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: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10747)

diff --git a/crypto/hmac/hmac.c b/crypto/hmac/hmac.c
index 6d4d24d77b44e92dc99295c9aa848ac4996bd178..a94550a37a96ced3c5266572ac1f94e5c67e70d5 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 788721ef1fb23cf1d4fd623d4a6f5a651af5c6c0..d6fd4f39e06617491f6250fbdecceae919f9a227 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 893e6169b4b0b92feeb23baf835cf75e80ca8f50..a4a9c849b9145d675c766d4f2fb4ee0288bf9d8c 100644 (file)
--- a/test/hmactest.c
+++ b/test/hmactest.c
@@ -169,6 +169,27 @@ static int test_hmac_run(void)
     if (!TEST_str_eq(p, 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, 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, test[6].digest))
+        goto err;
+
     ret = 1;
 err:
     HMAC_CTX_free(ctx);