Network Working Group S. Thomas
Internet-Draft R. Reginelli
Intended status: Standards Track A. Hope-Bailie
Expires: April 27, 2017 Ripple
October 24, 2016
Crypto-Conditions
draft-thomas-crypto-conditions-01
Abstract
Crypto-conditions provide a mechanism to describe a signed message
such that multiple actors in a distributed system can all verify the
same signed message and agree on whether it matches the description.
This provides a useful primitive for event-based systems that are
distributed on the Internet since we can describe events in a
standard deterministic manner (represented by signed messages) and
therefore define generic authenticated event handlers.
Feedback
This specification is a part of the Interledger Protocol [1] work.
Feedback related to this specification should be sent to public-
interledger@w3.org [2].
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-
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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 April 27, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Features . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1. Multi-Algorithm . . . . . . . . . . . . . . . . . . . 4
1.2.2. Multi-Signature . . . . . . . . . . . . . . . . . . . 5
1.2.3. Multi-Level . . . . . . . . . . . . . . . . . . . . . 5
2. Format . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Binary Encoding . . . . . . . . . . . . . . . . . . . . . 5
2.2. String Types . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Bitmask . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Condition . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4.1. String Format . . . . . . . . . . . . . . . . . . . . 6
2.4.2. Binary Format . . . . . . . . . . . . . . . . . . . . 7
2.4.3. Fields . . . . . . . . . . . . . . . . . . . . . . . 7
2.4.4. Example Condition . . . . . . . . . . . . . . . . . . 8
2.5. Fulfillment . . . . . . . . . . . . . . . . . . . . . . . 9
2.5.1. String Format . . . . . . . . . . . . . . . . . . . . 9
2.5.2. Binary Format . . . . . . . . . . . . . . . . . . . . 9
2.5.3. Fields . . . . . . . . . . . . . . . . . . . . . . . 9
2.5.4. Example Fulfillment . . . . . . . . . . . . . . . . . 9
3. Feature Suites . . . . . . . . . . . . . . . . . . . . . . . 10
3.1. SHA-256 . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2. PREIMAGE . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3. PREFIX . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.4. THRESHOLD . . . . . . . . . . . . . . . . . . . . . . . . 12
3.5. RSA-PSS . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.6. ED25519 . . . . . . . . . . . . . . . . . . . . . . . . . 12
4. Condition Types . . . . . . . . . . . . . . . . . . . . . . . 12
4.1. PREIMAGE-SHA-256 . . . . . . . . . . . . . . . . . . . . 12
4.1.1. Condition . . . . . . . . . . . . . . . . . . . . . . 13
4.1.2. Fulfillment . . . . . . . . . . . . . . . . . . . . . 13
4.1.3. Example . . . . . . . . . . . . . . . . . . . . . . . 13
4.2. PREFIX-SHA-256 . . . . . . . . . . . . . . . . . . . . . 13
4.2.1. Condition . . . . . . . . . . . . . . . . . . . . . . 14
4.2.2. Fulfillment . . . . . . . . . . . . . . . . . . . . . 14
4.2.3. Example . . . . . . . . . . . . . . . . . . . . . . . 14
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4.3. THRESHOLD-SHA-256 . . . . . . . . . . . . . . . . . . . . 14
4.3.1. Condition . . . . . . . . . . . . . . . . . . . . . . 14
4.3.2. Fulfillment . . . . . . . . . . . . . . . . . . . . . 15
4.3.3. Example . . . . . . . . . . . . . . . . . . . . . . . 16
4.4. RSA-SHA-256 . . . . . . . . . . . . . . . . . . . . . . . 16
4.4.1. Condition . . . . . . . . . . . . . . . . . . . . . . 17
4.4.2. Fulfillment . . . . . . . . . . . . . . . . . . . . . 18
4.4.3. Implementation . . . . . . . . . . . . . . . . . . . 18
4.4.4. Example . . . . . . . . . . . . . . . . . . . . . . . 18
4.5. ED25519 . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.5.1. Condition . . . . . . . . . . . . . . . . . . . . . . 19
4.5.2. Fulfillment . . . . . . . . . . . . . . . . . . . . . 19
4.5.3. Example . . . . . . . . . . . . . . . . . . . . . . . 19
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.1. Normative References . . . . . . . . . . . . . . . . . . 19
5.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Security Considerations . . . . . . . . . . . . . . 21
Appendix B. Test Values . . . . . . . . . . . . . . . . . . . . 21
Appendix C. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 21
Appendix D. IANA Considerations . . . . . . . . . . . . . . . . 24
D.1. Crypto-Condition Type Registry . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
This specification describes a message format for crypto-conditions
and fulfillments, with binary and string-based encodings for each.
Crypto-conditions are *distributable event descriptions*. This means
crypto-conditions say how to recognize a message without saying
exactly what the message is. You can transmit a crypto-condition
freely, but you cannot forge the message it describes. For
convenience, we hash the description so that the crypto-condition can
be a fixed size.
Fulfillments are *cryptographically verifiable messages* that prove
an event occurred. If you transmit a fulfillment, then everyone who
has the condition can agree that the condition has been met.
In the Interledger protocol, crypto-conditions and fulfillments
provide irrepudiable proof that a transfer occurred in one ledger, as
messages that can be easily shared with other ledgers. This allows
ledgers to escrow funds or hold a transfer conditionally, then
execute the transfer automatically when the ledger sees the
fulfillment of the stated condition.
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Crypto-conditions may also be useful in other contexts where a system
needs to make a decision based on predefined criteria, such as smart
contracts.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Within this specification, the term "condition" refers to the hash of
a description of a signed message. The hash function must be
preimage-resistant.
The term "fulfillment" refers to a description of a signed message
and a signed message that matches the description. We hash the
description and compare that to the condition, and also compare the
signed message to the description. If the message matches the
description and the hash of the description matches the condition, we
say that the fulfillment fulfills the condition.
In the simplest case, the fulfillment can be a preimage that hashes
to the condition, in which case the preimage is both the description
and the message.
1.2. Features
Crypto-conditions are a standard format for expressing conditions and
fulfillments. The format supports multiple algorithms, including
different hash functions and cryptographic signing schemes. Crypto-
conditions can be nested in multiple levels, with each level possibly
having multiple signatures.
This format has been designed so that it can be expanded. For
example, you can add new cryptographic signature schemes or hash
functions. This is important because advances in cryptography
frequently render old algorithms insecure or invent newer, more
effective algorithms.
The Section 2.3 of a crypto-condition indicates which algorithms it
uses, so a compliant implementation can know whether it supports the
functionality required to interpret the crypto-condition.
1.2.1. Multi-Algorithm
The crypto-condition format contains a Section 2.3 that specifies
which hash function and signing scheme to use. Any message format
for a condition or a fulfillment contains such a bitmask.
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Implementations MAY state their supported algorithms by providing a
bitmask in the same format. To verify that a given implementation
can verify a fulfillment for a given condition, you compare the
bitmasks. If all bits set in the condition's bitmask are also set in
the implementation's bitmask, then the implementation can verify the
condition's fulfillment.
1.2.2. Multi-Signature
Crypto-conditions can abstract away many of the details of multi-
sign. When a party provides a condition, other parties can treat it
opaquely and do not need to know about its internal structure. That
allows parties to define arbitrary multi-signature setups without
breaking compatibility.
Protocol designers can use crypto-conditions as a drop-in replacement
for public key signature algorithms and add multi-signature support
to their protocols without adding any additional complexity.
1.2.3. Multi-Level
Crypto-conditions elegantly support weighted multi-signatures and
multi-level signatures. A threshold condition has a number of
weighted subconditions, and a target threshold. Each subcondition
can be a signature or another threshold condition. This provides
flexibility in forming complex conditions.
For example, consider a threshold condition that consists of two
subconditions, one each from Agnes and Bruce. Agnes's condition can
be a signature condition while Bruce's condition is a threshold
condition, requiring both Claude and Dan to sign for him.
Weighted signatures allow more complex relationships than simple
M-of-N signing. For example, a weighted condition can support an
arrangement of subconditions such as, "Either Ron, Adi, and Leonard
must approve; or Clifford must approve."
2. Format
2.1. Binary Encoding
A description of crypto-conditions is provided in this document using
Abstract Syntax Notation One (ASN.1) as defined in [itu.X680.2015].
Implementations of this spec MUST support encoding and decoding using
Octet Encoding Rules (OER) as defined in [itu.X696.2015].
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2.2. String Types
Crypto-conditions use the following types within string encoding:
BASE10 Variable-length integer encoded as a base-10 (decimal)
number. Implementations MUST reject encoded values that are too
large for them to parse. Implementations MUST be tested for
overflows.
BASE16 Variable-length integer encoded as a base-16 (hexadecimal)
number. Implementations MUST reject encoded values that are too
large for them to parse. Implementations MUST be tested for
overflows. No leading zeros.
BASE64URL Base64-URL encoding. See [RFC4648], Section 5.
2.3. Bitmask
Any system that accepts crypto-conditions must be able to state its
supported algorithms. It must be possible to verify that all
algorithms used in a certain condition are indeed supported even if
the fulfillment is not available yet. Therefore, all conditions and
fulfillments contain a bitmask to express the required features.
Implementations provide a bitmask of features they support.
Each bit represents a different suite of features. Each type of
crypto-condition depends on one or more feature suites. If an
implementation supports all feature suites that a certain type
depends on, the implementation MUST support that condition type. The
list of known types and feature suites is the IANA-maintained Crypto-
Condition Type Registry (Appendix D.1) .
To save space, the bitmask is encoded as a variable-length integer.
2.4. Condition
Below are the string and binary encoding formats for a condition.
2.4.1. String Format
Conditions are ASCII encoded as:
"cc:" BASE16(type) ":" BASE16(featureBitmask) ":"
BASE64URL(fingerprint) ":" BASE10(maxFulfillmentLength)
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2.4.2. Binary Format
Conditions are binary encoded as:
Condition ::= SEQUENCE {
type ConditionType,
featureBitmask INTEGER (0..MAX),
fingerprint OCTET STRING,
maxFulfillmentLength INTEGER (0..MAX)
}
ConditionType ::= INTEGER {
preimageSha256(0),
rsaSha256(1),
prefixSha256(2),
thresholdSha256(3),
ed25519(4)
} (0..65535)
2.4.3. Fields
type is the numeric type identifier representing the condition type.
featureBitmask is an unsigned integer encoding the set of feature
suites an implementation must support in order to be able to
successfully parse the fulfillment to this condition. This is the
boolean OR of the featureBitmask values of the top-level condition
type and all subcondition types, recursively.
fingerprint is an octet string uniquely representing the condition
with respect to other conditions of the same type.
Implementations which index conditions MUST use the entire string
or binary encoded condition as the key, not just the fingerprint -
as different conditions of different types may have the same
fingerprint. The length and contents of the fingerprint are
defined by the condition type. For most condition types, the
fingerprint is a cryptographically secure hash of the data which
defines the condition, such as a public key. This is encoded as a
variable length octet string as different condition types may use
different functions to produce the fingerprint which may therefore
have different lengths. While it would be possible to determine
the expected length of the fingerprint based on the type it is
useful to be able to decode a condition even if the type is not
recognized.
maxFulfillmentLength is the maximum length of the fulfillment
payload that can fulfill this condition, in bytes. The payload
size is measured unencoded. (The size of the payload is larger in
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BASE64URL format.) When a crypto-condition is submitted to an
implementation, this implementation MUST verify that it will be
able to process a fulfillment with a payload of size
maxFulfillmentLength.
2.4.4. Example Condition
An example condition in string format:
cc:0:3:dB-8fb14MdO75Brp_Pvh4d7ganckilrRl13RS_UmrXA:66
The example has the following attributes:
+---------------+---------------------------------+-----------------+
| Field | Value | Description |
+---------------+---------------------------------+-----------------+
| preface | "cc" | Constant. |
| | | Indicates this |
| | | is a condition. |
| | | |
| type | "0" | Type 0 is |
| | | [PREIMAGE- |
| | | SHA-256][]. |
| | | |
| featuresBitma | "3" | Boolean-OR |
| sk | | combination of |
| | | feature suites |
| | | SHA-256 |
| | | (feature bit |
| | | 0x01) and |
| | | PREIMAGE |
| | | (feature bit |
| | | 0x02). |
| | | |
| fingerprint | "dB-8fb14MdO75Brp_Pvh4d7ganckil | The hash of the |
| | rRl13RS_UmrXA" | fulfillment for |
| | | this condition. |
| | | |
| maxFulfillmen | "66" | The fulfillment |
| tLength | | payload is 66 |
| | | bytes long, |
| | | before being |
| | | BASE64URL- |
| | | encoded. |
+---------------+---------------------------------+-----------------+
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2.5. Fulfillment
Below are the string and binary encoding formats for a fulfillment.
2.5.1. String Format
Fulfillments are ASCII encoded as:
"cf:" BASE16(type) ":" BASE64URL(payload)
2.5.2. Binary Format
Fulfillments are binary encoded as:
Fulfillment ::= SEQUENCE {
type ConditionType,
payload OCTET STRING
}
2.5.3. Fields
type is the numeric type identifier representing the condition type.
For some condition types the fulfillment will contain further
subfulfillments, however the type field always represents the
outermost, or top-level, type.
payload The payload is an octet string whose internal format is
defined by each of the types.
2.5.4. Example Fulfillment
The following is an example fulfillment in string format, for the
example condition (Section 2.4.4):
cf:0:VGhlIG9ubHkgYmFzaXMgZm9yIGdvb2QgU29jaWV0eSBpcyB1bmxpbWl0ZWQgY3JlZGl0LuKAlE9zY2FyIFdpbGRl
The example has the following attributes:
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+---------+------------------+--------------------------------------+
| Field | Value | Description |
+---------+------------------+--------------------------------------+
| preface | "cf" | Constant. Indicates this is a |
| | | fulfillment. |
| | | |
| type | "0" | Type 0 is [PREIMAGE-SHA-256][]. |
| | | |
| payload | "VGhlIG...pbGRl" | The BASE64URL-encoded SHA-256 |
| | | preimage of the condition, since |
| | | this is a PREIMAGE-SHA-256 type |
| | | fulfillment. In this case, it is an |
| | | arbitrary string. |
+---------+------------------+--------------------------------------+
3. Feature Suites
This specification defines a starting set of feature suites necessary
to support the [Condition Types][] also defined in this
specification. Future versions of this spec MAY introduce new
feature suites and condition types, which SHALL be registered in the
IANA maintained Crypto-Condition Type Registry (Appendix D.1).
Support for a condition type MUST depend on one or more feature
suites. However, all new condition types MUST depend on at least one
of the new feature suites. This ensures that all previously created
implementations correctly recognize that they do not support the new
type.
Feature suites are chosen such that they represent reasonable
clusters of functionality. For instance, it is reasonable to require
that an implementation which supports SHA-256 in one context MUST
support it in all contexts, since it already needed to implement the
algorithm.
An implementation which supports a certain set of feature suites MUST
accept all condition types which depend only on that set or any
subset of feature suites.
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+-----------+---------+-----------+---------------------------------+
| Suite | Feature | Feature | Summary |
| Name | Bit | Bit | |
| | | (BASE16) | |
+-----------+---------+-----------+---------------------------------+
| SHA-256 | 2^0 | 0x01 | The SHA-256 hashing algorithm. |
| | | | |
| PREIMAGE | 2^1 | 0x02 | The functionality of comparing |
| | | | a hash to a preimage. |
| | | | |
| PREFIX | 2^2 | 0x04 | The functionality of prefixing |
| | | | the fulfillment with a prefix |
| | | | before generating the |
| | | | condition. |
| | | | |
| THRESHOLD | 2^3 | 0x08 | The functionality of composing |
| | | | a condition out of several |
| | | | weighted subconditions. |
| | | | |
| RSA-PSS | 2^4 | 0x10 | The RSA-PSS signature |
| | | | algorithm. |
| | | | |
| ED25519 | 2^5 | 0x20 | The ED25519 signature |
| | | | algorithm. |
+-----------+---------+-----------+---------------------------------+
3.1. SHA-256
The SHA-256 feature suite provides the SHA-256 hash function.
SHA-256 is a cryptographic hash function published by the US National
Institute of Standards and Technology that produces 256 bits of
output. This feature suite is assigned the feature bit 2^0 = 0x01.
3.2. PREIMAGE
The PREIMAGE feature suite provides conditions that use a preimage as
a one-time signature. This feature suite is assigned the feature bit
2^1 = 0x02.
The fingerprint of a preimage condition is the hash of an arbitrary
value. The payload of a preimage fulfillment is the hashed arbitrary
value before hashing, also known as the preimage. Conditions that
use this preimage MUST also rely on a cryptographically secure
hashing algorithm. Since cryptographically secure hashing functions
are preimage-resistant, only the original creator of a preimage
condition can produce the preimage, as long as it contains a large
amount of random entropy.
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3.3. PREFIX
The PREFIX feature suite provides conditions that prepend a fixed
message to a subcondition. This feature suite is assigned the
feature bit 2^2 = 0x04.
A prefix condition prepends the message to be validated with a
constant string before passing it on to the subcondition's validation
function.
3.4. THRESHOLD
The THRESHOLD feature suite provides conditions that have several
weighted subconditions and a threshold number. This feature suite is
assigned the feature bit 2^3 = 0x08.
Threshold conditions provide flexible multi-signing, such as
requiring "M-of-N" subconditions be fulfilled. Subconditions can
also be weighted so that one subcondition can count multiple times
towards meeting the threshold.
3.5. RSA-PSS
The RSA-PSS feature suite provides the RSS-PSA signature algorithm.
RSA-PSS is a signature algorithm based on the RSA cryptosystem, which
relates to the problem of factoring the product of two large prime
numbers. This feature suite is assigned the feature bit 2^4 = 0x10.
3.6. ED25519
The ED25519 feature suite provides the Ed25519 signature algorithm.
Ed25519 is a signature algorithm based on the EdDSA signing scheme
and the compact elliptic curve known as Ed25519. This feature suite
is assigned the feature bit 2^5 = 0x20.
4. Condition Types
The following condition types are defined in this version of the
specification. Future versions of this spec MAY introduce new
feature suites and condition types, which SHALL be registered in the
IANA maintained Crypto-Condition Type Registry (Appendix D.1).
4.1. PREIMAGE-SHA-256
PREIMAGE-SHA-256 is assigned the type ID 0. It relies on the SHA-256
and PREIMAGE feature suites which corresponds to a feature bitmask of
0x03.
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This type of condition is also called a "hashlock". By creating a
hash of a difficult-to-guess 256-bit random or pseudo-random integer
it is possible to create a condition which the creator can trivially
fulfill by publishing the random value. However, for anyone else,
the condition is cryptographically hard to fulfill, because they
would have to find a preimage for the given condition hash.
Implementations MUST ignore any input message when validating a
PREIMAGE-SHA-256 fulfillment.
4.1.1. Condition
The fingerprint of a PREIMAGE-SHA-256 condition is the SHA-256 hash
of the preimage.
4.1.2. Fulfillment
The fulfillment payload of a PREIMAGE-SHA-256 condition is the
preimage.
4.1.3. Example
Example condition:
cc:0:3:dB-8fb14MdO75Brp_Pvh4d7ganckilrRl13RS_UmrXA:66
Example fulfillment:
cf:0:VGhlIG9ubHkgYmFzaXMgZm9yIGdvb2QgU29jaWV0eSBpcyB1bmxpbWl0ZWQgY3JlZGl0LuKAlE9zY2FyIFdpbGRl
4.2. PREFIX-SHA-256
PREFIX-SHA-256 is assigned the type ID 1. It relies on the SHA-256
and PREFIX feature suites which corresponds to a feature bitmask of
0x05.
Prefix conditions provide a way to effective narrow the scope of
other conditions. A condition can be used as the fingerprint of a
public key to sign an arbitrary message. By creating a prefix
subcondition we can narrow the scope from signing an arbitrary
message to signing a message with a specific prefix.
When a prefix fulfillment is validated against a message, it will
prepend the prefix to the provided message and will use the result as
the message to validate against the subfulfillment.
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4.2.1. Condition
The fingerprint of a PREFIX-SHA-256 condition is the SHA-256 digest
of the fingerprint contents given below:
PrefixSha256FingerprintContents ::= SEQUENCE {
prefix OCTET STRING,
condition Condition
}
prefix is an arbitrary octet string which will be prepended to the
message during validation.
condition is the subcondition which the subfulfillment must match.
4.2.2. Fulfillment
PrefixSha256FulfillmentPayload ::= SEQUENCE {
prefix OCTET STRING,
subfulfillment Fulfillment
}
prefix is an arbitrary octet string which will be prepended to the
message during validation.
subfulfillment is the fulfilled subcondition.
4.2.3. Example
Example condition:
cc:1:25:7myveZs3EaZMMuez-3kq6u69BDNYMYRMi_VF9yIuFLc:102
Example fulfillment:
cf:1:DUhlbGxvIFdvcmxkISAABGDsFyuTrV5WO_STLHDhJFA0w1Rn7y79TWTr-BloNGfiv7YikfrZQy-PKYucSkiV2-KT9v_aGmja3wzN719HoMchKl_qPNqXo_TAPqny6Kwc7IalHUUhJ6vboJ0bbzMcBwo
4.3. THRESHOLD-SHA-256
THRESHOLD-SHA-256 is assigned the type ID 2. It relies on the
SHA-256 and THRESHOLD feature suites which corresponds to a feature
bitmask of 0x09.
4.3.1. Condition
The fingerprint of a THRESHOLD-SHA-256 condition is the SHA-256
digest of the fingerprint contents given below:
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ThresholdSha256FingerprintContents ::= SEQUENCE {
threshold INTEGER (0..4294967295),
subconditions SEQUENCE OF ThresholdSubcondition
}
ThresholdSubcondition ::= SEQUENCE {
weight INTEGER (0..4294967295),
condition Condition
}
The list of conditions is sorted first based on length, shortest
first. Elements of the same length are sorted in lexicographic (big-
endian) order, smallest first.
threshold threshold MUST be an integer in the range 1 ... 2^32 - 1.
In order to fulfill a threshold condition, the weights of the
provided fulfillments MUST be greater than or equal to the
threshold.
subconditions is the set of subconditions, each provided as a tuple
of weight and condition.
weight is the numeric weight of this subcondition, i.e. how many
times it counts against the threshold.
condition is the subcondition.
4.3.2. Fulfillment
ThresholdSha256FulfillmentPayload ::= SEQUENCE {
threshold INTEGER (0..4294967295),
subfulfillments SEQUENCE OF ThresholdSubfulfillment
}
ThresholdSubfulfillment ::= SEQUENCE {
weight INTEGER (0..4294967295) DEFAULT 1,
condition Condition OPTIONAL,
fulfillment Fulfillment OPTIONAL
}
threshold is a number and MUST be an integer in the range 1 ... 2^32
- 1. In order to fulfill a threshold condition, the weights of
the provided fulfillments MUST be greater than or equal to the
threshold.
subfulfillments is the set of subconditions and subfulfillments,
each provided as a tuple of weight and condition or fulfillment.
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weight is the numeric weight of this subcondition, i.e. how many
times it counts against the threshold. It MUST be an integer in
the range 1 ... 2^32 - 1.
condition is the subcondition if this subcondition is not fulfilled.
fulfillment is the subfulfillment if this subcondition is fulfilled.
4.3.3. Example
Example condition:
cc:2:2b:mJUaGKCuF5n-3tfXM2U81VYtHbX-N8MP6kz8R-ASwNQ:146
Example fulfillment:
cf:2:AQEBAgEBAwAAAAABAQAnAAQBICDsFyuTrV5WO_STLHDhJFA0w1Rn7y79TWTr-BloNGfivwFg
4.4. RSA-SHA-256
RSA-SHA-256 is assigned the type ID 3. It relies on the SHA-256 and
RSA-PSS feature suites which corresponds to a feature bitmask of
0x11.
The signature algorithm used is RSASSA-PSS as defined in PKCS#1 v2.2.
[RFC3447]
Implementations MUST NOT use the default RSASSA-PSS-params.
Implementations MUST use the SHA-256 hash algorithm and therefor, the
same algorithm in the mask generation algorithm, as recommended in
[RFC3447]. Implementations MUST also use a salt length of 32 bytes
(equal to the size of the output from the SHA-256 algorithm).
Therefore the algorithm identifier will have the following value:
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rSASSA-PSS-Crypto-Conditions-Identifier RSASSA-AlgorithmIdentifier ::= {
algorithm id-RSASSA-PSS,
parameters RSASSA-PSS-params : {
hashAlgorithm sha256,
maskGenAlgorithm mgf1SHA256,
saltLength 32,
trailerField trailerFieldBC
}
}
sha256 HashAlgorithm ::= {
algorithm id-sha256,
parameters NULL
}
mgf1SHA256 MaskGenAlgorithm ::= {
algorithm id-mgf1,
parameters HashAlgorithm : sha256
}
4.4.1. Condition
The fingerprint of a RSA-SHA-256 condition is the SHA-256 digest of
the fingerprint contents given below:
RsaSha256FingerprintContents ::= SEQUENCE {
modulus OCTET STRING (SIZE(128..512))
}
modulus is an octet string representing the RSA public modulus in
big-endian byte order. The first byte of the modulus MUST NOT be
zero.
The corresponding public exponent e is assumed to be 65537 as
recommended in [RFC4871] . Very large exponents can be a DoS
vector [LARGE-RSA-EXPONENTS] and 65537 is the largest Fermat
prime, which has some nice properties
[USING-RSA-EXPONENT-OF-65537] .
Implementations MUST reject moduli smaller than 128 bytes (1017
bits) or greater than 512 bytes (4096 bits.) Large moduli slow
down signature verification which can be a denial-of-service
vector. DNSSEC also limits the modulus to 4096 bits [RFC3110] .
OpenSSL supports up to 16384 bits [OPENSSL-X509-CERT-EXAMPLES] .
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4.4.2. Fulfillment
RsaSha256FulfillmentPayload ::= SEQUENCE {
modulus OCTET STRING (SIZE(128..512)),
signature OCTET STRING (SIZE(128..512))
}
modulus is an octet string representing the RSA public modulus in
big-endian byte order. See Section 4.4.1
signature is an octet string representing the RSA signature. It
MUST be encoded in big-endian byte order with the exact same
number of octets as the modulus, even if this means adding leading
zeros. This ensures that the fulfillment size is constant and
known ahead of time. Note that the field is still binary encoded
with a length prefix for consistency.
Implementations MUST verify that the signature and modulus consist
of the same number of octets and that the signature is numerically
less than the modulus.
The message to be signed is provided separately. If no message is
provided, the message is assumed to be an octet string of length
zero.
4.4.3. Implementation
The recommended modulus size as of 2016 is 2048 bits
[KEYLENGTH-RECOMMENDATION] . In the future we anticipate an upgrade
to 3072 bits which provides approximately 128 bits of security
[NIST-KEYMANAGEMENT] (p. 64), about the same level as SHA-256.
4.4.4. Example
Example condition:
cc:3:11:Bw-r77AGqSCL0huuMQYj3KW0Jh67Fpayeq9h_4UJctg:260
Example fulfillment:
cf:3:gYCzDnqTh4O6v4NoUP9J4U-H4_ktXEbjP-yj5PCyI1hYCxF2WZX0uO6n-0cSwuHjFvf3dalT0jIhahadmmTdwAcSCkALN_KvwHe2L-ME3nTeahGexAdrUpxPYJawuq1PUz3wFzubgi_YXWX6S--pLY9ST2nLygE2vYDQlcFprsDglYGAjQM0-Z5B-953uQtJ5dXL1D5TWpM0s0eFF0Zty7J2Y3Nb0PqsR5I47a2wYlA7-106vjC8gHFdHVeSR6JksSrhj8YaMWfV0A6qhPz6hq-TqSKCXd4mf3eCpyyFYR_EyH5zXd56sJEU3snWlFbB_bKAW4si_qdfY9dT87YGUp_Grm0
4.5. ED25519
ED25519 is assigned the type ID 4. It relies only on the ED25519
feature suite which corresponds to a bitmask of 0x20.
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The exact algorithm and encodings used for public key and signature
are defined in [I-D.irtf-cfrg-eddsa] as Ed25519. SHA-512 is used as
the hashing function.
Note: This document is not defining the SHA-512 versions of other
condition types. In addition, the Ed25519 condition type is already
uniquely identified by a corresponding Ed25519 feature suite.
Therefore we intentionally do not introduce a SHA-512 feature suite
in this document.
4.5.1. Condition
The fingerprint of a ED25519 condition is the 32 byte Ed25519 public
key. Since the public key is already very small, we do not hash it.
4.5.2. Fulfillment
Ed25519FulfillmentPayload ::= SEQUENCE {
publicKey OCTET STRING (SIZE(32)),
signature OCTET STRING (SIZE(64))
}
publicKey is an octet string containing the Ed25519 public key.
signature is an octet string containing the Ed25519 signature.
4.5.3. Example
Example condition:
cc:4:20:7Bcrk61eVjv0kyxw4SRQNMNUZ-8u_U1k6_gZaDRn4r8:96
Example fulfillment:
cf:4:7Bcrk61eVjv0kyxw4SRQNMNUZ-8u_U1k6_gZaDRn4r-2IpH62UMvjymLnEpIldvik_b_2hpo2t8Mze9fR6DHISpf6jzal6P0wD6p8uisHOyGpR1FISer26CdG28zHAcK
5. References
5.1. Normative References
[I-D.irtf-cfrg-eddsa]
Josefsson, S. and I. Liusvaara, "Edwards-curve Digital
Signature Algorithm (EdDSA)", draft-irtf-cfrg-eddsa-04
(work in progress), March 2016.
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[itu.X680.2015]
International Telecommunications Union, "Information
technology - Abstract Syntax Notation One (ASN.1):
Specification of basic notation", August 2015,
<https://www.itu.int/rec/T-REC-X.680-201508-I/>.
[itu.X696.2015]
International Telecommunications Union, "Information
technology - ASN.1 encoding rules: Specification of Octet
Encoding Rules (OER)", August 2015,
<http://handle.itu.int/11.1002/1000/12487>.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
2003, <http://www.rfc-editor.org/info/rfc3447>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
5.2. Informative References
[KEYLENGTH-RECOMMENDATION]
"BlueKrypt - Cryptographic Key Length Recommendation",
September 2015, <https://www.keylength.com/en/compare/>.
[LARGE-RSA-EXPONENTS]
"Imperial Violet - Very large RSA public exponents (17 Mar
2012)", March 2012,
<https://www.imperialviolet.org/2012/03/17/rsados.html>.
[NIST-KEYMANAGEMENT]
, , , , and , "NIST - Recommendation for Key Management -
Part 1 - General (Revision 3)", July 2012,
<http://csrc.nist.gov/publications/nistpubs/800-57/
sp800-57_part1_rev3_general.pdf>.
[OPENSSL-X509-CERT-EXAMPLES]
"OpenSSL - X509 certificate examples for testing and
verification", July 2012,
<http://fm4dd.com/openssl/certexamples.htm>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
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[RFC3110] Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the
Domain Name System (DNS)", RFC 3110, DOI 10.17487/RFC3110,
May 2001, <http://www.rfc-editor.org/info/rfc3110>.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, DOI 10.17487/RFC4871, May 2007,
<http://www.rfc-editor.org/info/rfc4871>.
[USING-RSA-EXPONENT-OF-65537]
"Cryptography - StackExchange - Impacts of not using RSA
exponent of 65537", November 2014,
<https://crypto.stackexchange.com/questions/3110/impacts-
of-not-using-rsa-exponent-of-65537>.
Appendix A. Security Considerations
This section to be expanded in a later draft.
Appendix B. Test Values
This section to be expanded in a later draft. For now, see the test
cases for the reference implementation:
https://github.com/interledger/five-bells-condition/tree/master/test
Appendix C. ASN.1 Module
--<ASN1.PDU CryptoConditions.Condition, CryptoConditions.Fulfillment>--
CryptoConditions
DEFINITIONS
AUTOMATIC TAGS ::=
BEGIN
/**
* CONTAINERS
*/
Condition ::= SEQUENCE {
type ConditionType,
featureBitmask OCTET STRING,
fingerprint OCTET STRING,
maxFulfillmentLength INTEGER (0..MAX)
}
Fulfillment ::= SEQUENCE {
type ConditionType,
payload OCTET STRING
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}
ConditionType ::= INTEGER {
preimageSha256(0),
rsaSha256(1),
prefixSha256(2),
thresholdSha256(3),
ed25519(4)
} (0..65535)
/**
* FULFILLMENT PAYLOADS
*/
-- For preimage conditions, the payload equals the preimage
PrefixSha256FulfillmentPayload ::= SEQUENCE {
prefix OCTET STRING,
subfulfillment Fulfillment
}
ThresholdSha256FulfillmentPayload ::= SEQUENCE {
threshold INTEGER (0..4294967295),
subfulfillments SEQUENCE OF ThresholdSubfulfillment
}
ThresholdSubfulfillment ::= SEQUENCE {
weight INTEGER (0..4294967295) DEFAULT 1,
condition Condition OPTIONAL,
fulfillment Fulfillment OPTIONAL
}
RsaSha256FulfillmentPayload ::= SEQUENCE {
modulus OCTET STRING (SIZE(128..512)),
signature OCTET STRING (SIZE(128..512))
}
Ed25519FulfillmentPayload ::= SEQUENCE {
publicKey OCTET STRING (SIZE(32)),
signature OCTET STRING (SIZE(64))
}
/**
* FINGERPRINTS
*/
-- SHA-256 hash of the fingerprint contents
Sha256Fingerprint ::= OCTET STRING (SIZE(32)) -- digest
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-- 32-byte Ed25519 public key
Ed25519Fingerprint ::= OCTET STRING (SIZE(32)) -- publicKey
/**
* FINGERPRINT CONTENTS
*
* The content that will be hashed to arrive at the fingerprint.
*/
-- The preimage type hashes the raw contents of the preimage
PrefixSha256FingerprintContents ::= SEQUENCE {
prefix OCTET STRING,
condition Condition
}
ThresholdSha256FingerprintContents ::= SEQUENCE {
threshold INTEGER (0..4294967295),
subconditions SEQUENCE OF ThresholdSubcondition
}
ThresholdSubcondition ::= SEQUENCE {
weight INTEGER (0..4294967295),
condition Condition
}
RsaSha256FingerprintContents ::= INTEGER (0..MAX) -- modulus
/**
* EXAMPLES
*/
exampleCondition Condition ::=
{
type preimageSha256,
featureBitmask '03'H,
fingerprint '
E3B0C442 98FC1C14 9AFBF4C8 996FB924 27AE41E4 649B934C A495991B 7852B855
'H,
maxFulfillmentLength 2
}
exampleFulfillment Fulfillment ::=
{
type preimageSha256,
payload '00'H
}
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exampleRsaSha256FulfillmentPayload RsaSha256FulfillmentPayload ::=
{
modulus '
B30E7A93 8783BABF 836850FF 49E14F87 E3F92D5C 46E33FEC A3E4F0B2 2358580B
11765995 F4B8EEA7 FB4712C2 E1E316F7 F775A953 D232216A 169D9A64 DDC00712
0A400B37 F2AFC077 B62FE304 DE74DE6A 119EC407 6B529C4F 6096B0BA AD4F533D
F0173B9B 822FD85D 65FA4BEF A92D8F52 4F69CBCA 0136BD80 D095C169 AEC0E095
'H,
signature '
48E8945E FE007556 D5BF4D5F 249E4808 F7307E29 511D3262 DAEF61D8 8098F9AA
4A8BC062 3A8C9757 38F65D6B F459D543 F289D73C BC7AF4EA 3A33FBF3 EC444044
7911D722 94091E56 1833628E 49A772ED 608DE6C4 4595A91E 3E17D6CF 5EC3B252
8D63D2AD D6463989 B12EEC57 7DF64709 60DF6832 A9D84C36 0D1C217A D64C8625
BDB594FB 0ADA086C DECBBDE5 80D424BF 9746D2F0 C312826D BBB00AD6 8B52C4CB
7D47156B A35E3A98 1C973863 792CC80D 04A18021 0A524158 65B64B3A 61774B1D
3975D78A 98B0821E E55CA0F8 6305D425 29E10EB0 15CEFD40 2FB59B2A BB8DEEE5
2A6F2447 D2284603 D219CD4E 8CF9CFFD D5498889 C3780B59 DD6A57EF 7D732620
'H
}
exampleEd25519FulfillmentPayload Ed25519FulfillmentPayload ::=
{
publicKey '
EC172B93 AD5E563B F4932C70 E1245034 C35467EF 2EFD4D64 EBF81968 3467E2BF
'H,
signature '
B62291FA D9432F8F 298B9C4A 4895DBE2 93F6FFDA 1A68DADF 0CCDEF5F 47A0C721
2A5FEA3C DA97A3F4 C03EA9F2 E8AC1CEC 86A51D45 2127ABDB A09D1B6F 331C070A
'H
}
END
Appendix D. IANA Considerations
D.1. Crypto-Condition Type Registry
The following initial entries should be added to the Crypto-Condition
Type registry to be created and maintained at (the suggested URI)
http://www.iana.org/assignments/crypto-condition-types :
The following feature suite bits are registered:
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+----------+------+------+-----------+
| Type Bit | Exp. | Hex | Feature |
+----------+------+------+-----------+
| 1 | 2^0 | 0x01 | SHA-256 |
| | | | |
| 10 | 2^1 | 0x02 | PREIMAGE |
| | | | |
| 100 | 2^2 | 0x04 | PREFIX |
| | | | |
| 1000 | 2^3 | 0x08 | THRESHOLD |
| | | | |
| 10000 | 2^4 | 0x10 | RSA |
| | | | |
| 100000 | 2^5 | 0x20 | ED25519 |
+----------+------+------+-----------+
Table 1: Crypto-Condition Feature Suites
The following types are registered:
+---------+------------------+-------------------+
| Type ID | Required Bitmask | Type Name |
+---------+------------------+-------------------+
| 0 | 0x03 | PREIMAGE-SHA-256 |
| | | |
| 1 | 0x05 | PREFIX-SHA-256 |
| | | |
| 2 | 0x09 | THRESHOLD-SHA-256 |
| | | |
| 3 | 0x11 | RSA-SHA-256 |
| | | |
| 4 | 0x20 | ED25519 |
+---------+------------------+-------------------+
Table 2: Crypto-Condition Types
Authors' Addresses
Stefan Thomas
Ripple
300 Montgomery Street
San Francisco, CA 94104
US
Phone: -----------------
Email: stefan@ripple.com
URI: https://www.ripple.com
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Rome Reginelli
Ripple
300 Montgomery Street
San Francisco, CA 94104
US
Phone: -----------------
Email: rome@ripple.com
URI: https://www.ripple.com
Adrian Hope-Bailie
Ripple
300 Montgomery Street
San Francisco, CA 94104
US
Phone: -----------------
Email: adrian@ripple.com
URI: https://www.ripple.com
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