diff --git a/Makefile b/Makefile
index 7845fffc1c..5262f9e07a 100644
--- a/Makefile
+++ b/Makefile
@@ -342,13 +342,16 @@ libgtest.files := $(libgtest.dir)/gtest-all.cc
 libgbenchmark := $(lib)
 libgbenchmark.dir := lib/gbenchmark
 libgbenchmark.files := $(libgbenchmark.dir)/gbenchmark_main.cc $(libgbenchmark.dir)/gbenchmark-all.cc
+libblake2 := $(lib)
+libblake2.dir := lib/blake2
+libblake2.files := $(libblake2.dir)/blake2b-ref.c
 
 # We don't add libponyrt here. It's a special case because it can be compiled
 # to LLVM bitcode.
 ifeq ($(OSTYPE), linux)
-  libraries := libponyc libponyrt-pic libgtest libgbenchmark
+  libraries := libponyc libponyrt-pic libgtest libgbenchmark libblake2
 else
-  libraries := libponyc libgtest libgbenchmark
+  libraries := libponyc libgtest libgbenchmark libblake2
 endif
 
 # Third party, but prebuilt. Prebuilt libraries are defined as
@@ -396,7 +399,8 @@ benchmarks := libponyc.benchmarks libponyrt.benchmarks
 
 # Define include paths for targets if necessary. Note that these include paths
 # will automatically apply to the test suite of a target as well.
-libponyc.include := -I src/common/ -I src/libponyrt/ $(llvm.include)
+libponyc.include := -I src/common/ -I src/libponyrt/ $(llvm.include) \
+  -isystem lib/blake2
 libponycc.include := -I src/common/ $(llvm.include)
 libponyrt.include := -I src/common/ -I src/libponyrt/
 libponyrt-pic.include := $(libponyrt.include)
@@ -413,6 +417,7 @@ libponyrt.benchmarks.include := -I src/common/ -I src/libponyrt/ -isystem \
 ponyc.include := -I src/common/ -I src/libponyrt/ $(llvm.include)
 libgtest.include := -isystem lib/gtest/
 libgbenchmark.include := -isystem lib/gbenchmark/include/
+libblake2.include := -isystem lib/blake2/
 
 ifneq (,$(filter $(OSTYPE), osx bsd))
   libponyrt.include += -I /usr/local/include
@@ -475,13 +480,14 @@ endif
 # target specific disabling of build options
 libgtest.disable = -Wconversion -Wno-sign-conversion -Wextra
 libgbenchmark.disable = -Wconversion -Wno-sign-conversion -Wextra
+libblake2.disable = -Wconversion -Wno-sign-conversion -Wextra
 
 # Link relationships.
-ponyc.links = libponyc libponyrt llvm
-libponyc.tests.links = libgtest libponyc llvm
+ponyc.links = libponyc libponyrt llvm libblake2
+libponyc.tests.links = libgtest libponyc llvm libblake2
 libponyc.tests.links.whole = libponyrt
 libponyrt.tests.links = libgtest libponyrt
-libponyc.benchmarks.links = libgbenchmark libponyc libponyrt llvm
+libponyc.benchmarks.links = libblake2 libgbenchmark libponyc libponyrt llvm
 libponyrt.benchmarks.links = libgbenchmark libponyrt
 
 ifeq ($(OSTYPE),linux)
@@ -516,7 +522,7 @@ all: $(targets)
 	@:
 
 # Dependencies
-libponyc.depends := libponyrt
+libponyc.depends := libponyrt libblake2
 libponyc.tests.depends := libponyc libgtest
 libponyrt.tests.depends := libponyrt libgtest
 libponyc.benchmarks.depends := libponyc libgbenchmark
diff --git a/lib/blake2/blake2-impl.h b/lib/blake2/blake2-impl.h
new file mode 100644
index 0000000000..5dff7fc7a3
--- /dev/null
+++ b/lib/blake2/blake2-impl.h
@@ -0,0 +1,160 @@
+/*
+   BLAKE2 reference source code package - reference C implementations
+
+   Copyright 2012, Samuel Neves <sneves@dei.uc.pt>.  You may use this under the
+   terms of the CC0, the OpenSSL Licence, or the Apache Public License 2.0, at
+   your option.  The terms of these licenses can be found at:
+
+   - CC0 1.0 Universal : http://creativecommons.org/publicdomain/zero/1.0
+   - OpenSSL license   : https://www.openssl.org/source/license.html
+   - Apache 2.0        : http://www.apache.org/licenses/LICENSE-2.0
+
+   More information about the BLAKE2 hash function can be found at
+   https://blake2.net.
+*/
+#ifndef BLAKE2_IMPL_H
+#define BLAKE2_IMPL_H
+
+#include <stdint.h>
+#include <string.h>
+
+#if !defined(__cplusplus) && (!defined(__STDC_VERSION__) || __STDC_VERSION__ < 199901L)
+  #if   defined(_MSC_VER)
+    #define BLAKE2_INLINE __inline
+  #elif defined(__GNUC__)
+    #define BLAKE2_INLINE __inline__
+  #else
+    #define BLAKE2_INLINE
+  #endif
+#else
+  #define BLAKE2_INLINE inline
+#endif
+
+static BLAKE2_INLINE uint32_t load32( const void *src )
+{
+#if defined(NATIVE_LITTLE_ENDIAN)
+  uint32_t w;
+  memcpy(&w, src, sizeof w);
+  return w;
+#else
+  const uint8_t *p = ( const uint8_t * )src;
+  return (( uint32_t )( p[0] ) <<  0) |
+         (( uint32_t )( p[1] ) <<  8) |
+         (( uint32_t )( p[2] ) << 16) |
+         (( uint32_t )( p[3] ) << 24) ;
+#endif
+}
+
+static BLAKE2_INLINE uint64_t load64( const void *src )
+{
+#if defined(NATIVE_LITTLE_ENDIAN)
+  uint64_t w;
+  memcpy(&w, src, sizeof w);
+  return w;
+#else
+  const uint8_t *p = ( const uint8_t * )src;
+  return (( uint64_t )( p[0] ) <<  0) |
+         (( uint64_t )( p[1] ) <<  8) |
+         (( uint64_t )( p[2] ) << 16) |
+         (( uint64_t )( p[3] ) << 24) |
+         (( uint64_t )( p[4] ) << 32) |
+         (( uint64_t )( p[5] ) << 40) |
+         (( uint64_t )( p[6] ) << 48) |
+         (( uint64_t )( p[7] ) << 56) ;
+#endif
+}
+
+static BLAKE2_INLINE uint16_t load16( const void *src )
+{
+#if defined(NATIVE_LITTLE_ENDIAN)
+  uint16_t w;
+  memcpy(&w, src, sizeof w);
+  return w;
+#else
+  const uint8_t *p = ( const uint8_t * )src;
+  return (( uint16_t )( p[0] ) <<  0) |
+         (( uint16_t )( p[1] ) <<  8) ;
+#endif
+}
+
+static BLAKE2_INLINE void store16( void *dst, uint16_t w )
+{
+#if defined(NATIVE_LITTLE_ENDIAN)
+  memcpy(dst, &w, sizeof w);
+#else
+  uint8_t *p = ( uint8_t * )dst;
+  *p++ = ( uint8_t )w; w >>= 8;
+  *p++ = ( uint8_t )w;
+#endif
+}
+
+static BLAKE2_INLINE void store32( void *dst, uint32_t w )
+{
+#if defined(NATIVE_LITTLE_ENDIAN)
+  memcpy(dst, &w, sizeof w);
+#else
+  uint8_t *p = ( uint8_t * )dst;
+  p[0] = (uint8_t)(w >>  0);
+  p[1] = (uint8_t)(w >>  8);
+  p[2] = (uint8_t)(w >> 16);
+  p[3] = (uint8_t)(w >> 24);
+#endif
+}
+
+static BLAKE2_INLINE void store64( void *dst, uint64_t w )
+{
+#if defined(NATIVE_LITTLE_ENDIAN)
+  memcpy(dst, &w, sizeof w);
+#else
+  uint8_t *p = ( uint8_t * )dst;
+  p[0] = (uint8_t)(w >>  0);
+  p[1] = (uint8_t)(w >>  8);
+  p[2] = (uint8_t)(w >> 16);
+  p[3] = (uint8_t)(w >> 24);
+  p[4] = (uint8_t)(w >> 32);
+  p[5] = (uint8_t)(w >> 40);
+  p[6] = (uint8_t)(w >> 48);
+  p[7] = (uint8_t)(w >> 56);
+#endif
+}
+
+static BLAKE2_INLINE uint64_t load48( const void *src )
+{
+  const uint8_t *p = ( const uint8_t * )src;
+  return (( uint64_t )( p[0] ) <<  0) |
+         (( uint64_t )( p[1] ) <<  8) |
+         (( uint64_t )( p[2] ) << 16) |
+         (( uint64_t )( p[3] ) << 24) |
+         (( uint64_t )( p[4] ) << 32) |
+         (( uint64_t )( p[5] ) << 40) ;
+}
+
+static BLAKE2_INLINE void store48( void *dst, uint64_t w )
+{
+  uint8_t *p = ( uint8_t * )dst;
+  p[0] = (uint8_t)(w >>  0);
+  p[1] = (uint8_t)(w >>  8);
+  p[2] = (uint8_t)(w >> 16);
+  p[3] = (uint8_t)(w >> 24);
+  p[4] = (uint8_t)(w >> 32);
+  p[5] = (uint8_t)(w >> 40);
+}
+
+static BLAKE2_INLINE uint32_t rotr32( const uint32_t w, const unsigned c )
+{
+  return ( w >> c ) | ( w << ( 32 - c ) );
+}
+
+static BLAKE2_INLINE uint64_t rotr64( const uint64_t w, const unsigned c )
+{
+  return ( w >> c ) | ( w << ( 64 - c ) );
+}
+
+/* prevents compiler optimizing out memset() */
+static BLAKE2_INLINE void secure_zero_memory(void *v, size_t n)
+{
+  static void *(*const volatile memset_v)(void *, int, size_t) = &memset;
+  memset_v(v, 0, n);
+}
+
+#endif
diff --git a/lib/blake2/blake2.h b/lib/blake2/blake2.h
new file mode 100644
index 0000000000..9104333f7e
--- /dev/null
+++ b/lib/blake2/blake2.h
@@ -0,0 +1,91 @@
+/*
+   BLAKE2 reference source code package - reference C implementations
+
+   Copyright 2012, Samuel Neves <sneves@dei.uc.pt>.  You may use this under the
+   terms of the CC0, the OpenSSL Licence, or the Apache Public License 2.0, at
+   your option.  The terms of these licenses can be found at:
+
+   - CC0 1.0 Universal : http://creativecommons.org/publicdomain/zero/1.0
+   - OpenSSL license   : https://www.openssl.org/source/license.html
+   - Apache 2.0        : http://www.apache.org/licenses/LICENSE-2.0
+
+   More information about the BLAKE2 hash function can be found at
+   https://blake2.net.
+*/
+#ifndef BLAKE2_H
+#define BLAKE2_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+#if defined(_MSC_VER)
+#define BLAKE2_PACKED(x) __pragma(pack(push, 1)) x __pragma(pack(pop))
+#else
+#define BLAKE2_PACKED(x) x __attribute__((packed))
+#endif
+
+#if defined(__cplusplus)
+extern "C" {
+#endif
+
+  enum blake2b_constant
+  {
+    BLAKE2B_BLOCKBYTES = 128,
+    BLAKE2B_OUTBYTES   = 64,
+    BLAKE2B_KEYBYTES   = 64,
+    BLAKE2B_SALTBYTES  = 16,
+    BLAKE2B_PERSONALBYTES = 16
+  };
+
+  typedef struct blake2b_state__
+  {
+    uint64_t h[8];
+    uint64_t t[2];
+    uint64_t f[2];
+    uint8_t  buf[BLAKE2B_BLOCKBYTES];
+    size_t   buflen;
+    size_t   outlen;
+    uint8_t  last_node;
+  } blake2b_state;
+
+  BLAKE2_PACKED(struct blake2b_param__
+  {
+    uint8_t  digest_length; /* 1 */
+    uint8_t  key_length;    /* 2 */
+    uint8_t  fanout;        /* 3 */
+    uint8_t  depth;         /* 4 */
+    uint32_t leaf_length;   /* 8 */
+    uint32_t node_offset;   /* 12 */
+    uint32_t xof_length;    /* 16 */
+    uint8_t  node_depth;    /* 17 */
+    uint8_t  inner_length;  /* 18 */
+    uint8_t  reserved[14];  /* 32 */
+    uint8_t  salt[BLAKE2B_SALTBYTES]; /* 48 */
+    uint8_t  personal[BLAKE2B_PERSONALBYTES];  /* 64 */
+  });
+
+  typedef struct blake2b_param__ blake2b_param;
+
+  /* Padded structs result in a compile-time error */
+  enum {
+    BLAKE2_DUMMY_2 = 1/(int)(sizeof(blake2b_param) == BLAKE2B_OUTBYTES)
+  };
+
+  /* Streaming API */
+  int blake2b_init( blake2b_state *S, size_t outlen );
+  int blake2b_init_key( blake2b_state *S, size_t outlen, const void *key, size_t keylen );
+  int blake2b_init_param( blake2b_state *S, const blake2b_param *P );
+  int blake2b_update( blake2b_state *S, const void *in, size_t inlen );
+  int blake2b_final( blake2b_state *S, void *out, size_t outlen );
+
+  /* Simple API */
+  int blake2b( void *out, size_t outlen, const void *in, size_t inlen, const void *key, size_t keylen );
+
+  /* This is simply an alias for blake2b */
+  int blake2( void *out, size_t outlen, const void *in, size_t inlen, const void *key, size_t keylen );
+
+#if defined(__cplusplus)
+}
+#endif
+
+#endif
diff --git a/lib/blake2/blake2b-ref.c b/lib/blake2/blake2b-ref.c
new file mode 100644
index 0000000000..cd38b1ba00
--- /dev/null
+++ b/lib/blake2/blake2b-ref.c
@@ -0,0 +1,379 @@
+/*
+   BLAKE2 reference source code package - reference C implementations
+
+   Copyright 2012, Samuel Neves <sneves@dei.uc.pt>.  You may use this under the
+   terms of the CC0, the OpenSSL Licence, or the Apache Public License 2.0, at
+   your option.  The terms of these licenses can be found at:
+
+   - CC0 1.0 Universal : http://creativecommons.org/publicdomain/zero/1.0
+   - OpenSSL license   : https://www.openssl.org/source/license.html
+   - Apache 2.0        : http://www.apache.org/licenses/LICENSE-2.0
+
+   More information about the BLAKE2 hash function can be found at
+   https://blake2.net.
+*/
+
+#include <stdint.h>
+#include <string.h>
+#include <stdio.h>
+
+#include "blake2.h"
+#include "blake2-impl.h"
+
+static const uint64_t blake2b_IV[8] =
+{
+  0x6a09e667f3bcc908ULL, 0xbb67ae8584caa73bULL,
+  0x3c6ef372fe94f82bULL, 0xa54ff53a5f1d36f1ULL,
+  0x510e527fade682d1ULL, 0x9b05688c2b3e6c1fULL,
+  0x1f83d9abfb41bd6bULL, 0x5be0cd19137e2179ULL
+};
+
+static const uint8_t blake2b_sigma[12][16] =
+{
+  {  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15 } ,
+  { 14, 10,  4,  8,  9, 15, 13,  6,  1, 12,  0,  2, 11,  7,  5,  3 } ,
+  { 11,  8, 12,  0,  5,  2, 15, 13, 10, 14,  3,  6,  7,  1,  9,  4 } ,
+  {  7,  9,  3,  1, 13, 12, 11, 14,  2,  6,  5, 10,  4,  0, 15,  8 } ,
+  {  9,  0,  5,  7,  2,  4, 10, 15, 14,  1, 11, 12,  6,  8,  3, 13 } ,
+  {  2, 12,  6, 10,  0, 11,  8,  3,  4, 13,  7,  5, 15, 14,  1,  9 } ,
+  { 12,  5,  1, 15, 14, 13,  4, 10,  0,  7,  6,  3,  9,  2,  8, 11 } ,
+  { 13, 11,  7, 14, 12,  1,  3,  9,  5,  0, 15,  4,  8,  6,  2, 10 } ,
+  {  6, 15, 14,  9, 11,  3,  0,  8, 12,  2, 13,  7,  1,  4, 10,  5 } ,
+  { 10,  2,  8,  4,  7,  6,  1,  5, 15, 11,  9, 14,  3, 12, 13 , 0 } ,
+  {  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15 } ,
+  { 14, 10,  4,  8,  9, 15, 13,  6,  1, 12,  0,  2, 11,  7,  5,  3 }
+};
+
+
+static void blake2b_set_lastnode( blake2b_state *S )
+{
+  S->f[1] = (uint64_t)-1;
+}
+
+/* Some helper functions, not necessarily useful */
+static int blake2b_is_lastblock( const blake2b_state *S )
+{
+  return S->f[0] != 0;
+}
+
+static void blake2b_set_lastblock( blake2b_state *S )
+{
+  if( S->last_node ) blake2b_set_lastnode( S );
+
+  S->f[0] = (uint64_t)-1;
+}
+
+static void blake2b_increment_counter( blake2b_state *S, const uint64_t inc )
+{
+  S->t[0] += inc;
+  S->t[1] += ( S->t[0] < inc );
+}
+
+static void blake2b_init0( blake2b_state *S )
+{
+  size_t i;
+  memset( S, 0, sizeof( blake2b_state ) );
+
+  for( i = 0; i < 8; ++i ) S->h[i] = blake2b_IV[i];
+}
+
+/* init xors IV with input parameter block */
+int blake2b_init_param( blake2b_state *S, const blake2b_param *P )
+{
+  const uint8_t *p = ( const uint8_t * )( P );
+  size_t i;
+
+  blake2b_init0( S );
+
+  /* IV XOR ParamBlock */
+  for( i = 0; i < 8; ++i )
+    S->h[i] ^= load64( p + sizeof( S->h[i] ) * i );
+
+  S->outlen = P->digest_length;
+  return 0;
+}
+
+
+
+int blake2b_init( blake2b_state *S, size_t outlen )
+{
+  blake2b_param P[1];
+
+  if ( ( !outlen ) || ( outlen > BLAKE2B_OUTBYTES ) ) return -1;
+
+  P->digest_length = (uint8_t)outlen;
+  P->key_length    = 0;
+  P->fanout        = 1;
+  P->depth         = 1;
+  store32( &P->leaf_length, 0 );
+  store32( &P->node_offset, 0 );
+  store32( &P->xof_length, 0 );
+  P->node_depth    = 0;
+  P->inner_length  = 0;
+  memset( P->reserved, 0, sizeof( P->reserved ) );
+  memset( P->salt,     0, sizeof( P->salt ) );
+  memset( P->personal, 0, sizeof( P->personal ) );
+  return blake2b_init_param( S, P );
+}
+
+
+int blake2b_init_key( blake2b_state *S, size_t outlen, const void *key, size_t keylen )
+{
+  blake2b_param P[1];
+
+  if ( ( !outlen ) || ( outlen > BLAKE2B_OUTBYTES ) ) return -1;
+
+  if ( !key || !keylen || keylen > BLAKE2B_KEYBYTES ) return -1;
+
+  P->digest_length = (uint8_t)outlen;
+  P->key_length    = (uint8_t)keylen;
+  P->fanout        = 1;
+  P->depth         = 1;
+  store32( &P->leaf_length, 0 );
+  store32( &P->node_offset, 0 );
+  store32( &P->xof_length, 0 );
+  P->node_depth    = 0;
+  P->inner_length  = 0;
+  memset( P->reserved, 0, sizeof( P->reserved ) );
+  memset( P->salt,     0, sizeof( P->salt ) );
+  memset( P->personal, 0, sizeof( P->personal ) );
+
+  if( blake2b_init_param( S, P ) < 0 ) return -1;
+
+  {
+    uint8_t block[BLAKE2B_BLOCKBYTES];
+    memset( block, 0, BLAKE2B_BLOCKBYTES );
+    memcpy( block, key, keylen );
+    blake2b_update( S, block, BLAKE2B_BLOCKBYTES );
+    secure_zero_memory( block, BLAKE2B_BLOCKBYTES ); /* Burn the key from stack */
+  }
+  return 0;
+}
+
+#define G(r,i,a,b,c,d)                      \
+  do {                                      \
+    a = a + b + m[blake2b_sigma[r][2*i+0]]; \
+    d = rotr64(d ^ a, 32);                  \
+    c = c + d;                              \
+    b = rotr64(b ^ c, 24);                  \
+    a = a + b + m[blake2b_sigma[r][2*i+1]]; \
+    d = rotr64(d ^ a, 16);                  \
+    c = c + d;                              \
+    b = rotr64(b ^ c, 63);                  \
+  } while(0)
+
+#define ROUND(r)                    \
+  do {                              \
+    G(r,0,v[ 0],v[ 4],v[ 8],v[12]); \
+    G(r,1,v[ 1],v[ 5],v[ 9],v[13]); \
+    G(r,2,v[ 2],v[ 6],v[10],v[14]); \
+    G(r,3,v[ 3],v[ 7],v[11],v[15]); \
+    G(r,4,v[ 0],v[ 5],v[10],v[15]); \
+    G(r,5,v[ 1],v[ 6],v[11],v[12]); \
+    G(r,6,v[ 2],v[ 7],v[ 8],v[13]); \
+    G(r,7,v[ 3],v[ 4],v[ 9],v[14]); \
+  } while(0)
+
+static void blake2b_compress( blake2b_state *S, const uint8_t block[BLAKE2B_BLOCKBYTES] )
+{
+  uint64_t m[16];
+  uint64_t v[16];
+  size_t i;
+
+  for( i = 0; i < 16; ++i ) {
+    m[i] = load64( block + i * sizeof( m[i] ) );
+  }
+
+  for( i = 0; i < 8; ++i ) {
+    v[i] = S->h[i];
+  }
+
+  v[ 8] = blake2b_IV[0];
+  v[ 9] = blake2b_IV[1];
+  v[10] = blake2b_IV[2];
+  v[11] = blake2b_IV[3];
+  v[12] = blake2b_IV[4] ^ S->t[0];
+  v[13] = blake2b_IV[5] ^ S->t[1];
+  v[14] = blake2b_IV[6] ^ S->f[0];
+  v[15] = blake2b_IV[7] ^ S->f[1];
+
+  ROUND( 0 );
+  ROUND( 1 );
+  ROUND( 2 );
+  ROUND( 3 );
+  ROUND( 4 );
+  ROUND( 5 );
+  ROUND( 6 );
+  ROUND( 7 );
+  ROUND( 8 );
+  ROUND( 9 );
+  ROUND( 10 );
+  ROUND( 11 );
+
+  for( i = 0; i < 8; ++i ) {
+    S->h[i] = S->h[i] ^ v[i] ^ v[i + 8];
+  }
+}
+
+#undef G
+#undef ROUND
+
+int blake2b_update( blake2b_state *S, const void *pin, size_t inlen )
+{
+  const unsigned char * in = (const unsigned char *)pin;
+  if( inlen > 0 )
+  {
+    size_t left = S->buflen;
+    size_t fill = BLAKE2B_BLOCKBYTES - left;
+    if( inlen > fill )
+    {
+      S->buflen = 0;
+      memcpy( S->buf + left, in, fill ); /* Fill buffer */
+      blake2b_increment_counter( S, BLAKE2B_BLOCKBYTES );
+      blake2b_compress( S, S->buf ); /* Compress */
+      in += fill; inlen -= fill;
+      while(inlen > BLAKE2B_BLOCKBYTES) {
+        blake2b_increment_counter(S, BLAKE2B_BLOCKBYTES);
+        blake2b_compress( S, in );
+        in += BLAKE2B_BLOCKBYTES;
+        inlen -= BLAKE2B_BLOCKBYTES;
+      }
+    }
+    memcpy( S->buf + S->buflen, in, inlen );
+    S->buflen += inlen;
+  }
+  return 0;
+}
+
+int blake2b_final( blake2b_state *S, void *out, size_t outlen )
+{
+  uint8_t buffer[BLAKE2B_OUTBYTES] = {0};
+  size_t i;
+
+  if( out == NULL || outlen < S->outlen )
+    return -1;
+
+  if( blake2b_is_lastblock( S ) )
+    return -1;
+
+  blake2b_increment_counter( S, S->buflen );
+  blake2b_set_lastblock( S );
+  memset( S->buf + S->buflen, 0, BLAKE2B_BLOCKBYTES - S->buflen ); /* Padding */
+  blake2b_compress( S, S->buf );
+
+  for( i = 0; i < 8; ++i ) /* Output full hash to temp buffer */
+    store64( buffer + sizeof( S->h[i] ) * i, S->h[i] );
+
+  memcpy( out, buffer, S->outlen );
+  secure_zero_memory(buffer, sizeof(buffer));
+  return 0;
+}
+
+/* inlen, at least, should be uint64_t. Others can be size_t. */
+int blake2b( void *out, size_t outlen, const void *in, size_t inlen, const void *key, size_t keylen )
+{
+  blake2b_state S[1];
+
+  /* Verify parameters */
+  if ( NULL == in && inlen > 0 ) return -1;
+
+  if ( NULL == out ) return -1;
+
+  if( NULL == key && keylen > 0 ) return -1;
+
+  if( !outlen || outlen > BLAKE2B_OUTBYTES ) return -1;
+
+  if( keylen > BLAKE2B_KEYBYTES ) return -1;
+
+  if( keylen > 0 )
+  {
+    if( blake2b_init_key( S, outlen, key, keylen ) < 0 ) return -1;
+  }
+  else
+  {
+    if( blake2b_init( S, outlen ) < 0 ) return -1;
+  }
+
+  blake2b_update( S, ( const uint8_t * )in, inlen );
+  blake2b_final( S, out, outlen );
+  return 0;
+}
+
+int blake2( void *out, size_t outlen, const void *in, size_t inlen, const void *key, size_t keylen ) {
+  return blake2b(out, outlen, in, inlen, key, keylen);
+}
+
+#if defined(SUPERCOP)
+int crypto_hash( unsigned char *out, unsigned char *in, unsigned long long inlen )
+{
+  return blake2b( out, BLAKE2B_OUTBYTES, in, inlen, NULL, 0 );
+}
+#endif
+
+#if defined(BLAKE2B_SELFTEST)
+#include <string.h>
+#include "blake2-kat.h"
+int main( void )
+{
+  uint8_t key[BLAKE2B_KEYBYTES];
+  uint8_t buf[BLAKE2_KAT_LENGTH];
+  size_t i, step;
+
+  for( i = 0; i < BLAKE2B_KEYBYTES; ++i )
+    key[i] = ( uint8_t )i;
+
+  for( i = 0; i < BLAKE2_KAT_LENGTH; ++i )
+    buf[i] = ( uint8_t )i;
+
+  /* Test simple API */
+  for( i = 0; i < BLAKE2_KAT_LENGTH; ++i )
+  {
+    uint8_t hash[BLAKE2B_OUTBYTES];
+    blake2b( hash, BLAKE2B_OUTBYTES, buf, i, key, BLAKE2B_KEYBYTES );
+
+    if( 0 != memcmp( hash, blake2b_keyed_kat[i], BLAKE2B_OUTBYTES ) )
+    {
+      goto fail;
+    }
+  }
+
+  /* Test streaming API */
+  for(step = 1; step < BLAKE2B_BLOCKBYTES; ++step) {
+    for (i = 0; i < BLAKE2_KAT_LENGTH; ++i) {
+      uint8_t hash[BLAKE2B_OUTBYTES];
+      blake2b_state S;
+      uint8_t * p = buf;
+      size_t mlen = i;
+      int err = 0;
+
+      if( (err = blake2b_init_key(&S, BLAKE2B_OUTBYTES, key, BLAKE2B_KEYBYTES)) < 0 ) {
+        goto fail;
+      }
+
+      while (mlen >= step) {
+        if ( (err = blake2b_update(&S, p, step)) < 0 ) {
+          goto fail;
+        }
+        mlen -= step;
+        p += step;
+      }
+      if ( (err = blake2b_update(&S, p, mlen)) < 0) {
+        goto fail;
+      }
+      if ( (err = blake2b_final(&S, hash, BLAKE2B_OUTBYTES)) < 0) {
+        goto fail;
+      }
+
+      if (0 != memcmp(hash, blake2b_keyed_kat[i], BLAKE2B_OUTBYTES)) {
+        goto fail;
+      }
+    }
+  }
+
+  puts( "ok" );
+  return 0;
+fail:
+  puts("error");
+  return -1;
+}
+#endif
diff --git a/packages/serialise/serialise.pony b/packages/serialise/serialise.pony
index 0a1542a647..4af2fe8a2d 100644
--- a/packages/serialise/serialise.pony
+++ b/packages/serialise/serialise.pony
@@ -20,8 +20,24 @@ possibly including recompilation of the same code. This is due to the use of
 type identifiers rather than a heavy-weight self-describing serialisation
 schema. This also means it isn't safe to deserialise something serialised by
 the same program compiled for a different platform.
+
+The Serialise.signature method is provided for the purposes of comparing
+communicating Pony binaries to determine if they are the same. Confirming this
+before deserialising data can help mitigate the risk of accidental serialisation
+across different Pony binaries, but does not on its own address the security
+issues of accepting data from untrusted sources.
 """
 
+primitive Serialise
+  fun signature(): Array[U8] val =>
+    """
+    Returns a byte array that is unique to this compiled Pony binary, for the
+    purposes of comparing before deserialising any data from that source.
+    It is statistically impossible for two serialisation-incompatible Pony
+    binaries to have the same serialise signature.
+    """
+    @"internal.signature"[Array[U8] val]()
+
 primitive SerialiseAuth
   """
   This is a capability that allows the holder to serialise objects. It does not
diff --git a/src/libponyc/ast/ast.c b/src/libponyc/ast/ast.c
index 9e4312dce5..8cbb4dc897 100644
--- a/src/libponyc/ast/ast.c
+++ b/src/libponyc/ast/ast.c
@@ -67,13 +67,22 @@ struct ast_t
   token_t* t;
   symtab_t* symtab;
   void* data;
-  struct ast_t* parent;
-  struct ast_t* child;
-  struct ast_t* sibling;
-  struct ast_t* annotation_type;
+  ast_t* parent;
+  ast_t* child;
+  ast_t* sibling;
+  ast_t* annotation_type;
   uint32_t flags;
 };
 
+// Minimal AST structure for signature computation.
+typedef struct ast_signature_t
+{
+  token_signature_t* t;
+  struct ast_signature_t* child;
+  struct ast_signature_t* sibling;
+  struct ast_signature_t* annotation_type;
+} ast_signature_t;
+
 static bool ast_cmp(ast_t* a, ast_t* b)
 {
   return a == b;
@@ -1676,6 +1685,194 @@ void ast_extract_children(ast_t* parent, size_t child_count,
   }
 }
 
+static void ast_signature_serialise_trace(pony_ctx_t* ctx, void* object)
+{
+  ast_t* ast = (ast_t*)object;
+
+  // Ignore the data. We don't want to cross package boundaries.
+  // The symtab, parent and type don't provide additional information to the
+  // signature so we ignore them as well.
+
+  token_id id = ast_id(ast);
+  bool docstring = false;
+
+  if(id == TK_STRING)
+  {
+    switch(ast_id(ast_parent(ast)))
+    {
+      case TK_MODULE:
+      case TK_NEW:
+      case TK_FUN:
+      case TK_BE:
+      case TK_TYPE:
+      case TK_INTERFACE:
+      case TK_TRAIT:
+      case TK_PRIMITIVE:
+      case TK_STRUCT:
+      case TK_CLASS:
+      case TK_ACTOR:
+        docstring = true;
+        break;
+
+      default: {}
+    }
+  }
+
+  pony_traceknown(ctx, ast->t, docstring ?
+    token_docstring_signature_pony_type() : token_signature_pony_type(),
+    PONY_TRACE_MUTABLE);
+
+  if(id == TK_NOMINAL)
+  {
+    pony_assert(ast->child != NULL);
+
+    pony_traceknown(ctx, ast->child, (ast_id(ast->child) == TK_ID) ?
+      ast_nominal_pkg_id_signature_pony_type() : ast_signature_pony_type(),
+      PONY_TRACE_MUTABLE);
+  } else if(ast->child != NULL) {
+    pony_traceknown(ctx, ast->child, ast_signature_pony_type(),
+      PONY_TRACE_MUTABLE);
+  }
+
+  if((id != TK_PACKAGE) && (ast->sibling != NULL))
+    pony_traceknown(ctx, ast->sibling, ast_signature_pony_type(),
+      PONY_TRACE_MUTABLE);
+
+  // Don't use ast->annotation_type directly. It could be a type, and we don't
+  // want to serialise types.
+  ast_t* annotation = ast_annotation(ast);
+
+  if(annotation != NULL)
+    pony_traceknown(ctx, annotation, ast_signature_pony_type(),
+      PONY_TRACE_MUTABLE);
+}
+
+static void ast_signature_serialise(pony_ctx_t* ctx, void* object, void* buf,
+  size_t offset, int mutability)
+{
+  (void)mutability;
+
+  ast_t* ast = (ast_t*)object;
+  ast_signature_t* dst = (ast_signature_t*)((uintptr_t)buf + offset);
+
+  dst->t = (token_signature_t*)pony_serialise_offset(ctx, ast->t);
+  dst->child = (ast_signature_t*)pony_serialise_offset(ctx, ast->child);
+
+  if(ast_id(ast) != TK_PACKAGE)
+    dst->sibling = (ast_signature_t*)pony_serialise_offset(ctx, ast->sibling);
+  else
+    dst->sibling = NULL;
+
+  ast_t* annotation = ast_annotation(ast);
+
+  dst->annotation_type = (ast_signature_t*)pony_serialise_offset(ctx,
+    annotation);
+}
+
+static pony_type_t ast_signature_pony =
+{
+  0,
+  sizeof(ast_signature_t),
+  0,
+  0,
+  NULL,
+  NULL,
+  ast_signature_serialise_trace,
+  ast_signature_serialise,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  0,
+  NULL,
+  NULL,
+  NULL
+};
+
+pony_type_t* ast_signature_pony_type()
+{
+  return &ast_signature_pony;
+}
+
+// Special case serialisation for package IDs nodes as TK_NOMINAL children.
+static void ast_nominal_pkg_id_signature_serialise_trace(pony_ctx_t* ctx,
+  void* object)
+{
+  ast_t* ast = (ast_t*)object;
+
+  pony_assert(ast_id(ast) == TK_ID);
+  ast_t* parent = ast->parent;
+  (void)parent;
+  pony_assert((parent != NULL) && (ast_id(parent) == TK_NOMINAL) &&
+    (ast == parent->child));
+
+  // Ignore the token. We'll setup a fake token directly referencing the
+  // associated package later.
+
+  pony_assert(ast->child == NULL);
+  pony_assert(ast->sibling != NULL);
+
+  pony_traceknown(ctx, ast->sibling, ast_signature_pony_type(),
+    PONY_TRACE_MUTABLE);
+
+  ast_t* annotation = ast_annotation(ast);
+
+  if(annotation != NULL)
+    pony_traceknown(ctx, annotation, ast_signature_pony_type(),
+      PONY_TRACE_MUTABLE);
+}
+
+static void ast_nominal_pkg_id_signature_serialise(pony_ctx_t* ctx,
+  void* object, void* buf, size_t offset, int mutability)
+{
+  (void)mutability;
+
+  ast_t* ast = (ast_t*)object;
+  ast_signature_t* dst = (ast_signature_t*)((uintptr_t)buf + offset);
+
+  ast_t* def = (ast_t*)ast_data(ast_parent(ast));
+  pony_assert(def != NULL);
+  ast_t* pkg_ast = ast_nearest(def, TK_PACKAGE);
+  package_t* pkg = (package_t*)ast_data(pkg_ast);
+  pony_assert(pkg != NULL);
+  dst->t = (token_signature_t*)pony_serialise_offset(ctx, pkg);
+
+  dst->child = (ast_signature_t*)pony_serialise_offset(ctx, ast->child);
+  dst->sibling = (ast_signature_t*)pony_serialise_offset(ctx, ast->sibling);
+
+  ast_t* annotation = ast_annotation(ast);
+
+  dst->annotation_type = (ast_signature_t*)pony_serialise_offset(ctx,
+    annotation);
+}
+
+static pony_type_t ast_nominal_pkg_id_signature_pony =
+{
+  0,
+  sizeof(ast_signature_t),
+  0,
+  0,
+  NULL,
+  NULL,
+  ast_nominal_pkg_id_signature_serialise_trace,
+  ast_nominal_pkg_id_signature_serialise,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  0,
+  NULL,
+  NULL,
+  NULL
+};
+
+pony_type_t* ast_nominal_pkg_id_signature_pony_type()
+{
+  return &ast_nominal_pkg_id_signature_pony;
+}
+
 static void ast_serialise_trace_data(pony_ctx_t* ctx, ast_t* ast)
 {
   if(ast->data == NULL)
diff --git a/src/libponyc/ast/ast.h b/src/libponyc/ast/ast.h
index bd12df150f..c24074b1a5 100644
--- a/src/libponyc/ast/ast.h
+++ b/src/libponyc/ast/ast.h
@@ -205,6 +205,10 @@ void ast_extract_children(ast_t* parent, size_t child_count,
       children); \
   }
 
+pony_type_t* ast_signature_pony_type();
+
+pony_type_t* ast_nominal_pkg_id_signature_pony_type();
+
 pony_type_t* ast_pony_type();
 
 #if defined(PLATFORM_IS_POSIX_BASED) && defined(__cplusplus)
diff --git a/src/libponyc/ast/token.c b/src/libponyc/ast/token.c
index f6074e3d01..c29c4dc7a6 100644
--- a/src/libponyc/ast/token.c
+++ b/src/libponyc/ast/token.c
@@ -8,7 +8,7 @@
 #include <stdlib.h>
 #include <string.h>
 
-typedef struct token_t
+struct token_t
 {
   token_id id;
   source_t* source;
@@ -27,7 +27,26 @@ typedef struct token_t
     double real;
     lexint_t integer;
   };
-} token_t;
+};
+
+
+// Minimal token structure for signature computation.
+struct token_signature_t
+{
+  token_id id;
+
+  union
+  {
+    struct
+    {
+      const char* string;
+      size_t str_length;
+    };
+
+    double real;
+    lexint_t integer;
+  };
+};
 
 
 token_t* token_new(token_id id)
@@ -316,6 +335,125 @@ void token_set_pos(token_t* token, source_t* source, size_t line, size_t pos)
 
 // Serialisation
 
+static void token_signature_serialise_trace(pony_ctx_t* ctx, void* object)
+{
+  token_t* token = (token_t*)object;
+
+  if((token->id == TK_STRING) || (token->id == TK_ID))
+    string_trace_len(ctx, token->string, token->str_length);
+}
+
+static void token_signature_serialise(pony_ctx_t* ctx, void* object, void* buf,
+  size_t offset, int mutability)
+{
+  (void)mutability;
+
+  token_t* token = (token_t*)object;
+  token_signature_t* dst = (token_signature_t*)((uintptr_t)buf + offset);
+
+  memset(dst, 0, sizeof(token_signature_t));
+
+  dst->id = token->id;
+
+  switch(token->id)
+  {
+    case TK_STRING:
+    case TK_ID:
+      dst->str_length = token->str_length;
+      dst->string = (const char*)pony_serialise_offset(ctx,
+        (char*)token->string);
+      break;
+
+    case TK_FLOAT:
+      dst->real = token->real;
+      break;
+
+    case TK_INT:
+      dst->integer = token->integer;
+      break;
+
+    default: {}
+  }
+}
+
+static pony_type_t token_signature_pony =
+{
+  0,
+  sizeof(token_signature_t),
+  0,
+  0,
+  NULL,
+  NULL,
+  token_signature_serialise_trace,
+  token_signature_serialise,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  0,
+  NULL,
+  NULL,
+  NULL
+};
+
+pony_type_t* token_signature_pony_type()
+{
+  return &token_signature_pony;
+}
+
+// Docstring-specific signature serialisation. We don't want docstrings to
+// influence signatures so we pretend them to be TK_NONE nodes.
+static void token_docstring_signature_serialise_trace(pony_ctx_t* ctx,
+  void* object)
+{
+  (void)ctx;
+
+  token_t* token = (token_t*)object;
+
+  pony_assert(token->id == TK_STRING);
+}
+
+static void token_docstring_signature_serialise(pony_ctx_t* ctx, void* object,
+  void* buf, size_t offset, int mutability)
+{
+  (void)ctx;
+  (void)object;
+  (void)mutability;
+
+  token_signature_t* dst = (token_signature_t*)((uintptr_t)buf + offset);
+
+  memset(dst, 0, sizeof(token_signature_t));
+
+  dst->id = TK_NONE;
+}
+
+static pony_type_t token_docstring_signature_pony =
+{
+  0,
+  sizeof(token_signature_t),
+  0,
+  0,
+  NULL,
+  NULL,
+  token_docstring_signature_serialise_trace,
+  token_docstring_signature_serialise,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  0,
+  NULL,
+  NULL,
+  NULL
+};
+
+pony_type_t* token_docstring_signature_pony_type()
+{
+  return &token_docstring_signature_pony;
+}
+
 static void token_serialise_trace(pony_ctx_t* ctx, void* object)
 {
   token_t* token = (token_t*)object;
diff --git a/src/libponyc/ast/token.h b/src/libponyc/ast/token.h
index 2f18ec333b..0a126def12 100644
--- a/src/libponyc/ast/token.h
+++ b/src/libponyc/ast/token.h
@@ -15,6 +15,8 @@ extern "C" {
 
 typedef struct token_t token_t;
 
+typedef struct token_signature_t token_signature_t;
+
 typedef enum token_id
 {
   TK_EOF,
@@ -291,6 +293,13 @@ token_t* token_dup_new_id(token_t* token, token_id id);
   */
 void token_free(token_t* token);
 
+/// Get a pony_type_t for token_t. Should only be used for signature computation.
+pony_type_t* token_signature_pony_type();
+
+/// Get a pony_type_t for a docstring token_t. Should only be used for signature
+/// computation.
+pony_type_t* token_docstring_signature_pony_type();
+
 /// Get a pony_type_t for token_t. Should only be used for serialisation.
 pony_type_t* token_pony_type();
 
diff --git a/src/libponyc/codegen/genprim.c b/src/libponyc/codegen/genprim.c
index d1fa836736..d698abd6b0 100644
--- a/src/libponyc/codegen/genprim.c
+++ b/src/libponyc/codegen/genprim.c
@@ -6,7 +6,9 @@
 #include "genopt.h"
 #include "genserialise.h"
 #include "gentrace.h"
+#include "../pkg/package.h"
 #include "../pkg/platformfuns.h"
+#include "../pkg/program.h"
 #include "../pass/names.h"
 #include "../type/assemble.h"
 #include "../type/cap.h"
@@ -1967,6 +1969,68 @@ static void make_rdtscp(compile_t* c)
   }
 }
 
+static LLVMValueRef make_signature_array(compile_t* c, compile_type_t* c_t,
+  const char* signature)
+{
+  LLVMValueRef args[SIGNATURE_LENGTH];
+
+  for(size_t i = 0; i < SIGNATURE_LENGTH; i++)
+    args[i] = LLVMConstInt(c->i8, signature[i], false);
+
+  LLVMValueRef sig = LLVMConstArray(c->i8, args, SIGNATURE_LENGTH);
+  LLVMValueRef g_sig = LLVMAddGlobal(c->module, LLVMTypeOf(sig), "");
+  LLVMSetLinkage(g_sig, LLVMPrivateLinkage);
+  LLVMSetInitializer(g_sig, sig);
+  LLVMSetGlobalConstant(g_sig, true);
+  LLVMSetUnnamedAddr(g_sig, true);
+
+  args[0] = LLVMConstInt(c->i32, 0, false);
+  args[1] = LLVMConstInt(c->i32, 0, false);
+
+  LLVMValueRef ptr = LLVMConstInBoundsGEP(g_sig, args, 2);
+
+  args[0] = c_t->desc;
+  args[1] = LLVMConstInt(c->intptr, SIGNATURE_LENGTH, false);
+  args[2] = args[1];
+  args[3] = ptr;
+
+  LLVMValueRef inst = LLVMConstNamedStruct(c_t->structure, args, 4);
+  LLVMValueRef g_inst = LLVMAddGlobal(c->module, c_t->structure, "");
+  LLVMSetInitializer(g_inst, inst);
+  LLVMSetGlobalConstant(g_inst, true);
+  LLVMSetLinkage(g_inst, LLVMPrivateLinkage);
+  LLVMSetUnnamedAddr(g_inst, true);
+
+  return g_inst;
+}
+
+void genprim_signature(compile_t* c)
+{
+  reach_type_t* t = reach_type_name(c->reach, "Array_U8_val");
+
+  if(t == NULL)
+    return;
+
+  compile_type_t* c_t = (compile_type_t*)t->c_type;
+
+  ast_t* def = (ast_t*)ast_data(t->ast);
+  pony_assert(def != NULL);
+
+  ast_t* program = ast_nearest(def, TK_PROGRAM);
+  pony_assert(program != NULL);
+
+  const char* signature = program_signature(program);
+  LLVMValueRef g_array = make_signature_array(c, c_t, signature);
+
+  // Array_U8_val* @internal.signature()
+  LLVMTypeRef f_type = LLVMFunctionType(c_t->use_type, NULL, 0, false);
+  LLVMValueRef fun = codegen_addfun(c, "internal.signature", f_type, false);
+  LLVMSetFunctionCallConv(fun, LLVMCCallConv);
+  codegen_startfun(c, fun, NULL, NULL, false);
+  LLVMBuildRet(c->builder, g_array);
+  codegen_finishfun(c);
+}
+
 void genprim_builtins(compile_t* c)
 {
   number_conversions(c);
diff --git a/src/libponyc/codegen/genprim.h b/src/libponyc/codegen/genprim.h
index 38e0c673e2..35af3efaec 100644
--- a/src/libponyc/codegen/genprim.h
+++ b/src/libponyc/codegen/genprim.h
@@ -29,6 +29,8 @@ void genprim_string_deserialise(compile_t* c, reach_type_t* t);
 
 void genprim_platform_methods(compile_t* c, reach_type_t* t);
 
+void genprim_signature(compile_t* c);
+
 void genprim_builtins(compile_t* c);
 
 void genprim_reachable_init(compile_t* c, ast_t* program);
diff --git a/src/libponyc/codegen/gentype.c b/src/libponyc/codegen/gentype.c
index b20d4036e7..66204a93e8 100644
--- a/src/libponyc/codegen/gentype.c
+++ b/src/libponyc/codegen/gentype.c
@@ -847,6 +847,8 @@ bool gentypes(compile_t* c)
     make_global_instance(c, t);
   }
 
+  genprim_signature(c);
+
   // Cache the instance of None, which is used as the return value for
   // behaviour calls.
   t = reach_type_name(c->reach, "None");
diff --git a/src/libponyc/pass/pass.c b/src/libponyc/pass/pass.c
index 3ea9513fc2..dbd546c4d4 100644
--- a/src/libponyc/pass/pass.c
+++ b/src/libponyc/pass/pass.c
@@ -15,6 +15,7 @@
 #include "serialisers.h"
 #include "docgen.h"
 #include "../codegen/codegen.h"
+#include "../pkg/program.h"
 #include "../../libponyrt/mem/pool.h"
 #include "ponyassert.h"
 
@@ -250,6 +251,15 @@ static bool ast_passes(ast_t** astp, pass_opt_t* options, pass_id last)
 
   if(options->check_tree)
     check_tree(*astp, options->check.errors);
+
+  if(ast_id(*astp) == TK_PROGRAM)
+  {
+    program_signature(*astp);
+
+    if(options->verbosity >= VERBOSITY_TOOL_INFO)
+      program_dump(*astp);
+  }
+
   return true;
 }
 
diff --git a/src/libponyc/pass/scope.c b/src/libponyc/pass/scope.c
index f73684a250..73f332c0c7 100644
--- a/src/libponyc/pass/scope.c
+++ b/src/libponyc/pass/scope.c
@@ -92,6 +92,10 @@ bool use_package(ast_t* ast, const char* path, ast_t* name,
   // again
   ast_setdata(ast, (void*)package);
 
+  ast_t* curr_package = ast_nearest(ast, TK_PACKAGE);
+  pony_assert(curr_package != NULL);
+  package_add_dependency(curr_package, package);
+
   if(name != NULL && ast_id(name) == TK_ID) // We have an alias
     return set_scope(options, ast, name, package, false);
 
diff --git a/src/libponyc/pkg/package.c b/src/libponyc/pkg/package.c
index dd9dd56f97..3162d38a2d 100644
--- a/src/libponyc/pkg/package.c
+++ b/src/libponyc/pkg/package.c
@@ -9,7 +9,9 @@
 #include "../expr/literal.h"
 #include "../../libponyrt/gc/serialise.h"
 #include "../../libponyrt/mem/pool.h"
+#include "../../libponyrt/sched/scheduler.h"
 #include "ponyassert.h"
+#include <blake2.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <stdbool.h>
@@ -68,17 +70,40 @@ static const char* simple_builtin =
   "  => None";
 
 
+DECLARE_HASHMAP_SERIALISE(package_set, package_set_t, package_t)
+
 // Per package state
-typedef struct package_t
+struct package_t
 {
   const char* path; // Absolute path
   const char* qualified_name; // For pretty printing, eg "builtin/U32"
   const char* id; // Hygienic identifier
-  const char* filename; // Filename if we are an executable
+  const char* filename; // Directory name
   const char* symbol; // Wart to use for symbol names
+  ast_t* ast;
+  package_set_t dependencies;
+  package_group_t* group;
+  size_t group_index;
   size_t next_hygienic_id;
+  size_t low_index;
   bool allow_ffi;
-} package_t;
+  bool on_stack;
+};
+
+// Minimal package data structure for signature computation.
+typedef struct package_signature_t
+{
+  const char* filename;
+  package_group_t* group;
+  size_t group_index;
+} package_signature_t;
+
+// A strongly connected component in the package dependency graph
+struct package_group_t
+{
+  char* signature;
+  package_set_t members;
+};
 
 // Per defined magic package sate
 typedef struct magic_package_t
@@ -98,6 +123,31 @@ static strlist_t* safe = NULL;
 static magic_package_t* magic_packages = NULL;
 static bool report_build = true;
 
+DECLARE_STACK(package_stack, package_stack_t, package_t)
+DEFINE_STACK(package_stack, package_stack_t, package_t)
+
+DEFINE_LIST_SERIALISE(package_group_list, package_group_list_t, package_group_t,
+  NULL, package_group_free, package_group_pony_type())
+
+
+static size_t package_hash(package_t* pkg)
+{
+  // Hash the full string instead of the stringtab pointer. We want a
+  // deterministic hash in order to enable deterministic hashmap iteration,
+  // which in turn enables deterministic package signatures.
+  return (size_t)ponyint_hash_str(pkg->qualified_name);
+}
+
+
+static bool package_cmp(package_t* a, package_t* b)
+{
+  return a->qualified_name == b->qualified_name;
+}
+
+
+DEFINE_HASHMAP_SERIALISE(package_set, package_set_t, package_t, package_hash,
+  package_cmp, NULL, package_pony_type())
+
 
 // Find the magic source code associated with the given path, if any
 static magic_package_t* find_magic_package(const char* path)
@@ -188,6 +238,12 @@ void path_cat(const char* part1, const char* part2, char result[FILENAME_MAX])
 }
 
 
+static int string_compare(const void* a, const void* b)
+{
+  return strcmp(*(const char**)a, *(const char**)b);
+}
+
+
 // Attempt to parse the source files in the specified directory and add them to
 // the given package AST
 // @return true on success, false on error
@@ -211,14 +267,16 @@ static bool parse_files_in_dir(ast_t* package, const char* dir_path,
     return false;
   }
 
+  size_t count = 0;
+  size_t buf_size = 4 * sizeof(const char*);
+  const char** entries = (const char**)ponyint_pool_alloc_size(buf_size);
   PONY_DIRINFO* d;
-  bool r = true;
 
   while((d = pony_dir_entry_next(dir)) != NULL)
   {
     // Handle only files with the specified extension that don't begin with
     // a dot. This avoids including UNIX hidden files in a build.
-    char* name = pony_dir_info_name(d);
+    const char* name = stringtab(pony_dir_info_name(d));
 
     if(name[0] == '.')
       continue;
@@ -227,13 +285,33 @@ static bool parse_files_in_dir(ast_t* package, const char* dir_path,
 
     if((p != NULL) && (strcmp(p, EXTENSION) == 0))
     {
-      char fullpath[FILENAME_MAX];
-      path_cat(dir_path, name, fullpath);
-      r &= parse_source_file(package, fullpath, opt);
+      if((count * sizeof(const char*)) == buf_size)
+      {
+        size_t new_buf_size = buf_size * 2;
+        entries = (const char**)ponyint_pool_realloc_size(buf_size,
+          new_buf_size, entries);
+        buf_size = new_buf_size;
+      }
+
+      entries[count++] = name;
     }
   }
 
   pony_closedir(dir);
+
+  // In order for package signatures to be deterministic, file parsing order
+  // must be deterministic too.
+  qsort(entries, count, sizeof(const char*), string_compare);
+  bool r = true;
+
+  for(size_t i = 0; i < count; i++)
+  {
+    char fullpath[FILENAME_MAX];
+    path_cat(dir_path, entries[i], fullpath);
+    r &= parse_source_file(package, fullpath, opt);
+  }
+
+  ponyint_pool_free_size(buf_size, entries);
   return r;
 }
 
@@ -512,7 +590,12 @@ static ast_t* create_package(ast_t* program, const char* name,
   else
     pkg->symbol = NULL;
 
+  pkg->ast = package;
+  package_set_init(&pkg->dependencies, 1);
+  pkg->group = NULL;
+  pkg->group_index = -1;
   pkg->next_hygienic_id = 0;
+  pkg->low_index = -1;
   ast_setdata(package, pkg);
 
   ast_scope(package);
@@ -525,6 +608,8 @@ static ast_t* create_package(ast_t* program, const char* name,
   else
     pkg->allow_ffi = true;
 
+  pkg->on_stack = false;
+
   return package;
 }
 
@@ -959,7 +1044,10 @@ ast_t* package_load(ast_t* from, const char* path, pass_opt_t* opt)
 void package_free(package_t* package)
 {
   if(package != NULL)
+  {
+    package_set_destroy(&package->dependencies);
     POOL_FREE(package_t, package);
+  }
 }
 
 
@@ -1068,6 +1156,449 @@ const char* package_alias_from_id(ast_t* module, const char* id)
 }
 
 
+void package_add_dependency(ast_t* package, ast_t* dep)
+{
+  pony_assert(ast_id(package) == TK_PACKAGE);
+  pony_assert(ast_id(dep) == TK_PACKAGE);
+
+  if(package == dep)
+    return;
+
+  package_t* pkg = (package_t*)ast_data(package);
+  package_t* pkg_dep = (package_t*)ast_data(dep);
+
+  pony_assert(pkg != NULL);
+  pony_assert(pkg_dep != NULL);
+
+  size_t index = HASHMAP_UNKNOWN;
+  package_t* in_set = package_set_get(&pkg->dependencies, pkg_dep, &index);
+
+  if(in_set != NULL)
+    return;
+
+  package_set_putindex(&pkg->dependencies, pkg_dep, index);
+}
+
+
+const char* package_signature(ast_t* package)
+{
+  pony_assert(ast_id(package) == TK_PACKAGE);
+
+  package_t* pkg = (package_t*)ast_data(package);
+  pony_assert(pkg->group != NULL);
+
+  return package_group_signature(pkg->group);
+}
+
+
+size_t package_group_index(ast_t* package)
+{
+  pony_assert(ast_id(package) == TK_PACKAGE);
+
+  package_t* pkg = (package_t*)ast_data(package);
+  pony_assert(pkg->group_index != (size_t)-1);
+
+  return pkg->group_index;
+}
+
+
+package_group_t* package_group_new()
+{
+  package_group_t* group = POOL_ALLOC(package_group_t);
+  group->signature = NULL;
+  package_set_init(&group->members, 1);
+  return group;
+}
+
+
+void package_group_free(package_group_t* group)
+{
+  if(group->signature != NULL)
+    ponyint_pool_free_size(SIGNATURE_LENGTH, group->signature);
+
+  package_set_destroy(&group->members);
+  POOL_FREE(package_group_t, group);
+}
+
+
+static void make_dependency_group(package_t* package,
+  package_group_list_t** groups, package_stack_t** stack, size_t* index)
+{
+  pony_assert(!package->on_stack);
+  package->group_index = package->low_index = (*index)++;
+  *stack = package_stack_push(*stack, package);
+  package->on_stack = true;
+
+  size_t i = HASHMAP_BEGIN;
+  package_t* dep;
+
+  while((dep = package_set_next(&package->dependencies, &i)) != NULL)
+  {
+    if(dep->group_index == (size_t)-1)
+    {
+      make_dependency_group(dep, groups, stack, index);
+
+      if(dep->low_index < package->low_index)
+        package->low_index = dep->low_index;
+    } else if(dep->on_stack && (dep->group_index < package->low_index)) {
+      package->low_index = dep->group_index;
+    }
+  }
+
+  if(package->group_index == package->low_index)
+  {
+    package_group_t* group = package_group_new();
+    package_t* member;
+    size_t i = 0;
+
+    do
+    {
+      *stack = package_stack_pop(*stack, &member);
+      member->on_stack = false;
+      member->group = group;
+      member->group_index = i++;
+      package_set_put(&group->members, member);
+    } while(package != member);
+
+    *groups = package_group_list_push(*groups, group);
+  }
+}
+
+
+// A dependency group is a strongly connected component in the dependency graph.
+package_group_list_t* package_dependency_groups(ast_t* first_package)
+{
+  package_group_list_t* groups = NULL;
+  package_stack_t* stack = NULL;
+  size_t index = 0;
+
+  while(first_package != NULL)
+  {
+    pony_assert(ast_id(first_package) == TK_PACKAGE);
+    package_t* package = (package_t*)ast_data(first_package);
+
+    if(package->group_index == (size_t)-1)
+      make_dependency_group(package, &groups, &stack, &index);
+
+    first_package = ast_sibling(first_package);
+  }
+
+  pony_assert(stack == NULL);
+  return package_group_list_reverse(groups);
+}
+
+
+static void print_signature(const char* sig)
+{
+  for(size_t i = 0; i < SIGNATURE_LENGTH; i++)
+    printf("%02hhX", sig[i]);
+}
+
+
+void package_group_dump(package_group_t* group)
+{
+  package_set_t deps;
+  package_set_init(&deps, 1);
+
+  fputs("Signature: ", stdout);
+
+  if(group->signature != NULL)
+    print_signature(group->signature);
+  else
+    fputs("(NONE)", stdout);
+
+  putchar('\n');
+
+  puts("Members:");
+
+  size_t i = HASHMAP_BEGIN;
+  package_t* member;
+
+  while((member = package_set_next(&group->members, &i)) != NULL)
+  {
+    printf("  %s\n", member->filename);
+
+    size_t j = HASHMAP_BEGIN;
+    package_t* dep;
+
+    while((dep = package_set_next(&member->dependencies, &j)) != NULL)
+    {
+      size_t k = HASHMAP_UNKNOWN;
+      package_t* in_set = package_set_get(&group->members, dep, &k);
+
+      if(in_set == NULL)
+      {
+        k = HASHMAP_UNKNOWN;
+        in_set = package_set_get(&deps, dep, &k);
+
+        if(in_set == NULL)
+          package_set_putindex(&deps, dep, k);
+      }
+    }
+  }
+
+  puts("Dependencies:");
+
+  i = HASHMAP_BEGIN;
+
+  while((member = package_set_next(&deps, &i)) != NULL)
+    printf("  %s\n", member->filename);
+
+  package_set_destroy(&deps);
+}
+
+
+// *_signature_* handles the current group, *_dep_signature_* handles the direct
+// dependencies. Indirect dependencies are ignored, they are covered by the
+// signature of the direct dependencies.
+// Some data is traced but not serialised. This is to avoid redundant
+// information.
+
+
+static void package_dep_signature_serialise_trace(pony_ctx_t* ctx,
+  void* object)
+{
+  package_t* package = (package_t*)object;
+
+  string_trace(ctx, package->filename);
+  pony_traceknown(ctx, package->group, package_group_dep_signature_pony_type(),
+    PONY_TRACE_MUTABLE);
+}
+
+static void package_signature_serialise_trace(pony_ctx_t* ctx,
+  void* object)
+{
+  package_t* package = (package_t*)object;
+
+  string_trace(ctx, package->filename);
+  // The group has already been traced.
+
+  size_t i = HASHMAP_BEGIN;
+  package_t* dep;
+
+  while((dep = package_set_next(&package->dependencies, &i)) != NULL)
+    pony_traceknown(ctx, dep, package_dep_signature_pony_type(),
+      PONY_TRACE_MUTABLE);
+
+  pony_traceknown(ctx, package->ast, ast_signature_pony_type(),
+    PONY_TRACE_MUTABLE);
+}
+
+
+static void package_signature_serialise(pony_ctx_t* ctx, void* object,
+  void* buf, size_t offset, int mutability)
+{
+  (void)mutability;
+
+  package_t* package = (package_t*)object;
+  package_signature_t* dst = (package_signature_t*)((uintptr_t)buf + offset);
+
+  dst->filename = (const char*)pony_serialise_offset(ctx,
+    (char*)package->filename);
+  dst->group = (package_group_t*)pony_serialise_offset(ctx, package->group);
+  dst->group_index = package->group_index;
+}
+
+
+static pony_type_t package_dep_signature_pony =
+{
+  0,
+  sizeof(package_signature_t),
+  0,
+  0,
+  NULL,
+  NULL,
+  package_dep_signature_serialise_trace,
+  package_signature_serialise, // Same function for both package and package_dep.
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  0,
+  NULL,
+  NULL,
+  NULL
+};
+
+
+pony_type_t* package_dep_signature_pony_type()
+{
+  return &package_dep_signature_pony;
+}
+
+
+static pony_type_t package_signature_pony =
+{
+  0,
+  sizeof(package_signature_t),
+  0,
+  0,
+  NULL,
+  NULL,
+  package_signature_serialise_trace,
+  package_signature_serialise,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  0,
+  NULL,
+  NULL,
+  NULL
+};
+
+
+pony_type_t* package_signature_pony_type()
+{
+  return &package_signature_pony;
+}
+
+
+static void package_group_dep_signature_serialise_trace(pony_ctx_t* ctx,
+  void* object)
+{
+  package_group_t* group = (package_group_t*)object;
+
+  pony_assert(group->signature != NULL);
+  pony_serialise_reserve(ctx, group->signature, SIGNATURE_LENGTH);
+}
+
+
+static void package_group_signature_serialise_trace(pony_ctx_t* ctx,
+  void* object)
+{
+  package_group_t* group = (package_group_t*)object;
+
+  pony_assert(group->signature == NULL);
+
+  size_t i = HASHMAP_BEGIN;
+  package_t* member;
+
+  while((member = package_set_next(&group->members, &i)) != NULL)
+  {
+    pony_traceknown(ctx, member, package_signature_pony_type(),
+      PONY_TRACE_MUTABLE);
+  }
+}
+
+
+static void package_group_signature_serialise(pony_ctx_t* ctx, void* object,
+  void* buf, size_t offset, int mutability)
+{
+  (void)ctx;
+  (void)mutability;
+
+  package_group_t* group = (package_group_t*)object;
+  package_group_t* dst = (package_group_t*)((uintptr_t)buf + offset);
+
+  if(group->signature != NULL)
+  {
+    uintptr_t ptr_offset = pony_serialise_offset(ctx, group->signature);
+    char* dst_sig = (char*)((uintptr_t)buf + ptr_offset);
+    memcpy(dst_sig, group->signature, SIGNATURE_LENGTH);
+    dst->signature = (char*)ptr_offset;
+  } else {
+    dst->signature = NULL;
+  }
+}
+
+
+static pony_type_t package_group_dep_signature_pony =
+{
+  0,
+  sizeof(const char*),
+  0,
+  0,
+  NULL,
+  NULL,
+  package_group_dep_signature_serialise_trace,
+  package_group_signature_serialise, // Same function for both group and group_dep.
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  0,
+  NULL,
+  NULL,
+  NULL
+};
+
+
+pony_type_t* package_group_dep_signature_pony_type()
+{
+  return &package_group_dep_signature_pony;
+}
+
+
+static pony_type_t package_group_signature_pony =
+{
+  0,
+  sizeof(const char*),
+  0,
+  0,
+  NULL,
+  NULL,
+  package_group_signature_serialise_trace,
+  package_group_signature_serialise,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  0,
+  NULL,
+  NULL,
+  NULL
+};
+
+
+pony_type_t* package_group_signature_pony_type()
+{
+  return &package_group_signature_pony;
+}
+
+
+static void* s_alloc_fn(pony_ctx_t* ctx, size_t size)
+{
+  (void)ctx;
+  return ponyint_pool_alloc_size(size);
+}
+
+
+static void s_throw_fn()
+{
+  pony_assert(false);
+}
+
+
+// TODO: Make group signature indiependent of package load order.
+const char* package_group_signature(package_group_t* group)
+{
+  if(group->signature == NULL)
+  {
+    pony_ctx_t ctx;
+    memset(&ctx, 0, sizeof(pony_ctx_t));
+    ponyint_array_t array;
+    memset(&array, 0, sizeof(ponyint_array_t));
+    char* buf = (char*)ponyint_pool_alloc_size(SIGNATURE_LENGTH);
+
+    pony_serialise(&ctx, group, package_group_signature_pony_type(), &array,
+      s_alloc_fn, s_throw_fn);
+    int status = blake2b(buf, SIGNATURE_LENGTH, array.ptr, array.size, NULL, 0);
+    (void)status;
+    pony_assert(status == 0);
+
+    group->signature = buf;
+    ponyint_pool_free_size(array.size, array.ptr);
+  }
+
+  return group->signature;
+}
+
+
 void package_done()
 {
   strlist_free(search);
@@ -1091,6 +1622,13 @@ static void package_serialise_trace(pony_ctx_t* ctx, void* object)
 
   if(package->symbol != NULL)
     string_trace(ctx, package->symbol);
+
+  pony_traceknown(ctx, package->ast, ast_pony_type(), PONY_TRACE_MUTABLE);
+  package_set_serialise_trace(ctx, &package->dependencies);
+
+  if(package->group != NULL)
+    pony_traceknown(ctx, package->group, package_group_pony_type(),
+      PONY_TRACE_MUTABLE);
 }
 
 
@@ -1110,8 +1648,16 @@ static void package_serialise(pony_ctx_t* ctx, void* object, void* buf,
     (char*)package->filename);
   dst->symbol = (const char*)pony_serialise_offset(ctx, (char*)package->symbol);
 
+  dst->ast = (ast_t*)pony_serialise_offset(ctx, package->ast);
+  package_set_serialise(ctx, &package->dependencies, buf,
+    offset + offsetof(package_t, dependencies), PONY_TRACE_MUTABLE);
+  dst->group = (package_group_t*)pony_serialise_offset(ctx, package->group);
+
+  dst->group_index = package->group_index;
   dst->next_hygienic_id = package->next_hygienic_id;
+  dst->low_index = package->low_index;
   dst->allow_ffi = package->allow_ffi;
+  dst->on_stack = package->on_stack;
 }
 
 
@@ -1126,6 +1672,12 @@ static void package_deserialise(pony_ctx_t* ctx, void* object)
   package->filename = string_deserialise_offset(ctx,
     (uintptr_t)package->filename);
   package->symbol = string_deserialise_offset(ctx, (uintptr_t)package->symbol);
+
+  package->ast = (ast_t*)pony_deserialise_offset(ctx, ast_pony_type(),
+    (uintptr_t)package->ast);
+  package_set_deserialise(ctx, &package->dependencies);
+  package->group = (package_group_t*)pony_deserialise_offset(ctx,
+    package_group_pony_type(), (uintptr_t)package->group);
 }
 
 
@@ -1157,6 +1709,78 @@ pony_type_t* package_pony_type()
 }
 
 
+static void package_group_serialise_trace(pony_ctx_t* ctx, void* object)
+{
+  package_group_t* group = (package_group_t*)object;
+
+  if(group->signature != NULL)
+    pony_serialise_reserve(ctx, group->signature, SIGNATURE_LENGTH);
+
+  package_set_serialise_trace(ctx, &group->members);
+}
+
+
+static void package_group_serialise(pony_ctx_t* ctx, void* object, void* buf,
+  size_t offset, int mutability)
+{
+  (void)ctx;
+  (void)mutability;
+
+  package_group_t* group = (package_group_t*)object;
+  package_group_t* dst = (package_group_t*)((uintptr_t)buf + offset);
+
+  uintptr_t ptr_offset = pony_serialise_offset(ctx, group->signature);
+  dst->signature = (char*)ptr_offset;
+
+  if(group->signature != NULL)
+  {
+    char* dst_sig = (char*)((uintptr_t)buf + ptr_offset);
+    memcpy(dst_sig, group->signature, SIGNATURE_LENGTH);
+  }
+
+  package_set_serialise(ctx, &group->members, buf,
+    offset + offsetof(package_group_t, members), PONY_TRACE_MUTABLE);
+}
+
+
+static void package_group_deserialise(pony_ctx_t* ctx, void* object)
+{
+  package_group_t* group = (package_group_t*)object;
+
+  group->signature = (char*)pony_deserialise_block(ctx,
+    (uintptr_t)group->signature, SIGNATURE_LENGTH);
+  package_set_deserialise(ctx, &group->members);
+}
+
+
+static pony_type_t package_group_pony =
+{
+  0,
+  sizeof(package_group_t),
+  0,
+  0,
+  NULL,
+  NULL,
+  package_group_serialise_trace,
+  package_group_serialise,
+  package_group_deserialise,
+  NULL,
+  NULL,
+  NULL,
+  NULL,
+  0,
+  NULL,
+  NULL,
+  NULL
+};
+
+
+pony_type_t* package_group_pony_type()
+{
+  return &package_group_pony;
+}
+
+
 bool is_path_absolute(const char* path)
 {
   // Begins with /
diff --git a/src/libponyc/pkg/package.h b/src/libponyc/pkg/package.h
index 5def1d2384..512ac1ab95 100644
--- a/src/libponyc/pkg/package.h
+++ b/src/libponyc/pkg/package.h
@@ -6,9 +6,15 @@
 #include "../ast/stringtab.h"
 #include "../pass/pass.h"
 
+#define SIGNATURE_LENGTH 64
+
 PONY_EXTERN_C_BEGIN
 
 typedef struct package_t package_t;
+typedef struct package_group_t package_group_t;
+
+DECLARE_LIST_SERIALISE(package_group_list, package_group_list_t,
+  package_group_t)
 
 /**
  * Cat together the 2 given path fragments into the given buffer.
@@ -139,13 +145,46 @@ bool package_allow_ffi(typecheck_t* t);
  */
 const char* package_alias_from_id(ast_t* module, const char* id);
 
+/**
+ * Adds a package to the dependency list of another package.
+ */
+void package_add_dependency(ast_t* package, ast_t* dep);
+
+const char* package_signature(ast_t* package);
+
+size_t package_group_index(ast_t* package);
+
+package_group_t* package_group_new();
+
+void package_group_free(package_group_t* group);
+
+/**
+ * Build a list of the dependency groups (the strongly connected components) in
+ * the package dependency graph. The list is topologically sorted.
+ */
+package_group_list_t* package_dependency_groups(ast_t* first_package);
+
+const char* package_group_signature(package_group_t* group);
+
+void package_group_dump(package_group_t* group);
+
 /**
  * Cleans up the list of search directories.
  */
 void package_done();
 
+pony_type_t* package_dep_signature_pony_type();
+
+pony_type_t* package_signature_pony_type();
+
+pony_type_t* package_group_dep_signature_pony_type();
+
+pony_type_t* package_group_signature_pony_type();
+
 pony_type_t* package_pony_type();
 
+pony_type_t* package_group_pony_type();
+
 bool is_path_absolute(const char* path);
 
 bool is_path_relative(const char* path);
diff --git a/src/libponyc/pkg/program.c b/src/libponyc/pkg/program.c
index 21a71436f9..38c738dc8a 100644
--- a/src/libponyc/pkg/program.c
+++ b/src/libponyc/pkg/program.c
@@ -4,12 +4,15 @@
 #include "../../libponyrt/gc/serialise.h"
 #include "../../libponyrt/mem/pool.h"
 #include "ponyassert.h"
+#include <blake2.h>
 #include <string.h>
 
 
 // Per program state.
 typedef struct program_t
 {
+  package_group_list_t* package_groups;
+  char* signature;
   uint32_t next_package_id;
   strlist_t* libpaths;
   strlist_t* libs;
@@ -46,6 +49,8 @@ static void append_to_args(program_t* program, const char* text)
 program_t* program_create()
 {
   program_t* p = POOL_ALLOC(program_t);
+  p->package_groups = NULL;
+  p->signature = NULL;
   p->next_package_id = 0;
   p->libpaths = NULL;
   p->libs = NULL;
@@ -60,6 +65,11 @@ void program_free(program_t* program)
 {
   pony_assert(program != NULL);
 
+  package_group_list_free(program->package_groups);
+
+  if(program->signature != NULL)
+    ponyint_pool_free_size(SIGNATURE_LENGTH, program->signature);
+
   strlist_free(program->libpaths);
   strlist_free(program->libs);
 
@@ -250,10 +260,91 @@ const char* program_lib_args(ast_t* program)
 }
 
 
+const char* program_signature(ast_t* program)
+{
+  pony_assert(program != NULL);
+  pony_assert(ast_id(program) == TK_PROGRAM);
+
+  program_t* data = (program_t*)ast_data(program);
+  pony_assert(data != NULL);
+
+  if(data->signature == NULL)
+  {
+    ast_t* first_package = ast_child(program);
+    pony_assert(first_package != NULL);
+
+    pony_assert(data->package_groups == NULL);
+    data->package_groups = package_dependency_groups(first_package);
+
+    blake2b_state hash_state;
+    int status = blake2b_init(&hash_state, SIGNATURE_LENGTH);
+    pony_assert(status == 0);
+
+    package_group_list_t* iter = data->package_groups;
+
+    while(iter != NULL)
+    {
+      package_group_t* group = package_group_list_data(iter);
+      const char* group_sig = package_group_signature(group);
+      blake2b_update(&hash_state, group_sig, SIGNATURE_LENGTH);
+      iter = package_group_list_next(iter);
+    }
+
+    data->signature = (char*)ponyint_pool_alloc_size(SIGNATURE_LENGTH);
+    status = blake2b_final(&hash_state, data->signature, SIGNATURE_LENGTH);
+    pony_assert(status == 0);
+  }
+
+  return data->signature;
+}
+
+
+static void print_signature(const char* sig)
+{
+  for(size_t i = 0; i < SIGNATURE_LENGTH; i++)
+    printf("%02hhX", sig[i]);
+}
+
+
+void program_dump(ast_t* program)
+{
+  pony_assert(program != NULL);
+  pony_assert(ast_id(program) == TK_PROGRAM);
+
+  program_t* data = (program_t*)ast_data(program);
+  pony_assert(data != NULL);
+
+  const char* signature = program_signature(program);
+  fputs("Program signature: ", stdout);
+  print_signature(signature);
+  puts("\n");
+
+  size_t i = 0;
+  package_group_list_t* iter = data->package_groups;
+
+  while(iter != NULL)
+  {
+    printf("Group " __zu "\n", i);
+    package_group_t* group = package_group_list_data(iter);
+    package_group_dump(group);
+    putchar('\n');
+    iter = package_group_list_next(iter);
+    i++;
+  }
+}
+
+
 static void program_serialise_trace(pony_ctx_t* ctx, void* object)
 {
   program_t* program = (program_t*)object;
 
+  if(program->package_groups != NULL)
+    pony_traceknown(ctx, program->package_groups,
+      package_group_list_pony_type(), PONY_TRACE_MUTABLE);
+
+  if(program->signature != NULL)
+    pony_serialise_reserve(ctx, program->signature, SIGNATURE_LENGTH);
+
   if(program->libpaths != NULL)
     pony_traceknown(ctx, program->libpaths, strlist_pony_type(),
       PONY_TRACE_MUTABLE);
@@ -274,30 +365,48 @@ static void program_serialise(pony_ctx_t* ctx, void* object, void* buf,
   program_t* program = (program_t*)object;
   program_t* dst = (program_t*)((uintptr_t)buf + offset);
 
+  dst->package_groups = (package_group_list_t*)pony_serialise_offset(ctx,
+    program->package_groups);
+
+  uintptr_t ptr_offset = pony_serialise_offset(ctx, program->signature);
+  dst->signature = (char*)ptr_offset;
+
+  if(program->signature != NULL)
+  {
+    char* dst_sig = (char*)((uintptr_t)buf + ptr_offset);
+    memcpy(dst_sig, program->signature, SIGNATURE_LENGTH);
+  }
+
+  dst->next_package_id = program->next_package_id;
   dst->libpaths = (strlist_t*)pony_serialise_offset(ctx, program->libpaths);
   dst->libs = (strlist_t*)pony_serialise_offset(ctx, program->libs);
-  dst->lib_args = (char*)pony_serialise_offset(ctx, program->lib_args);
   dst->lib_args_size = program->lib_args_size;
   dst->lib_args_alloced = program->lib_args_size + 1;
 
+  ptr_offset = pony_serialise_offset(ctx, program->lib_args);
+  dst->lib_args = (char*)ptr_offset;
+
   if(dst->lib_args != NULL)
-    memcpy(dst->lib_args, program->lib_args, program->lib_args_size + 1);
+  {
+    char* dst_lib = (char*)((uintptr_t)buf + ptr_offset);
+    memcpy(dst_lib, program->lib_args, program->lib_args_size + 1);
+  }
 }
 
 static void program_deserialise(pony_ctx_t* ctx, void* object)
 {
   program_t* program = (program_t*)object;
 
+  program->package_groups = (package_group_list_t*)pony_deserialise_offset(ctx,
+    package_group_list_pony_type(), (uintptr_t)program->package_groups);
+  program->signature = (char*)pony_deserialise_block(ctx,
+    (uintptr_t)program->signature, SIGNATURE_LENGTH);
   program->libpaths = (strlist_t*)pony_deserialise_offset(ctx,
     strlist_pony_type(), (uintptr_t)program->libpaths);
   program->libs = (strlist_t*)pony_deserialise_offset(ctx, strlist_pony_type(),
     (uintptr_t)program->libs);
-
-  if(program->lib_args != (char*)((size_t)(~0)))
-    program->lib_args = (char*)pony_deserialise_block(ctx,
-      (uintptr_t)program->lib_args, program->lib_args_size + 1);
-  else
-    program->lib_args = NULL;
+  program->lib_args = (char*)pony_deserialise_block(ctx,
+    (uintptr_t)program->lib_args, program->lib_args_size + 1);
 }
 
 
diff --git a/src/libponyc/pkg/program.h b/src/libponyc/pkg/program.h
index f791154fd7..7871ecbf50 100644
--- a/src/libponyc/pkg/program.h
+++ b/src/libponyc/pkg/program.h
@@ -42,6 +42,10 @@ void program_lib_build_args(ast_t* program, pass_opt_t* opt,
  */
 const char* program_lib_args(ast_t* program);
 
+const char* program_signature(ast_t* program);
+
+void program_dump(ast_t* program);
+
 pony_type_t* program_pony_type();
 
 PONY_EXTERN_C_END
diff --git a/src/libponyrt/ds/list.c b/src/libponyrt/ds/list.c
index 71cf0fea61..4fd6adbd62 100644
--- a/src/libponyrt/ds/list.c
+++ b/src/libponyrt/ds/list.c
@@ -178,3 +178,35 @@ void ponyint_list_free(list_t* list, free_fn f)
     list = next;
   }
 }
+
+void ponyint_list_serialise_trace(pony_ctx_t* ctx, void* object,
+  pony_type_t* list_type, pony_type_t* elem_type)
+{
+  list_t* list = (list_t*)object;
+
+  if(list->data != NULL)
+    pony_traceknown(ctx, list->data, elem_type, PONY_TRACE_MUTABLE);
+
+  if(list->next != NULL)
+    pony_traceknown(ctx, list->next, list_type, PONY_TRACE_MUTABLE);
+}
+
+void ponyint_list_serialise(pony_ctx_t* ctx, void* object, void* buf,
+  size_t offset)
+{
+  list_t* list = (list_t*)object;
+  list_t* dst = (list_t*)((uintptr_t)buf + offset);
+
+  dst->data = (void*)pony_serialise_offset(ctx, list->data);
+  dst->next = (list_t*)pony_serialise_offset(ctx, list->next);
+}
+
+void ponyint_list_deserialise(pony_ctx_t* ctx, void* object,
+  pony_type_t* list_type, pony_type_t* elem_type)
+{
+  list_t* list = (list_t*)object;
+
+  list->data = pony_deserialise_offset(ctx, elem_type, (uintptr_t)list->data);
+  list->next = (list_t*)pony_deserialise_offset(ctx, list_type,
+    (uintptr_t)list->next);
+}
diff --git a/src/libponyrt/ds/list.h b/src/libponyrt/ds/list.h
index 402a69a886..6d175f41dc 100644
--- a/src/libponyrt/ds/list.h
+++ b/src/libponyrt/ds/list.h
@@ -43,6 +43,15 @@ size_t ponyint_list_length(list_t* list);
 
 void ponyint_list_free(list_t* list, free_fn f);
 
+void ponyint_list_serialise_trace(pony_ctx_t* ctx, void* object,
+  pony_type_t* list_type, pony_type_t* elem_type);
+
+void ponyint_list_serialise(pony_ctx_t* ctx, void* object, void* buf,
+  size_t offset);
+
+void ponyint_list_deserialise(pony_ctx_t* ctx, void* object,
+  pony_type_t* list_type, pony_type_t* elem_type);
+
 #define DECLARE_LIST(name, name_t, elem) \
   typedef struct name_t name_t; \
   typedef bool (*name##_cmp_fn)(elem* a, elem* b); \
@@ -63,6 +72,14 @@ void ponyint_list_free(list_t* list, free_fn f);
   size_t name##_length(name_t* list); \
   void name##_free(name_t* list); \
 
+#define DECLARE_LIST_SERIALISE(name, name_t, type) \
+  DECLARE_LIST(name, name_t, type) \
+  void name##_serialise_trace(pony_ctx_t* ctx, void* object); \
+  void name##_serialise(pony_ctx_t* ctx, void* object, void* buf, \
+    size_t offset, int mutability); \
+  void name##_deserialise(pony_ctx_t* ctx, void* object); \
+  const pony_type_t* name##_pony_type(); \
+
 #define DEFINE_LIST(name, name_t, elem, cmpf, freef) \
   struct name_t {list_t contents;}; \
   \
@@ -128,6 +145,47 @@ void ponyint_list_free(list_t* list, free_fn f);
     ponyint_list_free((list_t*)list, (free_fn)free); \
   } \
 
+#define DEFINE_LIST_SERIALISE(name, name_t, elem, cmpf, freef, elem_type) \
+  DEFINE_LIST(name, name_t, elem, cmpf, freef) \
+  void name##_serialise_trace(pony_ctx_t* ctx, void* object) \
+  { \
+    ponyint_list_serialise_trace(ctx, object, name##_pony_type(), elem_type); \
+  } \
+  void name##_serialise(pony_ctx_t* ctx, void* object, void* buf, \
+    size_t offset, int mutability) \
+  { \
+    (void)mutability; \
+    ponyint_list_serialise(ctx, object, buf, offset); \
+  } \
+  void name##_deserialise(pony_ctx_t* ctx, void* object) \
+  { \
+    ponyint_list_deserialise(ctx, object, name##_pony_type(), elem_type); \
+  } \
+  static pony_type_t name##_pony = \
+  { \
+    0, \
+    sizeof(name_t), \
+    0, \
+    0, \
+    NULL, \
+    NULL, \
+    name##_serialise_trace, \
+    name##_serialise, \
+    name##_deserialise, \
+    NULL, \
+    NULL, \
+    NULL, \
+    NULL, \
+    0, \
+    NULL, \
+    NULL, \
+    NULL, \
+  }; \
+  const pony_type_t* name##_pony_type() \
+  { \
+    return &name##_pony; \
+  } \
+
 PONY_EXTERN_C_END
 
 #endif
diff --git a/src/libponyrt/gc/serialise.c b/src/libponyrt/gc/serialise.c
index 1fb04ef16a..b7ab1e4ce4 100644
--- a/src/libponyrt/gc/serialise.c
+++ b/src/libponyrt/gc/serialise.c
@@ -24,14 +24,6 @@ static pony_type_t** desc_table = NULL;
 
 PONY_EXTERN_C_END
 
-typedef struct
-{
-  pony_type_t* t;
-  size_t size;
-  size_t alloc;
-  char* ptr;
-} ponyint_array_t;
-
 struct serialise_t
 {
   uintptr_t key;
@@ -214,7 +206,8 @@ PONY_API size_t pony_serialise_offset(pony_ctx_t* ctx, void* p)
 }
 
 PONY_API void pony_serialise(pony_ctx_t* ctx, void* p, pony_type_t* t,
-  void* out, serialise_alloc_fn alloc_fn, serialise_throw_fn throw_fn)
+  ponyint_array_t* out, serialise_alloc_fn alloc_fn,
+  serialise_throw_fn throw_fn)
 {
   // This can raise an error.
   pony_assert(ctx->stack == NULL);
@@ -231,10 +224,9 @@ PONY_API void pony_serialise(pony_ctx_t* ctx, void* p, pony_type_t* t,
 
   ponyint_gc_handlestack(ctx);
 
-  ponyint_array_t* r = (ponyint_array_t*)out;
-  r->size = ctx->serialise_size;
-  r->alloc = r->size;
-  r->ptr = (char*)alloc_fn(ctx, r->size);
+  out->size = ctx->serialise_size;
+  out->alloc = out->size;
+  out->ptr = (char*)alloc_fn(ctx, out->size);
 
   size_t i = HASHMAP_BEGIN;
   serialise_t* s;
@@ -242,7 +234,7 @@ PONY_API void pony_serialise(pony_ctx_t* ctx, void* p, pony_type_t* t,
   while((s = ponyint_serialise_next(&ctx->serialise, &i)) != NULL)
   {
     if(!(s->block) && s->t != NULL && s->t->serialise != NULL)
-      s->t->serialise(ctx, (void*)s->key, r->ptr, s->value, s->mutability);
+      s->t->serialise(ctx, (void*)s->key, out->ptr, s->value, s->mutability);
   }
 
   serialise_cleanup(ctx);
@@ -388,14 +380,13 @@ PONY_API void* pony_deserialise_raw(pony_ctx_t* ctx, uintptr_t offset,
   return object;
 }
 
-PONY_API void* pony_deserialise(pony_ctx_t* ctx, pony_type_t* t, void* in,
-  serialise_alloc_fn alloc_fn, serialise_alloc_fn alloc_final_fn,
-  serialise_throw_fn throw_fn)
+PONY_API void* pony_deserialise(pony_ctx_t* ctx, pony_type_t* t,
+  ponyint_array_t* in, serialise_alloc_fn alloc_fn,
+  serialise_alloc_fn alloc_final_fn, serialise_throw_fn throw_fn)
 {
   // This can raise an error.
-  ponyint_array_t* r = (ponyint_array_t*)in;
-  ctx->serialise_buffer = r->ptr;
-  ctx->serialise_size = r->size;
+  ctx->serialise_buffer = in->ptr;
+  ctx->serialise_size = in->size;
   ctx->serialise_alloc = alloc_fn;
   ctx->serialise_alloc_final = alloc_final_fn;
   ctx->serialise_throw = throw_fn;
diff --git a/src/libponyrt/gc/serialise.h b/src/libponyrt/gc/serialise.h
index 37527c89dc..8b244fa59e 100644
--- a/src/libponyrt/gc/serialise.h
+++ b/src/libponyrt/gc/serialise.h
@@ -5,6 +5,14 @@
 
 PONY_EXTERN_C_BEGIN
 
+typedef struct
+{
+  pony_type_t* t;
+  size_t size;
+  size_t alloc;
+  char* ptr;
+} ponyint_array_t;
+
 typedef void* (*serialise_alloc_fn)(pony_ctx_t* ctx, size_t size);
 
 typedef void (*serialise_throw_fn)();
@@ -23,13 +31,14 @@ void ponyint_serialise_object(pony_ctx_t* ctx, void* p, pony_type_t* t,
 void ponyint_serialise_actor(pony_ctx_t* ctx, pony_actor_t* actor);
 
 PONY_API void pony_serialise(pony_ctx_t* ctx, void* p, pony_type_t* t,
-  void* out, serialise_alloc_fn alloc_fn, serialise_throw_fn throw_fn);
+  ponyint_array_t* out, serialise_alloc_fn alloc_fn,
+  serialise_throw_fn throw_fn);
 PONY_API size_t pony_serialise_offset(pony_ctx_t* ctx, void* p);
 PONY_API void pony_serialise_reserve(pony_ctx_t* ctx, void* p, size_t size);
 
-PONY_API void* pony_deserialise(pony_ctx_t* ctx, pony_type_t* t, void* in,
-  serialise_alloc_fn alloc_fn, serialise_alloc_fn alloc_final_fn,
-  serialise_throw_fn throw_fn);
+PONY_API void* pony_deserialise(pony_ctx_t* ctx, pony_type_t* t,
+  ponyint_array_t* in, serialise_alloc_fn alloc_fn,
+  serialise_alloc_fn alloc_final_fn, serialise_throw_fn throw_fn);
 PONY_API void* pony_deserialise_block(pony_ctx_t* ctx, uintptr_t offset,
   size_t size);
 PONY_API void* pony_deserialise_offset(pony_ctx_t* ctx, pony_type_t* t,
diff --git a/test/libponyc/compiler_serialisation.cc b/test/libponyc/compiler_serialisation.cc
index 172658cc46..3e4044523a 100644
--- a/test/libponyc/compiler_serialisation.cc
+++ b/test/libponyc/compiler_serialisation.cc
@@ -45,14 +45,6 @@ static void s_throw_fn()
   throw std::exception{};
 }
 
-typedef struct ponyint_array_t
-{
-  void* desc;
-  size_t size;
-  size_t alloc;
-  char* ptr;
-} ponyint_array_t;
-
 struct pool_size_deleter
 {
   size_t size;
diff --git a/test/libponyc/signature.cc b/test/libponyc/signature.cc
new file mode 100644
index 0000000000..ed91804b0a
--- /dev/null
+++ b/test/libponyc/signature.cc
@@ -0,0 +1,170 @@
+#include <gtest/gtest.h>
+#include <platform.h>
+
+#include <pkg/package.h>
+#include <pkg/program.h>
+
+#include "util.h"
+
+#define TEST_COMPILE(src) DO(test_compile(src, "ir"))
+
+class SignatureTest : public PassTest
+{};
+
+TEST_F(SignatureTest, RecompilationSameSignature)
+{
+  const char* src =
+    "actor Main\n"
+    "  new create(env: Env) =>\n"
+    "    None";
+
+  char first_signature[SIGNATURE_LENGTH];
+
+  TEST_COMPILE(src);
+
+  const char* signature = program_signature(program);
+  memcpy(first_signature, signature, SIGNATURE_LENGTH);
+
+  TEST_COMPILE(src);
+
+  signature = program_signature(program);
+
+  ASSERT_TRUE(memcmp(first_signature, signature, SIGNATURE_LENGTH) == 0);
+}
+
+TEST_F(SignatureTest, PackageReferences)
+{
+  const char* pkg =
+    "primitive Foo";
+
+  const char* src1 =
+    "use pkg1 = \"pkg1\"\n"
+    "use pkg2 = \"pkg2\"\n"
+
+    "actor Main\n"
+    "  new create(env: Env) =>\n"
+    "    pkg1.Foo";
+
+  const char* src2 =
+    "use pkg2 = \"pkg2\"\n"
+    "use pkg1 = \"pkg1\"\n"
+
+    "actor Main\n"
+    "  new create(env: Env) =>\n"
+    "    pkg2.Foo";
+
+  char main1_signature[SIGNATURE_LENGTH];
+  char pkg11_signature[SIGNATURE_LENGTH];
+  char pkg21_signature[SIGNATURE_LENGTH];
+
+  add_package("pkg1", pkg);
+  add_package("pkg2", pkg);
+
+  TEST_COMPILE(src1);
+
+  ast_t* main_ast = ast_child(program);
+  ast_t* pkg1_ast = ast_get(program, stringtab("pkg1"), nullptr);
+  ast_t* pkg2_ast = ast_get(program, stringtab("pkg2"), nullptr);
+
+  const char* signature = package_signature(main_ast);
+  memcpy(main1_signature, signature, SIGNATURE_LENGTH);
+  signature = package_signature(pkg1_ast);
+  memcpy(pkg11_signature, signature, SIGNATURE_LENGTH);
+  signature = package_signature(pkg2_ast);
+  memcpy(pkg21_signature, signature, SIGNATURE_LENGTH);
+
+  TEST_COMPILE(src2);
+
+  main_ast = ast_child(program);
+  pkg1_ast = ast_get(program, stringtab("pkg1"), nullptr);
+  pkg2_ast = ast_get(program, stringtab("pkg2"), nullptr);
+
+  signature = package_signature(main_ast);
+
+  ASSERT_FALSE(memcmp(main1_signature, signature, SIGNATURE_LENGTH) == 0);
+
+  signature = package_signature(pkg1_ast);
+
+  ASSERT_TRUE(memcmp(pkg11_signature, signature, SIGNATURE_LENGTH) == 0);
+
+  signature = package_signature(pkg2_ast);
+
+  ASSERT_TRUE(memcmp(pkg21_signature, signature, SIGNATURE_LENGTH) == 0);
+}
+
+TEST_F(SignatureTest, DocstringIgnored)
+{
+  const char* src1 =
+    "actor Main\n"
+    "  new create(env: Env) =>\n"
+    "    None";
+
+  const char* src2 =
+    "actor Main\n"
+    "  \"\"\"Foo\"\"\"\n"
+    "  new create(env: Env) =>\n"
+    "    None";
+
+  const char* src3 =
+    "actor Main\n"
+    "  \"\"\"Bar\"\"\"\n"
+    "  new create(env: Env) =>\n"
+    "    None";
+
+  char first_signature[SIGNATURE_LENGTH];
+
+  TEST_COMPILE(src1);
+
+  const char* signature = program_signature(program);
+  memcpy(first_signature, signature, SIGNATURE_LENGTH);
+
+  TEST_COMPILE(src2);
+
+  signature = program_signature(program);
+
+  ASSERT_TRUE(memcmp(first_signature, signature, SIGNATURE_LENGTH) == 0);
+
+  TEST_COMPILE(src3);
+
+  signature = program_signature(program);
+
+  ASSERT_TRUE(memcmp(first_signature, signature, SIGNATURE_LENGTH) == 0);
+}
+
+extern "C"
+{
+
+static char sig_in[SIGNATURE_LENGTH];
+
+EXPORT_SYMBOL char* signature_get()
+{
+  return sig_in;
+}
+
+}
+
+TEST_F(SignatureTest, SerialiseSignature)
+{
+  const char* src =
+    "use \"serialise\"\n"
+
+    "actor Main\n"
+    "  new create(env: Env) =>\n"
+    "    let sig_in = @signature_get[Pointer[U8]]()\n"
+    "    let sig = Serialise.signature()\n"
+    "    let ok = @memcmp[I32](sig_in, sig.cpointer(), sig.size())\n"
+    "    if ok == 0 then\n"
+    "      @pony_exitcode[None](I32(1))\n"
+    "    end";
+
+  set_builtin(nullptr);
+
+  TEST_COMPILE(src);
+
+  const char* signature = program_signature(program);
+  memcpy(sig_in, signature, SIGNATURE_LENGTH);
+
+  int exit_code = 0;
+  ASSERT_TRUE(run_program(&exit_code));
+  ASSERT_EQ(exit_code, 1);
+}
diff --git a/test/libponyrt/ds/hash.cc b/test/libponyrt/ds/hash.cc
index 6fb61407dd..e635c43277 100644
--- a/test/libponyrt/ds/hash.cc
+++ b/test/libponyrt/ds/hash.cc
@@ -386,14 +386,6 @@ TEST_F(HashMapTest, NotEmptyPutByIndex)
   ASSERT_EQ(e->val, m->val);
 }
 
-typedef struct ponyint_array_t
-{
-  void* desc;
-  size_t size;
-  size_t alloc;
-  char* ptr;
-} ponyint_array_t;
-
 struct testmap_deleter
 {
   void operator()(testmap_t* ptr)
diff --git a/wscript b/wscript
index ecbdcf2514..3e0c3e8b1c 100644
--- a/wscript
+++ b/wscript
@@ -259,6 +259,13 @@ def build(ctx):
         includes = [ 'lib/gbenchmark/include' ],
         defines  = [ 'HAVE_STD_REGEX' ]
     )
+    
+    # blake2
+    ctx(
+        features = 'c seq',
+        target   = 'blake2',
+        source   = ctx.path.ant_glob('lib/blake2/*.c'),
+    )
 
     # libponyc
     ctx(
@@ -266,7 +273,8 @@ def build(ctx):
         target    = 'libponyc',
         source    = ctx.path.ant_glob('src/libponyc/**/*.c') + \
                     ctx.path.ant_glob('src/libponyc/**/*.cc'),
-        includes  = [ 'src/common' ] + llvmIncludes + sslIncludes
+        includes  = [ 'src/common', 'lib/blake2' ] + llvmIncludes + sslIncludes,
+        use       = [ 'blake2' ]
     )
 
     # libponyc.benchmarks