,
-                                           &ctx->key_length))
+                    || !EVP_DigestFinal_ex(ctx->md_ctx, keytmp,
+                                           &keytmp_length))
                 return 0;
         } else {
-            if (len < 0 || len > (int)sizeof(ctx->key))
+            if (len < 0 || len > (int)sizeof(keytmp))
                 return 0;
-            memcpy(ctx->key, key, len);
-            ctx->key_length = len;
+            memcpy(keytmp, key, len);
+            keytmp_length = len;
         }
-        if (ctx->key_length != HMAC_MAX_MD_CBLOCK_SIZE)
-            memset(&ctx->key[ctx->key_length], 0,
-                   HMAC_MAX_MD_CBLOCK_SIZE - ctx->key_length);
-    }
+        if (keytmp_length != HMAC_MAX_MD_CBLOCK_SIZE)
+            memset(&keytmp[keytmp_length], 0,
+                   HMAC_MAX_MD_CBLOCK_SIZE - keytmp_length);
 
-    if (reset) {
         for (i = 0; i < HMAC_MAX_MD_CBLOCK_SIZE; i++)
-            pad[i] = 0x36 ^ ctx->key[i];
+            pad[i] = 0x36 ^ keytmp[i];
         if (!EVP_DigestInit_ex(ctx->i_ctx, md, impl)
                 || !EVP_DigestUpdate(ctx->i_ctx, pad, EVP_MD_block_size(md)))
             goto err;
 
         for (i = 0; i < HMAC_MAX_MD_CBLOCK_SIZE; i++)
-            pad[i] = 0x5c ^ ctx->key[i];
+            pad[i] = 0x5c ^ keytmp[i];
         if (!EVP_DigestInit_ex(ctx->o_ctx, md, impl)
                 || !EVP_DigestUpdate(ctx->o_ctx, pad, EVP_MD_block_size(md)))
             goto err;
@@ -81,8 +81,10 @@ int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int len,
         goto err;
     rv = 1;
  err:
-    if (reset)
+    if (reset) {
+        OPENSSL_cleanse(keytmp, sizeof(keytmp));
         OPENSSL_cleanse(pad, sizeof(pad));
+    }
     return rv;
 }
 
@@ -149,8 +151,6 @@ static void hmac_ctx_cleanup(HMAC_CTX *ctx)
     EVP_MD_CTX_reset(ctx->o_ctx);
     EVP_MD_CTX_reset(ctx->md_ctx);
     ctx->md = NULL;
-    ctx->key_length = 0;
-    OPENSSL_cleanse(ctx->key, sizeof(ctx->key));
 }
 
 void HMAC_CTX_free(HMAC_CTX *ctx)
@@ -201,8 +201,6 @@ int HMAC_CTX_copy(HMAC_CTX *dctx, HMAC_CTX *sctx)
         goto err;
     if (!EVP_MD_CTX_copy_ex(dctx->md_ctx, sctx->md_ctx))
         goto err;
-    memcpy(dctx->key, sctx->key, HMAC_MAX_MD_CBLOCK_SIZE);
-    dctx->key_length = sctx->key_length;
     dctx->md = sctx->md;
     return 1;
  err:

diff --git a/crypto/hmac/hmac_local.h b/crypto/hmac/hmac_local.h
index ed855e89a4e2529db9db49526acc844e6a5e53ff..1f40e01a594e682dd5cab3238c0b789674e8e5b2 100644 (file)
--- a/crypto/hmac/hmac_local.h
+++ b/crypto/hmac/hmac_local.h
@@ -18,8 +18,6 @@ struct hmac_ctx_st {
     EVP_MD_CTX *md_ctx;
     EVP_MD_CTX *i_ctx;
     EVP_MD_CTX *o_ctx;
-    unsigned int key_length;
-    unsigned char key[HMAC_MAX_MD_CBLOCK_SIZE];
 };
 
 #endif

diff --git a/test/hmactest.c b/test/hmactest.c
index ca775773a6e6b778a309270c6b206b9e5b56de85..e12c48fd8d40802395721efd7c289972fe4d4982 100644 (file)
--- a/test/hmactest.c
+++ b/test/hmactest.c
@@ -173,6 +173,27 @@ static int test_hmac_run(void)
     if (!TEST_str_eq(p, (char *)test[6].digest))
         goto err;
 
+    /* Test reusing a key */
+    if (!TEST_true(HMAC_Init_ex(ctx, NULL, 0, NULL, NULL))
+        || !TEST_true(HMAC_Update(ctx, test[6].data, test[6].data_len))
+        || !TEST_true(HMAC_Final(ctx, buf, &len)))
+        goto err;
+    p = pt(buf, len);
+    if (!TEST_str_eq(p, (char *)test[6].digest))
+        goto err;
+
+    /*
+     * Test reusing a key where the digest is provided again but is the same as
+     * last time
+     */
+    if (!TEST_true(HMAC_Init_ex(ctx, NULL, 0, EVP_sha256(), NULL))
+        || !TEST_true(HMAC_Update(ctx, test[6].data, test[6].data_len))
+        || !TEST_true(HMAC_Final(ctx, buf, &len)))
+        goto err;
+    p = pt(buf, len);
+    if (!TEST_str_eq(p, (char *)test[6].digest))
+        goto err;
+
     ret = 1;
 err:
     HMAC_CTX_free(ctx);