From 9579e103657288321166f88e7ea73209ac8fd580 Mon Sep 17 00:00:00 2001
From: Mattias Andrée <m@maandree.se>
Date: Wed, 23 Sep 2026 19:39:27 +0200
Subject: Fix bug that caused (custom) hash functions with fixed tag size to be
 unsupported
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Signed-off-by: Mattias Andrée <m@maandree.se>
---
 librecrypt_add_algorithm.c  | 232 ++++++++++++++++++++++++++++++++++++++++++--
 librecrypt_crypt.c          | 225 ++++++++++++++++++++++++++++++++++++++++--
 librecrypt_get_encoding.c   | 112 +++++++++++++++++++--
 librecrypt_hash.c           | 219 ++++++++++++++++++++++++++++++++++++++++-
 librecrypt_hash_.c          |  20 ++--
 librecrypt_hash_binary.c    | 219 ++++++++++++++++++++++++++++++++++++++++-
 librecrypt_make_settings.c  | 222 ++++++++++++++++++++++++++++++++++++++++++
 librecrypt_test_supported.c | 173 ++++++++++++++++++++++++++++++++-
 librecrypt_verify.c         | 206 +++++++++++++++++++++++++++++++++++++++
 9 files changed, 1582 insertions(+), 46 deletions(-)

diff --git a/librecrypt_add_algorithm.c b/librecrypt_add_algorithm.c
index 877e410..d561983 100644
--- a/librecrypt_add_algorithm.c
+++ b/librecrypt_add_algorithm.c
@@ -15,7 +15,7 @@ librecrypt_add_algorithm(char *out_buffer, size_t size, const char *augend,
 	size_t prefix1, prefix2, min, ret, len, phraselen;
 	size_t hashsize1, hashsize2;
 	char *phrase, pad;
-	int strict_pad, r_int, nul_term;
+	int strict_pad, r_int, nul_term, saved_errno;
 	const unsigned char *lut;
 	ssize_t r;
 
@@ -167,10 +167,12 @@ librecrypt_add_algorithm(char *out_buffer, size_t size, const char *augend,
 	/* Chain the hash algorithms: write `augment` and hash */
 	r = librecrypt_crypt(out_buffer, nul_term ? size + 1u : 0u, phrase, phraselen, augment, ctx);
 	if (r <= 0) {
+		saved_errno = errno;
 		librecrypt_wipe(phrase, phraselen);
 		free(phrase);
 		if (!r)
 			abort(); /* $covered$ (impossible) */
+		errno = saved_errno;
 		return -1;
 	}
 	if (ret > (size_t)(SSIZE_MAX - r)) {
@@ -192,6 +194,194 @@ librecrypt_add_algorithm(char *out_buffer, size_t size, const char *augend,
 #else
 
 
+#define ALPHABET "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
+#define ALT_ALPHABET "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ+/"
+
+NONSTRING static const char encoding_lut[256u] = MAKE_ENCODING_LUT(ALPHABET);
+NONSTRING static const char alt_encoding_lut[256u] = MAKE_ENCODING_LUT(ALT_ALPHABET);
+
+static const unsigned char decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX, XX,
+	XX,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
+	15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, XX, XX, XX, XX, XX,
+	XX, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+	41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static const unsigned char alt_decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, XX, XX, XX, XX, XX, XX,
+	XX, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50,
+	51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX,
+	XX, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
+	25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static unsigned
+rot4_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$rot4$") - 1u)
+		if (!strncmp(settings, "$rot4$", sizeof("$rot4$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static unsigned
+trunc_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$trunc$") - 1u)
+		if (!strncmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static int
+trunc_supported(const char *phrase, size_t len, int text, const char *settings,
+                size_t prefix, size_t *len_out)
+{
+	size_t i, digit, n = 0u, q, r;
+
+	(void) phrase;
+	(void) len;
+	(void) text;
+
+	if (prefix < sizeof("$trunc$") - 1u || memcmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+		return 0;
+
+	if (prefix == sizeof("$trunc$") - 1u) {
+		*len_out = 4u;
+		return 1;
+	}
+
+	if (settings[sizeof("$trunc$") - 1u] == '*') {
+		for (i = sizeof("$trunc$*") - 1u; i < prefix; i++) {
+			if ('0' > settings[i] || settings[i] > '9')
+				return 0;
+			digit = (size_t)(settings[i] - '0');
+			if (n > (SIZE_MAX - digit) / 10u)
+				return 0;
+			n = n * 10u + digit;
+		}
+	} else {
+		for (i = sizeof("$trunc$") - 1u; i < prefix; i++) {
+			if (settings[i] == '@')
+				break;
+			if (alt_decoding_lut[(unsigned char)settings[i]] == XX)
+				return 0;
+			n += 1u;
+		}
+		q = n / 4u;
+		r = n % 4u;
+		if (r == 1u)
+			return 0;
+		n = q * 3u + r;
+		if (r) {
+			n -= 1u;
+			for (; r < 4u && i < prefix; r++, i++)
+				if (settings[i] != '@')
+					break;
+			if (r != 4u || i != prefix)
+				return 0;
+		}
+	}
+	if (!n)
+		return 0;
+	*len_out = n;
+	return 1;
+}
+
+static int
+rot4_hash(char *restrict out_buffer, size_t size, const char *phrase, size_t len,
+          const char *settings, size_t prefix, LIBRECRYPT_CONTEXT *ctx)
+{
+	size_t i;
+
+	(void) settings;
+	(void) prefix;
+	(void) ctx;
+
+	for (i = 0u; i < 8u && i < size; i++) {
+		out_buffer[i] = '\0';
+		if (i < len)
+			out_buffer[i] = (char)((((int)phrase[i] & 0x0F) << 4) | (((int)phrase[i] >> 4) & 0x0F));
+	}
+	return 0;
+}
+
+static int
+trunc_hash(char *restrict out_buffer, size_t size, const char *phrase, size_t len,
+           const char *settings, size_t prefix, LIBRECRYPT_CONTEXT *ctx)
+{
+	size_t hash_size, i;
+
+	(void) ctx;
+
+	if (!trunc_supported(NULL, 0u, 1, settings, prefix, &hash_size)) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	size = MIN(hash_size, size);
+	for (i = 0u; i < size; i++)
+		out_buffer[i] = i < len ? phrase[i] : '\0';
+
+	return 0;
+}
+
+static const struct librecrypt_algorithm rot4_algo = {
+	.is_algorithm = &rot4_is_algorithm,
+	.hash = &rot4_hash,
+	.encoding_lut = encoding_lut,
+	.decoding_lut = decoding_lut,
+	.hash_size = 8u,
+	.flexible_hash_size = 0,
+	.strict_pad = 1,
+	.pad = '#'
+};
+
+static const struct librecrypt_algorithm trunc_algo = {
+	.is_algorithm = &trunc_is_algorithm,
+	.hash = &trunc_hash,
+	.encoding_lut = alt_encoding_lut,
+	.decoding_lut = alt_decoding_lut,
+	.hash_size = 4u,
+	.flexible_hash_size = 1,
+	.strict_pad = 1,
+	.pad = '@'
+};
+
+#define TEST_PHRASE64_ROT4 "EiM0RVZneIk#"
+#define TEST_HASH_ROT4 "ITJDVGV2h5g#"
+#define TEST_HASH_TRUNC "4ycQhg@@"
+#define TEST_HASH_TRUNC6 "4ycQhlpD"
+#define TEST_HASH_TRUNC_ROT4 "ITJDVAAAAAA#"
+#define TEST_HASH_TRUNC6_ROT4 "ITJDVGV2AAA#"
+#define TEST_HASH_ROT4_TRUNC "8j93l0@@"
+#define TEST_HASH_ROT4_TRUNC6 "8j93l6lS"
+#define TEST_HASH_TRUNC_TRUNC6 "4ycQhg00"
+
+
 int
 main(void)
 {
@@ -200,6 +390,8 @@ main(void)
 #define HASH1 "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTYVWXYZ/+"
 #define ASTRA "*48"
 
+	const struct librecrypt_algorithm custom[] = {rot4_algo, trunc_algo};
+	LIBRECRYPT_CONTEXT *ctx = NULL;
 	char buf[1024], phrase[sizeof(buf)], expected[sizeof(buf)], pad;
 	size_t i, min, phraselen;
 	int strict_pad;
@@ -213,7 +405,7 @@ main(void)
 #define CHECK(AUGEND, AUGMENT, RESULT)\
 	do {\
 		CANARY_FILL(buf);\
-		r = librecrypt_add_algorithm(buf, sizeof(buf), (AUGEND), (AUGMENT), NULL);\
+		r = librecrypt_add_algorithm(buf, sizeof(buf), (AUGEND), (AUGMENT), ctx);\
 		EXPECT(r > 0);\
 		EXPECT((size_t)r == strlen(RESULT));\
 		assert((size_t)r < sizeof(buf) + 1u);\
@@ -223,7 +415,7 @@ main(void)
 		\
 		for (i = (size_t)r + 2u;; i--) {\
 			CANARY_FILL(buf);\
-			EXPECT(librecrypt_add_algorithm(buf, i, (AUGEND), (AUGMENT), NULL) == r);\
+			EXPECT(librecrypt_add_algorithm(buf, i, (AUGEND), (AUGMENT), ctx) == r);\
 			if (!i) {\
 				CANARY_CHECK(buf, 0u);\
 				break;\
@@ -234,13 +426,13 @@ main(void)
 			CANARY_CHECK(buf, min + 1u);\
 		}\
 		\
-		EXPECT(librecrypt_add_algorithm(NULL, 0u, (AUGEND), (AUGMENT), NULL) == r);\
+		EXPECT(librecrypt_add_algorithm(NULL, 0u, (AUGEND), (AUGMENT), ctx) == r);\
 		\
 		assert(sizeof(buf) > strlen(AUGEND));\
 		\
 		CANARY_FILL(buf);\
 		stpcpy(buf, (AUGEND));\
-		EXPECT(librecrypt_add_algorithm(buf, sizeof(buf), buf, (AUGMENT), NULL) == r);\
+		EXPECT(librecrypt_add_algorithm(buf, sizeof(buf), buf, (AUGMENT), ctx) == r);\
 		EXPECT(!buf[r]);\
 		EXPECT(!memcmp(buf, (RESULT), (size_t)r));\
 		n = strlen(AUGEND) + 1u;\
@@ -251,7 +443,7 @@ main(void)
 			CANARY_FILL(buf);\
 			stpcpy(buf, (AUGEND));\
 			n = strlen(AUGEND) + 1u;\
-			EXPECT(librecrypt_add_algorithm(buf, i, buf, (AUGMENT), NULL) == r);\
+			EXPECT(librecrypt_add_algorithm(buf, i, buf, (AUGMENT), ctx) == r);\
 			if (!i) {\
 				CANARY_CHECK(buf, n);\
 				break;\
@@ -374,6 +566,34 @@ main(void)
 	EXPECT(errno == ENOSYS);
 	CANARY_CHECK(buf, 0u);
 
+	ctx = librecrypt_create_context();
+	assert(ctx != NULL);
+
+	librecrypt_set_custom_algorithms(ctx, custom, ELEMSOF(custom));
+	CHECK("$rot4$", "$rot4$", "$rot4$>$rot4$");
+	CHECK("$rot4$", "$trunc$", "$rot4$>$trunc$");
+	CHECK("$rot4$", "$trunc$*6", "$rot4$>$trunc$*6");
+	CHECK("$trunc$", "$trunc$", "$trunc$>$trunc$");
+	CHECK("$trunc$", "$trunc$*6", "$trunc$>$trunc$*6");
+	CHECK("$trunc$", "$rot4$", "$trunc$>$rot4$");
+	CHECK("$trunc$*4", "$trunc$", "$trunc$*4>$trunc$");
+	CHECK("$trunc$*4", "$trunc$*6", "$trunc$*4>$trunc$*6");
+	CHECK("$trunc$*4", "$rot4$", "$trunc$*4>$rot4$");
+	CHECK("$trunc$*6", "$trunc$", "$trunc$*6>$trunc$");
+	CHECK("$trunc$*6", "$trunc$*6", "$trunc$*6>$trunc$*6");
+	CHECK("$trunc$*6", "$rot4$", "$trunc$*6>$rot4$");
+	CHECK("$rot4$"TEST_HASH_ROT4, "$rot4$", "$rot4$>$rot4$"TEST_PHRASE64_ROT4);
+	CHECK("$rot4$"TEST_HASH_ROT4, "$trunc$", "$rot4$>$trunc$"TEST_HASH_ROT4_TRUNC);
+	CHECK("$rot4$"TEST_HASH_ROT4, "$trunc$*6", "$rot4$>$trunc$"TEST_HASH_ROT4_TRUNC6);
+	CHECK("$trunc$"TEST_HASH_TRUNC6, "$trunc$", "$trunc$*6>$trunc$"TEST_HASH_TRUNC);
+	CHECK("$trunc$"TEST_HASH_TRUNC6, "$trunc$*6", "$trunc$*6>$trunc$"TEST_HASH_TRUNC6);
+	CHECK("$trunc$"TEST_HASH_TRUNC6, "$rot4$", "$trunc$*6>$rot4$"TEST_HASH_TRUNC6_ROT4);
+	CHECK("$trunc$"TEST_HASH_TRUNC, "$trunc$", "$trunc$*4>$trunc$"TEST_HASH_TRUNC);
+	CHECK("$trunc$"TEST_HASH_TRUNC, "$trunc$*6", "$trunc$*4>$trunc$"TEST_HASH_TRUNC_TRUNC6);
+	CHECK("$trunc$"TEST_HASH_TRUNC, "$rot4$", "$trunc$*4>$rot4$"TEST_HASH_TRUNC_ROT4);
+
+	librecrypt_free_context(ctx);
+
 	STOP_RESOURCE_TEST();
 	return 0;
 }
diff --git a/librecrypt_crypt.c b/librecrypt_crypt.c
index fb93cd2..d837724 100644
--- a/librecrypt_crypt.c
+++ b/librecrypt_crypt.c
@@ -14,6 +14,194 @@ librecrypt_crypt(char *restrict out_buffer, size_t size, const char *phrase,
 #else
 
 
+#define ALPHABET "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
+#define ALT_ALPHABET "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ+/"
+
+NONSTRING static const char encoding_lut[256u] = MAKE_ENCODING_LUT(ALPHABET);
+NONSTRING static const char alt_encoding_lut[256u] = MAKE_ENCODING_LUT(ALT_ALPHABET);
+
+static const unsigned char decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX, XX,
+	XX,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
+	15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, XX, XX, XX, XX, XX,
+	XX, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+	41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static const unsigned char alt_decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, XX, XX, XX, XX, XX, XX,
+	XX, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50,
+	51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX,
+	XX, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
+	25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static unsigned
+rot4_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$rot4$") - 1u)
+		if (!strncmp(settings, "$rot4$", sizeof("$rot4$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static unsigned
+trunc_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$trunc$") - 1u)
+		if (!strncmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static int
+trunc_supported(const char *phrase, size_t len, int text, const char *settings,
+                size_t prefix, size_t *len_out)
+{
+	size_t i, digit, n = 0u, q, r;
+
+	(void) phrase;
+	(void) len;
+	(void) text;
+
+	if (prefix < sizeof("$trunc$") - 1u || memcmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+		return 0;
+
+	if (prefix == sizeof("$trunc$") - 1u) {
+		*len_out = 4u;
+		return 1;
+	}
+
+	if (settings[sizeof("$trunc$") - 1u] == '*') {
+		for (i = sizeof("$trunc$*") - 1u; i < prefix; i++) {
+			if ('0' > settings[i] || settings[i] > '9')
+				return 0;
+			digit = (size_t)(settings[i] - '0');
+			if (n > (SIZE_MAX - digit) / 10u)
+				return 0;
+			n = n * 10u + digit;
+		}
+	} else {
+		for (i = sizeof("$trunc$") - 1u; i < prefix; i++) {
+			if (settings[i] == '@')
+				break;
+			if (alt_decoding_lut[(unsigned char)settings[i]] == XX)
+				return 0;
+			n += 1u;
+		}
+		q = n / 4u;
+		r = n % 4u;
+		if (r == 1u)
+			return 0;
+		n = q * 3u + r;
+		if (r) {
+			n -= 1u;
+			for (; r < 4u && i < prefix; r++, i++)
+				if (settings[i] != '@')
+					break;
+			if (r != 4u || i != prefix)
+				return 0;
+		}
+	}
+	if (!n)
+		return 0;
+	*len_out = n;
+	return 1;
+}
+
+static int
+rot4_hash(char *restrict out_buffer, size_t size, const char *phrase, size_t len,
+          const char *settings, size_t prefix, LIBRECRYPT_CONTEXT *ctx)
+{
+	size_t i;
+
+	(void) settings;
+	(void) prefix;
+	(void) ctx;
+
+	for (i = 0u; i < 8u && i < size; i++) {
+		out_buffer[i] = '\0';
+		if (i < len)
+			out_buffer[i] = (char)((((int)phrase[i] & 0x0F) << 4) | (((int)phrase[i] >> 4) & 0x0F));
+	}
+	return 0;
+}
+
+static int
+trunc_hash(char *restrict out_buffer, size_t size, const char *phrase, size_t len,
+           const char *settings, size_t prefix, LIBRECRYPT_CONTEXT *ctx)
+{
+	size_t hash_size, i;
+
+	(void) ctx;
+
+	if (!trunc_supported(NULL, 0u, 1, settings, prefix, &hash_size)) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	size = MIN(hash_size, size);
+	for (i = 0u; i < size; i++)
+		out_buffer[i] = i < len ? phrase[i] : '\0';
+
+	return 0;
+}
+
+static const struct librecrypt_algorithm rot4_algo = {
+	.is_algorithm = &rot4_is_algorithm,
+	.hash = &rot4_hash,
+	.encoding_lut = encoding_lut,
+	.decoding_lut = decoding_lut,
+	.hash_size = 8u,
+	.flexible_hash_size = 0,
+	.strict_pad = 1,
+	.pad = '#'
+};
+
+static const struct librecrypt_algorithm trunc_algo = {
+	.is_algorithm = &trunc_is_algorithm,
+	.hash = &trunc_hash,
+	.encoding_lut = alt_encoding_lut,
+	.decoding_lut = alt_decoding_lut,
+	.hash_size = 4u,
+	.flexible_hash_size = 1,
+	.strict_pad = 1,
+	.pad = '@'
+};
+
+#define TEST_PHRASE "\x12\x23\x34\x45\x56\x67\x78\x89"
+#define TEST_PHRASE64_ROT4 "EiM0RVZneIk#"
+#define TEST_HASH_ROT4 "ITJDVGV2h5g#"
+#define TEST_HASH_TRUNC "4ycQhg@@"
+#define TEST_HASH_TRUNC6 "4ycQhlpD"
+#define TEST_HASH_TRUNC_ROT4 "ITJDVAAAAAA#"
+#define TEST_HASH_TRUNC6_ROT4 "ITJDVGV2AAA#"
+#define TEST_HASH_ROT4_TRUNC "8j93l0@@"
+#define TEST_HASH_ROT4_TRUNC6 "8j93l6lS"
+#define TEST_HASH_TRUNC_TRUNC6 "4ycQhg00"
+
 #define SP4 "    "
 #define SP20 SP4 SP4 SP4 SP4 SP4
 #define SP80 SP20 SP20 SP20 SP20
@@ -77,23 +265,27 @@ check(const char *phrase, const char *settings, const char *chain, size_t chain_
 	EXPECT(librecrypt_crypt(buf2, sizeof(buf2), phrase, len, chain, ctx) == (ssize_t)(hashlen - hash_prefix + chain_prefix));
 	EXPECT(!memcmp(buf2, chain, chain_prefix));
 	EXPECT(!memcmp(&buf[hash_prefix], &buf2[chain_prefix], hashlen - hash_prefix + 1u));
-	CANARY_X_CHECK(buf, hashlen, scratchsize);
 	CANARY_X_CHECK(buf2, hashlen - hash_prefix + chain_prefix, scratchsize);
+	CANARY_X_CHECK(buf, hashlen, scratchsize);
 }
 
 
-#define CHECK(PHRASE, CONF, HASHLEN, IS_DEFAULT_HASHLEN, HASH)\
+#define CHECK(PHRASE, CONF, HASHLEN, IS_DEFAULT_HASHLEN /* -1 if fixed */, HASH)\
 	do {\
 		size_t scratchsize = GET_SCRATCH_SIZE(HASHLEN);\
-		check(PHRASE, CONF HASH, CONF "*" #HASHLEN ">" CONF HASH,\
-		      sizeof(CONF "*" #HASHLEN ">" CONF) - 1u, CONF HASH,\
-		      sizeof(CONF) - 1u, scratchsize, ctx);\
-		check(PHRASE, CONF "*" #HASHLEN, CONF "*" #HASHLEN ">" CONF "*" #HASHLEN,\
-		      sizeof(CONF "*" #HASHLEN ">" CONF) - 1u, CONF HASH,\
-		      sizeof(CONF) - 1u, scratchsize, ctx);\
+		if (IS_DEFAULT_HASHLEN >= 0) {\
+			check(PHRASE, CONF HASH, CONF "*" #HASHLEN ">" CONF HASH,\
+			      sizeof(CONF "*" #HASHLEN ">" CONF) - 1u, CONF HASH,\
+			      sizeof(CONF) - 1u, scratchsize, ctx);\
+			check(PHRASE, CONF "*" #HASHLEN, CONF "*" #HASHLEN ">" CONF "*" #HASHLEN,\
+			      sizeof(CONF "*" #HASHLEN ">" CONF) - 1u, CONF HASH,\
+			      sizeof(CONF) - 1u, scratchsize, ctx);\
+		}\
 		if (IS_DEFAULT_HASHLEN) {\
 			check(PHRASE, CONF, CONF ">" CONF, sizeof(CONF ">" CONF) - 1u,\
 			      CONF HASH, sizeof(CONF) - 1u, scratchsize, ctx);\
+			check(PHRASE, CONF HASH, CONF ">" CONF HASH, sizeof(CONF ">" CONF) - 1u,\
+			      CONF HASH, sizeof(CONF) - 1u, scratchsize, ctx);\
 		}\
 	} while (0)
 
@@ -109,6 +301,7 @@ check(const char *phrase, const char *settings, const char *chain, size_t chain_
 int
 main(void)
 {
+	const struct librecrypt_algorithm custom[] = {rot4_algo, trunc_algo};
 	char buf[1024], buf2[1024], conf[256], nuls[256];
 	LIBRECRYPT_CONTEXT *ctx = NULL;
 	ssize_t r;
@@ -275,6 +468,22 @@ main(void)
 # undef GET_SCRATCH_SIZE
 #endif
 
+#define GET_SCRATCH_SIZE(HASHLEN) (HASHLEN)
+	librecrypt_set_custom_algorithms(ctx, custom, ELEMSOF(custom));
+	CHECK(TEST_PHRASE, "$trunc$", 4, 1, TEST_HASH_TRUNC);
+	CHECK(TEST_PHRASE, "$trunc$", 6, 0, TEST_HASH_TRUNC6);
+	CHECK(TEST_PHRASE, "$rot4$", 8, -1, TEST_HASH_ROT4);
+	CHECK(TEST_PHRASE, "$rot4$>$trunc$", 4, 1, TEST_HASH_ROT4_TRUNC);
+	CHECK(TEST_PHRASE, "$rot4$>$trunc$", 6, 0, TEST_HASH_ROT4_TRUNC6);
+	CHECK(TEST_PHRASE, "$trunc$>$rot4$", 8, -1, TEST_HASH_TRUNC_ROT4);
+	CHECK(TEST_PHRASE, "$trunc$*6>$rot4$", 8, -1, TEST_HASH_TRUNC6_ROT4);
+	CHECK(TEST_PHRASE, "$trunc$>$trunc$", 4, 1, TEST_HASH_TRUNC);
+	CHECK(TEST_PHRASE, "$trunc$>$trunc$", 6, 0, TEST_HASH_TRUNC_TRUNC6);
+	CHECK(TEST_PHRASE, "$trunc$*6>$trunc$", 4, 1, TEST_HASH_TRUNC);
+	CHECK(TEST_PHRASE, "$trunc$*6>$trunc$", 6, 0, TEST_HASH_TRUNC6);
+	CHECK(TEST_PHRASE, "$rot4$>$rot4$", 8, -1, TEST_PHRASE64_ROT4);
+#undef GET_SCRATCH_SIZE
+
 	librecrypt_free_context(ctx);
 
 	STOP_RESOURCE_TEST();
diff --git a/librecrypt_get_encoding.c b/librecrypt_get_encoding.c
index 7d0e62c..6d02d07 100644
--- a/librecrypt_get_encoding.c
+++ b/librecrypt_get_encoding.c
@@ -38,6 +38,88 @@ librecrypt_get_encoding(const char *settings, size_t len, char *pad_out, int *st
 # ifndef FUZZ
 
 
+#define ALPHABET "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
+#define ALT_ALPHABET "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ+/"
+
+NONSTRING static const char encoding_lut[256u] = MAKE_ENCODING_LUT(ALPHABET);
+NONSTRING static const char alt_encoding_lut[256u] = MAKE_ENCODING_LUT(ALT_ALPHABET);
+
+static const unsigned char decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX, XX,
+	XX,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
+	15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, XX, XX, XX, XX, XX,
+	XX, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+	41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static const unsigned char alt_decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, XX, XX, XX, XX, XX, XX,
+	XX, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50,
+	51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX,
+	XX, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
+	25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static unsigned
+rot4_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$rot4$") - 1u)
+		if (!strncmp(settings, "$rot4$", sizeof("$rot4$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static unsigned
+trunc_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$trunc$") - 1u)
+		if (!strncmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static const struct librecrypt_algorithm rot4_algo = {
+	.is_algorithm = &rot4_is_algorithm,
+	.encoding_lut = encoding_lut,
+	.decoding_lut = decoding_lut,
+	.hash_size = 8u,
+	.flexible_hash_size = 0,
+	.strict_pad = 1,
+	.pad = '#'
+};
+
+static const struct librecrypt_algorithm trunc_algo = {
+	.is_algorithm = &trunc_is_algorithm,
+	.encoding_lut = alt_encoding_lut,
+	.decoding_lut = alt_decoding_lut,
+	.hash_size = 4u,
+	.flexible_hash_size = 1,
+	.strict_pad = 1,
+	.pad = '@'
+};
+
 #define NSA "$~no~such~algorithm~$"
 
 
@@ -73,7 +155,7 @@ check_decoding_lut(const unsigned char *lut, const char *alpha)
 	do {\
 		pad = (char)~(PAD);\
 		strict_pad = -1;\
-		elut = librecrypt_get_encoding(PREFIX, sizeof(PREFIX) - 1u, &pad, &strict_pad, 0, NULL);\
+		elut = librecrypt_get_encoding(PREFIX, sizeof(PREFIX) - 1u, &pad, &strict_pad, 0, ctx);\
 		EXPECT(elut != NULL);\
 		EXPECT(pad == (PAD));\
 		EXPECT(strict_pad == (STRICT_PAD));\
@@ -81,7 +163,7 @@ check_decoding_lut(const unsigned char *lut, const char *alpha)
 		\
 		pad = (char)~(PAD);\
 		strict_pad = -1;\
-		dlut = librecrypt_get_encoding(PREFIX, sizeof(PREFIX) - 1u, &pad, &strict_pad, 1, NULL);\
+		dlut = librecrypt_get_encoding(PREFIX, sizeof(PREFIX) - 1u, &pad, &strict_pad, 1, ctx);\
 		EXPECT(dlut != NULL);\
 		EXPECT(pad == (PAD));\
 		EXPECT(strict_pad == (STRICT_PAD));\
@@ -89,7 +171,7 @@ check_decoding_lut(const unsigned char *lut, const char *alpha)
 		\
 		pad = (char)~(PAD);\
 		strict_pad = -1;\
-		elut = librecrypt_get_encoding(NSA">"PREFIX, sizeof(NSA">"PREFIX) - 1u, &pad, &strict_pad, 0, NULL);\
+		elut = librecrypt_get_encoding(NSA">"PREFIX, sizeof(NSA">"PREFIX) - 1u, &pad, &strict_pad, 0, ctx);\
 		EXPECT(elut != NULL);\
 		EXPECT(pad == (PAD));\
 		EXPECT(strict_pad == (STRICT_PAD));\
@@ -105,6 +187,8 @@ check_decoding_lut(const unsigned char *lut, const char *alpha)
 int
 main(void)
 {
+	const struct librecrypt_algorithm custom[] = {rot4_algo, trunc_algo};
+	LIBRECRYPT_CONTEXT *ctx = NULL;
 	const char *elut;
 	const unsigned char *dlut;
 	char pad;
@@ -114,19 +198,19 @@ main(void)
 	INIT_RESOURCE_TEST();
 
 	errno = 0;
-	EXPECT(librecrypt_get_encoding(NSA, sizeof(NSA) - 1u, &pad, &strict_pad, 0, NULL) == NULL);
+	EXPECT(librecrypt_get_encoding(NSA, sizeof(NSA) - 1u, &pad, &strict_pad, 0, ctx) == NULL);
 	EXPECT(errno == ENOSYS);
 
 	errno = 0;
-	EXPECT(librecrypt_get_encoding(NSA, sizeof(NSA) - 1u, &pad, &strict_pad, 1, NULL) == NULL);
+	EXPECT(librecrypt_get_encoding(NSA, sizeof(NSA) - 1u, &pad, &strict_pad, 1, ctx) == NULL);
 	EXPECT(errno == ENOSYS);
 
 	errno = 0;
-	EXPECT(librecrypt_get_encoding(">"NSA, sizeof(">"NSA) - 1u, &pad, &strict_pad, 0, NULL) == NULL);
+	EXPECT(librecrypt_get_encoding(">"NSA, sizeof(">"NSA) - 1u, &pad, &strict_pad, 0, ctx) == NULL);
 	EXPECT(errno == ENOSYS);
 
 	errno = 0;
-	EXPECT(librecrypt_get_encoding(">"NSA, sizeof(">"NSA) - 1u, &pad, &strict_pad, 1, NULL) == NULL);
+	EXPECT(librecrypt_get_encoding(">"NSA, sizeof(">"NSA) - 1u, &pad, &strict_pad, 1, ctx) == NULL);
 	EXPECT(errno == ENOSYS);
 
 	IF__argon2i_v1_0__SUPPORTED(CHECK("$argon2i$", UPPER LOWER DIGIT "+/", '=', 0);)
@@ -139,6 +223,20 @@ main(void)
 	IF__argon2id_v1_3__SUPPORTED(CHECK("$argon2id$v=19$", UPPER LOWER DIGIT "+/", '=', 0);)
 	IF__argon2ds_v1_3__SUPPORTED(CHECK("$argon2ds$v=19$", UPPER LOWER DIGIT "+/", '=', 0);)
 
+	ctx = librecrypt_create_context();
+	assert(ctx != NULL);
+
+	librecrypt_set_custom_algorithms(ctx, custom, ELEMSOF(custom));
+	CHECK("$rot4$", ALPHABET, '#', 1);
+	CHECK("$trunc$", ALT_ALPHABET, '@', 1);
+	CHECK("$trunc$*4", ALT_ALPHABET, '@', 1);
+	CHECK("$rot4$>$trunc$", ALT_ALPHABET, '@', 1);
+	CHECK("$rot4$>$trunc$*4", ALT_ALPHABET, '@', 1);
+	CHECK("$trunc$>$rot4$", ALPHABET, '#', 1);
+	CHECK("$trunc$*4>$rot4$", ALPHABET, '#', 1);
+
+	librecrypt_free_context(ctx);
+
 	STOP_RESOURCE_TEST();
 	return 0;
 }
diff --git a/librecrypt_hash.c b/librecrypt_hash.c
index 3d37d03..18627d1 100644
--- a/librecrypt_hash.c
+++ b/librecrypt_hash.c
@@ -14,6 +14,194 @@ librecrypt_hash(char *restrict out_buffer, size_t size, const char *phrase,
 #else
 
 
+#define ALPHABET "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
+#define ALT_ALPHABET "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ+/"
+
+NONSTRING static const char encoding_lut[256u] = MAKE_ENCODING_LUT(ALPHABET);
+NONSTRING static const char alt_encoding_lut[256u] = MAKE_ENCODING_LUT(ALT_ALPHABET);
+
+static const unsigned char decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX, XX,
+	XX,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
+	15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, XX, XX, XX, XX, XX,
+	XX, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+	41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static const unsigned char alt_decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, XX, XX, XX, XX, XX, XX,
+	XX, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50,
+	51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX,
+	XX, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
+	25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static unsigned
+rot4_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$rot4$") - 1u)
+		if (!strncmp(settings, "$rot4$", sizeof("$rot4$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static unsigned
+trunc_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$trunc$") - 1u)
+		if (!strncmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static int
+trunc_supported(const char *phrase, size_t len, int text, const char *settings,
+                size_t prefix, size_t *len_out)
+{
+	size_t i, digit, n = 0u, q, r;
+
+	(void) phrase;
+	(void) len;
+	(void) text;
+
+	if (prefix < sizeof("$trunc$") - 1u || memcmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+		return 0;
+
+	if (prefix == sizeof("$trunc$") - 1u) {
+		*len_out = 4u;
+		return 1;
+	}
+
+	if (settings[sizeof("$trunc$") - 1u] == '*') {
+		for (i = sizeof("$trunc$*") - 1u; i < prefix; i++) {
+			if ('0' > settings[i] || settings[i] > '9')
+				return 0;
+			digit = (size_t)(settings[i] - '0');
+			if (n > (SIZE_MAX - digit) / 10u)
+				return 0;
+			n = n * 10u + digit;
+		}
+	} else {
+		for (i = sizeof("$trunc$") - 1u; i < prefix; i++) {
+			if (settings[i] == '@')
+				break;
+			if (alt_decoding_lut[(unsigned char)settings[i]] == XX)
+				return 0;
+			n += 1u;
+		}
+		q = n / 4u;
+		r = n % 4u;
+		if (r == 1u)
+			return 0;
+		n = q * 3u + r;
+		if (r) {
+			n -= 1u;
+			for (; r < 4u && i < prefix; r++, i++)
+				if (settings[i] != '@')
+					break;
+			if (r != 4u || i != prefix)
+				return 0;
+		}
+	}
+	if (!n)
+		return 0;
+	*len_out = n;
+	return 1;
+}
+
+static int
+rot4_hash(char *restrict out_buffer, size_t size, const char *phrase, size_t len,
+          const char *settings, size_t prefix, LIBRECRYPT_CONTEXT *ctx)
+{
+	size_t i;
+
+	(void) settings;
+	(void) prefix;
+	(void) ctx;
+
+	for (i = 0u; i < 8u && i < size; i++) {
+		out_buffer[i] = '\0';
+		if (i < len)
+			out_buffer[i] = (char)((((int)phrase[i] & 0x0F) << 4) | (((int)phrase[i] >> 4) & 0x0F));
+	}
+	return 0;
+}
+
+static int
+trunc_hash(char *restrict out_buffer, size_t size, const char *phrase, size_t len,
+           const char *settings, size_t prefix, LIBRECRYPT_CONTEXT *ctx)
+{
+	size_t hash_size, i;
+
+	(void) ctx;
+
+	if (!trunc_supported(NULL, 0u, 1, settings, prefix, &hash_size)) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	size = MIN(hash_size, size);
+	for (i = 0u; i < size; i++)
+		out_buffer[i] = i < len ? phrase[i] : '\0';
+
+	return 0;
+}
+
+static const struct librecrypt_algorithm rot4_algo = {
+	.is_algorithm = &rot4_is_algorithm,
+	.hash = &rot4_hash,
+	.encoding_lut = encoding_lut,
+	.decoding_lut = decoding_lut,
+	.hash_size = 8u,
+	.flexible_hash_size = 0,
+	.strict_pad = 1,
+	.pad = '#'
+};
+
+static const struct librecrypt_algorithm trunc_algo = {
+	.is_algorithm = &trunc_is_algorithm,
+	.hash = &trunc_hash,
+	.encoding_lut = alt_encoding_lut,
+	.decoding_lut = alt_decoding_lut,
+	.hash_size = 4u,
+	.flexible_hash_size = 1,
+	.strict_pad = 1,
+	.pad = '@'
+};
+
+#define TEST_PHRASE "\x12\x23\x34\x45\x56\x67\x78\x89"
+#define TEST_PHRASE64_ROT4 "EiM0RVZneIk#"
+#define TEST_HASH_ROT4 "ITJDVGV2h5g#"
+#define TEST_HASH_TRUNC "4ycQhg@@"
+#define TEST_HASH_TRUNC6 "4ycQhlpD"
+#define TEST_HASH_TRUNC_ROT4 "ITJDVAAAAAA#"
+#define TEST_HASH_TRUNC6_ROT4 "ITJDVGV2AAA#"
+#define TEST_HASH_ROT4_TRUNC "8j93l0@@"
+#define TEST_HASH_ROT4_TRUNC6 "8j93l6lS"
+#define TEST_HASH_TRUNC_TRUNC6 "4ycQhg00"
+
 #define SP4 "    "
 #define SP20 SP4 SP4 SP4 SP4 SP4
 #define SP80 SP20 SP20 SP20 SP20
@@ -79,18 +267,22 @@ check(const char *phrase, const char *settings, const char *chain,
 	errno = 0;
 	EXPECT(librecrypt_hash(buf2, sizeof(buf2), phrase, len, chain, ctx) == (ssize_t)hashlen);
 	EXPECT(!memcmp(buf, buf2, hashlen + 1u));
-	CANARY_X_CHECK(buf, hashlen + 2, scratchsize);
 	CANARY_X_CHECK(buf2, hashlen + 2, scratchsize);
+	CANARY_X_CHECK(buf, hashlen + 2, scratchsize);
 }
 
 
-#define CHECK(PHRASE, CONF, HASHLEN, IS_DEFAULT_HASHLEN, HASH)\
+#define CHECK(PHRASE, CONF, HASHLEN, IS_DEFAULT_HASHLEN /* -1 if fixed */, HASH)\
 	do {\
 		size_t scratchsize = GET_SCRATCH_SIZE(HASHLEN);\
-		check(PHRASE, CONF HASH, CONF "*" #HASHLEN ">" CONF HASH, HASH, scratchsize, ctx);\
-		check(PHRASE, CONF "*" #HASHLEN, CONF "*" #HASHLEN ">" CONF "*" #HASHLEN, HASH, scratchsize, ctx);\
-		if (IS_DEFAULT_HASHLEN)\
+		if (IS_DEFAULT_HASHLEN >= 0) {\
+			check(PHRASE, CONF HASH, CONF "*" #HASHLEN ">" CONF HASH, HASH, scratchsize, ctx);\
+			check(PHRASE, CONF "*" #HASHLEN, CONF "*" #HASHLEN ">" CONF "*" #HASHLEN, HASH, scratchsize, ctx);\
+		}\
+		if (IS_DEFAULT_HASHLEN) {\
 			check(PHRASE, CONF, CONF ">" CONF, HASH, scratchsize, ctx);\
+			check(PHRASE, CONF HASH, CONF ">" CONF HASH, HASH, scratchsize, ctx);\
+		}\
 	} while (0)
 
 
@@ -129,6 +321,7 @@ check(const char *phrase, const char *settings, const char *chain,
 int
 main(void)
 {
+	const struct librecrypt_algorithm custom[] = {rot4_algo, trunc_algo};
 	char conf[256], nuls[256];
 	LIBRECRYPT_CONTEXT *ctx = NULL;
 	int r;
@@ -216,6 +409,22 @@ main(void)
 # undef GET_SCRATCH_SIZE
 #endif
 
+#define GET_SCRATCH_SIZE(HASHLEN) (HASHLEN)
+	librecrypt_set_custom_algorithms(ctx, custom, ELEMSOF(custom));
+	CHECK(TEST_PHRASE, "$trunc$", 4, 1, TEST_HASH_TRUNC);
+	CHECK(TEST_PHRASE, "$trunc$", 6, 0, TEST_HASH_TRUNC6);
+	CHECK(TEST_PHRASE, "$rot4$", 8, -1, TEST_HASH_ROT4);
+	CHECK(TEST_PHRASE, "$rot4$>$trunc$", 4, 1, TEST_HASH_ROT4_TRUNC);
+	CHECK(TEST_PHRASE, "$rot4$>$trunc$", 6, 0, TEST_HASH_ROT4_TRUNC6);
+	CHECK(TEST_PHRASE, "$trunc$>$rot4$", 8, -1, TEST_HASH_TRUNC_ROT4);
+	CHECK(TEST_PHRASE, "$trunc$*6>$rot4$", 8, -1, TEST_HASH_TRUNC6_ROT4);
+	CHECK(TEST_PHRASE, "$trunc$>$trunc$", 4, 1, TEST_HASH_TRUNC);
+	CHECK(TEST_PHRASE, "$trunc$>$trunc$", 6, 0, TEST_HASH_TRUNC_TRUNC6);
+	CHECK(TEST_PHRASE, "$trunc$*6>$trunc$", 4, 1, TEST_HASH_TRUNC);
+	CHECK(TEST_PHRASE, "$trunc$*6>$trunc$", 6, 0, TEST_HASH_TRUNC6);
+	CHECK(TEST_PHRASE, "$rot4$>$rot4$", 8, -1, TEST_PHRASE64_ROT4);
+#undef GET_SCRATCH_SIZE
+
 	librecrypt_free_context(ctx);
 
 	STOP_RESOURCE_TEST();
diff --git a/librecrypt_hash_.c b/librecrypt_hash_.c
index 71ec98d..164a47b 100644
--- a/librecrypt_hash_.c
+++ b/librecrypt_hash_.c
@@ -118,18 +118,15 @@ next:
 		/* Special case for bsdicrypt */
 		prefix = 1u; /* $covered$ (TODO we currently don't have an algorithm to trigger this) */
 	}
-	if (!algo->flexible_hash_size && prefix != n)
-		goto einval;
 
 	/* Get hash size */
-	if (!algo->flexible_hash_size) {
-		/* fixed */
-		hash_size = algo->hash_size;
-	} else if (prefix == n) {
+	if (prefix == n) {
 		/* default */
 		hash_size = algo->hash_size;
 	} else if (settings[prefix] == '*') {
-		/* hash length encoded */
+		/* hash length encoded, not allowed when fixed */
+		if (!algo->flexible_hash_size)
+			goto einval;
 		i = prefix + 1u;
 		hash_size = 0u;
 		for (; i < n; i++) {
@@ -169,6 +166,9 @@ next:
 		hash_size = quotient * 3u;
 		if (remainder)
 			hash_size += remainder - 1u;
+		/* Must align with fixed hash size when hash size is fixed */
+		if (!algo->flexible_hash_size && hash_size != algo->hash_size)
+			goto einval;
 	}
 
 	/* For `librecrypt_crypt`: copy hash configurations to output */
@@ -436,7 +436,7 @@ add1_hash(char *restrict out_buffer, size_t size, const char *phrase, size_t len
 	return 0;
 }
 
-static const struct librecrypt_algorithm rot4 = {
+static const struct librecrypt_algorithm rot4_algo = {
 	.is_algorithm = &rot4_is_algorithm,
 	.hash = &rot4_hash,
 	.encoding_lut = elut,
@@ -447,7 +447,7 @@ static const struct librecrypt_algorithm rot4 = {
 	.pad = '#'
 };
 
-static const struct librecrypt_algorithm add1 = {
+static const struct librecrypt_algorithm add1_algo = {
 	.is_algorithm = &add1_is_algorithm,
 	.hash = &add1_hash,
 	.encoding_lut = elut,
@@ -476,7 +476,7 @@ main(void)
 	size_t i, n;
 	ssize_t r, r1, r1b, r1c, r2, r3;
 	LIBRECRYPT_CONTEXT *ctx = NULL;
-	struct librecrypt_algorithm custom[] = {rot4, add1};
+	struct librecrypt_algorithm custom[] = {rot4_algo, add1_algo};
 
 	SET_UP_ALARM();
 	INIT_RESOURCE_TEST();
diff --git a/librecrypt_hash_binary.c b/librecrypt_hash_binary.c
index 8596160..398f7e3 100644
--- a/librecrypt_hash_binary.c
+++ b/librecrypt_hash_binary.c
@@ -14,6 +14,194 @@ librecrypt_hash_binary(void *restrict out_buffer, size_t size, const char *phras
 #else
 
 
+#define ALPHABET "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
+#define ALT_ALPHABET "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ+/"
+
+NONSTRING static const char encoding_lut[256u] = MAKE_ENCODING_LUT(ALPHABET);
+NONSTRING static const char alt_encoding_lut[256u] = MAKE_ENCODING_LUT(ALT_ALPHABET);
+
+static const unsigned char decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX, XX,
+	XX,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
+	15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, XX, XX, XX, XX, XX,
+	XX, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+	41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static const unsigned char alt_decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, XX, XX, XX, XX, XX, XX,
+	XX, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50,
+	51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX,
+	XX, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
+	25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static unsigned
+rot4_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$rot4$") - 1u)
+		if (!strncmp(settings, "$rot4$", sizeof("$rot4$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static unsigned
+trunc_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$trunc$") - 1u)
+		if (!strncmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static int
+trunc_supported(const char *phrase, size_t len, int text, const char *settings,
+                size_t prefix, size_t *len_out)
+{
+	size_t i, digit, n = 0u, q, r;
+
+	(void) phrase;
+	(void) len;
+	(void) text;
+
+	if (prefix < sizeof("$trunc$") - 1u || memcmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+		return 0;
+
+	if (prefix == sizeof("$trunc$") - 1u) {
+		*len_out = 4u;
+		return 1;
+	}
+
+	if (settings[sizeof("$trunc$") - 1u] == '*') {
+		for (i = sizeof("$trunc$*") - 1u; i < prefix; i++) {
+			if ('0' > settings[i] || settings[i] > '9')
+				return 0;
+			digit = (size_t)(settings[i] - '0');
+			if (n > (SIZE_MAX - digit) / 10u)
+				return 0;
+			n = n * 10u + digit;
+		}
+	} else {
+		for (i = sizeof("$trunc$") - 1u; i < prefix; i++) {
+			if (settings[i] == '@')
+				break;
+			if (alt_decoding_lut[(unsigned char)settings[i]] == XX)
+				return 0;
+			n += 1u;
+		}
+		q = n / 4u;
+		r = n % 4u;
+		if (r == 1u)
+			return 0;
+		n = q * 3u + r;
+		if (r) {
+			n -= 1u;
+			for (; r < 4u && i < prefix; r++, i++)
+				if (settings[i] != '@')
+					break;
+			if (r != 4u || i != prefix)
+				return 0;
+		}
+	}
+	if (!n)
+		return 0;
+	*len_out = n;
+	return 1;
+}
+
+static int
+rot4_hash(char *restrict out_buffer, size_t size, const char *phrase, size_t len,
+          const char *settings, size_t prefix, LIBRECRYPT_CONTEXT *ctx)
+{
+	size_t i;
+
+	(void) settings;
+	(void) prefix;
+	(void) ctx;
+
+	for (i = 0u; i < 8u && i < size; i++) {
+		out_buffer[i] = '\0';
+		if (i < len)
+			out_buffer[i] = (char)((((int)phrase[i] & 0x0F) << 4) | (((int)phrase[i] >> 4) & 0x0F));
+	}
+	return 0;
+}
+
+static int
+trunc_hash(char *restrict out_buffer, size_t size, const char *phrase, size_t len,
+           const char *settings, size_t prefix, LIBRECRYPT_CONTEXT *ctx)
+{
+	size_t hash_size, i;
+
+	(void) ctx;
+
+	if (!trunc_supported(NULL, 0u, 1, settings, prefix, &hash_size)) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	size = MIN(hash_size, size);
+	for (i = 0u; i < size; i++)
+		out_buffer[i] = i < len ? phrase[i] : '\0';
+
+	return 0;
+}
+
+static const struct librecrypt_algorithm rot4_algo = {
+	.is_algorithm = &rot4_is_algorithm,
+	.hash = &rot4_hash,
+	.encoding_lut = encoding_lut,
+	.decoding_lut = decoding_lut,
+	.hash_size = 8u,
+	.flexible_hash_size = 0,
+	.strict_pad = 1,
+	.pad = '#'
+};
+
+static const struct librecrypt_algorithm trunc_algo = {
+	.is_algorithm = &trunc_is_algorithm,
+	.hash = &trunc_hash,
+	.encoding_lut = alt_encoding_lut,
+	.decoding_lut = alt_decoding_lut,
+	.hash_size = 4u,
+	.flexible_hash_size = 1,
+	.strict_pad = 1,
+	.pad = '@'
+};
+
+#define TEST_PHRASE "\x12\x23\x34\x45\x56\x67\x78\x89"
+#define TEST_PHRASE64_ROT4 "EiM0RVZneIk#"
+#define TEST_HASH_ROT4 "ITJDVGV2h5g#"
+#define TEST_HASH_TRUNC "4ycQhg@@"
+#define TEST_HASH_TRUNC6 "4ycQhlpD"
+#define TEST_HASH_TRUNC_ROT4 "ITJDVAAAAAA#"
+#define TEST_HASH_TRUNC6_ROT4 "ITJDVGV2AAA#"
+#define TEST_HASH_ROT4_TRUNC "8j93l0@@"
+#define TEST_HASH_ROT4_TRUNC6 "8j93l6lS"
+#define TEST_HASH_TRUNC_TRUNC6 "4ycQhg00"
+
 #define SP4 "    "
 #define SP20 SP4 SP4 SP4 SP4 SP4
 #define SP80 SP20 SP20 SP20 SP20
@@ -67,18 +255,22 @@ check(const char *phrase, const char *settings, const char *chain, const char *h
 	errno = 0;
 	EXPECT(librecrypt_hash_binary(buf2, sizeof(buf2), phrase, len, chain, ctx) == (ssize_t)hashlen);
 	EXPECT(!memcmp(buf, buf2, hashlen));
-	CANARY_X_CHECK(buf, hashlen, scratchsize);
 	CANARY_X_CHECK(buf2, hashlen, scratchsize);
+	CANARY_X_CHECK(buf, hashlen, scratchsize);
 }
 
 
-#define CHECK(PHRASE, CONF, HASHLEN, IS_DEFAULT_HASHLEN, HASH)\
+#define CHECK(PHRASE, CONF, HASHLEN, IS_DEFAULT_HASHLEN /* -1 if fixed */, HASH)\
 	do {\
 		size_t scratchsize = GET_SCRATCH_SIZE(HASHLEN);\
-		check(PHRASE, CONF HASH, CONF "*" #HASHLEN ">" CONF HASH, HASH, (size_t)HASHLEN, scratchsize, ctx);\
-		check(PHRASE, CONF "*" #HASHLEN, CONF "*" #HASHLEN ">" CONF "*" #HASHLEN, HASH, (size_t)HASHLEN, scratchsize, ctx);\
-		if (IS_DEFAULT_HASHLEN)\
+		if (IS_DEFAULT_HASHLEN >= 0) {\
+			check(PHRASE, CONF HASH, CONF "*" #HASHLEN ">" CONF HASH, HASH, (size_t)HASHLEN, scratchsize, ctx);\
+			check(PHRASE, CONF "*" #HASHLEN, CONF "*" #HASHLEN ">" CONF "*" #HASHLEN, HASH, (size_t)HASHLEN, scratchsize, ctx);\
+		}\
+		if (IS_DEFAULT_HASHLEN) {\
 			check(PHRASE, CONF, CONF ">" CONF, HASH, (size_t)HASHLEN, scratchsize, ctx);\
+			check(PHRASE, CONF HASH, CONF ">" CONF HASH, HASH, (size_t)HASHLEN, scratchsize, ctx);\
+		}\
 	} while (0)
 
 
@@ -96,6 +288,7 @@ check(const char *phrase, const char *settings, const char *chain, const char *h
 int
 main(void)
 {
+	const struct librecrypt_algorithm custom[] = {rot4_algo, trunc_algo};
 	LIBRECRYPT_CONTEXT *ctx = NULL;
 	char nuls[256];
 
@@ -164,6 +357,22 @@ main(void)
 # undef GET_SCRATCH_SIZE
 #endif
 
+#define GET_SCRATCH_SIZE(HASHLEN) (HASHLEN)
+	librecrypt_set_custom_algorithms(ctx, custom, ELEMSOF(custom));
+	CHECK(TEST_PHRASE, "$trunc$", 4, 1, TEST_HASH_TRUNC);
+	CHECK(TEST_PHRASE, "$trunc$", 6, 0, TEST_HASH_TRUNC6);
+	CHECK(TEST_PHRASE, "$rot4$", 8, -1, TEST_HASH_ROT4);
+	CHECK(TEST_PHRASE, "$rot4$>$trunc$", 4, 1, TEST_HASH_ROT4_TRUNC);
+	CHECK(TEST_PHRASE, "$rot4$>$trunc$", 6, 0, TEST_HASH_ROT4_TRUNC6);
+	CHECK(TEST_PHRASE, "$trunc$>$rot4$", 8, -1, TEST_HASH_TRUNC_ROT4);
+	CHECK(TEST_PHRASE, "$trunc$*6>$rot4$", 8, -1, TEST_HASH_TRUNC6_ROT4);
+	CHECK(TEST_PHRASE, "$trunc$>$trunc$", 4, 1, TEST_HASH_TRUNC);
+	CHECK(TEST_PHRASE, "$trunc$>$trunc$", 6, 0, TEST_HASH_TRUNC_TRUNC6);
+	CHECK(TEST_PHRASE, "$trunc$*6>$trunc$", 4, 1, TEST_HASH_TRUNC);
+	CHECK(TEST_PHRASE, "$trunc$*6>$trunc$", 6, 0, TEST_HASH_TRUNC6);
+	CHECK(TEST_PHRASE, "$rot4$>$rot4$", 8, -1, TEST_PHRASE64_ROT4);
+#undef GET_SCRATCH_SIZE
+
 	librecrypt_free_context(ctx);
 
 	STOP_RESOURCE_TEST();
diff --git a/librecrypt_make_settings.c b/librecrypt_make_settings.c
index 84270d1..89a73fb 100644
--- a/librecrypt_make_settings.c
+++ b/librecrypt_make_settings.c
@@ -45,6 +45,201 @@ enosys:
 #else
 
 
+#define ALPHABET "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
+
+NONSTRING static const char encoding_lut[256u] = MAKE_ENCODING_LUT(ALPHABET);
+
+static const unsigned char decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX, XX,
+	XX,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
+	15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, XX, XX, XX, XX, XX,
+	XX, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+	41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static unsigned
+rot4_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$rot4$") - 1u)
+		if (!strncmp(settings, "$rot4$", sizeof("$rot4$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static unsigned
+trunc_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$trunc$") - 1u)
+		if (!strncmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static int
+rot4_supported(const char *phrase, size_t len, int text, const char *settings,
+               size_t prefix, size_t *len_out)
+{
+	size_t i, j, n;
+
+	(void) phrase;
+	(void) len;
+	(void) text;
+
+	i = sizeof("$rot4$") - 1u;
+	if (prefix < i || memcmp(settings, "$rot4$", prefix))
+		return 0;
+	n = i - prefix;
+	if (n && n != 12u)
+		return 0;
+	if (n == 12u) {
+		for (j = 0u; j < 11u; j++, i++)
+			if (decoding_lut[(unsigned char)settings[i]] == XX)
+				return 0;
+		if (decoding_lut[(unsigned char)settings[i]] != '#')
+			return 0;
+	}
+
+	*len_out = 8u;
+	return 1;
+}
+
+static int
+trunc_supported(const char *phrase, size_t len, int text, const char *settings,
+                size_t prefix, size_t *len_out)
+{
+	size_t i, digit, n = 0u, q, r;
+
+	(void) phrase;
+	(void) len;
+	(void) text;
+
+	if (prefix < sizeof("$trunc$") - 1u || memcmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+		return 0;
+
+	if (prefix == sizeof("$trunc$") - 1u) {
+		*len_out = 4u;
+		return 1;
+	}
+
+	if (settings[sizeof("$trunc$") - 1u] == '*') {
+		for (i = sizeof("$trunc$*") - 1u; i < prefix; i++) {
+			if ('0' > settings[i] || settings[i] > '9')
+				return 0;
+			digit = (size_t)(settings[i] - '0');
+			if (n > (SIZE_MAX - digit) / 10u)
+				return 0;
+			n = n * 10u + digit;
+		}
+	} else {
+		for (i = sizeof("$trunc$") - 1u; i < prefix; i++) {
+			if (settings[i] == '@')
+				break;
+			if (decoding_lut[(unsigned char)settings[i]] == XX)
+				return 0;
+			n += 1u;
+		}
+		q = n / 4u;
+		r = n % 4u;
+		if (r == 1u)
+			return 0;
+		n = q * 3u + r;
+		if (r) {
+			n -= 1u;
+			for (; r < 4u && i < prefix; r++, i++)
+				if (settings[i] != '@')
+					break;
+			if (r != 4u || i != prefix)
+				return 0;
+		}
+	}
+	if (!n)
+		return 0;
+	*len_out = n;
+	return 1;
+}
+
+static ssize_t
+rot4_make_settings(char *out_buffer, size_t size, const char *algorithm,
+                   size_t memcost, uintmax_t timecost, int gensalt,
+                   ssize_t (*rng)(void *out, size_t n, void *user), void *user)
+{
+	size_t ret = sizeof("$rot4$") - 1u;
+
+	(void) algorithm;
+	(void) memcost;
+	(void) timecost;
+	(void) gensalt;
+	(void) rng;
+	(void) user;
+
+	if (size) {
+		size = MIN(ret, size - 1u);
+		memcpy(out_buffer, "$rot4$", size);
+		out_buffer[size] = '\0';
+	}
+
+	return (ssize_t) ret;
+}
+
+static ssize_t
+trunc_make_settings(char *out_buffer, size_t size, const char *algorithm,
+                    size_t memcost, uintmax_t timecost, int gensalt,
+                    ssize_t (*rng)(void *out, size_t n, void *user), void *user)
+{
+	size_t ret = sizeof("$trunc$*4") - 1u;
+
+	(void) algorithm;
+	(void) memcost;
+	(void) timecost;
+	(void) gensalt;
+	(void) rng;
+	(void) user;
+
+	if (size) {
+		size = MIN(ret, size - 1u);
+		memcpy(out_buffer, "$trunc$*4", size);
+		out_buffer[size] = '\0';
+	}
+
+	return (ssize_t)ret;
+}
+
+static const struct librecrypt_algorithm rot4_algo = {
+	.is_algorithm = &rot4_is_algorithm,
+	.test_supported = &rot4_supported,
+	.make_settings = &rot4_make_settings,
+	.encoding_lut = encoding_lut,
+	.decoding_lut = decoding_lut,
+	.hash_size = 8u,
+	.flexible_hash_size = 0,
+	.strict_pad = 1,
+	.pad = '#'
+};
+
+static const struct librecrypt_algorithm trunc_algo = {
+	.is_algorithm = &trunc_is_algorithm,
+	.test_supported = &trunc_supported,
+	.make_settings = &trunc_make_settings,
+	.encoding_lut = encoding_lut,
+	.decoding_lut = decoding_lut,
+	.hash_size = 4u,
+	.flexible_hash_size = 1,
+	.strict_pad = 1,
+	.pad = '@'
+};
+
+
 static unsigned char saltbyte = 0u;
 
 
@@ -72,6 +267,8 @@ saltfail(void *out, size_t n, void *user)
 int
 main(void)
 {
+	const struct librecrypt_algorithm custom[] = {rot4_algo, trunc_algo};
+	LIBRECRYPT_CONTEXT *ctx = librecrypt_create_context();
 	char buf[1024];
 	char buf2[sizeof(buf)];
 	int any_supported = 0;
@@ -195,6 +392,31 @@ main(void)
 		EXPECT(errno == ENOSYS);
 	}
 
+	ctx = librecrypt_create_context();
+	assert(ctx != NULL);
+
+	librecrypt_set_custom_algorithms(ctx, custom, ELEMSOF(custom));
+
+	CANARY_FILL(buf);
+	r = librecrypt_make_settings(buf, sizeof(buf), "$rot4$", 0u, 0u, 0, NULL, NULL, ctx);
+	EXPECT(r == (ssize_t)(sizeof("$rot4$") - 1u));
+	EXPECT(!strcmp(buf, "$rot4$"));
+	CANARY_CHECK(buf, (size_t)r + 1u);
+
+	CANARY_FILL(buf);
+	r = librecrypt_make_settings(buf, sizeof(buf), "$trunc$", 0u, 0u, 0, NULL, NULL, ctx);
+	EXPECT(r == (ssize_t)(sizeof("$trunc$*4") - 1u));
+	EXPECT(!strcmp(buf, "$trunc$*4"));
+	CANARY_CHECK(buf, (size_t)r + 1u);
+
+	CANARY_FILL(buf);
+	r = librecrypt_make_settings(buf, sizeof(buf), "$trunc$*6", 0u, 0u, 0, NULL, NULL, ctx);
+	EXPECT(r == (ssize_t)(sizeof("$trunc$*4") - 1u));
+	EXPECT(!strcmp(buf, "$trunc$*4"));
+	CANARY_CHECK(buf, (size_t)r + 1u);
+
+	librecrypt_free_context(ctx);
+
 	STOP_RESOURCE_TEST();
 	return 0;
 }
diff --git a/librecrypt_test_supported.c b/librecrypt_test_supported.c
index f069d44..3e416c0 100644
--- a/librecrypt_test_supported.c
+++ b/librecrypt_test_supported.c
@@ -44,22 +44,171 @@ librecrypt_test_supported(const char *phrase, size_t len, int text, const char *
 # ifndef FUZZ
 
 
+#define ALPHABET "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
+
+NONSTRING static const char encoding_lut[256u] = MAKE_ENCODING_LUT(ALPHABET);
+
+static const unsigned char decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX, XX,
+	XX,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
+	15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, XX, XX, XX, XX, XX,
+	XX, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+	41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static unsigned
+rot4_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$rot4$") - 1u)
+		if (!strncmp(settings, "$rot4$", sizeof("$rot4$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static unsigned
+trunc_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$trunc$") - 1u)
+		if (!strncmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static int
+rot4_supported(const char *phrase, size_t len, int text, const char *settings,
+               size_t prefix, size_t *len_out)
+{
+	size_t i, j, n;
+
+	(void) phrase;
+	(void) len;
+	(void) text;
+
+	i = sizeof("$rot4$") - 1u;
+	if (prefix < i || memcmp(settings, "$rot4$", prefix))
+		return 0;
+	n = i - prefix;
+	if (n && n != 12u)
+		return 0;
+	if (n == 12u) {
+		for (j = 0u; j < 11u; j++, i++)
+			if (decoding_lut[(unsigned char)settings[i]] == XX)
+				return 0;
+		if (decoding_lut[(unsigned char)settings[i]] != '#')
+			return 0;
+	}
+
+	*len_out = 8u;
+	return 1;
+}
+
+static int
+trunc_supported(const char *phrase, size_t len, int text, const char *settings,
+                size_t prefix, size_t *len_out)
+{
+	size_t i, digit, n = 0u, q, r;
+
+	(void) phrase;
+	(void) len;
+	(void) text;
+
+	if (prefix < sizeof("$trunc$") - 1u || memcmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+		return 0;
+
+	if (prefix == sizeof("$trunc$") - 1u) {
+		*len_out = 4u;
+		return 1;
+	}
+
+	if (settings[sizeof("$trunc$") - 1u] == '*') {
+		for (i = sizeof("$trunc$*") - 1u; i < prefix; i++) {
+			if ('0' > settings[i] || settings[i] > '9')
+				return 0;
+			digit = (size_t)(settings[i] - '0');
+			if (n > (SIZE_MAX - digit) / 10u)
+				return 0;
+			n = n * 10u + digit;
+		}
+	} else {
+		for (i = sizeof("$trunc$") - 1u; i < prefix; i++) {
+			if (settings[i] == '@')
+				break;
+			if (decoding_lut[(unsigned char)settings[i]] == XX)
+				return 0;
+			n += 1u;
+		}
+		q = n / 4u;
+		r = n % 4u;
+		if (r == 1u)
+			return 0;
+		n = q * 3u + r;
+		if (r) {
+			n -= 1u;
+			for (; r < 4u && i < prefix; r++, i++)
+				if (settings[i] != '@')
+					break;
+			if (r != 4u || i != prefix)
+				return 0;
+		}
+	}
+	if (!n)
+		return 0;
+	*len_out = n;
+	return 1;
+}
+
+static const struct librecrypt_algorithm rot4_algo = {
+	.is_algorithm = &rot4_is_algorithm,
+	.test_supported = &rot4_supported,
+	.encoding_lut = encoding_lut,
+	.decoding_lut = decoding_lut,
+	.hash_size = 8u,
+	.flexible_hash_size = 0,
+	.strict_pad = 1,
+	.pad = '#'
+};
+
+static const struct librecrypt_algorithm trunc_algo = {
+	.is_algorithm = &trunc_is_algorithm,
+	.test_supported = &trunc_supported,
+	.encoding_lut = encoding_lut,
+	.decoding_lut = decoding_lut,
+	.hash_size = 4u,
+	.flexible_hash_size = 1,
+	.strict_pad = 1,
+	.pad = '@'
+};
+
 #define NSA "$~no~such~algorithm~$"
 
 
 #define CHECK(ALGO, VALID, INVALID)\
 	do {\
-		EXPECT(librecrypt_test_supported(NULL, 4096u, 0, ALGO VALID, NULL) == 1);\
-		EXPECT(librecrypt_test_supported(NULL, 4096u, 0, ALGO VALID">"NSA, NULL) == 0);\
-		EXPECT(librecrypt_test_supported(NULL, 4096u, 0, NSA">"ALGO VALID, NULL) == 0);\
-		EXPECT(librecrypt_test_supported(NULL, 4096u, 0, ALGO VALID">"ALGO VALID, NULL) == 1);\
-		EXPECT(librecrypt_test_supported(NULL, 4096u, 0, ALGO INVALID, NULL) == 0);\
+		EXPECT(librecrypt_test_supported(NULL, 4096u, 0, ALGO VALID, ctx) == 1);\
+		EXPECT(librecrypt_test_supported(NULL, 4096u, 0, ALGO VALID">"NSA, ctx) == 0);\
+		EXPECT(librecrypt_test_supported(NULL, 4096u, 0, NSA">"ALGO VALID, ctx) == 0);\
+		EXPECT(librecrypt_test_supported(NULL, 4096u, 0, ALGO VALID">"ALGO VALID, ctx) == 1);\
+		EXPECT(librecrypt_test_supported(NULL, 4096u, 0, ALGO INVALID, ctx) == 0);\
 	} while (0)
 
 
 int
 main(void)
 {
+	const struct librecrypt_algorithm custom[] = {rot4_algo, trunc_algo};
+	LIBRECRYPT_CONTEXT *ctx = NULL;
+
 	SET_UP_ALARM();
 	INIT_RESOURCE_TEST();
 
@@ -83,6 +232,20 @@ main(void)
 	IF__argon2id_v1_0__SUPPORTED(CHECK("$argon2id$", "m=8,t=1,p=1$*16$*40", "m=0,t=0,p=0$*1$*1"));
 	IF__argon2ds_v1_0__SUPPORTED(CHECK("$argon2ds$", "m=8,t=1,p=1$*16$*40", "m=0,t=0,p=0$*1$*1"));
 
+	ctx = librecrypt_create_context();
+	assert(ctx != NULL);
+
+	librecrypt_set_custom_algorithms(ctx, custom, ELEMSOF(custom));
+	CHECK("$rot4$", "", "x");
+	CHECK("$trunc$", "", "*0");
+	CHECK("$trunc$", "*6", "*0");
+	EXPECT(librecrypt_test_supported(NULL, 4096u, 0, "$rot4$>$trunc$", ctx) == 1);
+	EXPECT(librecrypt_test_supported(NULL, 4096u, 0, "$rot4$>$trunc$*8", ctx) == 1);
+	EXPECT(librecrypt_test_supported(NULL, 4096u, 0, "$trunc$*6>$rot4$", ctx) == 1);
+	EXPECT(librecrypt_test_supported(NULL, 4096u, 0, "$trunc$>$rot4$", ctx) == 1);
+
+	librecrypt_free_context(ctx);
+
 	STOP_RESOURCE_TEST();
 	return 0;
 }
diff --git a/librecrypt_verify.c b/librecrypt_verify.c
index fa51efd..aa304ca 100644
--- a/librecrypt_verify.c
+++ b/librecrypt_verify.c
@@ -64,9 +64,199 @@ librecrypt_verify(const char *phrase, size_t len, const char *settings, LIBRECRY
 #else
 
 
+#define ALPHABET "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
+#define ALT_ALPHABET "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ+/"
+
+NONSTRING static const char encoding_lut[256u] = MAKE_ENCODING_LUT(ALPHABET);
+NONSTRING static const char alt_encoding_lut[256u] = MAKE_ENCODING_LUT(ALT_ALPHABET);
+
+static const unsigned char decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX, XX,
+	XX,  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14,
+	15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, XX, XX, XX, XX, XX,
+	XX, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
+	41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static const unsigned char alt_decoding_lut[256u] = {
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, 62, XX, XX, XX, 63,
+	 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, XX, XX, XX, XX, XX, XX,
+	XX, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50,
+	51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, XX, XX, XX, XX, XX,
+	XX, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
+	25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX,
+	XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX, XX
+};
+
+static unsigned
+rot4_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$rot4$") - 1u)
+		if (!strncmp(settings, "$rot4$", sizeof("$rot4$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static unsigned
+trunc_is_algorithm(const char *settings, size_t len)
+{
+	if (len >= sizeof("$trunc$") - 1u)
+		if (!strncmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+			return 1u;
+	return 0u;
+}
+
+static int
+trunc_supported(const char *phrase, size_t len, int text, const char *settings,
+                size_t prefix, size_t *len_out)
+{
+	size_t i, digit, n = 0u, q, r;
+
+	(void) phrase;
+	(void) len;
+	(void) text;
+
+	if (prefix < sizeof("$trunc$") - 1u || memcmp(settings, "$trunc$", sizeof("$trunc$") - 1u))
+		return 0;
+
+	if (prefix == sizeof("$trunc$") - 1u) {
+		*len_out = 4u;
+		return 1;
+	}
+
+	if (settings[sizeof("$trunc$") - 1u] == '*') {
+		for (i = sizeof("$trunc$*") - 1u; i < prefix; i++) {
+			if ('0' > settings[i] || settings[i] > '9')
+				return 0;
+			digit = (size_t)(settings[i] - '0');
+			if (n > (SIZE_MAX - digit) / 10u)
+				return 0;
+			n = n * 10u + digit;
+		}
+	} else {
+		for (i = sizeof("$trunc$") - 1u; i < prefix; i++) {
+			if (settings[i] == '@')
+				break;
+			if (alt_decoding_lut[(unsigned char)settings[i]] == XX)
+				return 0;
+			n += 1u;
+		}
+		q = n / 4u;
+		r = n % 4u;
+		if (r == 1u)
+			return 0;
+		n = q * 3u + r;
+		if (r) {
+			n -= 1u;
+			for (; r < 4u && i < prefix; r++, i++)
+				if (settings[i] != '@')
+					break;
+			if (r != 4u || i != prefix)
+				return 0;
+		}
+	}
+	if (!n)
+		return 0;
+	*len_out = n;
+	return 1;
+}
+
+static int
+rot4_hash(char *restrict out_buffer, size_t size, const char *phrase, size_t len,
+          const char *settings, size_t prefix, LIBRECRYPT_CONTEXT *ctx)
+{
+	size_t i;
+
+	(void) settings;
+	(void) prefix;
+	(void) ctx;
+
+	for (i = 0u; i < 8u && i < size; i++) {
+		out_buffer[i] = '\0';
+		if (i < len)
+			out_buffer[i] = (char)((((int)phrase[i] & 0x0F) << 4) | (((int)phrase[i] >> 4) & 0x0F));
+	}
+	return 0;
+}
+
+static int
+trunc_hash(char *restrict out_buffer, size_t size, const char *phrase, size_t len,
+           const char *settings, size_t prefix, LIBRECRYPT_CONTEXT *ctx)
+{
+	size_t hash_size, i;
+
+	(void) ctx;
+
+	if (!trunc_supported(NULL, 0u, 1, settings, prefix, &hash_size)) {
+		errno = EINVAL;
+		return -1;
+	}
+
+	size = MIN(hash_size, size);
+	for (i = 0u; i < size; i++)
+		out_buffer[i] = i < len ? phrase[i] : '\0';
+
+	return 0;
+}
+
+static const struct librecrypt_algorithm rot4_algo = {
+	.is_algorithm = &rot4_is_algorithm,
+	.hash = &rot4_hash,
+	.encoding_lut = encoding_lut,
+	.decoding_lut = decoding_lut,
+	.hash_size = 8u,
+	.flexible_hash_size = 0,
+	.strict_pad = 1,
+	.pad = '#'
+};
+
+static const struct librecrypt_algorithm trunc_algo = {
+	.is_algorithm = &trunc_is_algorithm,
+	.hash = &trunc_hash,
+	.encoding_lut = alt_encoding_lut,
+	.decoding_lut = alt_decoding_lut,
+	.hash_size = 4u,
+	.flexible_hash_size = 1,
+	.strict_pad = 1,
+	.pad = '@'
+};
+
+#define TEST_PHRASE "\x12\x23\x34\x45\x56\x67\x78\x89"
+#define TEST_PHRASE64_ROT4 "EiM0RVZneIk#"
+#define TEST_HASH_ROT4 "ITJDVGV2h5g#"
+#define TEST_HASH_TRUNC "4ycQhg@@"
+#define TEST_HASH_TRUNC6 "4ycQhlpD"
+#define TEST_HASH_TRUNC_ROT4 "ITJDVAAAAAA#"
+#define TEST_HASH_TRUNC6_ROT4 "ITJDVGV2AAA#"
+#define TEST_HASH_ROT4_TRUNC "8j93l0@@"
+#define TEST_HASH_ROT4_TRUNC6 "8j93l6lS"
+#define TEST_HASH_TRUNC_TRUNC6 "4ycQhg00"
+
+
 int
 main(void)
 {
+	const struct librecrypt_algorithm custom[] = {rot4_algo, trunc_algo};
 	LIBRECRYPT_CONTEXT *ctx = NULL;
 	char conf[256], nuls[256], spaces[256];
 	int r;
@@ -153,6 +343,22 @@ main(void)
 	EXPECT(librecrypt_verify(spaces, 80u, "$argon2i$v=19$m=8,t=1,p=1$ICAgICAgICA$mzPlVOVjVos", ctx) == 1);
 #endif
 
+#define CHECK(CRYPT) EXPECT(librecrypt_verify(TEST_PHRASE, sizeof(TEST_PHRASE) - 1u, CRYPT, ctx) == 1);
+	librecrypt_set_custom_algorithms(ctx, custom, ELEMSOF(custom));
+	CHECK("$trunc$"TEST_HASH_TRUNC);
+	CHECK("$trunc$"TEST_HASH_TRUNC6);
+	CHECK("$rot4$"TEST_HASH_ROT4);
+	CHECK("$rot4$>$trunc$"TEST_HASH_ROT4_TRUNC);
+	CHECK("$rot4$>$trunc$"TEST_HASH_ROT4_TRUNC6);
+	CHECK("$trunc$>$rot4$"TEST_HASH_TRUNC_ROT4);
+	CHECK("$trunc$*6>$rot4$"TEST_HASH_TRUNC6_ROT4);
+	CHECK("$trunc$>$trunc$"TEST_HASH_TRUNC);
+	CHECK("$trunc$>$trunc$"TEST_HASH_TRUNC_TRUNC6);
+	CHECK("$trunc$*6>$trunc$"TEST_HASH_TRUNC);
+	CHECK("$trunc$*6>$trunc$"TEST_HASH_TRUNC6);
+	CHECK("$rot4$>$rot4$"TEST_PHRASE64_ROT4);
+#undef CHECK
+
 	librecrypt_free_context(ctx);
 
 	STOP_RESOURCE_TEST();
-- 
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