hex-to-base32z.py: add -r flag to do reverse conversion

contrib/hex-to-base32z.py

@@ -3,6 +3,7 @@
 import sys
 
 base32z_dict = 'ybndrfg8ejkmcpqxot1uwisza345h769'
+base32z_map = {base32z_dict[i]: i for i in range(len(base32z_dict))}
 
 def lokinet_snode_addr(pk_hex):
     """Returns the lokinet snode address from a hex ed25519 pubkey"""
@@ -21,10 +22,40 @@ def lokinet_snode_addr(pk_hex):
     result += base32z_dict[val << (5 - bits)]
     return result + ".snode"
 
-if len(sys.argv) < 2 or any(len(x) != 64 for x in sys.argv[1:]):
+
+def hex_from_snode(b32z):
+    """undoes what the above does; b32z should have '.snode' already stripped off"""
+    assert(len(b32z) == 52)
+    val = 0
+    bits = 0
+    for x in b32z:
+        val = (val << 5) | base32z_map[x]  # Arbitrary precision integers FTW
+
+    # `val` is now a 260 bit value (52 * 5 bits per char); but we only use the first bit of the last
+    # value (which is why lokinet addresses always end with y or o)
+    assert(b32z[-1] in 'yo')
+    val >>= 4
+
+    return "{:64x}".format(val)
+
+
+reverse = False
+if len(sys.argv) >= 2 and sys.argv[1] == '-r':
+    reverse = True
+    del sys.argv[1]
+
+if len(sys.argv) < 2 or (
+        any(len(x) not in (52, 58) for x in sys.argv[1:])
+        if reverse else
+        any(len(x) != 64 for x in sys.argv[1:])
+        ):
     print("Usage: {} PUBKEY [PUBKEY ...] -- converts ed25519 pubkeys to .snode addresses".format(sys.argv[0]))
+    print("Usage: {} -r SNODE [SNODE ...] -- converts snode addresses to ed25519 pubkeys".format(sys.argv[0]))
     sys.exit(1)
 
-for key in sys.argv[1:]:
-    print("{} -> {}".format(key, lokinet_snode_addr(key)))
-
+if reverse:
+    for key in sys.argv[1:]:
+        print("{}.snode -> {}".format(key[0:52], hex_from_snode(key[0:52])))
+else:
+    for key in sys.argv[1:]:
+        print("{} -> {}".format(key, lokinet_snode_addr(key)))