About
Both S/MIME and OpenPGP are end to end security mechanisms that facilitate
secure email transmission. S/MIME and OpenPGP are competing
technologies that operate in much the same way. This document aims to analyze the
difference between the two technologies, and to describe the costs and benefits
of each.
PGP is the common name for a security methodology first introduced by Philip
Zimmermann. PGP is an acronym for "Pretty Good Privacy". The IETF standard
for this methodology is referred to as OpenPGP. OpenPGP is described by
RFC 2440.
S/MIME is short for Secure/Multipurpose Internet Mail Extensions. It was
first developed by RSA Data Security, Inc. The S/MIME standard is described by
RFC 3851. Not all versions of the technology are part of the standard.
History
OpenPGP
PGP was first developed by Philip Zimmermann in 1991. The source code of the
project was distributed with the program and a team of developers emerged around
the project. In 1996, Philip and his team formed PGP Inc. PGP Inc. created
versions three, four and five of PGP.
In 1997, the technology branched in two directions. First, in July, amidst
growing concerns over algorithms with licensing difficulties, Philip and a
subset of the team proposed an "Unencumbered PGP" to the IETF for standards
consideration [WIKIP-OP]. The proposal was accepted and given the formal name
OpenPGP..
In December of 1997, PGP Inc. was aquired by Network Associates, Inc (NAI).
Philip left NAI in 2001 to join Hush Communications, an OpenPGP based mail service
provider. In 2002, NAI sold PGP assets to a group of original PGP team
members excluding Philip [WIKIP-OP]. The newly formed company was named PGP Corporation,
and it is still in business selling PGP based technolgogies. Philip serves
in an advisory role for the company.
OpenPGP is still on the standards track (as of this writing), and is being
actively developed. One primary implementation of the standard has emerged from
the GNU project, named GNU Privacy Guard (GnuPG).
S/MIME
S/MIME was first developed by the RSA Data Security Inc. in collaboration with
other private organizations [IMC]. Early versions of the technology were not part
of the standards track, primarily due to patent restrictions on the RSA encryption
algorithm used in the technology. Version three of the technology was the first
to be part of a standard in 1999. However it was defined by RFC 2633 which has
since been obsoleted by S/MIME version 3.1 described by RFC 3851. RFC 3851 is
the current standard and the topic of this document.
One Distinct Difference
OpenPGP is a general purpose encryption methodology. As stated by the OpenPGP
RFC it is "data integrity services for messages and data files." S/MIME
is specifically designed for MIME-encapsulated data. Generally this means email,
but technically it could mean any technology supporting MIME encoding. From the
S/MIME RFC, "S/MIME provides a consistent way to send and receive secure MIME
data."
This is an important distinction. OpenPGP can and does have practical application
outside of the context of Internet messaging. For example:
- A user may choose to encrypt their entire home directory to protect data
on disk. All documents stored inside the directory would be encrypted and
only accesible with the corresponding private key.
- A user could encrypt a single file, then use delivery mechanisms other
than email, such as FTP, to transfer the file to its destination.
Products exist for both the commerical PGP specification
and the OpenPGP specification. Additionally GnuPG can be used
as a library by third party programs to enable encryption and decryption for
many different scenerios.
Similarities
Both OpenPGP and S/MIME function in much the same way. We will ignore the
fact, for now, that OpenPGP has use outside of email and focus on its merits as
an email encryption methodology. Both OpenPGP and S/MIME are end-to-end
encryption methodologies. In other words, the data is encrypted at the origination
site, transmitted in a standard networking envelope (the traffic patterns are
not protected) and decrypted at the destination site. Additionally, both use
public-private key type encryption.
Process
The following list describes the common path taken by an email
message transformed by either technology:
- Compose message
- Encrypt/sign message with private key
- Sign message
- Encrypt data (requires recipients public key)
- Sign message and encrypt data (requires recipients public key)
- Transmit message
- Decrypt/verify message with senders public key
- Verify signed message
- Decrypt data (requires recipients private key)
- Verify message and decrypt data (requires recipients private key)
- Read message
Encryption and Signing Process
There are many details involved in describing the encryption and signing
process. For the purposes of this document we will explore where they are
different in the process. For simplicity, we will compare S/MIME to OpenPGP, not
OpenPGP/MIME (see below), and we will show situations where encyption and signing
are occuring.
OpenPGP Encryption & Signing
- Plaintext message created (M)
- M hashed (H)
- H encrypted (EH)
- EH and M concatenated (EHM)
- EHM compressed (Z)
- Z symettrically encrypted with session key (ZS)
- Session key encrypted with recipient's public key (SP)
- ZS and SP concatenated (EM)
- Encrypted and signed message, EM, complete
S/MIME Encryption & Signing
OpenPGP is assembled as shown with a couple of concatenations of parts. S/MIME
divides these parts into separate subtype MIME entities. These separate entities
can encapsulate each other and be applied in any order because they all result in
MIME encapuslated output. Following is a listing of the relevant "Content Types"
according to the RFC:
- Data
- SignedData
- EnvelopedData
- CompressedData
S/MIME prior to version 3.1 did not provide compression. Compression was
added to 3.1 as a MIME type. It is not clear how well current S/MIME client
programs support compression . The prevailing community information indicates
that compression is not well supported. Presumably, if compression were
implemented, OpenPGP and S/MIME would map roughly in the following way:
| OpenPGP Step | S/MIME Content Type |
|---|
- Plaintext message created (M)
|
Data |
- M hashed (H)
- H encrypted (EH)
- EH and M concatenated (EHM)
|
SignedData |
|
|
CompressedData (see above) |
- Z symettrically encrypted with session key (ZS)
- Session key encrypted with recipient's public key (SP)
- ZS and SP concatenated (EM)
- Encrypted and signed message, EM, complete
|
EnvelopedData |
Transmission Format
OpenPGP has two message formats: OpenPGP and OpenPGP/MIME. OpenPGP, by default,
offers a non-mime-encapsulated message transmission. In other words it is
equivalent to a standard plain text email without any attachements.
OpenPGP/MIME is similar in formatting to S/MIME. Both use MIME encapsulation
for the transmission of data and signtures. Following are actual transmissions
of encrypted and signed data (just signed or just encrypted would have similar
characteristics):
OpenPGP - Signed & Encrypted
From: sender@555.com
To: recipient@555.com
Subject: Test Sign and Enc
Content-Type: text/plain;
charset="iso-8859-1"
-----BEGIN PGP MESSAGE-----
Charset: ISO-8859-1
Version: GnuPG v1.4.7 (MingW32)
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org
hQIOA1D+V4G4ChJkEAf/Q9gyX3nshwfjJiwBZVoZ/RcRRDeDmZC1czezhw2IHERT
CUEPlOvis/s7X7iv/6Fs3ycdOEB0/B74oTeIOIK7hdHGkDXg5LL1fPj9eBm46ETa
fN1unMpzdXrk94R6eg+s0Pz1/+cR4Dv45BX2pqZJBTSg4WrC0TBT5U9uKWS4lXYQ
QMTRlvSl+K6ZlM5VCeB8FpRcKGqFs6mTIUNo/l+oTGFaDE/iWd4cd07WXbBekiNZ
...[segment snipped for readability]...
2f30QgYMWXnyAZAh//dapq7yg+UFIaiZinF4pYOIR5U2VmctEygXIm8+a433UypG
Ot0RErIE+gwUGyfkGG5aXrG1plcvrPuvR3pFkwvf19jpatlL4pMARr+eGHIG/rQN
HuE1M2OUOsIr7pKkpbZ423gcgE1MYoGo8YWGRRBwNTbsRdwaLGfJ1IjYCCRT5eap
os8XHKH2SITlYazz6gpG7htkSFDhj8a+UeoezXmyYnuYA13VLQHDKQ/eYMvQOpum
1sh9VmHjGp1jypC1pcY=3D
=3D1HFW
-----END PGP MESSAGE-----
OpenPGP/MIME - Signed & Encrypted
From: sender@555.com
To: recipient@555.com
Subject: test PGP/MIME
Content-Type: multipart/encrypted;
protocol="application/pgp-encrypted";
boundary="------------enig371AFEE0FF01CE1E773B775E"
This is an OpenPGP/MIME encrypted message (RFC 2440 and 3156)
--------------enig371AFEE0FF01CE1E773B775E
Content-Type: application/pgp-encrypted
Content-Description: PGP/MIME version identification
Version: 1
--------------enig371AFEE0FF01CE1E773B775E
Content-Type: application/octet-stream; name="encrypted.asc"
Content-Description: OpenPGP encrypted message
Content-Disposition: inline; filename="encrypted.asc"
-----BEGIN PGP MESSAGE-----
Version: GnuPG v1.4.7 (MingW32)
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org
hQIOA1D+V4G4ChJkEAf/R5yUQuWvLQY1XBdi9I3qCLIzKFCMv+XzOx1HUvs4kbGL
0FrQulEBzCtkNR9E6iflWkrlxZoPyrZ2Jec3wPidvVX6FxAoKFVdr/WW2pKd5iyW
KfKztEl35PjInwlqsT/XS9JXFL2yy83uzCr/R9K0KgRUUa0evr0IdhI7xCEXQoIj
5rYAoG8lxQzzkaMb+l3V/HwBiKu9unmQcrcTF9VU3IG9FqMkex9eiqbSjefoWOmm
...[segment snipped for readability]...
WAdF3IxeZbxLMDBlQCuvc8Kug6jXaKeONYrhva7rzaBHvdILECJeJXznd4hyPsuk
CCN3momgnC2y2RFWVosl0JxJiIjd2a8JALK2N07DVX7Z3qd4rw/Ra1fpnhcCX0ob
Nbs+ahvAccTegkfOeisUYQOGrrNz7uxDhSlM1JVd4dQd2mB9VA9eQwpHtIM7QOA1
pfNprvFq7vVP3GZ0Nbo71I2BgMklhnuVl919vyOXQjpkTaxh4i/6lztTV4ay+4yb
smiF0cgByg==
=1wu6
-----END PGP MESSAGE-----
--------------enig371AFEE0FF01CE1E773B775E--
S/MIME - Signed & Encrypted
From: sender@555.com
To: recipient@555.com
Subject: Testing S/MIME Enc
Content-Type: application/x-pkcs7-mime; name="smime.p7m"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="smime.p7m"
Content-Description: S/MIME Encrypted Message
MIAGCSqGSIb3DQEHA6CAMIACAQAxggMkMIIBjgIBADB2MGIxCzAJBgNVBAYTAlpBMSUwIwYD
VQQKExxUaGF3dGUgQ29uc3VsdGluZyAoUHR5KSBMdGQuMSwwKgYDVQQDEyNUaGF3dGUgUGVy
c29uYWwgRnJlZW1haWwgSXNzdWluZyBDQQIQTSqaMaVxLd7IQrJvKRr4vjANBgkqhkiG9w0B
AQEFAASCAQAFYfRWKLZvmLlehIFYRNb153/H07aveQvVpWIMB7IDVnJvuMfANSFFnn2Dr8CP
...[segment snipped for readability]...
RSfYf9wEj40OMZGWDHfHbwNPCIz0usiD4xhUdqEUmmQMb8JtZ+NNf72Lepla/l57n3YSoLUf
LX+RqhtCX56MI2ynbXRHIAC1519NDFVwJArF225km6lic3A7/GoxvQDMPHocMJUf7OW6Fs5T
mWj/DriO18fsdbDZbAM2FgTyaszWi8QhEglcSw5bDSfQPdYtU8HyBaA6lxrZcTjshp5vt8Or
UcaGG0NQINx/KQYxkksREExI3mnSPTaxapyl2JjXSAQIpNuen07JarwAAAAAAAAAAAAA
Algorithms
Both technologies share a number of algorithms. The following table
represents each function and its corresponding set of technologies as
described by their respective RFCs (shared algorithms have added
emphasis):
| Function | OpenPGP | S/MIME |
|---|
| Public Key |
RSA, Diffie-Hellman, DSA, Elgamal, Elliptic Curve, ECDSA |
RSA, Diffie-Hellman, DSA |
|---|
| Symmetric Key |
AES, Triple-DES, IDEA, CAST5, Blowfish, SAFER-SK128, DES/SK, |
AES, Triple-DES, RC2 |
|---|
| Compression |
ZLIB, ZIP |
ZLIB (as described in related RFC 3274) |
|---|
| Hash |
MD5, SHA-1, RIPE-MD/160, Double-Width SHA, MD2, TIGER, HAVAL |
MD5, SHA-1 |
|---|
Web of Trust and Public Key Infrastructure
OpenPGP and S/MIME both provide mechanisms for authenticating parties. OpenPGP
relies on a concept called a "Web of Trust." It is essentially a distributed peer
validating service. The concept was introduced by Philip Zimmermann.
It works much like social networking. Person A, Alice, sends her public key
to person B, Bob. Bob adds Alice's public key to his key ring and optionally
indicates his level of trust in Alice's key. Over time the
trust endorsements are calculated and the net effect is a decentralized "web of
trust."
S/MIME relies on Certificate Authorities (CA) to authenticate parties. This
is similar in nature to the way SSL certificates are issued for serving secure
web sites. Various models are used by CA's to authenticate a person. Generally
it is similar to authenticating with a government agency. A person must provide
several forms of identification.
Both OpenPGP and S/MIME allow you to use them relatively
untrusted. In other words, you may generate or obtain valid private and public keys that
are useable but are "untrusted". This is a good thing for getting started
quickly with each technology. For example if one wanted to communicate securely
with someone who had never used either technology the recipient could quickly be up and
running enabling them to communicate in a timely manner.
The primary difference between a Web of Trust and a Public Key Infrastructure
is a matter of trust. Do you trust a decentralized peer group or a centralized
organization?
Implementation
Both OpenPGP and S/MIME are well supported across many platforms and
applications. For the purposes of this document, a test was conducted on the
following platforms and applications:
- Linux
-
- OpenPGP
-
- S/MIME
-
- Mac OS X
-
- OpenPGP
-
- S/MIME
-
- Windows
-
- OpenPGP
-
- S/MIME
-
Evolution,
Mail.app and Outlook were considered, for the purposes of this paper, to be the most entrenched
mail applications on each platform. This is not a completely fair assertion for the Linux
platform as many good email clients exist for that platform. However,
Evolution is very similar to Outlook which increase its likely usage in business settings.
All tested software listed above were fairly easily installed and configured.
It is worth noting that all tested Mail User Agent's (MUA) support S/MIME by
default and all MUA's required a plugin to support OpenPGP. With the exception of
Evolution which came preconfigured for both technologies.
Problems only occurred with one tested program, Gpg4win's Outlook plugin. Specifically,
Outlook with Gpg4win could not encrypt plaintext formatted email messages. Working from a
bug report found online it was discovered that by composing HTML formatted email messages
worked as expected. For Outlook PGP integration PGP.com's PGP Desktop may be a better
choice.
A few notes from the tested products:
- S/MIME integration was very easy for all mail clients
- Enigmail makes OpenPGP integration very easy for Thunderbird.
- Evolution could use an integrated GPG key management window. External applications
exist for this purpose but an integrated window, similar to Enigmail, would improve
usability.
- Similar criticism applies to Mail.app. An integrated key management window would
improve usability.
- The easiest configuration was S/MIME for Mail.app. Though most S/MIME configurations
were simple, Mail.app was completely configured by adding the S/MIME key to the system
keyring, simply by double-clicking the key file.
- Thunderbird should be noted. It was the only cross platform mail application.
Additionally it was easily configured to use both OpenPGP and S/MIME. Certainly
Enigmail makes it the easiest OpenGPG implementation.
Summary
The largest difference between OpenPGP and S/MIME is the ability for OpenPGP
to encrypt a variety of data types not limited to MIME encoded data types. This gives
OpenPGP a decided edge over S/MIME in the way of versatility.
For large data streams, OpenPGP provides more efficient
compression. Though compression is available in the S/MIME specification it is
implemented as an MIME container. In other words, the contents to be compressed
have already been encoded per the MIME specification, usually Base64. Base64
actually grows the data by about 137% [WIKIP-B64]. In contrast, OpenPGP
compresses the original plaintext, and optionally the signature prior to
encryption or any type of encoding.
Both OpenPGP and S/MIME use industry standard encryption algorithms. Neither
technology is more secure than the other, barring esoteric arguments. Some
would rank each technology based on the trust model used. However, the security
aspect of each trust model is subjective.
The trust models used for each technology are very different in philosophy.
In practice, neither method is more difficult to use than the other. It is
arguable that S/MIME is more attractive due to its similarities to SSL
certificate distribution, a model already trusted by most web users implicitly.
S/MIME has an advantage when it comes to implementation. All mail
clients tested came with S/MIME support built in. As a result, the barriers to
entry for a new user are lower. Generating an S/MIME certificate
required a bit more effort than OpenPGP.
So the time commitment for setting up each initially is similar. However
the zero-footprint install may be attractive to those managing many workstations.
How would one choose between the two technologies? Many factors play a
part in this decision. The following table provides a visual representation
of the pros and cons of each technology:
| Function | OpenPGP | S/MIME |
|---|
| Versatility |
+ |
- |
|---|
| Space Efficiency |
+ |
- |
|---|
| Security |
+ |
+ |
|---|
| Trust Model |
+ |
+ |
|---|
| Implementation |
- |
+ |
|---|
