Check the parent DRBG's strength

We currently don't support the algorithm from NIST SP 800-90C
10.1.2 to use a weaker DRBG as source
Reviewed-by: NDr. Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
GH: #5506

crypto/err/openssl.txt

@@ -907,6 +907,7 @@ RAND_F_GET_ENTROPY:106:get_entropy
 RAND_F_RAND_BYTES:100:RAND_bytes
 RAND_F_RAND_DRBG_ENABLE_LOCKING:119:rand_drbg_enable_locking
 RAND_F_RAND_DRBG_GENERATE:107:RAND_DRBG_generate
+RAND_F_RAND_DRBG_GET_ENTROPY:120:rand_drbg_get_entropy
 RAND_F_RAND_DRBG_INSTANTIATE:108:RAND_DRBG_instantiate
 RAND_F_RAND_DRBG_NEW:109:RAND_DRBG_new
 RAND_F_RAND_DRBG_RESEED:110:RAND_DRBG_reseed
@@ -2300,6 +2301,7 @@ RAND_R_NOT_A_REGULAR_FILE:122:Not a regular file
 RAND_R_NOT_INSTANTIATED:115:not instantiated
 RAND_R_NO_DRBG_IMPLEMENTATION_SELECTED:128:no drbg implementation selected
 RAND_R_PARENT_LOCKING_NOT_ENABLED:130:parent locking not enabled
+RAND_R_PARENT_STRENGTH_TOO_WEAK:131:parent strength too weak
 RAND_R_PERSONALISATION_STRING_TOO_LONG:116:personalisation string too long
 RAND_R_PRNG_NOT_SEEDED:100:PRNG not seeded
 RAND_R_RANDOM_POOL_OVERFLOW:125:random pool overflow

crypto/rand/drbg_lib.c

@@ -178,6 +178,15 @@ static RAND_DRBG *rand_drbg_new(int secure,
     if (RAND_DRBG_set(drbg, type, flags) == 0)
         goto err;
 
+    if (parent != NULL && drbg->strength > parent->strength) {
+        /*
+         * We currently don't support the algorithm from NIST SP 800-90C
+         * 10.1.2 to use a weaker DRBG as source
+         */
+        RANDerr(RAND_F_RAND_DRBG_NEW, RAND_R_PARENT_STRENGTH_TOO_WEAK);
+        goto err;
+    }
+
     if (!RAND_DRBG_set_callbacks(drbg, rand_drbg_get_entropy,
                                  rand_drbg_cleanup_entropy,
                                  NULL, NULL))

crypto/rand/rand_err.c

@@ -23,6 +23,8 @@ static const ERR_STRING_DATA RAND_str_functs[] = {
      "rand_drbg_enable_locking"},
     {ERR_PACK(ERR_LIB_RAND, RAND_F_RAND_DRBG_GENERATE, 0),
      "RAND_DRBG_generate"},
+    {ERR_PACK(ERR_LIB_RAND, RAND_F_RAND_DRBG_GET_ENTROPY, 0),
+     "rand_drbg_get_entropy"},
     {ERR_PACK(ERR_LIB_RAND, RAND_F_RAND_DRBG_INSTANTIATE, 0),
      "RAND_DRBG_instantiate"},
     {ERR_PACK(ERR_LIB_RAND, RAND_F_RAND_DRBG_NEW, 0), "RAND_DRBG_new"},
@@ -86,6 +88,8 @@ static const ERR_STRING_DATA RAND_str_reasons[] = {
     "no drbg implementation selected"},
     {ERR_PACK(ERR_LIB_RAND, 0, RAND_R_PARENT_LOCKING_NOT_ENABLED),
     "parent locking not enabled"},
+    {ERR_PACK(ERR_LIB_RAND, 0, RAND_R_PARENT_STRENGTH_TOO_WEAK),
+    "parent strength too weak"},
     {ERR_PACK(ERR_LIB_RAND, 0, RAND_R_PERSONALISATION_STRING_TOO_LONG),
     "personalisation string too long"},
     {ERR_PACK(ERR_LIB_RAND, 0, RAND_R_PRNG_NOT_SEEDED), "PRNG not seeded"},

crypto/rand/rand_lib.c

@@ -176,8 +176,18 @@ size_t rand_drbg_get_entropy(RAND_DRBG *drbg,
 {
     size_t ret = 0;
     size_t entropy_available = 0;
-    RAND_POOL *pool = RAND_POOL_new(entropy, min_len, max_len);
+    RAND_POOL *pool;
+
+    if (drbg->parent && drbg->strength > drbg->parent->strength) {
+        /*
+         * We currently don't support the algorithm from NIST SP 800-90C
+         * 10.1.2 to use a weaker DRBG as source
+         */
+        RANDerr(RAND_F_RAND_DRBG_GET_ENTROPY, RAND_R_PARENT_STRENGTH_TOO_WEAK);
+        return 0;
+    }
 
+    pool = RAND_POOL_new(entropy, min_len, max_len);
     if (pool == NULL)
         return 0;
 

include/openssl/randerr.h

@@ -26,6 +26,7 @@ int ERR_load_RAND_strings(void);
 # define RAND_F_RAND_BYTES                                100
 # define RAND_F_RAND_DRBG_ENABLE_LOCKING                  119
 # define RAND_F_RAND_DRBG_GENERATE                        107
+# define RAND_F_RAND_DRBG_GET_ENTROPY                     120
 # define RAND_F_RAND_DRBG_INSTANTIATE                     108
 # define RAND_F_RAND_DRBG_NEW                             109
 # define RAND_F_RAND_DRBG_RESEED                          110
@@ -67,6 +68,7 @@ int ERR_load_RAND_strings(void);
 # define RAND_R_NOT_INSTANTIATED                          115
 # define RAND_R_NO_DRBG_IMPLEMENTATION_SELECTED           128
 # define RAND_R_PARENT_LOCKING_NOT_ENABLED                130
+# define RAND_R_PARENT_STRENGTH_TOO_WEAK                  131
 # define RAND_R_PERSONALISATION_STRING_TOO_LONG           116
 # define RAND_R_PRNG_NOT_SEEDED                           100
 # define RAND_R_RANDOM_POOL_OVERFLOW                      125