Accept PubKey bit length when constructing TKey generator

Instead of passing safe prime bit length which is a kind of an
implementation detail, we should pass a public key bit length. Public
key is a Paillier modulus defining plaintext space. This information is
more important than the safe prime value/bit length used under the hood.

thresholdkey_generator.go

@@ -19,7 +19,7 @@ import (
 //               with Applications to Electronic Voting
 //               Aarhus University, Dept. of Computer Science, BRICS
 type ThresholdKeyGenerator struct {
-	nbits                          int
+	publicKeyBitLength             int
 	TotalNumberOfDecryptionServers int
 	Threshold                      int
 	Random                         io.Reader
@@ -49,11 +49,16 @@ type ThresholdKeyGenerator struct {
 // is done on the input values.  You need to be sure that nbits is big enough
 // and that Threshold > TotalNumberOfDecryptionServers / 2.
 // The plaintext space for the key will be Z_n.
-func GetThresholdKeyGenerator(nbits, TotalNumberOfDecryptionServers, Threshold int, random io.Reader) *ThresholdKeyGenerator {
+func GetThresholdKeyGenerator(
+	publicKeyBitLength int,
+	totalNumberOfDecryptionServers int,
+	threshold int,
+	random io.Reader,
+) *ThresholdKeyGenerator {
 	ret := new(ThresholdKeyGenerator)
-	ret.nbits = nbits
-	ret.TotalNumberOfDecryptionServers = TotalNumberOfDecryptionServers
-	ret.Threshold = Threshold
+	ret.publicKeyBitLength = publicKeyBitLength // TODO: at least 32
+	ret.TotalNumberOfDecryptionServers = totalNumberOfDecryptionServers
+	ret.Threshold = threshold
 	ret.Random = random
 	return ret
 }
@@ -61,7 +66,9 @@ func GetThresholdKeyGenerator(nbits, TotalNumberOfDecryptionServers, Threshold i
 func (tkg *ThresholdKeyGenerator) generateSafePrimes() (*big.Int, *big.Int, error) {
 	concurrencyLevel := 4
 	timeout := 120 * time.Second
-	return GenerateSafePrime(tkg.nbits, concurrencyLevel, timeout, tkg.Random)
+	safePrimeBitLength := tkg.publicKeyBitLength / 2
+
+	return GenerateSafePrime(safePrimeBitLength, concurrencyLevel, timeout, tkg.Random)
 }
 
 func (tkg *ThresholdKeyGenerator) initPandP1() error {

thresholdkey_generator_test.go

@@ -13,42 +13,42 @@ var MockGenerateSafePrimes = func() (*big.Int, *big.Int, error) {
 
 func TestGenerateSafePrimesOfThresholdKeyGenerator(t *testing.T) {
 	tkh := new(ThresholdKeyGenerator)
-	tkh.nbits = 10
+	tkh.publicKeyBitLength = 32
 	tkh.Random = rand.Reader
 	p, q, err := tkh.generateSafePrimes()
 	if err != nil {
 		t.Error(err)
 	}
-	AreSafePrimes(p, q, 10, t)
+	AreSafePrimes(p, q, 16, t)
 }
 
 func TestInitPandP1(t *testing.T) {
 	tkh := new(ThresholdKeyGenerator)
-	tkh.nbits = 10
+	tkh.publicKeyBitLength = 32
 	tkh.Random = rand.Reader
 
 	tkh.initPandP1()
-	AreSafePrimes(tkh.p, tkh.p1, 10, t)
+	AreSafePrimes(tkh.p, tkh.p1, 16, t)
 }
 
 func TestInitQandQ1(t *testing.T) {
 	tkh := new(ThresholdKeyGenerator)
-	tkh.nbits = 10
+	tkh.publicKeyBitLength = 32
 	tkh.Random = rand.Reader
 
 	tkh.initQandQ1()
-	AreSafePrimes(tkh.q, tkh.q1, 10, t)
+	AreSafePrimes(tkh.q, tkh.q1, 16, t)
 }
 
 func TestInitPsAndQs(t *testing.T) {
 	tkh := new(ThresholdKeyGenerator)
-	tkh.nbits = 10
+	tkh.publicKeyBitLength = 32
 	tkh.Random = rand.Reader
 
 	tkh.initPsAndQs()
 
-	AreSafePrimes(tkh.p, tkh.p1, 10, t)
-	AreSafePrimes(tkh.q, tkh.q1, 10, t)
+	AreSafePrimes(tkh.p, tkh.p1, 16, t)
+	AreSafePrimes(tkh.q, tkh.q1, 16, t)
 }
 
 func TestArePsAndQsGood(t *testing.T) {
@@ -103,7 +103,7 @@ func TestInitD(t *testing.T) {
 
 func TestInitNumerialValues(t *testing.T) {
 	tkh := new(ThresholdKeyGenerator)
-	tkh.nbits = 10
+	tkh.publicKeyBitLength = 32
 	tkh.Random = rand.Reader
 
 	if err := tkh.initNumerialValues(); err != nil {
@@ -113,7 +113,7 @@ func TestInitNumerialValues(t *testing.T) {
 
 func TestGenerateHidingPolynomial(t *testing.T) {
 	tkh := new(ThresholdKeyGenerator)
-	tkh.nbits = 10
+	tkh.publicKeyBitLength = 32
 	tkh.Threshold = 10
 	tkh.Random = rand.Reader
 	if err := tkh.initNumerialValues(); err != nil {
@@ -138,7 +138,7 @@ func TestGenerateHidingPolynomial(t *testing.T) {
 
 func TestComputeShare(t *testing.T) {
 	tkh := new(ThresholdKeyGenerator)
-	tkh.nbits = 10
+	tkh.publicKeyBitLength = 32
 	tkh.Threshold = 3
 	tkh.TotalNumberOfDecryptionServers = 5
 	tkh.nm = b(103)
@@ -151,7 +151,7 @@ func TestComputeShare(t *testing.T) {
 
 func TestCreateShares(t *testing.T) {
 	tkh := new(ThresholdKeyGenerator)
-	tkh.nbits = 10
+	tkh.publicKeyBitLength = 32
 	tkh.Threshold = 10
 	tkh.TotalNumberOfDecryptionServers = 100
 	tkh.Random = rand.Reader

thresholdkey_test.go

@@ -184,7 +184,7 @@ func TestupdateCprime(t *testing.T) {
 }
 
 func TestEncryptingDecryptingSimple(t *testing.T) {
-	tkh := GetThresholdKeyGenerator(10, 2, 1, rand.Reader)
+	tkh := GetThresholdKeyGenerator(32, 2, 1, rand.Reader)
 	tpks, err := tkh.Generate()
 	if err != nil {
 		t.Error(err)
@@ -205,7 +205,7 @@ func TestEncryptingDecryptingSimple(t *testing.T) {
 }
 
 func TestEncryptingDecrypting(t *testing.T) {
-	tkh := GetThresholdKeyGenerator(10, 2, 2, rand.Reader)
+	tkh := GetThresholdKeyGenerator(32, 2, 2, rand.Reader)
 	tpks, err := tkh.Generate()
 	if err != nil {
 		t.Error(err)
@@ -227,7 +227,7 @@ func TestEncryptingDecrypting(t *testing.T) {
 }
 
 func TestHomomorphicThresholdEncryption(t *testing.T) {
-	tkh := GetThresholdKeyGenerator(10, 2, 2, rand.Reader)
+	tkh := GetThresholdKeyGenerator(32, 2, 2, rand.Reader)
 	tpks, _ := tkh.Generate()
 
 	plainText1 := b(13)
@@ -278,7 +278,7 @@ func TestValidate(t *testing.T) {
 }
 
 func TestCombinePartialDecryptionsZKP(t *testing.T) {
-	tkh := GetThresholdKeyGenerator(10, 2, 2, rand.Reader)
+	tkh := GetThresholdKeyGenerator(32, 2, 2, rand.Reader)
 	tpks, err := tkh.Generate()
 	if err != nil {
 		t.Error(err)
@@ -311,7 +311,7 @@ func TestCombinePartialDecryptionsZKP(t *testing.T) {
 }
 
 func TestCombinePartialDecryptionsWith100Shares(t *testing.T) {
-	tkh := GetThresholdKeyGenerator(10, 100, 50, rand.Reader)
+	tkh := GetThresholdKeyGenerator(32, 100, 50, rand.Reader)
 	tpks, err := tkh.Generate()
 	if err != nil {
 		t.Error(err)
@@ -338,7 +338,7 @@ func TestCombinePartialDecryptionsWith100Shares(t *testing.T) {
 }
 
 func TestVerifyDecryption(t *testing.T) {
-	tkh := GetThresholdKeyGenerator(10, 2, 2, rand.Reader)
+	tkh := GetThresholdKeyGenerator(32, 2, 2, rand.Reader)
 	tpks, err := tkh.Generate()
 
 	pk := &tpks[0].ThresholdPublicKey