Stateless OpenPGP Command Line Interface
draft-dkg-openpgp-stateless-cli-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Active".
|
|
|---|---|---|---|
| Author | Daniel Kahn Gillmor | ||
| Last updated | 2019-10-29 (Latest revision 2019-10-28) | ||
| RFC stream | (None) | ||
| Formats | |||
| Additional resources | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-dkg-openpgp-stateless-cli-01
openpgp D. Gillmor
Internet-Draft ACLU
Intended status: Informational October 29, 2019
Expires: May 1, 2020
Stateless OpenPGP Command Line Interface
draft-dkg-openpgp-stateless-cli-01
Abstract
This document defines a generic stateless command-line interface for
dealing with OpenPGP messages, known as "sop". It aims for a
minimal, well-structured API covering OpenPGP object security.
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 https://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."
This Internet-Draft will expire on May 1, 2020.
Copyright Notice
Copyright (c) 2019 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
(https://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.
Gillmor Expires May 1, 2020 [Page 1]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Subcommands . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. version: Version Information . . . . . . . . . . . . . . 5
3.2. generate-key: Generate a Secret Key . . . . . . . . . . . 5
3.3. extract-cert: Extract a Certificate from a Secret Key . . 5
3.4. sign: Create a Detached Signature . . . . . . . . . . . . 6
3.5. verify: Verify a Detached Signature . . . . . . . . . . . 6
3.6. encrypt: Encrypt a Message . . . . . . . . . . . . . . . 7
3.7. decrypt: Decrypt a Message . . . . . . . . . . . . . . . 8
3.8. armor: Add ASCII Armor . . . . . . . . . . . . . . . . . 10
3.9. dearmor: Remove ASCII Armor . . . . . . . . . . . . . . . 11
4. Input String Types . . . . . . . . . . . . . . . . . . . . . 11
4.1. DATE . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2. USERID . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Input/Output Indirect Types . . . . . . . . . . . . . . . . . 12
5.1. CERTS . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. KEY . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3. CIPHERTEXT . . . . . . . . . . . . . . . . . . . . . . . 13
5.4. SIGNATURE . . . . . . . . . . . . . . . . . . . . . . . . 13
5.5. SESSIONKEY . . . . . . . . . . . . . . . . . . . . . . . 13
5.6. PASSWORD . . . . . . . . . . . . . . . . . . . . . . . . 14
5.7. VERIFICATIONS . . . . . . . . . . . . . . . . . . . . . . 14
5.8. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Failure modes . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Guidance for Implementors . . . . . . . . . . . . . . . . . . 15
7.1. One OpenPGP Message At a Time . . . . . . . . . . . . . . 15
7.2. Simplified Subset of OpenPGP Message . . . . . . . . . . 16
7.3. Validate Signatures Only From Known Signers . . . . . . . 16
7.4. Detached Signatures . . . . . . . . . . . . . . . . . . . 16
7.5. Reliance on Supplied Certs and Keys . . . . . . . . . . . 16
8. Guidance for Consumers . . . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9.1. Signature Verification . . . . . . . . . . . . . . . . . 17
9.2. Compression . . . . . . . . . . . . . . . . . . . . . . . 18
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18
10.1. Object Security vs. Transport Security . . . . . . . . . 18
11. Document Considerations . . . . . . . . . . . . . . . . . . . 18
11.1. Document History . . . . . . . . . . . . . . . . . . . . 19
11.2. Future Work . . . . . . . . . . . . . . . . . . . . . . 19
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
13.1. Normative References . . . . . . . . . . . . . . . . . . 21
13.2. Informative References . . . . . . . . . . . . . . . . . 21
Gillmor Expires May 1, 2020 [Page 2]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
Different OpenPGP implementations have many different requirements,
which typically break down in two main categories: key/certificate
management and object security.
The purpose of this document is to provide a "stateless" interface
that primarily handles the object security side of things, and
assumes that secret key management and certificate management will be
handled some other way.
This separation should make it easier to provide interoperability
testing for the object security work, and to allow implementations to
consume and produce new cryptographic primitives as needed.
This document defines a generic stateless command-line interface for
dealing with OpenPGP messages, known here by the placeholder "sop".
It aims for a minimal, well-structured API.
An OpenPGP implementation should not name its executable "sop" to
implement this specification, of course. It just needs to provide a
binary that conforms to this interface.
A "sop" implementation should leave no trace on the system, and its
behavior should not be affected by anything other than command-line
arguments and input.
Obviously, the user will need to manage their secret keys (and their
peers' certificates) somehow, but the goal of this interface is to
separate out that task from the task of interacting with OpenPGP
messages.
While this document identifies a command-line interface, the rough
outlines of this interface should also be amenable to relatively
straightforward library implementations in different languages.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Gillmor Expires May 1, 2020 [Page 3]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
1.2. Terminology
This document uses the term "key" to refer exclusively to OpenPGP
Transferable Secret Keys (see section 11.2 of [RFC4880]).
It uses the term "certificate" to refer to OpenPGP Transferable
Public Key (see section 11.1 of [RFC4880]).
"Stateless" in "Stateless OpenPGP" means avoiding secret key and
certificate state. The user is responsible for managing all OpenPGP
certificates and secret keys themselves, and passing them to "sop" as
needed. The user should also not be concerned that any state could
affect the underlying operations.
OpenPGP revocations can have "Reason for Revocation" (section
5.2.3.23 of [RFC4880]), which can be either "soft" or "hard". The
set of "soft" reasons is: "Key is superseded" and "Key is retired and
no longer used". All other reasons (and revocations that do not
state a reason) are "hard" revocations.
2. Examples
These examples show no error checking, but give a flavor of how "sop"
might be used in practice from a shell.
The key and certificate files described in them (e.g. "alice.sec")
could be for example those found in
[I-D.draft-bre-openpgp-samples-00].
sop generate-key "Alice Lovelace <alice@openpgp.example>" > alice.sec
sop extract-cert < alice.sec > alice.pgp
sop sign --as=text alice.sec < announcement.txt > announcement.txt.asc
sop verify announcement.txt.asc alice.pgp < announcement.txt
sop encrypt --sign-with=alice.sec --as=mime bob.pgp < msg.eml > encrypted.asc
sop decrypt alice.sec < ciphertext.asc > cleartext.out
3. Subcommands
"sop" uses a subcommand interface, similar to those popularized by
systems like "git" and "svn".
If the user supplies a subcommand that "sop" does not implement, it
fails with a return code of 69. If a "sop" implementation does not
handle a supplied option for a given subcommand, it fails with a
return code of 37.
Gillmor Expires May 1, 2020 [Page 4]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
For all commands that have an "--armor|--no-armor" option, it
defaults to "--armor", meaning that any output OpenPGP material
should be ASCII-armored (section 6 of [I-D.ietf-openpgp-rfc4880bis])
by default.
3.1. version: Version Information
sop version
o Standard Input: ignored
o Standard Output: version string
The version string emitted should contain the name of the "sop"
implementation, followed by a single space, followed by the version
number.
Example:
$ sop version
ExampleSop 0.2.1
$
3.2. generate-key: Generate a Secret Key
sop generate-key [--armor|--no-armor] [--] [USERID...]
o Standard Input: ignored
o Standard Output: "KEY" (Section 5.2)
Generate a single default OpenPGP certificate with zero or more User
IDs.
Example:
$ sop generate-key 'Alice Lovelace <alice@openpgp.example>' > alice.sec
$ head -n1 < alice.sec
-----BEGIN PGP PRIVATE KEY BLOCK-----
$
3.3. extract-cert: Extract a Certificate from a Secret Key
sop extract-cert [--armor|--no-armor]
o Standard Input: "KEY" (Section 5.2)
o Standard Output: "CERTS" (Section 5.1)
Gillmor Expires May 1, 2020 [Page 5]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
Note that the resultant "CERTS" object will only ever contain one
OpenPGP certificate.
Example:
$ sop extract-cert < alice.sec > alice.pgp
$ head -n1 < alice.pgp
-----BEGIN PGP PUBLIC KEY BLOCK-----
$
3.4. sign: Create a Detached Signature
sop sign [--armor|--no-armor]
[--as={binary|text}] [--] KEY [KEY...]
o Standard Input: "DATA" (Section 5.8)
o Standard Output: "SIGNATURE" (Section 5.4)
"--as" defaults to "binary". If "--as=text" and the input "DATA" is
not valid "UTF-8", "sop sign" fails with a return code of 53.
Example:
$ sop sign --as=text alice.sec < message.txt > message.txt.asc
$ head -n1 < message.txt.asc
-----BEGIN PGP SIGNATURE-----
$
3.5. verify: Verify a Detached Signature
sop verify [--not-before=DATE] [--not-after=DATE]
[--] SIGNATURE CERTS [CERTS...]
o Standard Input: "DATA" (Section 5.8)
o Standard Output: "VERIFICATIONS" (Section 5.7)
"--not-before" and "--not-after" indicate that signatures with dates
outside certain range MUST NOT be considered valid.
"--not-before" defaults to the beginning of time. Accepts the
special value "-" to indicate the beginning of time (i.e. no lower
boundary).
"--not-after" defaults to the current system time ("now"). Accepts
the special value "-" to indicate the end of time (i.e. no upper
boundary).
Gillmor Expires May 1, 2020 [Page 6]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
"sop verify" only returns 0 if at least one certificate included in
any "CERTS" object made a valid signature in the range over the
"DATA" supplied.
For details about the valid signatures, the user MUST inspect the
"VERIFICATIONS" output.
If no "CERTS" are supplied, "sop verify" fails with a return code of
19.
If no valid signatures are found, "sop verify" fails with a return
code of 3.
See Section 9.1 for more details about signature verification.
Example:
(In this example, we see signature verification succeed first, and
then fail on a modified version of the message.)
$ sop verify message.txt.asc alice.pgp < message.txt
2019-10-29T18:36:45Z EB85BB5FA33A75E15E944E63F231550C4F47E38E EB85BB5FA33A75E15E944E63F231550C4F47E38E signed by alice.pgp
$ echo $?
0
$ tr a-z A-Z < message.txt | sop verify message.txt.asc alice.pgp
$ echo $?
3
$
3.6. encrypt: Encrypt a Message
sop encrypt [--as={binary|text|mime}]
[--armor|--no-armor]
[--with-password=PASSWORD...]
[--sign-with=KEY...]
[--] [CERTS...]
o Standard Input: "DATA" (Section 5.8)
o Standard Output: "CIPHERTEXT" (Section 5.3)
"--as" defaults to "binary".
"--with-password" enables symmetric encryption (and can be used
multiple times if multiple passwords are desired). If "sop encrypt"
encounters a "PASSWORD" which is not a valid "UTF-8" string, it fails
with a return code of 31. If "sop encrypt" sees trailing whitespace
Gillmor Expires May 1, 2020 [Page 7]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
at the end of a "PASSWORD", it will trim the trailing whitespace
before using the password.
"--sign-with" enables signing by a secret key (and can be used
multiple times if multiple signatures are desired).
If "--as" is set to either "text" or "mime", then "--sign-with" will
sign as a canonical text document. In this case, if the input "DATA"
is not valid "UTF-8", "sop encrypt" fails with a return code of 53.
The resulting "CIPHERTEXT" should be decryptable by the secret keys
corresponding to every certificate included in all "CERTS", as well
as each password given with "--with-password".
If no "CERTS" or "--with-password" options are present, "sop encrypt"
fails with a return code of 19.
If at least one of the identified certificates requires encryption to
an unsupported asymmetric algorithm, "sop encrypt" fails with a
return code of 13.
If at least one of the identified certificates is not encryption-
capable (e.g., revoked, expired, no encryption-capable flags on
primary key and valid subkeys), "sop encrypt" fails with a return
code of 17.
If "sop encrypt" fails for any reason, it emits no "CIPHERTEXT".
Example:
(In this example, "bob.bin" is a file containing Bob's binary-
formatted OpenPGP certificate. Alice is encrypting a message to both
herself and Bob.)
$ sop encrypt --as=mime --sign-with=alice.key alice.asc bob.bin < message.eml > encrypted.asc
$ head -n1 encrypted.asc
-----BEGIN PGP MESSAGE-----
$
3.7. decrypt: Decrypt a Message
Gillmor Expires May 1, 2020 [Page 8]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
sop decrypt [--session-key-out=SESSIONKEY]
[--with-session-key=SESSIONKEY...]
[--with-password=PASSWORD...]
[--verify-out=VERIFICATIONS
[--verify-with=CERTS...]
[--verify-not-before=DATE]
[--verify-not-after=DATE] ]
[--] [KEY...]
o Standard Input: "CIPHERTEXT" (Section 5.3)
o Standard Output: "DATA" (Section 5.8)
"--session-key-out" can be used to learn the session key on
successful decryption.
If "sop decrypt" fails for any reason and the identified "--session-
key-out" file already exists in the filesystem, the file will be
unlinked.
"--with-session-key" enables decryption of the "CIPHERTEXT" using the
session key directly against the "SEIPD" packet. This option can be
used multiple times if several possible session keys should be tried.
"--with-password" enables decryption based on any "SKESK" packets in
the "CIPHERTEXT". This option can be used multiple times if the user
wants to try more than one password.
If "sop decrypt" tries and fails to use a supplied "PASSWORD", and it
observes that there is trailing "UTF-8" whitespace at the end of the
"PASSWORD", it will retry with the trailing whitespace stripped.
"--verify-out" produces signature verification status to the
designated file.
"sop decrypt" does not fail (that is, the return code is not
modified) based on the results of signature verification. The caller
MUST check the returned "VERIFICATIONS" to confirm signature status.
An empty "VERIFICATIONS" output indicates that no valid signatures
were found. If "sop decrypt" itself fails for any reason, and the
identified "VERIFICATIONS" file already exists in the filesystem, the
file will be unlinked.
"--verify-with" identifies a set of certificates whose signatures
would be acceptable for signatures over this message.
If the caller is interested in signature verification, both "--
verify-out" and at least one "--verify-with" must be supplied. If
Gillmor Expires May 1, 2020 [Page 9]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
only one of these arguments is supplied, "sop decrypt" fails with a
return code of 23.
"--verify-not-before" and "--verify-not-after" provide a date range
for acceptable signatures, by analogy with the options for "sop
verify" (see Section 3.5). They should only be supplied when doing
signature verification.
See Section 9.1 for more details about signature verification.
If no "KEY" or "--with-password" or "--with-session-key" options are
present, "sop decrypt" fails with a return code of 19.
If unable to decrypt, "sop decrypt" fails with a return code of 29.
"sop decrypt" only returns cleartext to Standard Output that was
successfully decrypted.
Example:
(In this example, Alice stashes and re-uses the session key of an
encrypted message.)
$ sop decrypt --session-key-out=session.key alice.sec < ciphertext.asc > cleartext.out
$ ls -l ciphertext.asc cleartext.out
-rw-r--r-- 1 user user 321 Oct 28 01:34 ciphertext.asc
-rw-r--r-- 1 user user 285 Oct 28 01:34 cleartext.out
$ sop decrypt --with-session-key=session.key < ciphertext.asc > cleartext2.out
$ diff cleartext.out cleartext2.out
$
3.8. armor: Add ASCII Armor
sop armor [--label={auto|sig|key|cert|message}]
[--allow-nested]
o Standard Input: 8-bit, unarmored OpenPGP material ("SIGNATURE",
"CERTS", "KEY", or "CIPHERTEXT")
o Standard Output: the same material with ASCII-armoring added
The user can choose to specify the label used in the header and tail
of the armoring. The default is "auto", in which case, "sop"
inspects the input and chooses the label appropriately. In this
case, if "sop" cannot select a label on the basis of the input, it
treats it as literal data, and labels it as a "message".
Gillmor Expires May 1, 2020 [Page 10]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
If the incoming data is already armored, and the "--allow-nested"
flag is not specified, the data MUST be output with no modifications.
Data is considered ASCII armored iff the first 14 bytes are exactly "
-----BEGIN PGP". This operation is thus idempotent by default.
Example:
$ sop armor < bob.bin > bob.pgp
$ head -n1 bob.pgp
-----BEGIN PGP PUBLIC KEY BLOCK-----
$
3.9. dearmor: Remove ASCII Armor
sop dearmor
o Standard Input: ASCII-armored OpenPGP material ("CIPHERTEXT",
"SIGNATURE", "CERTS", or "KEY")
o Standard Output: the same material with ASCII-armoring removed
Example:
$ sop dearmor < message.txt.asc > message.txt.sig
$
4. Input String Types
Some material is passed to "sop" directly as a string on the command
line.
4.1. DATE
An ISO-8601 formatted timestamp with time zone, or the special value
"now" to indicate the current system time.
Examples:
now
2019-10-29T12:11:04+00:00
2019-10-24T23:48:29Z
20191029T121104Z
In some cases where used to specify lower and upper boundaries, a
"DATE" value can be set to "-" to indicate "no time limit".
A flexible implementation of "sop" MAY accept date inputs in other
unambiguous forms.
Gillmor Expires May 1, 2020 [Page 11]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
4.2. USERID
This is an arbitrary "UTF-8" string. By convention, most User IDs
are of the form "Display Name <email.address@example.com>", but they
do not need to be.
5. Input/Output Indirect Types
Some material is passed to "sop" indirectly, typically by referring
to a filename containing the data in question. This type of data may
also be passed to "sop" on Standard Input, or delivered by "sop" to
Standard Output.
If the filename for any indirect material used as input has the
special form "@ENV:xxx", then contents of environment variable "$xxx"
is used instead of looking in the filesystem.
If the filename for any indirect material used as either input or
output has the special form "@FD:nnn" where "nnn" is a decimal
integer, then the associated data is read from file descriptor "nnn".
If any input data does not meet the requirements described below,
"sop" will fail with a return code of 41.
5.1. CERTS
One or more OpenPGP certificates (section 11.1 of
[I-D.ietf-openpgp-rfc4880bis]), aka "Transferable Public Key". May
be armored.
Although some existing workflows may prefer to use one "CERTS" object
with multiple certificates in it (a "keyring"), supplying exactly one
certificate per "CERTS" input will make error reporting clearer and
easier.
5.2. KEY
Exactly one OpenPGP Transferable Secret Key (section 11.2 of
[I-D.ietf-openpgp-rfc4880bis]). May be armored.
Secret key material should be in cleartext (that is, it should not be
locked with a password). If the secret key maerial is locked with a
password, "sop" may fail to use the key.
Gillmor Expires May 1, 2020 [Page 12]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
5.3. CIPHERTEXT
"sop" accepts only a restricted subset of the arbitrarily-nested
grammar allowed by the OpenPGP Messages definition (section 11.3 of
[I-D.ietf-openpgp-rfc4880bis]).
In particular, it accepts and generates only:
An OpenPGP message, consisting of a sequence of PKESKs (section 5.1
of [I-D.ietf-openpgp-rfc4880bis]) and SKESKs (section 5.3 of
[I-D.ietf-openpgp-rfc4880bis]), followed by one SEIPD (section 5.14
of [I-D.ietf-openpgp-rfc4880bis]).
The SEIPD can decrypt into one of two things:
o "Maybe Signed Data" (see below), or
o Compressed data packet that contains "Maybe Signed Data"
"Maybe Signed Data" is a sequence of:
o N (zero or more) one-pass signature packets, followed by
o zero or more signature packets, followed by
o one Literal data packet, followed by
o N signature packets (corresponding to the outer one-pass
signatures packets)
FIXME: does any tool do compression inside signing? Do we need to
handle that?
May be armored.
5.4. SIGNATURE
One or more OpenPGP Signature packets. May be armored.
5.5. SESSIONKEY
This documentation uses the GnuPG defacto "ASCII" representation:
"ALGONUM:HEXKEY"
where "ALGONUM" is the decimal value associated with the OpenPGP
Symmetric Key Algorithms (section 9.3 of
[I-D.ietf-openpgp-rfc4880bis]).
Gillmor Expires May 1, 2020 [Page 13]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
Example AES-256 session key:
9:FCA4BEAF687F48059CACC14FB019125CD57392BAB7037C707835925CBF9F7BCD
5.6. PASSWORD
This is expected to be a "UTF-8" string, but for "sop decrypt", any
bytestring that the user supplies will be accepted. Note the details
in "sop encrypt" and "sop decrypt" about trailing whitespace!
5.7. VERIFICATIONS
One line per successful signature verification. Each line has three
structured fields delimited by a single space, followed by arbitrary
text to the end of the line.
o ISO-8601 UTC datestamp
o Fingerprint of the signing key (may be a subkey)
o Fingerprint of primary key of signing certificate (if signed by
primary key, same as the previous field)
o arbitrary text
Example:
2019-10-24T23:48:29Z C90E6D36200A1B922A1509E77618196529AE5FF8 C4BC2DDB38CCE96485EBE9C2F20691179038E5C6 certificate from dkg.asc
5.8. DATA
Cleartext, arbitrary data. This is either a bytestream or "UTF-8"
text.
It MUST only be "UTF-8" text in the case of input supplied to "sop
sign --as=text" or "sop encrypt --as={mime|text}". If "sop" receives
"DATA" containing non-"UTF-8" octets in this case, it will fail with
return code 53.
6. Failure modes
When "sop" succeeds, it will return 0 and emit nothing to Standard
Error. When "sop" fails, it fails with a non-zero return code, and
emits one or more warning messages on Standard Error. Known return
codes include:
Gillmor Expires May 1, 2020 [Page 14]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
+--------+----------------------------------------------------------+
| Return | Meaning |
+--------+----------------------------------------------------------+
| 0 | Success |
| | |
| 3 | No acceptable signatures found ("sop verify") |
| | |
| 13 | Asymmetric algorithm unsupported ("sop encrypt") |
| | |
| 17 | Certificate not encryption-capable (e.g., expired, |
| | revoked, unacceptable usage flags) ("sop encrypt") |
| | |
| 19 | Missing required argument |
| | |
| 23 | Incomplete verification instructions ("sop decrypt") |
| | |
| 29 | Unable to decrypt ("sop decrypt") |
| | |
| 31 | Non-"UTF-8" password ("sop encrypt") |
| | |
| 37 | Unsupported option |
| | |
| 41 | Invalid data type (no secret key where "KEY" expected, |
| | etc) |
| | |
| 53 | Non-text input where text expected |
| | |
| 69 | Unsupported subcommand |
+--------+----------------------------------------------------------+
A "sop" implementation MAY return other error codes than those listed
above.
7. Guidance for Implementors
"sop" uses a few assumptions that implementers might want to
consider.
7.1. One OpenPGP Message At a Time
"sop" is intended to be a simple tool that operates on one OpenPGP
object at a time. It should be composable, if you want to use it to
deal with multiple OpenPGP objects
FIXME: discuss what this means for streaming. The stdio interface
doesn't necessarily imply streamed output.
Gillmor Expires May 1, 2020 [Page 15]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
7.2. Simplified Subset of OpenPGP Message
While the formal grammar for OpenPGP Message is arbitrarily
nestable,"sop" constrains itself to what it sees as a single "layer"
(see Section 5.3).
This is a deliberate choice, because it is what most consumers
expect, and runaway recursion is bad news.
Note that an implementation of "sop decrypt" MAY choose to handle
more complex structures, but if it does, it should document the other
structures it handles and why it chooses to do so. We can use such
documentation to improve future versions of this spec.
7.3. Validate Signatures Only From Known Signers
There are generally only a few signers who are relevant for a given
OpenPGP message. When verifying signatures, "sop" expects that the
caller can identify those relevant signers ahead of time.
7.4. Detached Signatures
"sop" deals with detached signatures as the baseline form of OpenPGP
signatures.
The main problem this avoids is the trickiness of handling a
signature that is mixed inline into the data that it is signing.
7.5. Reliance on Supplied Certs and Keys
A truly stateless implementation may find that it spends more time
validating the internal consistency of certificates and keys than it
does on the actual object security operations.
For performance reasons, an implementation may choose to ignore
validation on certificate and key material supplied to it. The
security implications are of doing so depend on how the certs and
keys are managed outside of "sop".
8. Guidance for Consumers
While "sop" is originally conceived of as an interface for
interoperability testing, it's conceivable that an application that
uses OpenPGP for object security would want to use it.
FIXME: more guidance for how to use such a tool safely and
efficiently goes here.
Gillmor Expires May 1, 2020 [Page 16]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
FIXME: if an encrypted OpenPGP message arrives without metadata, it
is difficult to know which signers to consider when decrypting. How
do we do this efficiently without invoking "sop decrypt" twice, once
without "--verify-*" and again with the expected identity material?
9. Security Considerations
The OpenPGP object security model is typically used for
confidentiality and authenticity purposes.
9.1. Signature Verification
In many contexts, an OpenPGP signature is verified, to prove the
origin and integrity of an underlying object.
When "sop" checks a signature (e.g. via "sop verify" or "sop decrypt
--verify-with", it MUST NOT consider it to be verified unless all of
these conditions are met:
o The signature must be made by a signing-capable public key that is
present in one of the supplied certificates
o The certificate and signing subkey must have been created before
or at the signature time
o The cetificate and signing subkey must not have been expired at
the signature time
o The certificate and signing subkey must not be revoked with a
"hard" revocation
o If the certificate or signing subkey is revoked with a "soft"
revocation, then the signature time must predate the revocation
o The signing subkey must be properly bound to the primary key, and
cross-signed
o The signature (and any dependent signature, such as the cross-sig
or subkey binding signatures) must be made with strong
cryptographic algorithms (e.g., not "MD5" or a 1024-bit "RSA" key)
Implementers MAY also consider other factors in addition to the
origin and authenticity, including application-specific information.
For example, consider the application domain of checking software
updates.
Gillmor Expires May 1, 2020 [Page 17]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
If software package Foo version 13.3.2 was signed on 2019-10-04, and
the user receives a copy of Foo version 12.4.8 that was signed on
2019-10-16, it may be authentic and have a more recent signature
date. But it is not an upgrade (12.4.8 < 13.3.2), and therefore it
should not be applied automatically.
In such cases, it is critical that the application confirms that the
other information verified is _also_ protected by the relevant
OpenPGP signature.
Signature validity is a complex topic, and this documentation cannot
list all possible details.
9.2. Compression
The interface as currently specified does not allow for control of
compression. Compressing and encrypting data that may contain both
attacker-supplied material and sensitive material could leak
information about the sensitive material (see the CRIME attack).
Unless an application knows for sure that no attacker-supplied
material is present on the input, it should not compress during
encryption.
10. Privacy Considerations
Material produced by "sop encrypt" may be placed on an untrusted
machine (e.g., sent through the public "SMTP" network). That
material may contain metadata that leaks associational information
(e.g., recipient identifiers in PKESK packets). FIXME: document
things like PURBs and "--hidden-recipient")
10.1. Object Security vs. Transport Security
OpenPGP offers an object security model, but says little to nothing
about how the secured objects get to the relevant parties.
When sending or receiving OpenPGP material, the implementer should
consider what privacy leakage is implicit with the transport.
11. Document Considerations
[ RFC Editor: please remove this section before publication ]
This document is currently edited as markdown. Minor editorial
changes can be suggested via merge requests at
https://gitlab.com/dkg/openpgp-stateless-cli or by e-mail to the
Gillmor Expires May 1, 2020 [Page 18]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
authors. Please direct all significant commentary to the public IETF
OpenPGP mailing list: openpgp@ietf.org
11.1. Document History
substantive changes between -00 and -01:
o Changed "generate" subcommand to "generate-key"
o Changed "convert" subcommand to "extract-cert"
o Added "Input String Types" section as distinct from indirect I/O
o Made implicit arguments potentially explicit (e.g. "sop armor
--label=auto")
o Added "--allow-nested" to "sop armor" to make it idempotent by
default
o Added fingerprint of signing (sub)key to "VERIFICATIONS" output
o Dropped "--mode" and "--session-key" arguments for "sop encrypt"
(no plausible use, not needed for interop)
o Added "--with-session-key" argument to "sop decrypt" to allow for
session-key-based decryption
o Added examples to each subcommand
o More detailed error codes for "sop encrypt"
o Move from "CERT" to "CERTS" (each "CERTS" argument might contain
multiple certificates)
11.2. Future Work
o "detach-inband-signature-and-message" subcommand (split a
clearsigned message into a message and a detached signature) (see
Section 7.4
o certificate transformation into popular publication forms:
* WKD
* DANE OPENPGPKEY
* Autocrypt
Gillmor Expires May 1, 2020 [Page 19]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
o "sop encrypt" - specify compression? (see Section 9.2)
o "sop encrypt" - specify padding policy/mechanism?
o "sop decrypt" - how can it more safely handle zip bombs?
o "sop decrypt" - what should it do when encountering weakly-
encrypted (or unencrypted) input?
o "sop encrypt" - minimize metadata (e.g. "--throw-keyids")?
o handling secret keys that are locked with passwords?
o specify an error if a "DATE" arrives as input without a time zone?
o specify an error if a "sop" invocation sees multiple copies of a
specific "@FD:n" input (e.g., "sop sign @FD:3 @FD:3")
o add considerations about what it means for armored "CERTS" to
contain multiple certificates - multiple armorings? one big blob?
o do we need an interface or option (for performance?) with the
semantics that "sop" doesn't validate certificates internally, it
just accepts whatever's given as legit data? (see Section 7.5)
12. Acknowledgements
This work was inspired by Justus Winter's
[OpenPGP-Interoperability-Test-Suite].
The following people contributed helpful feedback and considerations
to this draft, but are not responsible for its problems:
o Justus Winter
o Vincent Breitmoser
o Edwin Taylor
o Jameson Rollins
o Allan Nordhoey
13. References
Gillmor Expires May 1, 2020 [Page 20]
Internet-Draft Stateless OpenPGP Command Line Interface October 2019
13.1. Normative References
[I-D.ietf-openpgp-rfc4880bis]
Koch, W., carlson, b., Tse, R., Atkins, D., and D.
Gillmor, "OpenPGP Message Format", draft-ietf-openpgp-
rfc4880bis-08 (work in progress), September 2019.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<https://www.rfc-editor.org/info/rfc4880>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
13.2. Informative References
[I-D.draft-bre-openpgp-samples-00]
Einarsson, B., juga, j., and D. Gillmor, "OpenPGP Example
Keys and Certificates", draft-bre-openpgp-samples-00 (work
in progress), October 2019.
[OpenPGP-Interoperability-Test-Suite]
"OpenPGP Interoperability Test Suite", October 2019,
<https://tests.sequoia-pgp.org/>.
Author's Address
Daniel Kahn Gillmor
American Civil Liberties Union
125 Broad St.
New York, NY 10004
USA
Email: dkg@fifthhorseman.net
Gillmor Expires May 1, 2020 [Page 21]