diff --git a/dtlp.c b/dtlp.c
index 616235d..6461e89 100644
--- a/dtlp.c
+++ b/dtlp.c
@@ -27,7 +27,7 @@ main (int argc, char *argv[])
 		if (!strcmp(argv[i], "-h"))
 			usage();
 		else if (!strcmp(argv[i], "-v")) {
-			puts("simple time lock puzzle decrypter v0.2");
+			puts("simple time lock puzzle decrypter v0.3");
 			exit(EX_OK);
 		} else if (!strcmp(argv[i], "-f")) {
 			fp = fopen(argv[++i], "r");
@@ -44,15 +44,15 @@ main (int argc, char *argv[])
 	mpz_init(n);
 	mpz_init(key);
 
-	/* read crypted key from stdin or file */
-	mpz_inp_str(Ck, fp, 16);
-	mpz_inp_str(a, fp, 16);
-	mpz_inp_str(t, fp, 16);
-	mpz_inp_str(n, fp, 16);
+	/* read encrypted key from stdin or file */
+	mpz_inp_str(Ck, fp, BASE16);
+	mpz_inp_str(a, fp, BASE16);
+	mpz_inp_str(t, fp, BASE16);
+	mpz_inp_str(n, fp, BASE16);
 	if (fp)
 		fclose(fp);
 
-	/* resolv time lock puzzle */
+	/* resolve time lock puzzle */
 	while (mpz_cmp_ui(t, 0UL)) {
 		mpz_powm_ui(a, a, 2UL, n);
 		mpz_sub_ui(t, t, 1UL);
diff --git a/etlp.1 b/etlp.1
index 5bc9100..5191c93 100644
--- a/etlp.1
+++ b/etlp.1
@@ -8,16 +8,18 @@ etlp \- encrypt time lock puzzle
 .IR test_time ]
 .RB [ \-f
 .IR key_file ]
+.RB [ \-b
+.IR bits ]
 .IR time_encrypted
 .SH DESCRIPTION
 .B etlp
 is a tool from the time lock puzzle cryptosystem suite
 .B stlp
-(simple time lock puzzle) that encrypts a hexadecimal key (read from stdin by default) and prints the time lock puzzle to stdout so, when decrypted with
+(simple time lock puzzle) that encrypts a hexadecimal key (read from stdin by default) and prints the time lock puzzle to stdout so decryption by
 .IR dtlp(1)
-it will take
+will take approximately
 .IR time_encrypted
-seconds in host machine.
+seconds on the host machine.
 .SH OPTIONS
 .TP
 .B \-h
@@ -27,19 +29,23 @@ prints usage format to stdout as help.
 prints version information to stdout, then exits.
 .TP
 .B \-t " test_time"
-time in seconds to invest in performance test.
+time in seconds to invest in the performance test.
 .IP
-This encryption requires to do a performance test on the machine with the random values generated. By default the test time will be 1 second. But this can be increased with an arbitrary
+To estimate the time it will take this machine to decrypt, a performance test is run prior to encryption. By default the test time will be 1 second, but this can be changed to
 .IR test_time
-seconds. It is recommended to increase the
-.IR test_time
-value for enough time to include the average workload in the performance test so the decryption time will be more close to the desired
+seconds. To get a better estimate, which is mostly relevant for large values of
 .IR time_encrypted
+it is recommended to increase
+.IR test_time
+to get a more accurate result.
 .
 .TP
 .B \-f " key_file"
 reads the key to encrypt from a file instead of stdin.
 .TP
+.B \-b " bits"
+length of the RSA modulus in bits. Must be a multiple of 2.
+.TP
 .B "time_encrypted"
 sets the time lock puzzle challenge so when decrypted it will require those seconds to obtain the key back.
 .SH EXAMPLES
diff --git a/etlp.c b/etlp.c
index 971b168..ab3f0af 100644
--- a/etlp.c
+++ b/etlp.c
@@ -7,22 +7,22 @@
 #include <gmp.h>
 
 #define DEF_TESTIME 1 /* seconds */
-#define SQUARES_PER_CICLE 1000UL /* squares to do each cicle */
-#define PRIME_LEN 512 /* prime bit length */
+#define SQUARES_PER_CYCLE 1000UL /* squares to do each cycle */
 #define BASE16 16
 #define BASE10 10
 
 static FILE *key_file;
 static unsigned long int time_enc, test_time = DEF_TESTIME;
+static unsigned long int bit_length = 1024; /* modulus length */
 static unsigned long int S;
 static gmp_randstate_t random_gen;
-static mpz_t n, fi_n;
+static mpz_t n, phi_n;
 static mpz_t Ck, b, a, e, t;
 
 static void
 usage(void)
 {
-	fputs("usage: etlp [-h] [-v] [-t test_time] [-f key_file] time_encrypted\n", stderr);
+	fputs("usage: etlp [-h] [-v] [-t test_time] [-f key_file] [-b bits] time_encrypted\n", stderr);
 	exit(EX_USAGE);
 }
 
@@ -31,7 +31,7 @@ setup(void)
 {
 	/* Initialize mpz_t global variables */
 	mpz_init(n);
-	mpz_init(fi_n);
+	mpz_init(phi_n);
 	mpz_init(Ck);
 	mpz_init(a);
 	mpz_init(b);
@@ -50,20 +50,20 @@ setup(void)
 	gmp_randseed_ui(random_gen, random_seed);
 }
 
-/* Obtain the modulus n and fi(n) by creating two big random primes values
- * (p, q) and multiply them.
+/* Obtain the modulus n and phi(n) by creating two large random primes
+ * (p, q) and multiplying them.
  *
  * n = p * q
- * fi(n) = (p - 1) * (q - 1) */
+ * phi(n) = (p - 1) * (q - 1) */
 static void
-gen_modulos(void)
+gen_modulus(void)
 {
 	mpz_t p, q;
 	mpz_init(p);
 	mpz_init(q);
 
-	mpz_urandomb(p, random_gen, PRIME_LEN);
-	mpz_urandomb(q, random_gen, PRIME_LEN);
+	mpz_urandomb(p, random_gen, bit_length/2);
+	mpz_urandomb(q, random_gen, bit_length/2);
 
 	mpz_nextprime(p, p);
 	mpz_nextprime(q, q);
@@ -73,7 +73,7 @@ gen_modulos(void)
 	mpz_sub_ui(p, p, 1UL);
 	mpz_sub_ui(q, q, 1UL);
 
-	mpz_mul(fi_n, p, q);
+	mpz_mul(phi_n, p, q);
 
 	mpz_clear(p);
 	mpz_clear(q);
@@ -83,7 +83,7 @@ gen_modulos(void)
 static void
 gen_base(void)
 {
-	mpz_urandomb(a, random_gen, PRIME_LEN*2);
+	mpz_urandomb(a, random_gen, bit_length);
 
 	mpz_sub_ui(n, n, 2UL);
 	mpz_mod(a, a, n);
@@ -95,7 +95,7 @@ gen_base(void)
 static void
 test_perf(void)
 {
-	unsigned long cicles = 0;
+	unsigned long cycles = 0;
 	clock_t t0, ticks, t_test;
 
 	mpz_set(b, a);
@@ -103,26 +103,26 @@ test_perf(void)
 	t0 = clock();
 	t_test = t0 + ticks;
 	do {
-		mpz_set_ui(t, SQUARES_PER_CICLE);
+		mpz_set_ui(t, SQUARES_PER_CYCLE);
 		while(mpz_cmp_ui(t, 0) > 0) {
 			mpz_powm_ui(b, b, 2UL, n);
 			mpz_sub_ui(t, t, 1UL);
 		}
-		cicles++;
+		cycles++;
 	} while(clock() < t_test);
-	S = (unsigned long)((SQUARES_PER_CICLE * cicles)/test_time);
+	S = (unsigned long)((SQUARES_PER_CYCLE * cycles)/test_time);
 }
 
 /* Encrypt efficiently by solving: Ck = k + b
  * b = a ^ e mod n
- * e = 2 ^ t mod fi_n */
+ * e = 2 ^ t mod phi_n */
 static void
 encrypt(void)
 {
-	unsigned int key;
-	mpz_t T, two;
+	mpz_t T, two, key;
 	mpz_init(T);
 	mpz_init(two);
+	mpz_init(key);
 
 	/* calculate challenge to reach desired time */
 	mpz_set_ui(T, time_enc);
@@ -130,17 +130,22 @@ encrypt(void)
 
 	/* calculate b */
 	mpz_set_ui(two, 2UL);
-	mpz_powm(e, two, t, fi_n);
+	mpz_powm(e, two, t, phi_n);
 	mpz_powm(b, a, e, n);
 
 	/* read key from stdin */
-	if (fscanf(key_file, "%x", &key) == EOF) {
+	if (mpz_inp_str(key, key_file, BASE16) == 0) {
 		fputs("Error reading key from stdin", stderr);
 		exit(EX_IOERR);
 	}
 
+	if (mpz_cmp(key, n) >= 0) {
+		fputs("Key is too large for modulus length", stderr);
+		exit(EX_USAGE);
+	}
+
 	/* encrypt key with b */
-	mpz_add_ui(Ck, b, key);
+	mpz_add(Ck, b, key);
 	
 	mpz_clear(T);
 	mpz_clear(two);
@@ -152,7 +157,7 @@ unsetup(void)
 	if(key_file != stdin)
 		fclose(key_file);
 	mpz_clear(n);
-	mpz_clear(fi_n);
+	mpz_clear(phi_n);
 	mpz_clear(Ck);
 	mpz_clear(t);
 	mpz_clear(a);
@@ -168,27 +173,38 @@ main (int argc, char *argv[])
 	int i;
 	key_file = stdin;
 
+	time_enc = 0;
 	/* process input */
 	for (i = 1; i < argc; i++)
 		if (!strcmp(argv[i], "-h"))
 			usage();
 		else if (!strcmp(argv[i], "-v")) {
-			puts("simple time lock puzzle encrypter v0.2");
+			puts("simple time lock puzzle encrypter v0.3");
 			exit(EX_OK);
 		} else if (!strcmp(argv[i], "-t"))
 			test_time = strtoul(argv[++i], NULL, BASE10);
-		else if (!strcmp(argv[i], "-f")) {
+		else if (!strcmp(argv[i], "-b")) {
+			bit_length = strtoul(argv[++i], NULL, BASE10);
+			if (bit_length % 2) {
+				fputs("Bit length should be a multiple of 2", stderr);
+				exit(EX_USAGE);
+			}
+		} else if (!strcmp(argv[i], "-f")) {
 			key_file = fopen(argv[++i], "r");
 			if (!key_file) {
 				fprintf(stderr, "Error opening %s\n", argv[i]);
 				exit(EX_NOINPUT);
 			}
-		}
-	time_enc = strtoul(argv[argc-1], NULL, BASE10);
+		} else if (time_enc == 0)
+			time_enc = strtoul(argv[i], NULL, BASE10);
+		else
+			usage();
+	
+	if (time_enc == 0) usage();
 
 	setup();
 
-	gen_modulos();
+	gen_modulus();
 	gen_base();
 	test_perf();