diff --git a/gtests/ssl_gtest/tls_subcerts_unittest.cc b/gtests/ssl_gtest/tls_subcerts_unittest.cc
--- a/gtests/ssl_gtest/tls_subcerts_unittest.cc
+++ b/gtests/ssl_gtest/tls_subcerts_unittest.cc
@@ -371,16 +371,21 @@ static void GenerateWeakRsaKey(ScopedSEC
 // Fail to connect with a weak RSA key.
 TEST_P(TlsConnectTls13, DCWeakKey) {
   Reset(kPssDelegatorId);
   EnsureTlsSetup();
   static const SSLSignatureScheme kSchemes[] = {ssl_sig_rsa_pss_rsae_sha256,
                                                 ssl_sig_rsa_pss_pss_sha256};
   client_->SetSignatureSchemes(kSchemes, PR_ARRAY_SIZE(kSchemes));
   server_->SetSignatureSchemes(kSchemes, PR_ARRAY_SIZE(kSchemes));
+  PRInt32 keySizeFlags;
+  ASSERT_EQ(SECSuccess, NSS_OptionGet(NSS_KEY_SIZE_POLICY_FLAGS, &keySizeFlags));
+  // turn off the signing key sizes so we actually test the ssl tests
+  ASSERT_EQ(SECSuccess,
+            NSS_OptionSet(NSS_KEY_SIZE_POLICY_FLAGS, NSS_KEY_SIZE_POLICY_SSL_FLAG ));
 #if RSA_MIN_MODULUS_BITS > RSA_WEAK_KEY
   // save the MIN POLICY length.
   PRInt32 minRsa;
 
   ASSERT_EQ(SECSuccess, NSS_OptionGet(NSS_RSA_MIN_KEY_SIZE, &minRsa));
 #if RSA_MIN_MODULUS_BITS >= 2048
   ASSERT_EQ(SECSuccess,
             NSS_OptionSet(NSS_RSA_MIN_KEY_SIZE, RSA_MIN_MODULUS_BITS + 1024));
@@ -408,16 +413,17 @@ TEST_P(TlsConnectTls13, DCWeakKey) {
   client_->EnableDelegatedCredentials();
 
   auto cfilter = MakeTlsFilter<TlsExtensionCapture>(
       client_, ssl_delegated_credentials_xtn);
   ConnectExpectAlert(client_, kTlsAlertInsufficientSecurity);
 #if RSA_MIN_MODULUS_BITS > RSA_WEAK_KEY
   ASSERT_EQ(SECSuccess, NSS_OptionSet(NSS_RSA_MIN_KEY_SIZE, minRsa));
 #endif
+  ASSERT_EQ(SECSuccess, NSS_OptionSet(NSS_KEY_SIZE_POLICY_FLAGS, keySizeFlags));
 }
 
 class ReplaceDCSigScheme : public TlsHandshakeFilter {
  public:
   ReplaceDCSigScheme(const std::shared_ptr<TlsAgent>& a)
       : TlsHandshakeFilter(a, {ssl_hs_certificate_verify}) {}
 
  protected:
diff --git a/lib/cryptohi/seckey.c b/lib/cryptohi/seckey.c
--- a/lib/cryptohi/seckey.c
+++ b/lib/cryptohi/seckey.c
@@ -1134,22 +1134,31 @@ SECKEY_PrivateKeyStrengthInBits(const SE
         return 0;
     }
 
     /* interpret modulus length as key strength */
     switch (privk->keyType) {
         case rsaKey:
         case rsaPssKey:
         case rsaOaepKey:
-            /* some tokens don't export CKA_MODULUS on the private key,
-             * PK11_SignatureLen works around this if necessary */
-            bitSize = PK11_SignatureLen((SECKEYPrivateKey *)privk) * PR_BITS_PER_BYTE;
-            if (bitSize == -1) {
-                bitSize = 0;
+            rv = PK11_ReadAttribute(privk->pkcs11Slot, privk->pkcs11ID,
+                                    CKA_MODULUS, NULL, &params);
+            if ((rv != SECSuccess) || (params.data == NULL)) {
+                /* some tokens don't export CKA_MODULUS on the private key,
+                 * PK11_SignatureLen works around this if necessary. This
+                 * method is less percise because it returns bytes instead
+                 * bits, so we only do it if we can't get the modulus */
+                bitSize = PK11_SignatureLen((SECKEYPrivateKey *)privk) * PR_BITS_PER_BYTE;
+                if (bitSize == -1) {
+                    return 0;
+                }
+                return bitSize;
             }
+            bitSize = SECKEY_BigIntegerBitLength(&params);
+            PORT_Free(params.data);
             return bitSize;
         case dsaKey:
         case fortezzaKey:
         case dhKey:
         case keaKey:
             rv = PK11_ReadAttribute(privk->pkcs11Slot, privk->pkcs11ID,
                                     CKA_PRIME, NULL, &params);
             if ((rv != SECSuccess) || (params.data == NULL)) {
diff --git a/lib/ssl/ssl3con.c b/lib/ssl/ssl3con.c
--- a/lib/ssl/ssl3con.c
+++ b/lib/ssl/ssl3con.c
@@ -1277,27 +1277,39 @@ ssl3_SignHashesWithPrivKey(SSL3Hashes *h
             PORT_SetError(SEC_ERROR_INVALID_KEY);
             goto done;
     }
     PRINT_BUF(60, (NULL, "hash(es) to be signed", hashItem.data, hashItem.len));
 
     if (useRsaPss || hash->hashAlg == ssl_hash_none) {
         CK_MECHANISM_TYPE mech = PK11_MapSignKeyType(key->keyType);
         int signatureLen = PK11_SignatureLen(key);
+        PRInt32 optval;
 
         SECItem *params = NULL;
         CK_RSA_PKCS_PSS_PARAMS pssParams;
         SECItem pssParamsItem = { siBuffer,
                                   (unsigned char *)&pssParams,
                                   sizeof(pssParams) };
 
         if (signatureLen <= 0) {
             PORT_SetError(SEC_ERROR_INVALID_KEY);
             goto done;
         }
+        /* since we are calling PK11_SignWithMechanism directly, we need to check the
+         * key policy ourselves (which is already checked in SGN_Digest */
+        rv = NSS_OptionGet(NSS_KEY_SIZE_POLICY_FLAGS, &optval);
+        if ((rv == SECSuccess) &&
+            ((optval & NSS_KEY_SIZE_POLICY_SIGN_FLAG) == NSS_KEY_SIZE_POLICY_SIGN_FLAG)) {
+            rv = SECKEY_EnforceKeySize(key->keyType, SECKEY_PrivateKeyStrengthInBits(key),
+                                       SEC_ERROR_SIGNATURE_ALGORITHM_DISABLED);
+            if (rv != SECSuccess) {
+                goto done; /* error code already set */
+            }
+        }
 
         buf->len = (unsigned)signatureLen;
         buf->data = (unsigned char *)PORT_Alloc(signatureLen);
         if (!buf->data)
             goto done; /* error code was set. */
 
         if (useRsaPss) {
             pssParams.hashAlg = ssl3_GetHashMechanismByHashType(hash->hashAlg);