Provide a single OTP function that accepts a configuration struct

cli.c

@@ -44,6 +44,7 @@ main(int argc, char *argv[])
 	const char *errstr;
 	unsigned char *key;
 	char *in, *line;
+	struct otpcfg otpcfg;
 	size_t inlen, linesz;
 	ssize_t linelen;
 	uint64_t counter;
@@ -132,17 +133,23 @@ main(int argc, char *argv[])
 	if (!b32_decode(key, keylen, in, inlen))
 		errx(1, "error decoding base32 string");
 
+	otpcfg.algorithm = hmac;
+	otpcfg.digits = digits;
+	otpcfg.secret = key;
+	otpcfg.secretlen = keylen;
 	if (do_hotp) {
-		r = hotp(hmac, key, keylen, counter, digits);
-		if (r == -1)
-			errx(1, "couldn't calculate HOTP");
-		printf("%0*" PRId32 "\n", digits, r);
+		otpcfg.type = OTP_HOTP;
+		otpcfg.u.hotp.counter = counter;
 	} else {
-		r = totp(hmac, key, keylen, counter, period, digits);
-		if (r == -1)
-			errx(1, "couldn't calculate TOTP");
-		printf("%0*" PRId32 "\n", digits, r);
+		otpcfg.type = OTP_TOTP;
+		otpcfg.u.totp.time = counter;
+		otpcfg.u.totp.period = period;
 	}
 
+	r = otp(&otpcfg);
+	if (r == -1)
+		errx(1, "couldn't calculate %cOTP", do_hotp ? 'H' : 'T');
+	printf("%0*" PRId32 "\n", digits, r);
+
 	return 0;
 }

otp.c

@@ -18,19 +18,47 @@
 
 #include "otp.h"
 
-static int32_t
-otp(const EVP_MD *evp_md, const void *key, int key_len, uint64_t counter,
-    unsigned int digits)
+int32_t
+otp(const struct otpcfg *otpcfg)
 {
+	const EVP_MD *evp_md;
 	unsigned char md_value[EVP_MAX_MD_SIZE];
 	uint8_t buf[8];
+	uint64_t counter;
 	uint32_t res;
 	int32_t modulo;
-	unsigned int md_len, offset;
+	unsigned int digits, md_len, offset;
 
-	if (digits < 6 || digits > 10)
+	switch (otpcfg->type) {
+	case OTP_HOTP:
+		counter = otpcfg->u.hotp.counter;
+		break;
+	case OTP_TOTP:
+		if (otpcfg->u.totp.period == 0)
+			return -1;
+		counter = otpcfg->u.totp.time / otpcfg->u.totp.period;
+		break;
+	default:
+		return -1;
+	}
+
+	switch (otpcfg->algorithm) {
+	case OTP_HMAC_SHA1:
+		evp_md = EVP_sha1();
+		break;
+	case OTP_HMAC_SHA256:
+		evp_md = EVP_sha256();
+		break;
+	case OTP_HMAC_SHA512:
+		evp_md = EVP_sha512();
+		break;
+	default:
 		return -1;
+	}
 
+	digits = otpcfg->digits;
+	if (digits < 6 || digits > 10)
+		return -1;
 	/*
 	 * 10 digits would be 10_000_000_000, which overflows 32 bits.
 	 * Nevertheless, given the number construction, 10 digits OTP are
@@ -50,8 +78,8 @@ otp(const EVP_MD *evp_md, const void *key, int key_len, uint64_t counter,
 	buf[6] = (counter >> 8) & 0xff;
 	buf[7] = counter & 0xff;
 
-	if (HMAC(evp_md, key, key_len, buf, sizeof(buf), md_value, &md_len) ==
-	    NULL)
+	if (HMAC(evp_md, otpcfg->secret, otpcfg->secretlen, buf, sizeof(buf),
+	    md_value, &md_len) == NULL)
 		return -1;
 
 	offset = md_value[md_len - 1] & 0xf;
@@ -63,36 +91,3 @@ otp(const EVP_MD *evp_md, const void *key, int key_len, uint64_t counter,
 
 	return digits == 10 ? (int32_t)res : (int32_t)res % modulo;
 }
-
-int32_t
-hotp(enum otp_hmac hmac, const void *key, size_t key_len, uint64_t counter,
-    unsigned int digits)
-{
-	const EVP_MD *evp_md;
-
-	if (key_len > INT_MAX)
-		return -1;
-
-	switch (hmac) {
-	case OTP_HMAC_SHA1:
-		evp_md = EVP_sha1();
-		break;
-	case OTP_HMAC_SHA256:
-		evp_md = EVP_sha256();
-		break;
-	case OTP_HMAC_SHA512:
-		evp_md = EVP_sha512();
-		break;
-	default:
-		return -1;
-	}
-
-	return otp(evp_md, key, (int)key_len, counter, digits);
-}
-
-int32_t
-totp(enum otp_hmac hmac, const void *key, size_t key_len, uint64_t t,
-    unsigned int period, unsigned int digits)
-{
-	return hotp(hmac, key, key_len, t / period, digits);
-}

otp.h

@@ -14,12 +14,33 @@
 
 #include <stdint.h>
 
+enum otp_type {
+	OTP_HOTP,
+	OTP_TOTP,
+};
+
 enum otp_hmac {
 	OTP_HMAC_SHA1,
 	OTP_HMAC_SHA256,
 	OTP_HMAC_SHA512,
 };
 
-int32_t hotp(enum otp_hmac, const void *, size_t, uint64_t, unsigned int);
-int32_t totp(enum otp_hmac, const void *, size_t, uint64_t, unsigned int,
-    unsigned int);
+struct otpcfg {
+	enum otp_type type;
+	enum otp_hmac algorithm;
+	unsigned int digits;
+	const void *secret;
+	size_t secretlen;
+
+	union {
+		struct {
+			uint64_t counter;
+		} hotp;
+		struct {
+			uint64_t time;
+			unsigned int period;
+		} totp;
+	} u;
+};
+
+int32_t otp(const struct otpcfg *);