diff --git a/cmd_sign.c b/cmd_sign.c
index 5ad4eda..6a5d185 100644
--- a/cmd_sign.c
+++ b/cmd_sign.c
@@ -16,12 +16,9 @@
 #include <openssl/pkcs7.h>
 #include <stdlib.h>
 #include <string.h>
-#include <sys/stat.h>
-#include <unistd.h>
 
 #include "commands.h"
 #include "libfsverity.h"
-#include "hash_algs.h"
 
 /*
  * Format in which verity file measurements are signed.  This is the same as
@@ -337,179 +334,6 @@ static bool write_signature(const char *filename, const u8 *sig, u32 sig_size)
 	return ok;
 }
 
-#define FS_VERITY_MAX_LEVELS	64
-
-struct block_buffer {
-	u32 filled;
-	u8 *data;
-};
-
-/*
- * Hash a block, writing the result to the next level's pending block buffer.
- * Returns true if the next level's block became full, else false.
- */
-static bool hash_one_block(struct hash_ctx *hash, struct block_buffer *cur,
-			   u32 block_size, const u8 *salt, u32 salt_size)
-{
-	struct block_buffer *next = cur + 1;
-
-	/* Zero-pad the block if it's shorter than block_size. */
-	memset(&cur->data[cur->filled], 0, block_size - cur->filled);
-
-	hash_init(hash);
-	hash_update(hash, salt, salt_size);
-	hash_update(hash, cur->data, block_size);
-	hash_final(hash, &next->data[next->filled]);
-
-	next->filled += hash->alg->digest_size;
-	cur->filled = 0;
-
-	return next->filled + hash->alg->digest_size > block_size;
-}
-
-static int full_read_fd(int fd, void *buf, size_t count)
-{
-	while (count) {
-		int n = read(fd, buf, min(count, INT_MAX));
-
-		if (n < 0) {
-			error_msg_errno("reading from file");
-			return n;
-		}
-		if (n == 0) {
-			error_msg("unexpected end-of-file");
-			return -ENODATA;
-		}
-		buf += n;
-		count -= n;
-	}
-	return 0;
-}
-
-/*
- * Compute the file's Merkle tree root hash using the given hash algorithm,
- * block size, and salt.
- */
-static bool compute_root_hash(int fd, u64 file_size,
-			      struct hash_ctx *hash, u32 block_size,
-			      const u8 *salt, u32 salt_size, u8 *root_hash)
-{
-	const u32 hashes_per_block = block_size / hash->alg->digest_size;
-	const u32 padded_salt_size = roundup(salt_size, hash->alg->block_size);
-	u8 *padded_salt = xzalloc(padded_salt_size);
-	u64 blocks;
-	int num_levels = 0;
-	int level;
-	struct block_buffer _buffers[1 + FS_VERITY_MAX_LEVELS + 1] = {};
-	struct block_buffer *buffers = &_buffers[1];
-	u64 offset;
-	bool ok = false;
-
-	if (salt_size != 0)
-		memcpy(padded_salt, salt, salt_size);
-
-	/* Compute number of levels */
-	for (blocks = DIV_ROUND_UP(file_size, block_size); blocks > 1;
-	     blocks = DIV_ROUND_UP(blocks, hashes_per_block)) {
-		ASSERT(num_levels < FS_VERITY_MAX_LEVELS);
-		num_levels++;
-	}
-
-	/*
-	 * Allocate the block buffers.  Buffer "-1" is for data blocks.
-	 * Buffers 0 <= level < num_levels are for the actual tree levels.
-	 * Buffer 'num_levels' is for the root hash.
-	 */
-	for (level = -1; level < num_levels; level++)
-		buffers[level].data = xmalloc(block_size);
-	buffers[num_levels].data = root_hash;
-
-	/* Hash each data block, also hashing the tree blocks as they fill up */
-	for (offset = 0; offset < file_size; offset += block_size) {
-		buffers[-1].filled = min(block_size, file_size - offset);
-
-		if (full_read_fd(fd, buffers[-1].data, buffers[-1].filled))
-			goto out;
-
-		level = -1;
-		while (hash_one_block(hash, &buffers[level], block_size,
-				      padded_salt, padded_salt_size)) {
-			level++;
-			ASSERT(level < num_levels);
-		}
-	}
-	/* Finish all nonempty pending tree blocks */
-	for (level = 0; level < num_levels; level++) {
-		if (buffers[level].filled != 0)
-			hash_one_block(hash, &buffers[level], block_size,
-				       padded_salt, padded_salt_size);
-	}
-
-	/* Root hash was filled by the last call to hash_one_block() */
-	ASSERT(buffers[num_levels].filled == hash->alg->digest_size);
-	ok = true;
-out:
-	for (level = -1; level < num_levels; level++)
-		free(buffers[level].data);
-	free(padded_salt);
-	return ok;
-}
-
-/*
- * Compute the fs-verity measurement of the given file.
- *
- * The fs-verity measurement is the hash of the fsverity_descriptor, which
- * contains the Merkle tree properties including the root hash.
- */
-static bool compute_file_measurement(int fd,
-				     const struct fsverity_hash_alg *hash_alg,
-				     u32 block_size, const u8 *salt,
-				     u32 salt_size, u8 *measurement)
-{
-	struct hash_ctx *hash = hash_alg->create_ctx(hash_alg);
-	u64 file_size;
-	struct fsverity_descriptor desc;
-	struct stat stbuf;
-	bool ok = false;
-
-	if (fstat(fd, &stbuf) != 0) {
-		error_msg_errno("can't stat input file");
-		goto out;
-	}
-	file_size = stbuf.st_size;
-
-	memset(&desc, 0, sizeof(desc));
-	desc.version = 1;
-	desc.hash_algorithm = hash_alg->hash_num;
-
-	ASSERT(is_power_of_2(block_size));
-	desc.log_blocksize = ilog2(block_size);
-
-	if (salt_size != 0) {
-		if (salt_size > sizeof(desc.salt)) {
-			error_msg("Salt too long (got %u bytes; max is %zu bytes)",
-				  salt_size, sizeof(desc.salt));
-			goto out;
-		}
-		memcpy(desc.salt, salt, salt_size);
-		desc.salt_size = salt_size;
-	}
-
-	desc.data_size = cpu_to_le64(file_size);
-
-	/* Root hash of empty file is all 0's */
-	if (file_size != 0 &&
-	    !compute_root_hash(fd, file_size, hash, block_size, salt,
-			       salt_size, desc.root_hash))
-		goto out;
-
-	hash_full(hash, &desc, sizeof(desc), measurement);
-	ok = true;
-out:
-	hash_free(hash);
-	return ok;
-}
-
 enum {
 	OPT_HASH_ALG,
 	OPT_BLOCK_SIZE,
@@ -538,7 +362,8 @@ int fsverity_cmd_sign(const struct fsverity_command *cmd,
 	u32 salt_size = 0;
 	const char *keyfile = NULL;
 	const char *certfile = NULL;
-	struct fsverity_signed_digest *digest = NULL;
+	struct libfsverity_digest *digest = NULL;
+	struct libfsverity_merkle_tree_params params;
 	char digest_hex[FS_VERITY_MAX_DIGEST_SIZE * 2 + 1];
 	u8 *sig = NULL;
 	u32 sig_size;
@@ -608,16 +433,17 @@ int fsverity_cmd_sign(const struct fsverity_command *cmd,
 	if (certfile == NULL)
 		certfile = keyfile;
 
-	digest = xzalloc(sizeof(*digest) + hash_alg->digest_size);
-	memcpy(digest->magic, "FSVerity", 8);
-	digest->digest_algorithm = cpu_to_le16(hash_alg->hash_num);
-	digest->digest_size = cpu_to_le16(hash_alg->digest_size);
-
 	if (!open_file(&file, argv[0], O_RDONLY, 0))
 		goto out_err;
 
-	if (!compute_file_measurement(file.fd, hash_alg, block_size,
-				      salt, salt_size, digest->digest))
+	memset(&params, 0, sizeof(struct libfsverity_merkle_tree_params));
+	params.version = 1;
+	params.hash_algorithm = hash_alg->hash_num;
+	params.block_size = block_size;
+	params.salt_size = salt_size;
+	params.salt = salt;
+
+	if (libfsverity_compute_digest(file.fd, &params, &digest))
 		goto out_err;
 
 	filedes_close(&file);
diff --git a/libverity.c b/libverity.c
index 6821aa2..4f01992 100644
--- a/libverity.c
+++ b/libverity.c
@@ -8,3 +8,210 @@
  * Written by Eric Biggers and Jes Sorensen.
  */
 
+#include <openssl/bio.h>
+#include <openssl/err.h>
+#include <openssl/pem.h>
+#include <openssl/pkcs7.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#include "commands.h"
+#include "libfsverity.h"
+#include "hash_algs.h"
+
+#define FS_VERITY_MAX_LEVELS	64
+
+struct block_buffer {
+	u32 filled;
+	u8 *data;
+};
+
+/*
+ * Hash a block, writing the result to the next level's pending block buffer.
+ * Returns true if the next level's block became full, else false.
+ */
+static bool hash_one_block(struct hash_ctx *hash, struct block_buffer *cur,
+			   u32 block_size, const u8 *salt, u32 salt_size)
+{
+	struct block_buffer *next = cur + 1;
+
+	/* Zero-pad the block if it's shorter than block_size. */
+	memset(&cur->data[cur->filled], 0, block_size - cur->filled);
+
+	hash_init(hash);
+	hash_update(hash, salt, salt_size);
+	hash_update(hash, cur->data, block_size);
+	hash_final(hash, &next->data[next->filled]);
+
+	next->filled += hash->alg->digest_size;
+	cur->filled = 0;
+
+	return next->filled + hash->alg->digest_size > block_size;
+}
+
+static int full_read_fd(int fd, void *buf, size_t count)
+{
+	while (count) {
+		int n = read(fd, buf, min(count, INT_MAX));
+
+		if (n < 0) {
+			error_msg_errno("reading from file");
+			return n;
+		}
+		if (n == 0) {
+			error_msg("unexpected end-of-file");
+			return -ENODATA;
+		}
+		buf += n;
+		count -= n;
+	}
+	return 0;
+}
+
+/*
+ * Compute the file's Merkle tree root hash using the given hash algorithm,
+ * block size, and salt.
+ */
+static bool compute_root_hash(int fd, u64 file_size,
+			      struct hash_ctx *hash, u32 block_size,
+			      const u8 *salt, u32 salt_size, u8 *root_hash)
+{
+	const u32 hashes_per_block = block_size / hash->alg->digest_size;
+	const u32 padded_salt_size = roundup(salt_size, hash->alg->block_size);
+	u8 *padded_salt = xzalloc(padded_salt_size);
+	u64 blocks;
+	int num_levels = 0;
+	int level;
+	struct block_buffer _buffers[1 + FS_VERITY_MAX_LEVELS + 1] = {};
+	struct block_buffer *buffers = &_buffers[1];
+	u64 offset;
+	bool ok = false;
+
+	if (salt_size != 0)
+		memcpy(padded_salt, salt, salt_size);
+
+	/* Compute number of levels */
+	for (blocks = DIV_ROUND_UP(file_size, block_size); blocks > 1;
+	     blocks = DIV_ROUND_UP(blocks, hashes_per_block)) {
+		ASSERT(num_levels < FS_VERITY_MAX_LEVELS);
+		num_levels++;
+	}
+
+	/*
+	 * Allocate the block buffers.  Buffer "-1" is for data blocks.
+	 * Buffers 0 <= level < num_levels are for the actual tree levels.
+	 * Buffer 'num_levels' is for the root hash.
+	 */
+	for (level = -1; level < num_levels; level++)
+		buffers[level].data = xmalloc(block_size);
+	buffers[num_levels].data = root_hash;
+
+	/* Hash each data block, also hashing the tree blocks as they fill up */
+	for (offset = 0; offset < file_size; offset += block_size) {
+		buffers[-1].filled = min(block_size, file_size - offset);
+
+		if (full_read_fd(fd, buffers[-1].data, buffers[-1].filled))
+			goto out;
+
+		level = -1;
+		while (hash_one_block(hash, &buffers[level], block_size,
+				      padded_salt, padded_salt_size)) {
+			level++;
+			ASSERT(level < num_levels);
+		}
+	}
+	/* Finish all nonempty pending tree blocks */
+	for (level = 0; level < num_levels; level++) {
+		if (buffers[level].filled != 0)
+			hash_one_block(hash, &buffers[level], block_size,
+				       padded_salt, padded_salt_size);
+	}
+
+	/* Root hash was filled by the last call to hash_one_block() */
+	ASSERT(buffers[num_levels].filled == hash->alg->digest_size);
+	ok = true;
+out:
+	for (level = -1; level < num_levels; level++)
+		free(buffers[level].data);
+	free(padded_salt);
+	return ok;
+}
+
+/*
+ * Compute the fs-verity measurement of the given file.
+ *
+ * The fs-verity measurement is the hash of the fsverity_descriptor, which
+ * contains the Merkle tree properties including the root hash.
+ */
+int
+libfsverity_compute_digest(int fd,
+			   const struct libfsverity_merkle_tree_params *params,
+			   struct libfsverity_digest **digest_ret)
+{
+	const struct fsverity_hash_alg *hash_alg;
+	struct libfsverity_digest *digest;
+	struct hash_ctx *hash;
+	struct fsverity_descriptor desc;
+	struct stat stbuf;
+	u64 file_size;
+	int retval = -EINVAL;
+
+	hash_alg = libfsverity_find_hash_alg_by_num(params->hash_algorithm);
+	hash = hash_alg->create_ctx(hash_alg);
+
+	digest = malloc(sizeof(struct libfsverity_digest) +
+			hash_alg->digest_size);
+	if (!digest_ret)
+		return -ENOMEM;
+	memcpy(digest->magic, "FSVerity", 8);
+	digest->digest_algorithm = cpu_to_le16(hash_alg->hash_num);
+	digest->digest_size = cpu_to_le16(hash_alg->digest_size);
+	memset(digest->digest, 0, hash_alg->digest_size);
+
+	if (fstat(fd, &stbuf) != 0) {
+		error_msg_errno("can't stat input file");
+		retval = -EBADF;
+		goto error_out;
+	}
+	file_size = stbuf.st_size;
+
+	memset(&desc, 0, sizeof(desc));
+	desc.version = 1;
+	desc.hash_algorithm = params->hash_algorithm;
+
+	ASSERT(is_power_of_2(params->block_size));
+	desc.log_blocksize = ilog2(params->block_size);
+
+	if (params->salt_size != 0) {
+		if (params->salt_size > sizeof(desc.salt)) {
+			error_msg("Salt too long (got %u bytes; max is %zu bytes)",
+				  params->salt_size, sizeof(desc.salt));
+			retval = EINVAL;
+			goto error_out;
+		}
+		memcpy(desc.salt, params->salt, params->salt_size);
+		desc.salt_size = params->salt_size;
+	}
+
+	desc.data_size = cpu_to_le64(file_size);
+
+	/* Root hash of empty file is all 0's */
+	if (file_size != 0 &&
+	    !compute_root_hash(fd, file_size, hash, params->block_size,
+			       params->salt, params->salt_size,
+			       desc.root_hash)) {
+		retval = -EAGAIN;
+		goto error_out;
+	}
+
+	hash_full(hash, &desc, sizeof(desc), digest->digest);
+	hash_free(hash);
+	*digest_ret = digest;
+
+	return 0;
+
+ error_out:
+	free(digest);
+	return retval;
+}
