diff --git a/573file/lz.c b/573file/lz.c
new file mode 100644
index 0000000..51c6045
--- /dev/null
+++ b/573file/lz.c
@@ -0,0 +1,169 @@
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "573file/lz.h"
+
+#include "util/iobuf.h"
+
+#define LZCOMP_MIN_MATCH 3
+#define LZCOMP_MAX_MATCH (LZCOMP_MIN_MATCH + 15)
+#define LZCOMP_NIL ((uint16_t)-1)
+
+struct lz_dec {
+  const uint8_t *src_pos;
+  const uint8_t *src_end;
+  uint8_t ring[0x1000];
+  uint16_t ring_pos;
+  uint16_t flags;
+  uint16_t copy_pos;
+  uint8_t copy_len;
+};
+
+static int lz_dec_backref_begin(struct lz_dec *lz);
+static int lz_dec_backref_getc(struct lz_dec *lz);
+static int lz_dec_get_flag(struct lz_dec *lz);
+static int lz_dec_getc(struct lz_dec *lz);
+
+int lz_dec_decompress(const uint8_t *in_bytes, size_t in_nbytes,
+                      struct iobuf *out) {
+  struct lz_dec *lz;
+  int byte;
+
+  assert(in_bytes != NULL);
+  assert(out != NULL);
+
+  lz = calloc(1, sizeof(*lz));
+
+  if (lz == NULL) {
+    return -ENOMEM;
+  }
+
+  lz->src_pos = in_bytes;
+  lz->src_end = in_bytes + in_nbytes;
+  lz->flags = 0x0001;
+
+  for (;;) {
+    byte = lz_dec_getc(lz);
+
+    if (byte < 0) {
+      break;
+    }
+
+    if (out->bytes != NULL) {
+      assert(out->pos < out->nbytes);
+
+      out->bytes[out->pos] = (uint8_t)byte;
+    }
+
+    out->pos++;
+  }
+
+  free(lz);
+
+  return 0;
+}
+
+static int lz_dec_getc(struct lz_dec *lz) {
+  int byte;
+  int flag;
+
+  assert(lz != NULL);
+
+  if (lz->src_pos == lz->src_end) {
+    return -1;
+  }
+
+  if (lz->copy_len > 0) {
+    byte = lz_dec_backref_getc(lz);
+  } else {
+    flag = lz_dec_get_flag(lz);
+
+    if (flag < 0) {
+      byte = -1;
+    } else if (flag) {
+      if (lz->src_pos < lz->src_end) {
+        byte = *lz->src_pos++;
+      } else {
+        byte = -1;
+      }
+    } else {
+      byte = lz_dec_backref_begin(lz);
+    }
+  }
+
+  if (byte < 0) {
+    return byte;
+  }
+
+  lz->ring[lz->ring_pos] = (uint8_t)byte;
+  lz->ring_pos = (lz->ring_pos + 1) & 0x0FFF;
+
+  return byte;
+}
+
+static int lz_dec_backref_getc(struct lz_dec *lz) {
+  int byte;
+
+  assert(lz != NULL);
+  assert(lz->copy_len > 0);
+
+  byte = lz->ring[lz->copy_pos];
+
+  lz->copy_pos = (lz->copy_pos + 1) & 0x0FFF;
+  lz->copy_len -= 1;
+
+  return byte;
+}
+
+static int lz_dec_backref_begin(struct lz_dec *lz) {
+  uint16_t copy_len;
+  uint16_t copy_off;
+  uint8_t hi;
+  uint8_t lo;
+
+  assert(lz != NULL);
+
+  if (lz->src_pos + 1 >= lz->src_end) {
+    return -1;
+  }
+
+  hi = *lz->src_pos++;
+  lo = *lz->src_pos++;
+
+  copy_len = lo & 0x0F;
+  copy_off = (hi << 4) | (lo >> 4);
+
+  /* This use of a backref marker as an explicit EOF is really weird */
+  if (copy_off == 0) {
+    lz->src_pos = NULL;
+    lz->src_end = NULL;
+
+    return -1;
+  }
+
+  lz->copy_len = copy_len + 3;
+  lz->copy_pos = (lz->ring_pos - copy_off) & 0x0FFF;
+
+  return lz_dec_backref_getc(lz);
+}
+
+static int lz_dec_get_flag(struct lz_dec *lz) {
+  int result;
+
+  assert(lz != NULL);
+
+  if (lz->flags == 0x0001) {
+    if (lz->src_pos == lz->src_end) {
+      return -1;
+    }
+
+    lz->flags = 0x0100 | *lz->src_pos++;
+  }
+
+  result = lz->flags & 1;
+  lz->flags >>= 1;
+
+  return result;
+}
diff --git a/573file/lz.h b/573file/lz.h
new file mode 100644
index 0000000..daeefdb
--- /dev/null
+++ b/573file/lz.h
@@ -0,0 +1,9 @@
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "util/iobuf.h"
+
+int lz_dec_decompress(const uint8_t *in_bytes, size_t in_nbytes,
+                      struct iobuf *out);
diff --git a/573file/meson.build b/573file/meson.build
index caed79c..20b6dd0 100644
--- a/573file/meson.build
+++ b/573file/meson.build
@@ -5,6 +5,8 @@ _573file_lib = static_library(
   sources: [
     'ifs.c',
     'ifs.h',
+    'lz.c',
+    'lz.h',
     'prop-binary-reader.c',
     'prop-binary-reader.h',
     'prop-type.c',