goodkey: default to 110 rounds of Fermat factorization (#7579)

This change guarantees compliance with CA/BF Ballot SC-073 "Compromised
and Weak Keys", which requires that at least 100 rounds of Fermat
Factorization be attempted:

> Section 6.1.1.3 Subscriber Key Pair Generation
> The CA SHALL reject a certificate request if... The Public Key
corresponds to an industry-demonstrated weak Private Key. For requests
submitted on or after November 15, 2024,... In the case of Close Primes
vulnerability (https://fermatattack.secvuln.info/), the CA SHALL reject
weak keys which can be factored within 100 rounds using Fermat’s
factorization method.

We choose 110 rounds to ensure a margin above and beyond the requirements.

Fixes https://github.com/letsencrypt/boulder/issues/7558

cmd/ceremony/main.go

@@ -34,7 +34,7 @@ var kp goodkey.KeyPolicy
 
 func init() {
 	var err error
-	kp, err = goodkey.NewPolicy(&goodkey.Config{FermatRounds: 100}, nil)
+	kp, err = goodkey.NewPolicy(nil, nil)
 	if err != nil {
 		log.Fatal("Could not create goodkey.KeyPolicy")
 	}

cmd/cert-checker/main_test.go

@@ -59,7 +59,7 @@ func init() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	kp, err = sagoodkey.NewPolicy(&goodkey.Config{FermatRounds: 100}, nil)
+	kp, err = sagoodkey.NewPolicy(nil, nil)
 	if err != nil {
 		log.Fatal(err)
 	}

goodkey/good_key.go

@@ -53,7 +53,7 @@ type Config struct {
 	// FermatRounds is an integer number of rounds of Fermat's factorization
 	// method that should be performed to attempt to detect keys whose modulus can
 	// be trivially factored because the two factors are very close to each other.
-	// If this config value is empty (0), no factorization will be attempted.
+	// If this config value is empty or 0, it will default to 110 rounds.
 	FermatRounds int
 }
 
@@ -122,7 +122,7 @@ type KeyPolicy struct {
 // defaults. If the config's AllowedKeys is nil, the LetsEncryptCPS AllowedKeys
 // is used. If the config's WeakKeyFile or BlockedKeyFile paths are empty, those
 // checks are disabled. If the config's FermatRounds is 0, Fermat Factorization
-// is disabled.
+// defaults to attempting 110 rounds.
 func NewPolicy(config *Config, bkc BlockedKeyCheckFunc) (KeyPolicy, error) {
 	if config == nil {
 		config = &Config{}
@@ -149,10 +149,14 @@ func NewPolicy(config *Config, bkc BlockedKeyCheckFunc) (KeyPolicy, error) {
 		}
 		kp.blockedList = blocked
 	}
-	if config.FermatRounds < 0 {
-		return KeyPolicy{}, fmt.Errorf("Fermat factorization rounds cannot be negative: %d", config.FermatRounds)
-	}
+	if config.FermatRounds == 0 {
+		// The BRs require 100 rounds, so give ourselves a margin above that.
+		kp.fermatRounds = 110
+	} else if config.FermatRounds < 100 {
+		return KeyPolicy{}, fmt.Errorf("Fermat factorization rounds must be at least 100: %d", config.FermatRounds)
+	} else {
 		kp.fermatRounds = config.FermatRounds
+	}
 	return kp, nil
 }
 
@@ -354,13 +358,12 @@ func (policy *KeyPolicy) goodKeyRSA(key *rsa.PublicKey) error {
 	if rocacheck.IsWeak(key) {
 		return badKey("key generated by vulnerable Infineon-based hardware")
 	}
+
 	// Check if the key can be easily factored via Fermat's factorization method.
-	if policy.fermatRounds > 0 {
-		err := checkPrimeFactorsTooClose(modulus, policy.fermatRounds)
+	err = checkPrimeFactorsTooClose(modulus, policy.fermatRounds)
 	if err != nil {
 		return badKey("key generated with factors too close together: %w", err)
 	}
-	}
 
 	return nil
 }

goodkey/good_key_test.go

@@ -300,7 +300,7 @@ func TestDefaultAllowedKeys(t *testing.T) {
 	test.Assert(t, policy.allowedKeys.ECDSAP384, "NIST P384 should be allowed")
 	test.Assert(t, !policy.allowedKeys.ECDSAP521, "NIST P521 should not be allowed")
 
-	policy, err = NewPolicy(&Config{FermatRounds: 100}, nil)
+	policy, err = NewPolicy(&Config{}, nil)
 	test.AssertNotError(t, err, "NewPolicy with nil config.AllowedKeys failed")
 	test.Assert(t, policy.allowedKeys.RSA2048, "RSA 2048 should be allowed")
 	test.Assert(t, policy.allowedKeys.RSA3072, "RSA 3072 should be allowed")

test/config-next/ca.json

@@ -143,8 +143,7 @@
 		"lifespanOCSP": "96h",
 		"goodkey": {
 			"weakKeyFile": "test/example-weak-keys.json",
-			"blockedKeyFile": "test/example-blocked-keys.yaml",
-			"fermatRounds": 100
+			"blockedKeyFile": "test/example-blocked-keys.yaml"
 		},
 		"ocspLogMaxLength": 4000,
 		"ocspLogPeriod": "500ms",

test/config-next/cert-checker.json

@@ -5,9 +5,6 @@
 			"maxOpenConns": 10
 		},
 		"hostnamePolicyFile": "test/hostname-policy.yaml",
-		"goodkey": {
-			"fermatRounds": 100
-		},
 		"workers": 16,
 		"unexpiredOnly": true,
 		"badResultsOnly": true,

test/config-next/ra.json

@@ -8,8 +8,7 @@
 		"pendingAuthorizationLifetimeDays": 7,
 		"goodkey": {
 			"weakKeyFile": "test/example-weak-keys.json",
-			"blockedKeyFile": "test/example-blocked-keys.yaml",
-			"fermatRounds": 100
+			"blockedKeyFile": "test/example-blocked-keys.yaml"
 		},
 		"orderLifetime": "168h",
 		"finalizeTimeout": "30s",