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.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.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 ", 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, . [RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R. Thayer, "OpenPGP Message Format", RFC 4880, DOI 10.17487/RFC4880, November 2007, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 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, . 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]