opmsg is a replacement for gpg which can encrypt/sign/verify your mails or create/verify detached signatures of local files. Even though the opmsg output looks similar, the concept is entirely different.
Features:
- Perfect Forward Secrecy (PFS) by means of ECDH or DH Kex
- native EC or RSA fallback if no (EC)DH keys left
- fully compliant to existing SMTP/IMAP/POP etc. standards; no need to touch any mail daemon/client/agent code
- signing messages is mandatory
- easy creation and throw-away of ids
- support for 1:1 key bindings to auto-select source key per destination
- adds the possibility to (re-)route messages different from mail address to defeat meta data collection
- configurable well-established hash and crypto algorithms and key lengths (RSA, DH, EC, AES)
- straight forward and open key storage, basically also managable via
cat,shred -uandlson the cmdline - seamless mutt integration
opmsg requires the crypto primitives from OpenSSL. Just relax, its
not using the SSL/TLS proto, just the ciphering and hash algorithms.
For standard Linux distros, just type make.
The compilation requires a C++ compiler that supports -std=c++11.
This can be configured with e.g. make CXX=eg++ LD=eg++ on OpenBSD.
This project supports both BN_GENCB_new and BN_GENCB for big number
generation. To disable BN_GENCB_new, set HAVE_BN_GENCB_NEW to false:
make DEFS=-DHAVE_BN_GENCB_NEW=0. So on OpenBSD, you would run
make CXX=eg++ LD=eg++ DEFS=-DHAVE_BN_GENCB_NEW=0. On OSX you should install
your own OpenSSL, as Apple marks OpenSSL as deprecated in favor of their own
crypto libs. You may also set all these options in the Makefile.
$ make
[...]
$ cp opmsg /usr/local/bin/
$ opmsg
opmsg: version=1.5 -- (C) 2015 opmsg-team: https://github.com/stealth/opmsg
Usage: opmsg [--confdir dir] [--native] [--encrypt dst-ID] [--decrypt] [--sign]
[--verify file] <--persona ID> [--import] [--list] [--listpgp]
[--short] [--long] [--split] [--new(ec)p] [--newdhp] [--calgo name]
[--phash name [--name name] [--in infile] [--out outfile]
[--link target id] [--burn]
--confdir, -c (must come first) defaults to ~/.opmsg
--native, -R EC/RSA override (dont use existing (EC)DH keys)
--encrypt, -E recipients persona hex id (-i to -o, needs -P)
--decrypt, -D decrypt --in to --out
--sign, -S create detached signature file from -i via -P
--verify, -V vrfy hash contained in detached file against -i
--persona, -P your persona hex id as used for signing
--import, -I import new persona from --in
--list, -l list all personas
--listpgp, -L list personas in PGP format (for mutt etc.)
--short short view of hex ids
--long long view of hex ids
--split split view of hex ids
--newp, -N create new RSA persona (should add --name)
--newecp create new EC persona (should add --name)
--link link (your) --persona as default src to this
target id
--newdhp create new DHparams for persona (rarely needed)
--calgo, -C use this algo for encryption
--phash, -p use this hash algo for hashing personas
--in, -i input file (stdin)
--out, -o output file (stdout)
--name, -n use this name for newly created personas
--burn (!dangerous!) burn private (EC)DH key after
decryption to achieve 'full' PFS
It successfully builds on Linux, OSX, OpenBSD and probably a lot of others (Solaris, FreeBSD,...).
The key concept of opmsg is the use of personas. Personas are an identity with either an EC or RSA key bound to it. Communication happens between two personas (which could be the same) which are uniquely identified by the hashsum of their EC/RSA keys:
$ opmsg --newp --name stealth
opmsg: version=1.2 -- (C) 2015 opmsg-team: https://github.com/stealth/opmsg
opmsg: creating new persona
.......[...].........................o..o..o..o..o..oO
opmsg: Successfully generated persona (RSA + DHparams) with id
opmsg: 1cb7992f96663853 1d33e59e83cd0542 95fb8016e5d9e35f b409630694571aba
opmsg: Tell your remote peer to add the following RSA pubkey like this:
opmsg: opmsg --import --phash sha256 --name stealth
-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC4Xds/bPlkdqA9VhDBOIEV/Dc9
4EfL5aPBOQAdTIaZKE69SJdwakFhqOY1PeaeGRDcGTVNLBQ1Udgbc2YCgQh1X5Dn
veRIGJoGfqWC7zeq/mx6yRer3PTUOA0gr30Uu7IO128fVDxNLYYUuvzhzcdysZAa
WkmRflKuaCEMQ3RjcQIDAQAB
-----END PUBLIC KEY-----
opmsg: Check (by phone, otr, twitter, id-selfie etc.) that above id matches
opmsg: the import message from your peer.
opmsg: AFTER THAT, you can go ahead, safely exchanging op-messages.
opmsg: SUCCESS.
This is pretty self-explaining. A new persona with name stealth is
created. The public RSA key of this persona has to be imported by
the remote peer that you want to opmsg-mail with via
opmsg --import --phash sha256 --name stealth
just as hinted above.
opmsg does not rely on a web-of-trust which in fact never really worked. Rather, due to ubiquious messenging, its much simpler today to verify the hashsum of the persona via additional communication paths. E.g. if you send the pubkey via plain mail, use SMS and twitter to distribute the hash, or send a picture/selfie with the hash and something that uniquely identifies you. Using two additional communication paths, which are unrelated to the path that you sent the key along, you have a high degree of trust. Side-note: If you want to stay anonymous, do not send selfies with your persona id and dont use communication paths that can be mapped to you.
By default sha256 is used to hash the pubkey blob but you may also specify ripemd160
or sha512. Whichever you choose, its important that your peer knows
about it during import, because you will be referenced with this hex hash value
in future.
The private part of the keys which are stored inside ~/.opmsg
are NOT encrypted. It is believed that once someone gained access
to your account, its all lost anyway (except for PFS as explained later),
so a passpharse just add a wrong feeling of security here. Keep
your account/box unpwned! Otherwise end2end encryption makes little
sense.
opmsg encourages users for easy persona creation and throwaway.
The directory structure below ~/.opmsg is easy and straight
forward. It just maps the hex ids of the personas and (EC)DH keys
to directories and can in fact be edited by hand.
Creation of RSA personas might take some time. Not just an RSA key is generated in that case - which is not very time consuming - but also DH parameters (2048bit by default) that are used to implement PFS in later messenging (see later chapter).
In order to speed up persona generation and to encourage use- and throwaway
and per-project personas, EC support was added to opmsg as of version=1.5.
Instead of --newp you would just use --newecp and everything else is the
same. opmsg will pick the right crypto transparently to you. No need to add
any further switches for encryption or alike. EC personas use the brainpool curves
(RFC 5639). The NIST curve secp521r1 may also be used as a fallback if your
libcrypto is outdated, but its recommended to use the brainpool curves which
dont keep any secrets about how their group parameters were selected.
Note: opmsgs printing versions earlier than opmsg: version=1.2 have had a weakness
that left the personas RSA's (public) e value integrity-unprotected during --import.
This could lallow attackers to mount MiM attacks that
downgrade e to 1. Its recommended to update to the newest version, which
correctly hashes the whole RSA pubkey and detects such tampering during --import. It is unlikely that such
an attack goes unnoticed, as wrong message signatures would most likely be detected
during operation, but if in doubt you can use the rsa-check script to check
your local keystore to not contain any Exponent values of 1. In either case, update
your opmsg to be on the safe side. As a bonus you would also get the new and
shiny aes-gcm and aes-ctr cipher modes!
Although this step is not strictly necessary, it is recommended. As personas are easily
created, you can (should) create a dedicated persona for each of your "projects" or
contacts. That is, if you have 7350 communication partners/peers, you should have
created 7350 personas; one RSA key for each of them. To handle that easily with your
mailer (see later for mutt integration), you should add a proper --name, describing your
id. Additionally, you should --link your source persona (each of the 7350 you created)
to the particular destination persona that you wish to communicate with using this source id:
$ opmsg --link b3c32d47dc8b58a6 --persona 1cb7992f96663853
opmsg: version=1.2 -- (C) 2015 opmsg-team: https://github.com/stealth/opmsg
opmsg: linking personas
opmsg: SUCCESS.
This is to avoid the need to specify --persona each time you
send a message to a different target persona, changing your local
id back and forth inside the config file or at the command line.
Above command says, that each time you send an opmsg to b3c32d47dc8b58a6,
the keying material from your local id 1cb7992f96663853 is used. To unlink
your id from b3c32d47dc8b58a6, remove the srclink file inside this personas
directory.
If no link is found for a persona, the config file and given
--persona argument is evaluated. Command line arguments
override the config file settings.
Given proper mail provider support (e.g. inboxes are created on the fly for addresses like hexid@example.com), the global surveillance meta graph would just contain pairs of communication partners. No clusters, just islands of 1:1 mappings.
Now for the coolest feature of opmsg: Perfect Forward Secrecy (PFS).
Without any need to re-code your mail clients or add bloat to the SMTP protocol, opmsg supports PFS by means of (EC)DH Kex out of the box.
For RSA personas, DH Kex is used. For EC personas ECDH Kex is used to derive the secret, hence the term (EC)DH.
As op-messages are always signed by its source persona,
whenever you send an opmsg to some other persona, a couple of
(EC)DH keys are generated and attached to the message. The remote
opmsg will verify its integrity (as it has this persona imported)
and add it to this persona's keystore. So whenever this remote peer
sends you a mail next time, it can choose one of the (EC)DH keys it has
got beforehand. If your peer runs out of (EC)DH keys, opmsg falls
back to native RSA or EC encryption, depending of the type of persona.
The peer deletes used (EC)DH pubkeys to not
use them twice and the local peer marks used keys with a
used file within the apropriate key-directory. Once again,
sha256 is used by default to index and to (worldwide) uniquely
identify (EC)DH keys.
Attention: If you keep encrypted op-messages in your mailbox,
do not throw away this persona. You wont be able to decrypt these mails
afterwards! Throwing away a persona also means to throw away all keying
material. Thats why opmsg has no switch to erase personas. You have
to do it by hand, by rm-ing the subdirectory of your choice. Thats
easily done, but keep in mind that any dangling op-messages in your
inbox will become unreadable, as all keys will be lost. If you want to
benefit from PFS, you have to archive the decrypted messages and
throw away used keys. After all opmsg is not a crypto container
or a replacement for FDE (which is recommended anyway). opmsg is
about to protect your messages in transit, not on disk.
As of version=1.3 there is a --burn option that nukes used DH
keys from storage. Be aware: you can only decrypt the message once.
Once the message is successfully decrypted, the (EC)DH key that was used
is overwritten and deleted from storage.
Just add to your .muttrc:
# Add a header so to easy pick opmsg via procmail rules
my_hdr X-opmsg: version1
set pgp_long_ids
# use this listing mode if the 'name' aliases are a substring of the email address
# to avoid huge listings
set pgp_list_pubring_command="/usr/local/bin/opmsg --listpgp --short --name %r"
# otherwise, skip the --name option to list all personas
#set pgp_list_pubring_command="/usr/local/bin/opmsg --listpgp --short"
set pgp_encrypt_sign_command="/usr/local/bin/opmsg --encrypt %r -i %f"
set pgp_encrypt_only_command="/usr/local/bin/opmsg --encrypt %r -i %f"
set pgp_decrypt_command="/usr/local/bin/opmsg --decrypt -i %f"
set pgp_verify_command="/usr/local/bin/opmsg --decrypt -i %f"
and work with your mails as you would it with PGP/GPG before. If you use a mix of GPG and opmsg peers, its probably wise to create a dedicated .muttrc file for opmsg and route opmsg mails to a different inbox, so you can easily work with GPG and opmsg in parallel.
You need to setp up your local ~/.opmsg/config to reflect
the source persona you are using when sending your mail via mutt,
unless you specify it via -P on the commandline or used --link:
# Should use long format to avoid loading of whole keystore.
# this is above generated persona so it owns the RSA private key
my_id = 1cb7992f966638531d33e59e83cd054295fb8016e5d9e35fb409630694571aba
# default
rsa_len = 4096
# default
dh_plen = 2048
calgo = aes128ctr
# split (default), long, short
idformat = short
# number of new DH keys attached to msg, default is 3
new_dh_keys = 3
However, any option could also be passed as a commandline argument to opmsg.
$ opmsg -C inv -D
opmsg: version=1.2 -- (C) 2015 opmsg-team: https://github.com/stealth/opmsg
opmsg: Invalid crypto algorithm. Valid crypto algorithms are:
opmsg: aes128cbc (default)
opmsg: aes128cfb
opmsg: aes128ctr
opmsg: aes128gcm
opmsg: aes256cbc
opmsg: aes256cfb
opmsg: aes256ctr
opmsg: aes256gcm
opmsg: bfcbc
opmsg: bfcfb
opmsg: cast5cbc
opmsg: cast5cfb
opmsg: null
opmsg: FAILED.
$ opmsg --list --short
opmsg: version=1.2 -- (C) 2015 opmsg-team: https://github.com/stealth/opmsg
opmsg: persona list:
opmsg: Successfully loaded 1 personas.
opmsg: (id) (name) (has-RSA-priv) (#DHkeys)
opmsg: 1cb7992f96663853 stealth 1 0
opmsg: SUCCESS.
Creating a detached signature for a file:
$ echo foo>foo
$ opmsg --sign -i foo --persona 1cb7992f96663853|tee -a foo.sign
opmsg: version=1.2 -- (C) 2015 opmsg-team: https://github.com/stealth/opmsg
opmsg: detached file-signing by persona 1cb7992f96663853
opmsg: SUCCESS.
-----BEGIN OPMSG-----
version=1
-----BEGIN SIGNATURE-----
U822A12k1IZiWqRKAr6uLKT/7HGR4inKpkqzz49xLNjBf4mo91HUxcPMFGQTDB/MbE9HqtdCgHNexfIy
GCC6Jb6egt2D70nIyhWfksW9KljdqwQzUbXp9CubxRAz5EqTS0n0ze092LuXxV4SuKV628CTBr5siIcf
za6g3Sfh+vg=
-----END SIGNATURE-----
rythmz=sha256:sha256:sha256:null:DOauqyrqoH4zslO4gr3FFI7EMbcLtRzU
src-id=1cb7992f966638531d33e59e83cd054295fb8016e5d9e35fb409630694571aba
dst-id=1cb7992f966638531d33e59e83cd054295fb8016e5d9e35fb409630694571aba
kex-id=00000000
-----BEGIN OPMSG DATA-----
b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c
-----END OPMSG-----
Verifying it:
$ sha256sum foo
b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c foo
$ opmsg -V foo -i foo.sign
opmsg: version=1.2 -- (C) 2015 opmsg-team: https://github.com/stealth/opmsg
opmsg: verifying detached file
opmsg: GOOD signature and hash via persona 1cb7992f96663853 1d33e59e83cd0542 95fb8016e5d9e35f b409630694571aba
opmsg: SUCCESS.
$ opmsg -V foo -i foo.sign --short
opmsg: version=1.2 -- (C) 2015 opmsg-team: https://github.com/stealth/opmsg
opmsg: verifying detached file
opmsg: GOOD signature and hash via persona 1cb7992f96663853
opmsg: SUCCESS.
As opmsg adds additional ids to the mail, it is possible to send all mail to random address of the destination persona provider who then could route it to the real mailbox or even offer everything as a single blob to download for everyone. This may be useful to defeat mail meta data collection. op-messages are self contained and ASCII-armored, so they could also be pasted to OTR, newsgroups, chats or paste-sites and picked up by the target persona from within a swarm. Also see above discussion of persona linking.