diff --git a/Makefile.am b/Makefile.am
index f3212b4a..88c64b30 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -309,6 +309,7 @@ EXTRA_DIST=								\
 	tests/08-passphrase-file.sh					\
 	tests/09-explicit-params.sh					\
 	tests/10-check-params.sh					\
+	tests/12-same-file.sh						\
 	tests/shared_test_functions.sh					\
 	tests/shared_valgrind_functions.sh				\
 	tests/test_scrypt.sh						\
diff --git a/main.c b/main.c
index 2b05e252..150d7fca 100644
--- a/main.c
+++ b/main.c
@@ -25,6 +25,9 @@
  */
 #include "platform.h"
 
+#include <sys/types.h>
+#include <sys/stat.h>
+
 #include <errno.h>
 #include <math.h>
 #include <stdint.h>
@@ -96,6 +99,37 @@ scrypt_mode_info(const char * infilename)
 	return (-1);
 }
 
+/**
+ * same_file(infile, outfilename):
+ * Return non-zero if the already-open ${infile} and the path ${outfilename}
+ * refer to the same regular file.  If we cannot tell -- most importantly if
+ * ${outfilename} does not exist yet -- return zero.
+ */
+static int
+same_file(FILE * infile, const char * outfilename)
+{
+	struct stat sb_in;
+	struct stat sb_out;
+
+	/* If we can't stat either file, assume that they're different. */
+	if (fstat(fileno(infile), &sb_in))
+		return (0);
+	if (stat(outfilename, &sb_out))
+		return (0);
+
+	/*
+	 * Only a regular file can be destroyed this way; opening a character
+	 * device such as /dev/null for writing does not discard the data we
+	 * are about to read.
+	 */
+	if (!S_ISREG(sb_in.st_mode) || !S_ISREG(sb_out.st_mode))
+		return (0);
+
+	/* The same device and inode is the same file. */
+	return ((sb_in.st_dev == sb_out.st_dev) &&
+	    (sb_in.st_ino == sb_out.st_ino));
+}
+
 /**
  * scrypt_mode_enc_dec(params, passphrase_entry, passphrase_arg, dec, verbose,
  *     force_resources, infilename, outfilename):
@@ -129,6 +163,17 @@ scrypt_mode_enc_dec(struct scryptenc_params params,
 		infile = stdin;
 	}
 
+	/*
+	 * Refuse to write the output into the file we are reading.  Opening
+	 * it for writing truncates it, so we would destroy the data we were
+	 * asked to encrypt or decrypt and then "succeed" in writing an
+	 * encryption of nothing.
+	 */
+	if ((outfilename != NULL) && same_file(infile, outfilename)) {
+		warn0("Input and output files are the same: %s", outfilename);
+		goto err1;
+	}
+
 	/* Get the password. */
 	if (passphrase_entry_readpass(&passwd, passphrase_entry,
 	    passphrase_arg, "Please enter passphrase",
diff --git a/scrypt.1 b/scrypt.1
index b81bb365..8b22ed4a 100644
--- a/scrypt.1
+++ b/scrypt.1
@@ -63,6 +63,16 @@ if specified, or the standard output otherwise.
 The user will be prompted to enter the passphrase used at
 encryption time to generate the derived encryption key.
 .Pp
+.Ar outfile
+cannot be the same file as
+.Ar infile :
+.Nm
+truncates
+.Ar outfile
+before it reads
+.Ar infile ,
+so it exits with an error instead of destroying the input.
+.Pp
 .Nm Cm info
 provides information about the encryption parameters used for
 .Ar infile .
diff --git a/tests/12-same-file.sh b/tests/12-same-file.sh
new file mode 100644
index 00000000..d23a5658
--- /dev/null
+++ b/tests/12-same-file.sh
@@ -0,0 +1,153 @@
+#!/bin/sh
+
+### Constants
+c_valgrind_min=1
+reference_file="${scriptdir}/verify-strings/test_scrypt.good"
+encrypted_reference_file="${scriptdir}/verify-strings/test_scrypt_good.enc"
+plaintext_target="${s_basename}-plaintext.txt"
+encrypted_target="${s_basename}-encrypted.enc"
+normal_output="${s_basename}-normal.enc"
+stderr_enc="${s_basename}-enc.stderr"
+stderr_dec="${s_basename}-dec.stderr"
+
+scenario_cmd() {
+	# "scrypt enc" must not write into the file it is reading; opening
+	# the output file truncates it.
+	cp "${reference_file}" "${plaintext_target}"
+
+	setup_check "scrypt enc same file"
+	(
+		echo "${password}" | ${c_valgrind_cmd} "${bindir}/scrypt" \
+		    enc --passphrase dev:stdin-once			\
+		    "${plaintext_target}" "${plaintext_target}"		\
+		    2> "${stderr_enc}"
+		expected_exitcode 1 $? > "${c_exitfile}"
+	)
+
+	setup_check "scrypt enc same file error"
+	grep -q "scrypt: Input and output files are the same"		\
+	    "${stderr_enc}"
+	echo $? > "${c_exitfile}"
+
+	setup_check "scrypt enc same file leaves input alone"
+	cmp -s "${plaintext_target}" "${reference_file}"
+	echo $? > "${c_exitfile}"
+
+	# The same applies to "scrypt dec".
+	cp "${encrypted_reference_file}" "${encrypted_target}"
+
+	setup_check "scrypt dec same file"
+	(
+		echo "${password}" | ${c_valgrind_cmd} "${bindir}/scrypt" \
+		    dec --passphrase dev:stdin-once			\
+		    "${encrypted_target}" "${encrypted_target}"		\
+		    2> "${stderr_dec}"
+		expected_exitcode 1 $? > "${c_exitfile}"
+	)
+
+	setup_check "scrypt dec same file error"
+	grep -q "scrypt: Input and output files are the same"		\
+	    "${stderr_dec}"
+	echo $? > "${c_exitfile}"
+
+	setup_check "scrypt dec same file leaves input alone"
+	cmp -s "${encrypted_target}" "${encrypted_reference_file}"
+	echo $? > "${c_exitfile}"
+
+	# Writing to a different file must still work.
+	setup_check "scrypt enc different file"
+	echo "${password}" | ${c_valgrind_cmd} "${bindir}/scrypt"	\
+	    enc --passphrase dev:stdin-once				\
+	    "${plaintext_target}" "${normal_output}"
+	echo $? > "${c_exitfile}"
+
+	# Extended static-alias coverage. This block belongs inside scenario_cmd.
+	alias_password="${s_basename}-alias-password.txt"
+	printf '%s\n' "${password}" > "${alias_password}"
+	for alias_mode in enc dec; do
+		alias_params=""
+		alias_reference="${encrypted_reference_file}"
+		if [ "${alias_mode}" = enc ]; then
+			alias_params="--logN 10 -r 1 -p 1"
+			alias_reference="${reference_file}"
+		fi
+		for alias_kind in hardlink output-symlink input-symlink stdin dotdot; do
+			alias_input="${s_basename}-${alias_mode}-${alias_kind}-input"
+			alias_link="${s_basename}-${alias_mode}-${alias_kind}-link"
+			alias_stderr="${s_basename}-${alias_mode}-${alias_kind}.stderr"
+			cp "${alias_reference}" "${alias_input}"
+			alias_source="${alias_input}"
+			alias_dest="${alias_link}"
+			case "${alias_kind}" in
+			hardlink)
+				ln "${alias_input}" "${alias_link}"
+				;;
+			output-symlink)
+				ln -s "${alias_input}" "${alias_link}"
+				;;
+			input-symlink)
+				ln -s "${alias_input}" "${alias_link}"
+				alias_source="${alias_link}"
+				alias_dest="${alias_input}"
+				;;
+			stdin)
+				alias_source="-"
+				alias_dest="${alias_input}"
+				;;
+			dotdot)
+				alias_dir="${s_basename}-${alias_mode}-${alias_kind}-dir"
+				mkdir "${alias_dir}"
+				alias_dest="${alias_dir}/../$(basename "${alias_input}")"
+				;;
+			esac
+
+			setup_check "scrypt ${alias_mode} ${alias_kind} rejects alias"
+			${c_valgrind_cmd} "${bindir}/scrypt" "${alias_mode}" \
+			    ${alias_params} --passphrase file:"${alias_password}" \
+			    "${alias_source}" "${alias_dest}" \
+			    < "${alias_input}" 2> "${alias_stderr}"
+			expected_exitcode 1 $? > "${c_exitfile}"
+
+			setup_check "scrypt ${alias_mode} ${alias_kind} error"
+			grep -q "scrypt: Input and output files are the same" \
+			    "${alias_stderr}"
+			echo $? > "${c_exitfile}"
+
+			setup_check "scrypt ${alias_mode} ${alias_kind} preserves input"
+			cmp -s "${alias_input}" "${alias_reference}"
+			echo $? > "${c_exitfile}"
+		done
+	done
+
+	# A character device is not a destructive same-regular-file alias.
+	setup_check "scrypt enc permits same null device"
+	${c_valgrind_cmd} "${bindir}/scrypt" enc --logN 10 -r 1 -p 1 \
+	    --passphrase file:"${alias_password}" /dev/null /dev/null
+	echo $? > "${c_exitfile}"
+
+	# Equal contents are not the same inode; preserve and roundtrip the input.
+	independent_input="${s_basename}-independent-input"
+	independent_output="${s_basename}-independent-output"
+	independent_plain="${s_basename}-independent-plain"
+	cp "${reference_file}" "${independent_input}"
+	cp "${reference_file}" "${independent_output}"
+	setup_check "scrypt enc permits identical contents in independent files"
+	${c_valgrind_cmd} "${bindir}/scrypt" enc --logN 10 -r 1 -p 1 \
+	    --passphrase file:"${alias_password}" \
+	    "${independent_input}" "${independent_output}"
+	echo $? > "${c_exitfile}"
+
+	setup_check "scrypt enc independent files preserves input"
+	cmp -s "${independent_input}" "${reference_file}"
+	echo $? > "${c_exitfile}"
+
+	setup_check "scrypt dec independent output roundtrip"
+	${c_valgrind_cmd} "${bindir}/scrypt" dec \
+	    --passphrase file:"${alias_password}" \
+	    "${independent_output}" "${independent_plain}"
+	echo $? > "${c_exitfile}"
+
+	setup_check "scrypt dec independent output matches input"
+	cmp -s "${independent_plain}" "${reference_file}"
+	echo $? > "${c_exitfile}"
+}