| ACE Working Group | S. Raza |
| Internet-Draft | J. Höglund |
| Intended status: Standards Track | RISE AB |
| Expires: September 10, 2020 | G. Selander |
| J. Mattsson | |
| Ericsson AB | |
| M. Furuhed | |
| Nexus Group | |
| March 09, 2020 |
CBOR Profile of X.509 Certificates
draft-raza-ace-cbor-certificates-04
This document specifies a CBOR encoding and profiling of X.509 public key certificate suitable for Internet of Things (IoT) deployments. The full X.509 public key certificate format and commonly used ASN.1 DER encoding is overly verbose for constrained IoT environments. Profiling together with CBOR encoding reduces the certificate size significantly with associated known performance benefits.
The CBOR certificates are compatible with the existing X.509 standard, enabling the use of profiled and compressed X.509 certificates without modifications in the existing X.509 standard.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 10, 2020.
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One of the challenges with deploying a Public Key Infrastructure (PKI) for the Internet of Things (IoT) is the size and encoding of X.509 public key certificates [RFC5280], since those are not optimized for constrained environments [RFC7228]. More compact certificate representations are desirable. Due to the current PKI usage of X.509 certificates, keeping X.509 compatibility is necessary at least for a transition period. However, the use of a more compact encoding with the Concise Binary Object Representation (CBOR) [RFC7049] reduces the certificate size significantly which has known performance benefits in terms of decreased communication overhead, power consumption, latency, storage, etc.
CBOR is a data format designed for small code size and small message size. CBOR builds on the JSON data model but extends it by e.g. encoding binary data directly without base64 conversion. In addition to the binary CBOR encoding, CBOR also has a diagnostic notation that is readable and editable by humans. The Concise Data Definition Language (CDDL) [RFC8610] provides a way to express structures for protocol messages and APIs that use CBOR. [RFC8610] also extends the diagnostic notation.
CBOR data items are encoded to or decoded from byte strings using a type-length-value encoding scheme, where the three highest order bits of the initial byte contain information about the major type. CBOR supports several different types of data items, in addition to integers (int, uint), simple values (e.g. null), byte strings (bstr), and text strings (tstr), CBOR also supports arrays [] of data items, maps {} of pairs of data items, and sequences of data items. For a complete specification and examples, see [RFC7049], [RFC8610], and [I-D.ietf-cbor-sequence].
This document specifies the CBOR certificate profile, which is a CBOR based encoding and compression of the X.509 certificate format. The profile is based on previous work on profiling of X.509 certificates for Internet of Things deployments [RFC7925] [X.509-IoT] which retains backwards compatibility with X.509, and can be applied for lightweight certificate based authentication with e.g. TLS [RFC8446], DTLS [I-D.ietf-tls-dtls13], or EDHOC [I-D.selander-ace-cose-ecdhe]. The same profile can be used for “native” CBOR encoded certificates, which further optimizes the performance in constrained environments but are not backwards compatible with X.509, see Section 6.
Other work has looked at reducing size of X.509 certificates. The purpose of this document is to stimulate a discussion on CBOR based certificates.
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.
This specification makes use of the terminology in [RFC7228].
This section specifies the content and encoding for CBOR certificates. The CBOR certificate can be a native CBOR certificate, in which case the signature is calculated on the CBOR encoded data, or a CBOR compressed X.509 certificates in which case the signature is calculated on the DER encoded ASN.1 data in the X.509 certificate. In both cases the certificate content is adhering to the restrictions given by [RFC7925]. The corresponding ASN.1 schema is given in Appendix A.
The encoding and compression has several components including: ASN.1 DER and base64 encoding are replaced with CBOR encoding, static fields are elided, and elliptic curve points are compressed. The X.509 fields and there CBOR encodings are listed below. Combining these different components reduces the certificate size significantly, something that is not possible with general purpose compressions algorithms, see Figure 1.
CBOR certificates are defined in terms of RFC 7925 profiled X.509 certificates:
subjectAltName = 1
basicConstraints = 2 + cA
keyUsage = 3 + digitalSignature
+ 2 * keyAgreement + 4 * keyCertSign
extKeyUsage = 10 + id-kp-serverAuth + 2 * id-kp-clientAuth
+ 4 * id-kp-codeSigning + 8 * id-kp-OCSPSigning
In addition to the above fields present in X.509, the CBOR ecoding introduces an additional field
The Concise Data Definition Language (CDDL) for CBOR certificate is:
certificate = (
type : int,
serialNumber : bytes,
issuer : { + int => bytes } / text,
validity_notBefore: uint,
validity_notAfter: uint,
subject : text / bytes
subjectPublicKey : bytes
extensions : [ *4 int, ? text / bytes ] / int,
signatureValue : bytes,
? ( signatureAlgorithm : int,
subjectPublicKeyInfo_algorithm : int )
)
The signatureValue for native CBOR certificates is calculated over the CBOR sequence:
(
type : int,
serialNumber : bytes,
issuer : { + int => bytes } / text,
validity_notBefore: uint,
validity_notAfter: uint,
subject : text / bytes
subjectPublicKey : bytes
extensions : [ *4 int, ? text / bytes ] / int,
? ( signatureAlgorithm : int,
subjectPublicKeyInfo_algorithm : int )
)
TODO - Specify exactly how issuer is encoded into a map / text and back again.
CBOR certificates can be deployed with legacy X.509 certificates and CA infrastructure. In order to verify the signature, the CBOR certificate is used to recreate the original X.509 data structure to be able to verify the signature.
For the currently used DTLS v1.2 protocol, where the handshake is sent unencrypted, the actual encoding and compression can be done at different locations depending on the deployment setting. For example, the mapping between CBOR certificate and standard X.509 certificate can take place in a 6LoWPAN border gateway which allows the server side to stay unmodified. This case gives the advantage of the low overhead of a CBOR certificate over a constrained wireless links. The conversion to X.509 within an IoT device will incur a computational overhead, however, this is negligible compared to the reduced communication overhead.
For the setting with constrained server and server-only authentication, the server only needs to be provisioned with the CBOR certificate and does not perform the conversion to X.509. This option is viable when client authentication can be asserted by other means.
For DTLS v1.3, because certificates are encrypted, the proposed encoding needs to be done fully end-to-end, through adding the encoding/decoding functionality to the server. This corresponds to the proposed native mode, a new certificate compression scheme. The required changes on the server side are in line with recent protocols utilizing cbor encoding for communication with resource constrained devices [RFC8613].
The CBOR encoding of the sample certificate given in Appendix A results in the numbers shown in Figure 1. After RFC 7925 profiling, most duplicated information has been removed, and the remaining text strings are minimal in size. Therefore the further size reduction reached with general compression mechanisms will be small, mainly corresponding to making the ASN.1 endcoding more compact. The zlib number was calculated with zlib-flate.
zlib-flate -compress < cert.der > cert.compressed
+------------------+--------------+------------+--------------------+ | | RFC 7925 | zlib | CBOR Certificate | +------------------+---------------------------+--------------------+ | Certificate Size | 314 | 295 | 136 | +------------------+--------------+------------+--------------------+
Figure 1: Comparing Sizes of Certificates (bytes)
Further performance improvements can be achieved with the use of native CBOR certificates. In this case the signature is calculated over the CBOR encoded structure rather than the ASN.1 encoded structure. This removes entirely the need for ASN.1 and reduces the processing in the authenticating devices.
This solution applies when the devices are only required to authenticate with a set of native CBOR certificate compatible servers, which may become a preferred approach for future deployments. The mapping between X.509 and CBOR certificates enables a migration path between the backwards compatible format and the fully optimized format. This motivates introducing a type flag to indicate if the certificate should be restored to X.509 or kept cbor encoded.
The CBOR profiling of X.509 certificates does not change the security assumptions needed when deploying standard X.509 certificates but decreases the number of fields transmitted, which reduces the risk for implementation errors.
Conversion between the certificate formats can be made in constant time to reduce risk of information leakage through side channels.
The current version of the format hardcodes the signature algorithm which does not allow for crypto agility. A COSE crypto algorithm can be specified with small overhead, and this changed is proposed for a future version of the draft.
The mechanism in this draft does not reveal any additional information compared to X.509.
Because of difference in size, it will be possible to detect that this profile is used.
The gateway solution described in Section 4 requires unencrypted certificates.
IANA has created a new registry titled “CBOR Certificate Types” under the new heading “CBOR Certificate”. The registration procedure is “Expert Review”. The columns of the registry are Value, Description, and Reference, where Value is an integer and the other columns are text strings. The initial contents of the registry are:
+-------+---------------------------------------+-------------------+ | Value | Description | Reference | +-------+---------------------------------------+-------------------+ | 0 | Native CBOR Certificate. | [[this document]] | | 1 | CBOR Compressed X.509 Certificate | [[this document]] | +-------+---------------------------------------+-------------------+
Figure 2: CBOR Certificate Types
IANA has created a new registry titled “CBOR Certificate Signature Algorithms” under the new heading “CBOR Certificate”. The registration procedure is “Expert Review”. The columns of the registry are Value, X.509 Algorithm, and Reference, where Value is an integer and the other columns are text strings. The initial contents of the registry are:
+-------+---------------------------------------+-------------------+ | Value | X.509 Signature Algorithm | Reference | +-------+---------------------------------------+-------------------+ | 0 | ecdsa-with-SHA384 | [[this document]] | | 1 | ecdsa-with-SHA512 | [[this document]] | | 2 | id-ecdsa-with-shake128 | [[this document]] | | 3 | id-ecdsa-with-shake256 | [[this document]] | | 4 | id-Ed25519 | [[this document]] | | 5 | id-Ed448 | [[this document]] | +-------+---------------------------------------+-------------------+
Figure 3: CBOR Certificate Signature Algorithms
IANA has created a new registry titled “CBOR Certificate Public Key Algorithms” under the new heading “CBOR Certificate”. The registration procedure is “Expert Review”. The columns of the registry are Value, X.509 Algorithm, and Reference, where Value is an integer and the other columns are text strings. The initial contents of the registry are:
+-------+---------------------------------------+-------------------+ | Value | X.509 Public Key Algorithm | Reference | +-------+---------------------------------------+-------------------+ | 0 | id-ecPublicKey + prime384v1 | [[this document]] | | 1 | id-ecPublicKey + prime512v1 | [[this document]] | | 2 | id-X25519 | [[this document]] | | 3 | id-X448 | [[this document]] | | 4 | id-Ed25519 | [[this document]] | | 5 | id-Ed448 | [[this document]] | +-------+---------------------------------------+-------------------+
Figure 4: CBOR Certificate Public Key Algorithms
Example RFC 7925 profiled X.509 certificate parsed with OpenSSL
Certificate:
Data:
Version: 3 (0x2)
Serial Number: 128269 (0x1f50d)
Signature Algorithm: ecdsa-with-SHA256
Issuer: CN=RFC test CA
Validity
Not Before: Jan 1 00:00:00 2020 GMT
Not After : Feb 2 00:00:00 2021 GMT
Subject: CN=01-23-45-FF-FE-67-89-AB
Subject Public Key Info:
Public Key Algorithm: id-ecPublicKey
Public-Key: (256 bit)
pub:
04:ae:4c:db:01:f6:14:de:fc:71:21:28:5f:dc:7f:
5c:6d:1d:42:c9:56:47:f0:61:ba:00:80:df:67:88:
67:84:5e:e9:a6:9f:d4:89:31:49:da:e3:d3:b1:54:
16:d7:53:2c:38:71:52:b8:0b:0d:f3:e1:af:40:8a:
95:d3:07:1e:58
ASN1 OID: prime256v1
NIST CURVE: P-256
X509v3 extensions:
X509v3 Key Usage:
Digital Signature
Signature Algorithm: ecdsa-with-SHA256
30:44:02:20:37:38:73:ef:87:81:b8:82:97:ef:23:5c:1f:ac:
cf:62:da:4e:44:74:0d:c2:a2:e6:a3:c6:c8:82:a3:23:8d:9c:
02:20:3a:d9:35:3b:a7:88:68:3b:06:bb:48:fe:ca:16:ea:71:
17:17:34:c6:75:c5:33:2b:2a:f1:cb:73:38:10:a1:fc
The DER encoding of the above certificate is 314 bytes
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
The CBOR certificate compression of the X.509 in CBOR diagnostic format is
(
1,
h'128269',
"RFC test CA",
1577836800,
1612224000,
h'0123456789AB',
h'02ae4cdb01f614defc7121285fdc7f5c6d1d42c95647f061ba
0080df678867845e',
5,
h'373873EF8781B88297EF235C1FACCF62DA4E44740DC2A2E6A3
C6C882A3238D9C3AD9353BA788683B06BB48FECA16EA711717
34C675C5332B2AF1CB733810A1FC'
)
The CBOR encoding (CBOR sequence) of the CBOR certificate is 136 bytes
01431282696B52464320746573742043411A5E0BE1001A601896004601234567 89AB582102AE4CDB01F614DEFC7121285FDC7F5C6D1D42C95647F061BA0080DF 678867845E055840373873EF8781B88297EF235C1FACCF62DA4E44740DC2A2E6 A3C6C882A3238D9C3AD9353BA788683B06BB48FECA16EA71171734C675C5332B 2AF1CB733810A1FC
The corresponfing native CBOR certificate in CBOR diagnostic format is equal execpt for type and signatureValue
(
0,
h'128269',
"RFC test CA",
1577836800,
1612224000,
h'0123456789AB',
h'02ae4cdb01f614defc7121285fdc7f5c6d1d42c95647f061
ba0080df678867845e',
5,
h'7F10A063DA8DB2FD49414440CDF85070AC22A266C7F1DFB1
577D9A35A295A8742E794258B76968C097F85542322A0796
0199C13CC0220A9BC729EF2ECA638CFE'
)
The CBOR encoding (CBOR sequence) of the CBOR certificate is 136 bytes
00431282696B52464320746573742043411A5E0BE1001A601896004601234567 89AB582102AE4CDB01F614DEFC7121285FDC7F5C6D1D42C95647F061BA0080DF 678867845E0558407F10A063DA8DB2FD49414440CDF85070AC22A266C7F1DFB1 577D9A35A295A8742E794258B76968C097F85542322A07960199C13CC0220A9B C729EF2ECA638CFE
IOTCertificate DEFINITIONS EXPLICIT TAGS ::= BEGIN
Certificate ::= SEQUENCE {
tbsCertificate TBSCertificate,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING
}
TBSCertificate ::= SEQUENCE {
version [0] INTEGER {v3(2)},
serialNumber INTEGER (1..MAX),
signature AlgorithmIdentifier,
issuer Name,
validity Validity,
subject Name,
subjectPublicKeyInfo SubjectPublicKeyInfo,
extensions [3] Extensions OPTIONAL
}
Name ::= SEQUENCE SIZE (1) OF DistinguishedName
DistinguishedName ::= SET SIZE (1) OF CommonName
CommonName ::= SEQUENCE {
type OBJECT IDENTIFIER (id-at-commonName),
value UTF8String
}
Validity ::= SEQUENCE {
notBefore UTCTime,
notAfter UTCTime
}
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING
}
AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER,
parameters ANY DEFINED BY algorithm OPTIONAL }
}
Extensions ::= SEQUENCE SIZE (1..MAX) OF Extension
Extension ::= SEQUENCE {
extnId OBJECT IDENTIFIER,
critical BOOLEAN DEFAULT FALSE,
extnValue OCTET STRING
}
id-at-commonName OBJECT IDENTIFIER ::=
{joint-iso-itu-t(2) ds(5) attributeType(4) 3}
END