It is possible to encrypt/decrypt messages with the bitcoin keys.
Maybe you are aware of the PyBitmessage project by Atheros:
https://bitcointalksearch.org/topic/announce-bitmessage-p2p-messaging-system-based-partially-on-bitcoin-128230Its code can be used to do this:
#!/usr/bin/env python
# -*- coding: utf-8 -*-
from highlevelcrypto import *
from pyelliptic import arithmetic
from binascii import hexlify, unhexlify
# msg
plaintext = "Bitcoin is the new world currency."
# from Casascius btc utility
privkeyUtil = "0x29635CB46E2AA93EFF403448E1555DC99F4ECB75898BBF232D769EA5C029A1DF"
pubkeyUtil = "0x04AAC5327CEF4681FEDCA19C5D9C009172484A3497B616376947E81F2E9ED35968A707D3C4966AB49608F047A5F9E29D16ABD35FB4F613D5B5A119EBDD6799C45D"
pubkeyHashUtil = "702B9096868BA888D71326D1F55EC2B20CB84D3A"
addressUtil = "1BE6sRNQgQ28iiYPKBXeyHh1E6LPAdaxfH"
################################################################
# actual work
################################################################
pubkeyEcc = privToPub(privkeyUtil[2:]).upper()
cipher = encrypt(plaintext, pubkeyEcc)
match = bool(decrypt(cipher, privkeyUtil[2:]) == plaintext)
################################################################
# output
print "keys------------"
print "data from Casascius utility"
print "privkey:", privkeyUtil
print "pubkey:", pubkeyUtil
print "address:", addressUtil
print
print "data generated from above privkey with PyBitmessage highlevelcrypto (pyelliptic / openSSL)"
print "pubkey:", pubkeyEcc
print "address:", arithmetic.pubkey_to_address(pubkeyEcc)
print
print "encryption------------"
print "plaintext:", plaintext
print "cipher:", hexlify(cipher)
if match:
print "decrypt: match"
#output:
##keys------------
##data from Casascius utility
##privkey: 0x29635CB46E2AA93EFF403448E1555DC99F4ECB75898BBF232D769EA5C029A1DF
##pubkey: 0x04AAC5327CEF4681FEDCA19C5D9C009172484A3497B616376947E81F2E9ED35968A707D3C4966AB49608F047A5F9E29D16ABD35FB4F613D5B5A119EBDD6799C45D
##address: 1BE6sRNQgQ28iiYPKBXeyHh1E6LPAdaxfH
##
##data generated from above privkey with PyBitmessage highlevelcrypto (pyelliptic / openSSL)
##pubkey: 04AAC5327CEF4681FEDCA19C5D9C009172484A3497B616376947E81F2E9ED35968A707D3C4966AB49608F047A5F9E29D16ABD35FB4F613D5B5A119EBDD6799C45D
##address: 1BE6sRNQgQ28iiYPKBXeyHh1E6LPAdaxfH
##
##encryption------------
##plaintext: Bitcoin is the new world currency.
##cipher: f8f358b861c041e430024ecbecccaada02ca00203c00513c46364b3f08de80b202e658155093dfd6f0847daff53295a858aafb2000205fef717e5e6935a450930925d8da7aecd02a560a4f8c5ebdad1517e8480724b8fb6fb42db52256c14df788546bd32360604f9cb0ad6ce79d8e98274e54a55526baceb8d559e854f75300114d0148d310d70ad7515e8f8f2e81a416d93d26a4f06d185c69bd64a2384303e9fff393a8ee
##decrypt: match
[/quote]
I am not a cryptographer but from what I understand it should be possible to make this really secure (not sure about the state of the libraries at hand).
