Internet Engineering Task Force (IETF) Phillip Hallam-Baker
Internet-Draft Comodo Group Inc.
Intended Status: Standards Track October 17, 2013
Expires: April 20, 2014
PRISM_Proof Email Key Generation and Publication
draft-hallambaker-prismproof-key-00
Abstract
This document describes previous efforts and their deployment legacy
and the requirements for a successful email security infrastructure.
A gap analysis is performed and the tasks divided into problems that
are generally considered solved albeit possibly requiring improved
execution and problems that may be regarded as research.
This division of the problem space into 'execution' and 'research'
portions allows different groups of developers to address each
independently and avoid unnecessary duplication of effort. A testbed
for development and early adopter deployment that achieves this
separation is described.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
April 20, 2014 [Page 1]
Internet-Draft PPE: Key Generation & Publication October 2013
Table of Contents
1. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Legacy Infrastructure . . . . . . . . . . . . . . . . . . 3
2. Key Generation and Identification . . . . . . . . . . . . . . 3
2.1. Strong Key Identifier . . . . . . . . . . . . . . . . . . 3
2.1.1. Strong Email Addresses . . . . . . . . . . . . . . . 4
2.2. Private Key Backup and Controlled Recovery . . . . . . . 6
2.2.1. Encrypted Private Key . . . . . . . . . . . . . . . 7
2.2.2. Key Splitting . . . . . . . . . . . . . . . . . . . 8
2.3. Private Key Example . . . . . . . . . . . . . . . . . . . 8
2.3.1. Key Identifier . . . . . . . . . . . . . . . . . . . 8
2.3.2. Private Key Backup . . . . . . . . . . . . . . . . . 9
3. Public Key Infrastructure . . . . . . . . . . . . . . . . . . 11
3.1. Certificate Signing Request. . . . . . . . . . . . . . . 11
3.2. Self-Signed Certificate. . . . . . . . . . . . . . . . . 11
3.3. Peer Endorsement . . . . . . . . . . . . . . . . . . . . 11
4. Publication Service . . . . . . . . . . . . . . . . . . . . . 14
4.1. Initial Key Publication . . . . . . . . . . . . . . . . . 14
5. Registration Example . . . . . . . . . . . . . . . . . . . . . 15
5.1. Enabling a new Device . . . . . . . . . . . . . . . . . . 17
6. Recovery Example . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Revocation . . . . . . . . . . . . . . . . . . . . . . . 18
7. Revocation Example . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Key Endorsement . . . . . . . . . . . . . . . . . . . . . 18
8. Endorsement Example . . . . . . . . . . . . . . . . . . . . . 18
9. OmniAssertBroker . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Assert . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1.1. Structure: Service . . . . . . . . . . . . . . . . . 19
9.1.2. Structure: EncryptedKey . . . . . . . . . . . . . . 20
9.1.3. Message: AssertRequest . . . . . . . . . . . . . . . 20
9.1.4. Message: AssertResponse . . . . . . . . . . . . . . 21
9.2. Recover . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.2.1. Message: RecoverRequest . . . . . . . . . . . . . . 21
9.2.2. Message: RecoverResponse . . . . . . . . . . . . . . 21
9.3. Revoke . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.3.1. Message: RevokeRequest . . . . . . . . . . . . . . . 22
9.3.2. Message: RevokeResponse . . . . . . . . . . . . . . 22
10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
12.1. Normative References . . . . . . . . . . . . . . . . . . 23
12.2. Informative References . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
April 20, 2014 [Page 2]
Internet-Draft PPE: Key Generation & Publication October 2013
1. Problem Statement
Generating a public keypair and registering it for use should be the
only occasion on which a user is required to think about their
cryptographic security. Nor should the user be required to think too
much in this circumstance either.
To enable others to send encrypted email to them, a user must at
minimum generate at least one public keypair and make the public key
portion available to the intended communit of potential senders. The
precise means by which this is achieved may be considered a hard
research problem. Accordingly this specification anticipates such
processing being performed 'in the cloud' (i.e. by magic) and
describes a Web Service interface that may be used to
1.1. Legacy Infrastructure
Twenty years of effort attempting to deploy secure email has left a
considerable legacy of deployed code. While this deployed code base
is not ideally suited to the task (or the problem would be solved
already) it is generally better to support use of such deployed
resources where they exist rather than attempt to build everything
from scratch.
One significant design consequence that flows from this approach is
to adopt ASN.1 encoding for cryptagraphic data objects, including the
Key Endorsement object described in this document. While there are
many better choices of data encoding and remarkably few that are
worse, most cryptographic toolkits provide support for parsing
X.509v3 certificates and generating Certificate Signing Requests and
many provide comprehensive support for a wide range of ASN.1 encoded
objects.
2. Key Generation and Identification
2.1. Strong Key Identifier
A Strong Key Identifier is an identifier that identifies a unique
public key formed using a strong Message digest function over the
public key parameter values.
This definition of Key Identifiers is considerably more restrictive
than the PKIX definition which allows an issuer to use any unique
string for the subjectKeyIdentifier and authorityKeyIdentifier
extensions.
Compliant certificate issuers SHOULD use Strong Key Identifiers as
specified in this document for PKIX Key Identifiers.
April 20, 2014 [Page 3]
Internet-Draft PPE: Key Generation & Publication October 2013
A strong Key Identifier takes one of the two following forms:
If the length of the Key Identifier is exactly 20 octets.
The Key Identifier is an OpenPGP v4 Key fingerprint calculated
as specified in [!RFC4880]
Otherwise
The first byte specifies the digest algorithm and the following
bytes the digest value calculated over the DER encoded
SubjectPublicKeyInfo.
The following algorithm values are assigned in this document:
0
SHA-2-512 truncated to 128 bits.
1
SHA-2-512 truncated to 224 bits.
2
SHA-2-512 truncated to 256 bits.
3
SHA-2-512 without truncation
128-255
Reserved for use in a future multi-byte algorithm identifier
scheme.
To prevent a downgrade attack in which an attacker truncates a longer
Key Identifier, the input to the message digest function is prepared
as follows:
Let V be the algorithm identifier value and D be the DER encoded
SubjectPublicKeyInfo and + stand for simple concatenation.
Key Identifier = H (V + D)
If it is necessary to present a Key Identifier to an end user, Base32
encoding is used. Additional dash (-) characters MAY be added to
improve readability and MUST be ignored by compliant applications.
2.1.1. Strong Email Addresses
To establish encrypted communications it is necessary to know a
public key for the recipient and the recipient's security policy. The
fact that a recipient is capable of receiving encrypted email does
not mean that they are capable of receiving encrypted email on every
device they use or that they are willing to accept encrypted email
from every sender.
April 20, 2014 [Page 4]
Internet-Draft PPE: Key Generation & Publication October 2013
A similar problem was faced when using Transport Layer Security
[RFC5246] with HTTP [RFC2616]. By default, Web requests are sent
without use of security. To force use of TLS, the URI method https is
used in place of http. The security policy is encoded in the URI.
Strong email addresses allow an email sender to encode the security
policy in an RFC822 [RFC2822] compliant email address. RFC822 defines
the 'user name' portion of an email address as follows:
addr-spec = local-part "@" domain
local-part = dot-atom / quoted-string / obs-local-part
atext = ALPHA / DIGIT /
"!" / "#" /
"$" / "%" /
"&" / "'" /
"*" / "+" /
"-" / "/" /
"=" / "?" /
"^" / "_" /
"`" / "{" /
"|" / "}" /
"~"
atom = [CFWS] 1*atext [CFWS]
dot-atom = [CFWS] dot-atom-text [CFWS]
In a Strong Email Address, the character '?' is reserved. Although
this is a legitimate account name in some operating systems, use is
prohibited in current editions of Windows and most UNIX based
operating systems.
April 20, 2014 [Page 5]
Internet-Draft PPE: Key Generation & Publication October 2013
The address syntax is modified as follows:
addr-spec = local-part "@" domain
local-part = dot-atom / quoted-string /
obs-local-part / strong-local
atext = ALPHA / DIGIT /
"!" / "#" /
"$" / "%" /
"&" / "'" /
"*" / "+" /
"-" / "/" /
"=" /
"^" / "_" /
"`" / "{" /
"|" / "}" /
"~"
strong-local = indirect-key / direct-key / nokey
ktext = ALPHA / DIGIT / "-"
key-identifier = 1*ktext
indirect-key = key-identifier "??" dot-atom
direct-key = key-identifier "?" dot-atom
nokey = "?" dot-atom
Addresses of the form indirect-key, direct-key and nokey are
interpreted as follows:
nokey
Messages sent to the address MUST be encrypted under an
encryption key that the sender determines to be trustworthy.
direct-key
If the public key specified by the Key Identifier is an
encryption key, messages sent to the address MUST be encrypted
under the specified key. Otherwise messages sent to the address
MUST be encrypted under an encryption key that has a direct key
endorsement under the specified key.
indirect-key
Messages sent to the address MUST be encrypted under an
encryption key that has a key endorsement under the specified
key.
2.2. Private Key Backup and Controlled Recovery
A frequently overlooked hazzard of using encryption is the risk of
data loss should the private key be lost or otherwise become
unavailable. Another practical difficulty that must be faced is the
need to enable encrypted email to be read on more than one device.
Once published, a strong email identifier effectively becomes a
April 20, 2014 [Page 6]
Internet-Draft PPE: Key Generation & Publication October 2013
personal root of trust, the value of which may increase over time.
Each of these use cases requires some form of private key backup and
recovery mechanism. While such mechanisms have traditionally been
considered to be an implementation choice that is outside the scope
of a protocol specification, to do so incurs a substantial risk of a
large number of bad implementation choices. In particular the need to
enable receipt of email on multiple devices requires a standards
based approach or else applications provided by different vendors
will not be able to exchange keys.
While a Key Escrow capability provides a Key Backup capability, the
reverse is not true. A Key Escrow system is generally understood to
support recovery of the private key without notice to the private key
holder while a Key Backup system need not meet this requirement.
A publication service MAY support Key Backup and Recovery. A user MAY
choose to use the Key Backup and Recovery function supported by a
Publication service.
If Key Backup is used, the key management client encrypts the private
key under a strong symmetric key and sends the encrypted data to the
publication service. The information necessary to recover the private
key is presented to the user in a compact form that MAY be written
down and stored without risk of hardware failure rendering the key
inaccessible.
2.2.1. Encrypted Private Key
Private Keys are encrypted using the PKCS#8 format as specified in
[RFC5208].
This specification is prefered to the PKCS#12 [I-D.moriarty-pkcs12v1-
1] format as the latter is essentially a wrapper for multiple PKCS#8
keys and associated certificates that can be generated by a
publication service if necessary.
Key management tools MUST support the use of AES-256 to encrypt
private keys. AES is prefered over AES-128 for the greater number of
encryption cycles rather than the increased brute force work factor.
Applications MAY use encryption keys with lengths less than 256 bits
provided that the keys have a length of at least 128 bits.
If the key size used is shorter than the key size required by the
encryption algorithm, the HKDF-Expand function described in [RFC5869]
is used to expand the truncated key to provide the necessary number
of bits.
Keys are presented in BASE32 encoding [RFC4648] with optional
separators '-' to improve readability. Applications MUST ignore
separators when decoding the keys.
April 20, 2014 [Page 7]
Internet-Draft PPE: Key Generation & Publication October 2013
2.2.2. Key Splitting
Key Management tools MAY support the use of a key splitting scheme to
allow greater control over key recovery. For example, the user might
split their key into three parts with a requirement that two parts
are necessary to reconstruct the key.
At this point the author has a paper by Rober Blakely Snr on an out-
of-patent key splitting scheme but insufficient time to read the
paper let alone write and implement the specification. If anyone is
looking for something to do, that would be useful.
2.3. Private Key Example
Alice uses a key generation tool to generate a public keypair. The
public parameters in hexadecimal are:
Modulus :
c1 66 de 02 62 35 3f af 7a 22 11 66 62 5a 1b 8d
3b 85 14 65 32 5d 6c e0 b5 db 09 e0 fc e4 16 34
96 ac 5b 76 01 96 e4 37 d5 8b db 52 a7 71 68 1c
86 1a 61 58 a7 0a 91 14 f2 d9 cd 4a 6b a5 e2 b3
94 c9 0b f2 7b ff 3b 6e a8 7b bf ca 27 0e b2 28
b0 d5 4a 1b 59 9a 8b 40 4e 80 3b dd 79 57 25 52
7a 70 ba 22 02 45 7b 4c e8 95 69 34 79 77 86 5f
09 36 30 18 1b 77 be c5 dc d3 ea db 1b 0a a0 8f
Exponent :
01 00 01
2.3.1. Key Identifier
KeyIdentifier: ACAIEA-FONPAC-5AC6LFA-K4ACHC-EAJWAHN-VPAM4A-COYPAO-VAA
alice@example.com
Send email to Alice using encryption if and only if an
encryption key for Alice can be found and Alice has published
the email encryption policy 'encryption preferred' or stronger.
?alice@example.com
Send email to Alice using encryption if and only if an
encryption key for Alice can be found, otherwise report an
error.
ACAIEA-FONPAC-5AC6LFA-K4ACHC-EAJWAHN-VPAM4A-COYPAO-
VAA?alice@example.com
Send email to Alice using encryption if and only if an
encryption key for Alice can be found that is directly endorsed
under the specified key, otherwise report an error.
April 20, 2014 [Page 8]
Internet-Draft PPE: Key Generation & Publication October 2013
ACAIEA-FONPAC-5AC6LFA-K4ACHC-EAJWAHN-VPAM4A-COYPAO-
VAA??alice@example.com
Send email to Alice using encryption if and only if an
encryption key for Alice can be found that is (directly or
indierectly) endorsed under the specified key, otherwise report
an error.
2.3.2. Private Key Backup
The private key component of Alice's key is as follows:
P :
f3 85 24 7b 95 3d a1 77 7c a4 4d a8 b8 00 3e 73
b2 9d 36 52 dc 64 21 e2 90 56 3c 51 d6 24 0c 20
77 1e d1 35 b4 c8 77 00 86 96 af 66 b0 5e 31 ff
15 ef 40 5e 00 21 54 18 fb dd f6 c2 bc 93 c2 1d
Q :
cb 50 32 f4 eb b5 74 80 b0 d1 f6 41 8c 90 9f 56
50 19 4e 64 be 93 f0 a2 bc 3c e9 e6 48 56 99 4e
4e 33 9c 77 31 92 45 a6 aa 35 39 7c f8 aa f3 35
85 05 09 78 8a 9f 4d 90 e3 36 61 84 ec 39 2d 9b
DP :
ca fa 35 58 95 22 d3 cd 66 a5 04 de 16 d0 8d 3d
9e a9 8f b8 2d 5f 81 26 f9 ac 07 87 26 f8 d0 ea
d6 9f 67 3e 5e bb a1 05 5d 29 88 76 0d 97 d6 10
8a d5 eb 4e ee c8 d8 f2 22 2d f7 1a 86 58 9a b9
DQ :
73 74 37 7b 9d de 8d 2a 07 3f 33 f8 45 3a 5b 41
48 7b 16 69 5f 4f e3 76 86 2e 91 24 94 2f 99 1f
3e 89 50 70 df 55 90 f7 f3 f0 05 95 52 20 c1 bb
c2 ad f9 92 da 25 5c 86 ca 80 37 20 a4 84 53 c1
InverseQ :
c1 b9 4b bf ee 41 77 b9 dc 0c cd 97 c0 96 77 22
0d e0 ed b2 3f 02 25 63 c8 0d 86 d8 5c 44 df 4d
d5 d7 3e 78 4a 5f 3d cb 76 5a 9b a8 1a 68 8e 47
9c 47 f9 8f 81 8d 6b 99 ab 74 56 88 4f ac c3 88
The private key is encrypted under a randomly assigned symmetric key
using PKCS8 encoding.
April 20, 2014 [Page 9]
Internet-Draft PPE: Key Generation & Publication October 2013
30 82 02 88 30 02 05 00 04 82 02 80 f3 fa 63 b9
1c d3 60 54 2f 75 ca 99 fe 42 1f 21 0d 2c f9 bd
4a 72 e4 ba da 09 91 f3 96 b7 b8 4a b6 78 da bc
92 55 ce c0 77 7c 75 96 86 05 cf 21 1b 23 a6 c6
12 fc e6 2c a5 36 7f 14 b3 bc 53 70 8a 8e fe 7f
99 d6 1d da 00 5f 5b 43 b2 cf 2d eb 0f 23 9c ce
0b bd 9c 81 29 b9 b8 7d 78 35 55 f7 45 5e 7b e0
d6 ef 9b bd 79 51 be 6d 88 f7 63 bb ef c8 b9 5b
90 c2 e9 a1 b5 d2 7b dd 69 95 3b 55 3a 79 8f 70
f4 26 38 4e 40 50 43 14 8c 57 65 7c cc 37 6a e2
4d 2f 51 fe 06 05 3b 7c 60 47 58 01 ef c6 f1 ae
4c 3c 28 8c c5 f0 0b f6 dd 8f ff 5d 22 a8 b6 5e
1b 94 29 ad f5 63 2e b8 60 ec 96 c8 63 df 2b 50
8b 27 a9 da ff 4a f8 b1 7d 6f 30 4c 9b 3d f6 65
a0 3e 24 6d 6f 2e d5 37 a4 52 ef 5a ef 11 51 84
e3 7e d2 19 7e 86 34 22 c5 78 5e 9e 6f 40 10 76
b3 cd 34 dd ea 3b 0e fc f3 38 1e e6 a3 32 54 a1
1a 7e 51 8d 0c 99 2e e7 20 06 21 5e b9 f8 87 19
f1 cd 82 00 6f 72 fb b9 a3 85 21 fb ac 80 2b aa
3a 47 b0 5d 03 74 77 08 70 de 64 25 a3 f5 bb 97
a3 08 ff 29 db 17 7b fa f8 80 c0 4e 90 5d 9a 15
04 60 73 f6 47 ff 82 6b 16 ce 19 a2 a0 1e a1 a3
a0 b0 2a fa 5b 51 0b 0c ab 92 53 fe 1e 1d af d9
78 9f 70 26 a6 32 80 d1 ef 6d 67 2a 48 b1 4b 3d
76 cf bc 8e 48 0c b0 9d da cf 4c b2 be aa d3 5f
ed 91 36 76 ef 5e ef 95 9c fd 4f 72 46 1d cf d2
15 4c 1a 93 9f 52 84 ce a5 c8 17 ff bc 0e 70 83
fc d3 c9 98 b6 0d f0 a8 72 8d 2d d4 49 6d 9d 79
35 c7 48 36 b1 49 95 f0 02 77 52 7a 50 13 74 80
7b 9d 1b bf cf f2 1e 99 6e 92 8f 8b b2 d8 35 6c
c9 2f 5d 3d da 4c 53 49 03 d6 63 56 7b d6 80 5d
69 b6 8d 9b fd 50 f4 c8 5e 9a 3c 91 26 b7 0a 62
d3 a7 2d 99 49 b2 1b 0f 74 71 62 ce 41 8a 2c dd
8a b4 97 38 dc be b8 6f b6 ba 29 e4 73 2d c0 ee
23 d1 2b 97 eb 7c 8d 42 16 33 3d 93 84 31 59 2f
f7 26 78 f2 3f 9a af ff 26 81 da 34 a6 74 bf 35
a9 c5 4d 6a 9d 48 d8 4b 00 a5 56 c2 46 e2 ce 65
f8 86 22 75 07 32 df 69 2c e2 74 09 54 4a 1e 38
62 56 6e 8e be c4 23 78 c1 f4 ea 20 96 a7 ac 89
54 6f 2d 0f 73 53 3b 66 7a 61 69 e7 6a d0 00 00
00 00 00 00 00 00 00 00 00 00 00 00
The cipher (specified in the PKCS8 object) is AES-256. The password
value in Base-32 encoding is:
April 20, 2014 [Page 10]
Internet-Draft PPE: Key Generation & Publication October 2013
Passcode: H2AL6A-ESJYAE-JABNDMA-HAAANDG;
1/3: CFAHVA-FMUWAN-PAHXIZA-BAAH2FE
2/3: PCAP7A-BEBNAH-LACDFJA-OAAHZXE
3/3: DBABPA-EHTBAD-CAGVAZA-MIAGPXE
3. Public Key Infrastructure
The precise means by which a public key is validated by a relying
party is outside the scope of this specification. Keys MAY be
validated by a traditional Certificate Authority or through peer to
peer endorsement or any combination of the two.
In order to maximize the flexibility for the trust infrastructre
designers, two syntaxes for presenting public keys for use are
supported. Key Management tools SHOULD support both:
A Certificate Signing Request
May be presented to a CA or other signer.
A self signed certificate
Presents the public key in a form that many Internet
applications accept directly.
3.1. Certificate Signing Request.
Certificate Signing Requests SHOULD conform to the following profile:
* The Key Identifier MUST be specified and MUST be a strong key
identifier
* [[Prohibit various PKIX lunacies]
3.2. Self-Signed Certificate.
Self Signed Certificates SHOULD conform to the following profile:
* The Key Identifier MUST be specified and MUST be a strong key
identifier
* [[Prohibit various PKIX lunacies]
3.3. Peer Endorsement
Traditionally PKIX only permits use of Certification Authority
provided trust assertions while OpenPGP only permits use of peer
endorsement through key signing. PPE supports the use of a
combination of both approaches for reasons described in [I-
D.hallambaker-prismproof-trust]
April 20, 2014 [Page 11]
Internet-Draft PPE: Key Generation & Publication October 2013
To perform peer endorsement, the following data structure is used:
Class Endorsement
TBSEndorsement TBSEndorsement
SignatureAlgorithm AlgorithmIdentifier
Signature Bits
Class TBSEndorsement
Version Integer
Issued Time
IssuerKeyIdentifier Octets
SubjectKeyIdentifier Octets
Subject List Name
SubjectAltName List SubjectAltName
Extensions List Extension
Class AlgorithmIdentifier
Algorithm OIDRef
Parameters Any
Class Name
Member Set AttributeTypeValue
Class AttributeTypeValue
Type OIDRef
Value AnyString
Object SubjectAltName id_ce_subjectAltName
Names List GeneralName
Class GeneralName
Value Choice
RFC822Name IA5String
Code 1
Implicit
DNSName IA5String
Code 2
Implicit
Class Extension
ObjectIdentifier OIDRef
Critical Boolean
Default "false"
Optional
Data Octets
[[Note that although my tool generates ASN.1 encoding this is for
purely pragmatic reasons of providing consistency. It is not meant to
in any shape or fashion stand for an endorsement of this crackpot
technology.]
April 20, 2014 [Page 12]
Internet-Draft PPE: Key Generation & Publication October 2013
A new structure is introduced to support Key Endorsement rather than
attempting to re-use the X.509v3 Certificate format in recognition of
key endorsement having distinctly different semantics from issue of
PKIX certificates. PKIX certificates are either end entity
certificates or certificate signing certificates. A PKIX certificate
is expressly prohibited from being used for both purposes. In the
PKIX model, finding a certificate chain to a trusted anchor is
necessary and sufficient to establish the trustworthiness of an end
entity certificate. In the Key Endorsement model the reliance on a
single key endorsement MAY be qualified by the age of the
endorsement, the circumstances of issue, the number of independent
trust paths from the relying party to the subject and the lengths of
each path.
Most of the fields in the TBSEndorsement structure have the same
semantics as in PKIX with the exception of the Validity interval
which is replaced by the time of issue.
The precise mechanism by which endorsement is used requires further
development. At minimum, the endorsement mechanism should allow the
following forms of endorsement to be differentiated:
Direct Endorsement
A endorsement of a user's key identifier by another key held by
the same user. This form of endorsement allows a user to
establish a personal master key that is only used for the
purpose of endorsing keys for specific uses (email encryption,
email signature, endorsement, etc.)
Peer Endorsement
A user endorses the key identifier of another user (the
subject) and possibly other aspects of the subject's identity
such as their name, likeness etc. Such an endorsement SHOULD
specify the basis for the endorsement (in person, remote,
recent acquaintance, verification of government documents,
childhood friend, etc.)
Group Endorsement
One of the use practices that has emerged from attempts to
employ PGP is the 'key party' in which groups of users perform
mutual keysigning.
Withdrawing an Endorsement
In certain circumstances, it MAY be necessary to withdraw an
endorsement. The reason for withdrawing the endorsement SHOULD
be specified in the UnEndorsement notice and MAY include,
notification of the loss of the private key, the subject is
deceased, etc.)
April 20, 2014 [Page 13]
Internet-Draft PPE: Key Generation & Publication October 2013
4. Publication Service
The Publication Service is a JSON/REST Web Service layered over HTTP
transport. Although the publication service performs an important
service, it is not a service trusted by the user since the
publication service has no access to the user's private key (except
in encrypted form) and does not sign any data that is read by the
user.
The Publication Service is one of the two interfaces between the part
of the email message security problem that is well understood and the
part that is widely regarded to be 'research'.
Selection of the publication service MAY be left to individual user
choice or a domain name holder MAY specify that publication requests
be directed to a specific publication service. Users of a public
email service are likely to want to insist on their own choice of
publication service while a bank or government enterprise that has
deployed its own security infrastructure is likely to want to insist
that only credentials they approve are accepted for their site.
To allow researchers the widest possible lattitude in developing new
trust infrastructures, publication of three trust assertion formats
are supported together with support for key backup and recovery.
These assertion formats are:
Self Signed Certificate
A PKIX self signed certificate which MAY be used in conjunction
with an existing application that accepts public key
information in self signed certificate form.
Certificate Signing Request
A PKCS#10 Certificate Signing Request conforming to [!RFC2986].
A publication interface MAY forward the Certificate Signing
request to a Certificate Authority for issue of a PKIX end
entity certificate.
Key Endorsement
A Key Endorsement in the format described in this document.
4.1. Initial Key Publication
The first time that the Publication Service is used is after the user
generates a new keypair.
For example, Alice registers the keypair generated in the previous
example with her chosen Publication Service. Her key management tool
makes an Assert request to the service with the following
information:
April 20, 2014 [Page 14]
Internet-Draft PPE: Key Generation & Publication October 2013
* The Strong Key Identifier
* The Encrypted Private Key
* A Self-Signed Certificate
* A Signed Certificate Signing Request
* Service information describing the email service parameters to
be used when sending messages using the corresponding email
account. [[Which really should be encrypted, shouldn't they?]
April 20, 2014 [Page 15]
Internet-Draft PPE: Key Generation & Publication October 2013
5. Registration Example
Request
{
"AssertRequest": {
"KeyIdentifier": "
AJqCYq5r2i7DXllBrhJHESWbLe2rw84cTsPr6qo",
"EncryptedKey": {
"EncryptedPrivateKey": "
MIICiDACBQAEggKA8_pjuRzTYFQvdcqZ_kIfIQ0s-b1KcuS62gmR85a3uEq2eNq8
klXOwHd8dZaGBc8hGyOmxhL85iylNn8Us7xTcIqO_n-Z1h3aAF9bQ7LPLesPI5zO
C72cgSm5uH14NVX3RV574Nbvm715Ub5tiPdju-_IuVuQwumhtdJ73WmVO1U6eY9w
9CY4TkBQQxSMV2V8zDdq4k0vUf4GBTt8YEdYAe_G8a5MPCiMxfAL9t2P_10iqLZe
G5QprfVjLrhg7JbIY98rUIsnqdr_SvixfW8wTJs99mWgPiRtby7VN6RS71rvEVGE
437SGX6GNCLFeF6eb0AQdrPNNN3qOw788zge5qMyVKEaflGNDJku5yAGIV65-IcZ
8c2CAG9y-7mjhSH7rIArqjpHsF0DdHcIcN5kJaP1u5ejCP8p2xd7-viAwE6QXZoV
BGBz9kf_gmsWzhmioB6ho6CwKvpbUQsMq5JT_h4dr9l4n3AmpjKA0e9tZypIsUs9
ds-8jkgMsJ3az0yyvqrTX-2RNnbvXu-VnP1PckYdz9IVTBqTn1KEzqXIF_-8DnCD
_NPJmLYN8KhyjS3USW2deTXHSDaxSZXwAndSelATdIB7nRu_z_IemW6Sj4uy2DVs
yS9dPdpMU0kD1mNWe9aAXWm2jZv9UPTIXpo8kSa3CmLTpy2ZSbIbD3RxYs5Biizd
irSXONy-uG-2uinkcy3A7iPRK5frfI1CFjM9k4QxWS_3JnjyP5qv_yaB2jSmdL81
qcVNap1I2EsApVbCRuLOZfiGInUHMt9pLOJ0CVRKHjhiVm6OvsQjeMH06iCWp6yJ
VG8tD3NTO2Z6YWnnatAAAAAAAAAAAAAAAAAAAA"},
"Certificate": ["
MIICKjCCAZ4CAQICEQDPmUDPAObF9gNRAiPqkCswMAIFADAEMAIxADAeFw0xMzEw
MTYxMjAwMDFaFw0zMzEwMzEwNjA2MDJaMAQwAjEAMIGUMAIFAAOBjQAwgYkCgYEA
wWbeAmI1P696IhFmYlobjTuFFGUyXWzgtdsJ4PzkFjSWrFt2AZbkN9WL21KncWgc
hhphWKcKkRTy2c1Ka6Xis5TJC_J7_ztuqHu_yicOsiiw1UobWZqLQE6AO915VyVS
enC6IgJFe0zolWk0eXeGXwk2MBgbd77F3NPq2xsKoI8CAwEAAQUABQAwgbowKAMO
HVUCAQAEHwQdAJqCYq5r2i7DXllBrhJHESWbLe2rw84cTsPr6qowLwMjHVUCAQAE
JjAkBB0AmoJirmvaLsNeWUGuEkcRJZst7avDzhxOw-vqqgUAAgEAMCQDER1VAgEA
BBswGTAXMBUFABYRYWxpY2VAZXhhbXBsZS5jb20wDwMPHVUCAf8EBgMEAAcAgDAW
AyUdVQIBAAQNMAswCQgEAwcFBQEGKzAOAxMdVQIBAAQFMAMCAQAwAgUAA4GBAGMo
0Ky-ccYSHWqRLbd4JFns3UVgEbcbGUzm-H29DEJq1WUuihR03dfzeXQv9BY271o_
Q_RsuFIbOYpEhpP2OG_5v6DdLOrvE6GsjydN7isLo0E6F-rxkVP6GfyMiDI5cr9z
1IR9b--DZUx_C8QK1c4JcASVANMc_Yt7_yn7kDkD"],
"CertificateRequest": ["
MIIBLTCBogIBADAEMAIxADCBlDACBQADgY0AMIGJAoGBAMFm3gJiNT-veiIRZmJa
G407hRRlMl1s4LXbCeD85BY0lqxbdgGW5DfVi9tSp3FoHIYaYVinCpEU8tnNSmul
4rOUyQvye_87bqh7v8onDrIosNVKG1mai0BOgDvdeVclUnpwuiICRXtM6JVpNHl3
hl8JNjAYG3e-xdzT6tsbCqCPAgMBAAEFADACBQADgYEABExEqqopLQXfVWZr5MJ0
digUmdcugrfykTnNMkLx3En8fVLMbrgBEu0Ndax_TqOk36_gjnyyjg2XGCI5BaTd
jHp13C8dsIdcfdePc3droSGLuPzMosZqzyN1qLhf5dEfXwp32gBwteXPV-YE9Nf3
rDEZ_vc32sK-09766Fbitz0"],
"Service": [{
"Email": "alice@example.com",
"Name": "smtp.example.com",
"Protocol": "_smtp._tls",
"Port": 587,
April 20, 2014 [Page 16]
Internet-Draft PPE: Key Generation & Publication October 2013
"TLS": true},
{
"Email": "alice@example.com",
"Name": "imap.example.com",
"Protocol": "_smtp._tls",
"Port": 993,
"TLS": true}]}}
Response
{
"AssertResponse": {}}
5.1. Enabling a new Device
Alice uses several different devices to read her email and she would
like to be able to read encrypted emails on all of them. This
requires that the private key be installed on each of the devices
that she might want to use.
Alice provides either the Key Recovery Passcode or a sufficient
number of Key Shares to reconstruct the passcode to the key
management tool running on each device. The device then requests
recovery of the private key and associated service information:
6. Recovery Example
Request
{
"RecoverRequest": {
"KeyIdentifier": "
AJqCYq5r2i7DXllBrhJHESWbLe2rw84cTsPr6qo"}}
Response
{
"RecoverResponse": {}}
Providing the service information with the private key allows the key
recovery tool to automate configuration of the user's email account
on the device if this has not been done already.
Using the key recovery mechanism to support key transport between
devices simplifies the initial coding task at the cost of a sub-
optimal user experience for the user with a large number of devices
in use and/or frequent key updates.
April 20, 2014 [Page 17]
Internet-Draft PPE: Key Generation & Publication October 2013
Future versions of the specification may adopt a different approach
to key recovery in which each device in which keys are to be
installed establishes a device specific keypair which is in turn used
to automate the key transport. A key concern in the design of such a
scheme being to prevent a weak random number generator on one device
causing the private key to be compromised.
6.1. Revocation
Should the private key be lost, the subject be deceased or some other
event occur that renders the key no longer servicable, a revocation
statement is generated and issued. Such revocation statements use the
Revoke request and the key endorsement message format:
7. Revocation Example
Request
{
"RevokeRequest": {}}
Response
{
"RevokeResponse": {}}
7.1. Key Endorsement
From time to time, Alice meets other PPE users and they endorse each
other's keys. The AssertRequest is used to submit one or more signed
key endorsements:
April 20, 2014 [Page 18]
Internet-Draft PPE: Key Generation & Publication October 2013
8. Endorsement Example
Request
{
"AssertRequest": {
"Endorsement": ["
MIH5MG8CAQAXDTEzMTAxNjA2MDYwM1oEHQCagmKua9ouw15ZQa4SRxElmy3tq8PO
HE7D6-qqBB0AGXoKrrHJ0-qMHfed6IOFC9Y_-V8bWp6Zmi_g3wUAMBkwFzAVMBMF
ABYPYm9iQGV4YW1wbGUuY29tBQAwAgUAA4GBAEPmx2IAjnlpR0z1V3K51HmjpYY3
dpJvsE0M41uAxPvhnnz-yCX5XYfa9MJzILag0eiVrVgTbE7CVH-ccRDgsr73sEri
LOc3vre32JWU2Cg1Y0s1sh1GMWJTj8DGPFLR-uOHCsFAxWK8XD6Y7hSlwZrh3EFu
SQfWoqzMtYkCY9wd"]}}
Response
{
"AssertResponse": {}}
A key endorsement MAY be submitted to the Publication Interface by
any party including the signer or the subject.
9. OmniAssertBroker
9.1. Assert
Register an assertion set.
The Assert transaction is used when a keypair is first created to
register the new Key Identifier, Self Signed Certificate and
Certificate Signing Request and to request revision of embedded
attributes such as the email security policy.
The Assert transaction is also used to request registration of Key
Endorsements.
9.1.1. Structure: Service
Email :
String [0..1] Principal Email address associated with the
account
OtherEmail :
String [0..Many] Additional Email addresses associated with the
account.
Name :
String [0..1] DNS Address of Service
April 20, 2014 [Page 19]
Internet-Draft PPE: Key Generation & Publication October 2013
Protocol :
String [0..1] SRV format protocol identification prefix.
Port :
Integer [0..1] IP Port number
TLS :
Boolean [0..1] If true, use of TLS is required
Security :
String [0..1] Security policy description
9.1.2. Structure: EncryptedKey
EncryptedPrivateKey :
Binary [1..1] PKCS#8 Encrypted Private Key as specified in
[!RFC5208].
ReleaseCode :
Binary [0..1] Release Code value for authorizing private key
recovery requests. If specified the service MUST NOT release
the encrypted private key unless the requestor satisfies a
challenge-response request that establishes knowledge of the
Release Code.
9.1.3. Message: AssertRequest
Register an assertion set
At present only a single Key Identifier may be registered per request
and no provision is made to link related requests. This is likely to
become necessary when different keys are being used for key
endorsement, signature, encryption and master purposes.
KeyIdentifier :
Binary [1..1] Strong Key Identifier formed using a message
digest function over the DER encoded Public Key Info block.
EncryptedKey :
EncryptedKey [0..1] Encrypted Private Key and associated
attributes.
Certificate :
Binary [0..Many] PKIX Certificates to be registered, comply
with [!RFC5280] and additional profile constraints specified
here.
CertificateRequest :
Binary [0..Many] Certificate Request in [!RFC2986] format.
April 20, 2014 [Page 20]
Internet-Draft PPE: Key Generation & Publication October 2013
Endorsement :
Binary [0..Many] Key Endorsements as specified in this
document.
Service :
Service [0..Many] Service connection information for associated
services. For example, email IMAP [!RFC3501], POP3 [!RFC5034]
and SUBMIT [!RFC4409] accounts.
9.1.4. Message: AssertResponse
Response to an assertion registration request.
It may be useful to expand the response to allow the gateway to
provide information such as certificates issued in response to the
certification request but these will typically require some form of
validation and thus be returned asynchronously.
9.2. Recover
Recover a previously registered encrypted private key file from the
service
If the Key Identifier cannot be found or there is no release code
associated with the encrypted private key, the transaction is
complete after the first response. Otherwise the service returns the
status code 'ChallengeResponse' in response to the initial request
and the client MUST make a second request in which it establishes
proof of knowledge of the release code to complete the transaction.
9.2.1. Message: RecoverRequest
Request recovery of a previously registered encrypted private key.
KeyIdentifier :
Binary [1..1] Key Identifier of key pair for which recovery of
the private key is being requested.
Challenge :
Binary [0..1] Client challenge value for proof of knowledge of
the release code.
Answer :
Binary [0..1] Answer value for proof of knowledge of the
release code.
April 20, 2014 [Page 21]
Internet-Draft PPE: Key Generation & Publication October 2013
9.2.2. Message: RecoverResponse
Respond to a recovery request.
If the encrypted private key associated with the specified Key
Identifier has an associated
EncryptedPrivateKey :
Binary [0..1] PKCS#8 Encrypted Private Key as specified in
[!RFC5208].
Challenge :
Binary [0..1] Server challenge value for proof of knowledge of
the release code.
Algorithm :
String [0..1] Digest algorithm for proof of knowledge of the
release code.
9.3. Revoke
Publish a revocation meta-assertion
9.3.1. Message: RevokeRequest
Regquest revocation of a previously registered key and all related
certificates and endorsements.
Note that whil key revocation necessarily entails revocation of all
the certificates and endorsement associated with the key, the reverse
is not the case. A user may revoke a certificate granting use of a
key for encrypted email without wishing to revoke a certificate for
the same key granting use for signed email.
KeyIdentifier :
Binary [1..1] Key Identifier of Key to be revoked.
Notice :
Binary [0..1] Signed Key Endorsement object with the 'revoke'
attribute specified.
9.3.2. Message: RevokeResponse
Response to revocation request.
10. Security Considerations
I am sure there are some.
April 20, 2014 [Page 22]
Internet-Draft PPE: Key Generation & Publication October 2013
11. Acknowledgments
Thanks to the many people who have encouraged me in this work and in
particular the members of the IETF PERPASS list and the Cryptography
mailing list. Future versions of the draft will have a more complete
list.
12. References
12.1. Normative References
[RFC5280] Cooper, D.,Santesson, S.,Farrell, S.,Boeyen, S.,Housley,
R.,Polk, W., "Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL)
Profile", RFC 5280, May 2008.
[RFC2986] Nystrom, M.,Kaliski, B., "PKCS #10: Certification Request
Syntax Specification Version 1.7", RFC 2986, November
2000.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC4409] Gellens, R.,Klensin, J., "Message Submission for Mail",
RFC 4409, April 2006.
[RFC5034] Siemborski, R.,Menon-Sen, A., "The Post Office Protocol
(POP3) Simple Authentication and Security Layer (SASL)
Authentication Mechanism", RFC 5034, July 2007.
[I-D.hallambaker-prismproof-trust] Hallam-Baker, P, "PRISM Proof
Trust Model", Internet-Draft draft-hallambaker-prismproof-
trust-00, 16 October 2013.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.
[RFC4880] Callas, J.,Donnerhacke, L.,Finney, H.,Shaw, D.,Thayer, R.,
"OpenPGP Message Format", RFC 4880, November 2007.
[RFC5208] Kaliski, B., "Public-Key Cryptography Standards (PKCS) #8:
Private-Key Information Syntax Specification Version 1.2",
RFC 5208, May 2008.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5869] Krawczyk, H.,Eronen, P., "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, May 2010.
April 20, 2014 [Page 23]
Internet-Draft PPE: Key Generation & Publication October 2013
12.2. Informative References
[I-D.moriarty-pkcs12v1-1] Moriarty, K,Nystrom, M,Parkinson, S,Rusch,
A,Scott, M, "PKCS 12 v1: Personal Information Exchange
Syntax", Internet-Draft draft-moriarty-pkcs12v1-1-01, 25
March 2013.
[RFC2616] Fielding, R.,Gettys, J.,Mogul, J.,Frystyk, H.,Masinter,
L.,Leach, P.,Berners-Lee, T., "Hypertext Transfer Protocol
-- HTTP/1.1", RFC 2616, June 1999.
[RFC5246] Dierks, T.,Rescorla, E., "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
Author's Address
Phillip Hallam-Baker
Comodo Group Inc.
philliph@comodo.com
April 20, 2014 [Page 24]