diff --git a/modules/pam_oath/oath.h b/modules/pam_oath/oath.h
index ffd28c0..f19849d 100644
--- a/modules/pam_oath/oath.h
+++ b/modules/pam_oath/oath.h
@@ -101,7 +101,18 @@ struct oath_key *oath_key_from_uri(const char *);
 struct oath_key *oath_key_from_file(const char *);
 char *oath_key_to_uri(const struct oath_key *);
 
+#define DUMMY_LABEL	("oath-dummy-key")
+#define DUMMY_LABELLEN	(sizeof DUMMY_LABEL)
+#define DUMMY_KEYLEN	80
+
+struct oath_key *oath_dummy_key(enum oath_mode, enum oath_hash, unsigned int);
+
 unsigned int oath_hotp(const uint8_t *, size_t, uint64_t, unsigned int);
+int oath_hotp_current(struct oath_key *);
+int oath_hotp_match(struct oath_key *, unsigned int, int);
+
 unsigned int oath_totp(const uint8_t *, size_t, unsigned int);
+int oath_totp_match(const struct oath_key *, unsigned int, int);
+unsigned int oath_totp_current(const struct oath_key *);
 
 #endif
diff --git a/modules/pam_oath/oath_hotp.c b/modules/pam_oath/oath_hotp.c
index 741719e..e247f78 100644
--- a/modules/pam_oath/oath_hotp.c
+++ b/modules/pam_oath/oath_hotp.c
@@ -37,6 +37,7 @@
 #include <openssl/hmac.h>
 
 #include <stdint.h>
+#include <string.h>
 
 #include "oath.h"
 
@@ -87,3 +88,52 @@ oath_hotp(const uint8_t *K, size_t Klen, uint64_t seq, unsigned int Digit)
 	D = Snum % mod;
 	return (D);
 }
+
+/*
+ * Computes the current code for the given key and advances the counter.
+ */
+int
+oath_hotp_current(struct oath_key *k)
+{
+	unsigned int code;
+
+	if (k == NULL)
+		return (-1);
+	if (k->mode != om_hotp)
+		return (-1);
+	if (k->counter == UINT64_MAX)
+		return (-1);
+	code = oath_hotp(k->key, k->keylen, k->counter, k->digits);
+	k->counter += 1;
+	return (code);
+}
+
+/*
+ * Compares the code provided by the user with expected values within a
+ * given window.  Returns 1 if there was a match, 0 if not, and -1 if an
+ * error occurred.
+ */
+int
+oath_hotp_match(struct oath_key *k, unsigned int response, int window)
+{
+	unsigned int code;
+	int dummy;
+
+	if (k == NULL)
+		return (-1);
+	if (window < 1)
+		return (-1);
+	if (k->mode != om_hotp)
+		return (-1);
+	if (k->counter >= UINT64_MAX - window)
+		return (-1);
+	dummy = (memcmp(k->label, DUMMY_LABEL, DUMMY_LABELLEN) == 0);
+	for (int i = 0; i < window; ++i) {
+		code = oath_hotp(k->key, k->keylen, k->counter + i, k->digits);
+		if (code == response && !dummy) {
+			k->counter = k->counter + i;
+			return (1);
+		}
+	}
+	return (0);
+}
diff --git a/modules/pam_oath/oath_key.c b/modules/pam_oath/oath_key.c
index eccab9d..43a714a 100644
--- a/modules/pam_oath/oath_key.c
+++ b/modules/pam_oath/oath_key.c
@@ -112,10 +112,8 @@ oath_key_from_uri(const char *uri)
 	if ((q = strchr(p, '/')) == NULL)
 		goto invalid;
 	if (strlcmp("hotp", p, q - p) == 0) {
-		openpam_log(PAM_LOG_DEBUG, "OATH mode: HOTP");
 		key->mode = om_hotp;
 	} else if (strlcmp("totp", p, q - p) == 0) {
-		openpam_log(PAM_LOG_DEBUG, "OATH mode: TOTP");
 		key->mode = om_totp;
 	} else {
 		goto invalid;
@@ -132,7 +130,7 @@ oath_key_from_uri(const char *uri)
 	p = q + 1;
 
 	/* extract parameters */
-	key->counter = UINTMAX_MAX;
+	key->counter = UINT64_MAX;
 	while (*p != '\0') {
 		if ((q = strchr(p, '=')) == NULL)
 			goto invalid;
@@ -152,6 +150,7 @@ oath_key_from_uri(const char *uri)
 			    base32_declen(r - q) > OATH_MAX_KEYLEN)
 				goto invalid;
 			key->key = key->data + key->labellen;
+			key->keylen = key->datalen - key->labellen;
 			if (base32_dec(q, r - q, key->key, &key->keylen) != 0)
 				goto invalid;
 			if (base32_enclen(key->keylen) != (size_t)(r - q))
@@ -179,11 +178,11 @@ oath_key_from_uri(const char *uri)
 				goto invalid;
 			key->digits = *q - '0';
 		} else if (strlcmp("counter=", p, q - p) == 0) {
-			if (key->counter != UINTMAX_MAX)
+			if (key->counter != UINT64_MAX)
 				/* dupe */
 				goto invalid;
 			n = strtoumax(q, &e, 10);
-			if (e != r || n >= UINTMAX_MAX)
+			if (e != r || n >= UINT64_MAX)
 				goto invalid;
 			key->counter = (uint64_t)n;
 		} else if (strlcmp("period=", p, q - p) == 0) {
@@ -226,6 +225,7 @@ oath_key_from_uri(const char *uri)
 		key->digits = 6;
 	if (key->keylen == 0)
 		goto invalid;
+	return (key);
 
 invalid:
 	openpam_log(PAM_LOG_NOTICE, "invalid OATH URI: %s", uri);
@@ -295,8 +295,8 @@ oath_key_to_uri(const struct oath_key *key)
 	}
 
 	/* compute length of base32-encoded key and append it */
-	kslen = base32_enclen(key->keylen);
-	if ((tmp = realloc(uri, urilen + kslen + 1)) == NULL) {
+	kslen = base32_enclen(key->keylen) + 1;
+	if ((tmp = realloc(uri, urilen + kslen)) == NULL) {
 		free(uri);
 		return (NULL);
 	}
@@ -308,3 +308,22 @@ oath_key_to_uri(const struct oath_key *key)
 
 	return (uri);
 }
+
+struct oath_key *
+oath_dummy_key(enum oath_mode mode, enum oath_hash hash, unsigned int digits)
+{
+	struct oath_key *key;
+
+	if ((key = oath_key_alloc(DUMMY_LABELLEN + DUMMY_KEYLEN)) == NULL)
+		return (NULL);
+	key->mode = mode;
+	key->digits = digits;
+	key->counter = 0;
+	key->timestep = 30;
+	key->hash = hash;
+	key->label = (char *)key->data;
+	memcpy(key->label, DUMMY_LABEL, DUMMY_LABELLEN);
+	key->key = key->data + DUMMY_LABELLEN;
+	key->keylen = DUMMY_KEYLEN;
+	return (key);
+}
diff --git a/modules/pam_oath/oath_totp.c b/modules/pam_oath/oath_totp.c
index 5cfc6b6..f499967 100644
--- a/modules/pam_oath/oath_totp.c
+++ b/modules/pam_oath/oath_totp.c
@@ -34,6 +34,7 @@
 #endif
 
 #include <stdint.h>
+#include <string.h>
 #include <time.h>
 
 #include "oath.h"
@@ -48,3 +49,45 @@ oath_totp(const uint8_t *K, size_t Klen, unsigned int Digit)
 	time(&now);
 	return (oath_hotp(K, Klen, now / TOTP_TIME_STEP, Digit));
 }
+
+unsigned int
+oath_totp_current(const struct oath_key *k)
+{
+	unsigned int code;
+	uint64_t seq;
+
+	if (k == NULL)
+		return (-1);
+	if (k->mode != om_totp)
+		return (-1);
+	if (k->timestep == 0)
+		return (-1);
+	seq = time(NULL) / k->timestep;
+	code = oath_hotp(k->key, k->keylen, seq, k->digits);
+	return (code);
+}
+
+int
+oath_totp_match(const struct oath_key *k, unsigned int response, int window)
+{
+	unsigned int code;
+	uint64_t seq;
+	int dummy;
+
+	if (k == NULL)
+		return (-1);
+	if (window < 1)
+		return (-1);
+	if (k->mode != om_totp)
+		return (-1);
+	if (k->timestep == 0)
+		return (-1);
+	seq = time(NULL) / k->timestep;
+	dummy = (memcmp(k->label, DUMMY_LABEL, DUMMY_LABELLEN) == 0);
+	for (int i = -window; i <= window; ++i) {
+		code = oath_hotp(k->key, k->keylen, seq + i, k->digits);
+		if (code == response && !dummy)
+			return (1);
+	}
+	return (0);
+}