Make sure public key bit length is an even number

For an odd n-bit number, we can't find two n-1-bit numbers which
multiplied gives an n-bit number. That's why public key length n must be
an even number.

I have also renamed AreSafePrimes to IsSafePrime since only the first
function argument is considered to be a safe prime according to a safe
prime definition.

safe_prime_generator_test.go

@@ -60,7 +60,7 @@ func TestAsyncGenerator(t *testing.T) {
 					t.Fatal(err)
 				}
 
-				AreSafePrimes(p, q, test.bitLen, t)
+				IsSafePrime(p, q, test.bitLen, t)
 			}
 		})
 	}

thresholdkey_generator.go

@@ -49,7 +49,8 @@ type ThresholdKeyGenerator struct {
 // GetThresholdKeyGenerator is a preferable way to construct the
 // ThresholdKeyGenerator.
 // Due to the various properties that must be met for the threshold key to be
-// considered valid, the minimum public key `N` bit length is 18 bits.
+// considered valid, the minimum public key `N` bit length is 18 bits and the
+// public key bit length should be an even number.
 // The plaintext space for the key will be `Z_N`.
 func GetThresholdKeyGenerator(
 	publicKeyBitLength int,
@@ -57,7 +58,14 @@ func GetThresholdKeyGenerator(
 	threshold int,
 	random io.Reader,
 ) (*ThresholdKeyGenerator, error) {
+	if publicKeyBitLength%2 == 1 {
+		// For an odd n-bit number, we can't find two n-1-bit numbers which
+		// multiplied gives an n-bit number.
+		return nil, errors.New("Public key bit length must be an even number")
+	}
 	if publicKeyBitLength < 18 {
+		// We need to find two n-bit safe primes, P and Q which are not equal.
+		// This is not possible for n<18.
 		return nil, errors.New("Public key bit length must be at least 18 bits")
 	}
 

thresholdkey_generator_test.go

@@ -20,11 +20,17 @@ func TestCreateThresholdKeyGenerator(t *testing.T) {
 		"generator successfully created for 20 bit key length": {
 			keyLength: 20,
 		},
-
+		"generator can't be created for 19 bit key length": {
+			keyLength:     19,
+			expectedError: errors.New("Public key bit length must be an even number"),
+		},
 		"generator successfully created for 18 bit key length": {
 			keyLength: 18,
 		},
-
+		"generator can't be created for 17 bit key length": {
+			keyLength:     17,
+			expectedError: errors.New("Public key bit length must be an even number"),
+		},
 		"generator can't be created for 16 bit key length": {
 			keyLength:     16,
 			expectedError: errors.New("Public key bit length must be at least 18 bits"),
@@ -60,7 +66,7 @@ func TestGenerateSafePrimesOfThresholdKeyGenerator(t *testing.T) {
 	if err != nil {
 		t.Error(err)
 	}
-	AreSafePrimes(p, q, 16, t)
+	IsSafePrime(p, q, 16, t)
 }
 
 func TestInitPandP1(t *testing.T) {
@@ -70,7 +76,7 @@ func TestInitPandP1(t *testing.T) {
 	}
 
 	tkh.initPandP1()
-	AreSafePrimes(tkh.p, tkh.p1, 16, t)
+	IsSafePrime(tkh.p, tkh.p1, 16, t)
 }
 
 func TestInitQandQ1(t *testing.T) {
@@ -80,7 +86,7 @@ func TestInitQandQ1(t *testing.T) {
 	}
 
 	tkh.initQandQ1()
-	AreSafePrimes(tkh.q, tkh.q1, 16, t)
+	IsSafePrime(tkh.q, tkh.q1, 16, t)
 }
 
 func TestInitPsAndQs(t *testing.T) {
@@ -91,8 +97,8 @@ func TestInitPsAndQs(t *testing.T) {
 
 	tkh.initPsAndQs()
 
-	AreSafePrimes(tkh.p, tkh.p1, 16, t)
-	AreSafePrimes(tkh.q, tkh.q1, 16, t)
+	IsSafePrime(tkh.p, tkh.p1, 16, t)
+	IsSafePrime(tkh.q, tkh.q1, 16, t)
 }
 
 func TestArePsAndQsGood(t *testing.T) {

utils_test.go

@@ -59,7 +59,9 @@ func TestFactorial(t *testing.T) {
 	}
 }
 
-func AreSafePrimes(p, q *big.Int, expectedLength int, t *testing.T) {
+// IsSafePrime checks whether `p` is a safe prime. A safe prime is a prime
+// number of the form `2q + 1`, where `q` is also a prime.
+func IsSafePrime(p, q *big.Int, expectedLength int, t *testing.T) {
 	if l := p.BitLen(); l != expectedLength {
 		t.Error("p does not have the good length. ", l)
 	}