This document describes and discuss the text encodings of Public-Key Infrastructure using X.509 (PKIX) Certificates, PKIX Certificate Revocation Lists (CRLs), PKCS #10 Certification Request Syntax, PKCS #7 structures, Cryptographic Message Syntax (CMS), PKCS #8 Private-Key Information Syntax, and Attribute Certificates. The text encodings are well-known, are implemented by several applications and libraries, and are widely deployed. This document is intended to articulate the de-facto rules that existing implementations operate by, and to give recommendations that will promote interoperability going forward.

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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1. Introduction

Several security-related standards used on the Internet define data formats that are normally encoded using Distinguished Encoding Rules (DER) [CCITT.X690.2002], which is a binary data format. This document is about text encodings of some of these formats:

A disadvantage of a binary data format is that it cannot be interchanged in textual transports, such as e-mail or text documents. One advantage with text encodings is that they are easy to modify using common text editors; for example, a user may concatenate several certificates to form a certificate chain with copy-and-paste operations.

The tradition within the RFC series can be traced back to PEM [RFC1421], based on a proposal by M. Rose in Message Encapsulation [RFC0934]. Originally called "PEM encapsulation mechanism", "encapsulated PEM message", or (arguably) "PEM printable encoding", today the format is sometimes referred to as "PEM encoding". Variations include OpenPGP ASCII Armor [RFC2015] and OpenSSH Key File Format [RFC4716].

For reasons that basically boil down to non-coordination or inattention, many PKIX and CMS libraries implement a text encoding that is similar to--but not identical with--PEM encoding. This document specifies the "PKIX text encoding" format, articulates the de-facto rules that most implementations operate by, and provides recommendations that will promote interoperability going forward. This document also provides common nomenclature for syntax elements, reflecting the evolution of this de-facto standard format. Peter Gutmann's X.509 Style Guide [X509SG] contains a section "base64 Encoding" that describes the formats and contains suggestions similar to what is in this document.

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 RFC 2119 [RFC2119].

2. General Considerations

PKIX text encoding begins with a line starting with "-----BEGIN" and ends with a line starting with "-----END". Between these lines, or "encapsulation boundaries", are base64-encoded [RFC4648] data. Data before the "-----BEGIN" and after the "-----END" encapsulation boundaries are permitted and MUST NOT cause parsers to malfunction. Furthermore, parsers MUST ignore whitespace and other non-base64 characters and MUST handle different newline conventions.

The type of data encoded is labeled depending on the type label in the "-----BEGIN" line (pre-encapsulation boundary). For example, the line may be "-----BEGIN CERTIFICATE-----" to indicate that the content is a PKIX certificate (see further below). Generators MUST put the same label on the "-----END" line (post-encapsulation boundary) as the corresponding "-----BEGIN" line. Parsers MAY disregard the label on the "-----END" line instead of signaling an error if there is a label mismatch.

The label type implies that the encoded data follows the specified syntax. Parsers MUST handle non-conforming data gracefully. However, not all parsers or generators prior to this Internet-Draft behave consistently. A conforming parser MAY interpret the contents as another label type, but ought to be aware of the security implications discussed in the Security Considerations section.

Unlike legacy PEM encoding [RFC1421], OpenPGP ASCII armor, and the OpenSSH key file format, PKIX text encoding does NOT define or permit attributes to be encoded alongside the PKIX or CMS data. Whitespace MAY appear between the pre-encapsulation boundary and the base64, but generators SHOULD NOT emit such whitespace.

Files MAY contain multiple PKIX text encoding instances. This is used, for example, when a file contains several certificates. Whether the instances are ordered or unordered depends on the context.

Generators MUST wrap the base64 encoded lines so that each line consists of exactly 64 characters except for the final line which will encode the remainder of the data (within the 64 character line boundary). Parsers MAY handle other line sizes. These requirements are consistent with PEM [RFC1421].

3. ABNF

pkixmsg    ::= preeb
               *eolWSP
               base64text
               posteb

preeb      ::= "-----BEGIN " label "-----" eol

posteb     ::= "-----END " label "-----" eol

base64char ::= ALPHA / DIGIT / "+" / "/"

base64pad  ::= "="

base64line ::= 1*base64char eol

base64finl ::= *base64char (base64pad eol base64pad /
               *2base64pad) eol
               ; ...AB= <CRLF> = <CRLF> is not good, but is valid

base64text ::= *base64line base64finl
        ; we could also use <encbinbody> from RFC 1421, which requires
        ; 16 groups of 4 chars, which means exactly 64 chars per
        ; line, except the final line, but this is more accurate

labelchar  ::= %x21-2C / %x2E-%7E   ; any printable character,
                                    ; except hyphen

label      ::= labelchar *(labelchar / labelchar "-" / SP) labelchar

eol        ::= CRLF / CR / LF

eolWSP     ::= WSP / CR / LF    ; compare with LWSP

Figure 1: ABNF

pkixmsgstrict    ::= preeb
                     strictbase64text
                     posteb

strictbase64finl ::= *15(4base64char) (4base64char / 3base64char
                     base64pad / 2base64char 2base64pad) eol

base64fullline   ::= 64base64char eol

strictbase64text ::= *base64fullline strictbase64finl

Figure 2: ABNF (Strict)

This specification RECOMMENDS that new implementations emit the strict format [abnf-strict-fig] specified above.

4. Text Encoding of PKIX Certificates

4.1. Encoding

PKIX certificates are encoded using the "CERTIFICATE" label. The encoded data MUST be a DER encoded ASN.1 "Certificate" structure as described in section 4 of [RFC5280].

Figure 3: Certificate Example

Historically the label "X509 CERTIFICATE" and also, less common, "X.509 CERTIFICATE" have been used. Generators conforming to this document MUST generate "CERTIFICATE" labels and MUST NOT generate "X509 CERTIFICATE" or "X.509 CERTIFICATE" labels. Parsers are NOT RECOMMENDED to treat "X509 CERTIFICATE" or "X.509 CERTIFICATE" as equivalent to "CERTIFICATE", but a valid exception may be for backwards compatibility (potentially together with a warning).

4.2. Explanatory Text

Many tools are known to emit explanatory text before the BEGIN and after the END lines for PKIX certificates, more than any other type. If emitted, such text SHOULD be related to the certificate, such as providing a textual representation of key data elements in the certificate.

Subject: CN=Atlantis
Issuer: CN=Atlantis
Validity: from 7/9/2012 3:10:38 AM UTC to 7/9/2013 3:10:37 AM UTC
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

Figure 4: Certificate Example with Explanatory Text

4.3. File Extension

Although text encodings of PKIX structures can occur anywhere, many tools are known to offer an option to encode PKIX structures in this text encoding. To promote interoperability and to separate DER encodings from text encodings, This Internet-Draft RECOMMENDS that the extension ".crt" be used for this text encoding. Implementations should be aware that in spite of this recommendation, many tools still default to encode certificates in this text encoding with the extension ".cer".

5. Text Encoding of PKIX CRLs

PKIX CRLs are encoded using the "X509 CRL" label. The encoded data MUST be a DER encoded ASN.1 "CertificateList" structure as described in Section 5 of [RFC5280].

Figure 5: CRL Example

Historically the label "CRL" has rarely been used. Today it is not common and many popular tools do not understand the label. Therefore, this document standardizes "X509 CRL" in order to promote interoperability and backwards-compatibility. Generators conforming to this document MUST generate "X509 CRL" labels and MUST NOT generate "CRL" labels. Parsers are NOT RECOMMENDED to treat "CRL" as equivalent to "X509 CRL".

6. Text Encoding of PKCS #10 Certification Request Syntax

PKCS #10 Certification Requests are encoded using the "CERTIFICATE REQUEST" label. The encoded data MUST be a DER encoded ASN.1 "CertificationRequest" structure as described in [RFC2986].

Figure 6: PKCS #10 Example

The label "NEW CERTIFICATE REQUEST" is also in wide use. Generators conforming to this document MUST generate "CERTIFICATE REQUEST" labels. Parsers MAY treat "NEW CERTIFICATE REQUEST" as equivalent to "CERTIFICATE REQUEST".

7. Text Encoding of PKCS #7 Cryptographic Message Syntax

PKCS #7 Cryptographic Message Syntax structures are encoded using the "PKCS7" label. The encoded data MUST be a DER encoded ASN.1 "ContentInfo" structure as described in [RFC2315].

Figure 7: PKCS #7 Example

The label "CERTIFICATE CHAIN" has been in use to denote a degenerative PKCS #7 structure that contains only a list of certificates. Several modern tools do not support this label. Generators MUST NOT generate the "CERTIFICATE CHAIN" label. Parsers are NOT RECOMMENDED to treat "CERTIFICATE CHAIN" as equivalent to "PKCS7".

PKCS #7 is an old standard that has long been superseded by CMS [RFC5652]. Implementations SHOULD NOT generate PKCS #7 when CMS is an alternative.

8. Text Encoding of Cryptographic Message Syntax

Cryptographic Message Syntax structures are encoded using the "CMS" label. The encoded data MUST be a DER encoded ASN.1 "ContentInfo" structure as described in [RFC5652].

-----BEGIN CMS-----
MIGDBgsqhkiG9w0BCRABCaB0MHICAQAwDQYLKoZIhvcNAQkQAwgwXgYJKoZIhvcN
AQcBoFEET3icc87PK0nNK9ENqSxItVIoSa0o0S/ISczMs1ZIzkgsKk4tsQ0N1nUM
dvb05OXi5XLPLEtViMwvLVLwSE0sKlFIVHAqSk3MBkkBAJv0Fx0=
-----END CMS-----

Figure 8: CMS Example

CMS is the IETF successor to PKCS #7. Section 1.1.1 of [RFC5652] describes the changes since PKCS #7 v1.5. Implementations SHOULD generate CMS when it is an alternative, promoting interoperability and forwards-compatibility.

9. Text Encoding of PKCS #8 Private Key Info, and One Asymmetric Key

The PrivateKeyInfo structure of PKCS #8 Private Key Information Syntax, renamed to OneAsymmetricKey in [RFC5958], is encoded using the "PRIVATE KEY" label. The encoded data MUST be a DER encoded ASN.1 "PrivateKeyInfo" structure as described in PKCS #8 [RFC5208], or the "OneAsymmetricKey" structure as described in [RFC5958]. The two are semantically identical, and can be distinguished by version number.

Figure 9: PKCS #8 PrivateKeyInfo Example

10. Text Encoding of PKCS #8 Encrypted Private Key Info

The EncryptedPrivateKeyInfo structure of PKCS #8 Private Key Information Syntax, called the same in [RFC5958], is encoded using the "ENCRYPTED PRIVATE KEY" label. The encoded data MUST be a DER encoded ASN.1 "EncryptedPrivateKeyInfo" structure as described in PKCS #8 [RFC5208] and [RFC5958].

Figure 10: PKCS #8 EncryptedPrivateKeyInfo Example

11. Text Encoding of Attribute Certificates

Attribute certificates are encoded using the "ATTRIBUTE CERTIFICATE" label. The encoded data MUST be a DER encoded ASN.1 "AttributeCertificate" structure as described in [RFC5755].

-----BEGIN ATTRIBUTE CERTIFICATE-----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-----END ATTRIBUTE CERTIFICATE-----

Figure 11: Attribute Certificate Example

12. Security Considerations

Data in this format often originates from untrusted sources, thus parsers must be prepared to handle unexpected data without causing security vulnerabilities.

Ambiguities are introduced by having more than one canonical encoding of the same data. The first ambiguity is introduced by permitting the text encoded representation instead of the binary DER encoding, but further ambiguities arise when multiple labels are treated as similar. Variations of whitespace and non-base64 alphabetic characters can create further ambiguities. Implementations that rely on canonical representation or the ability to fingerprint a particular data format need to understand that this Internet-Draft does not define canonical encodings. If canonical encodings are desired, the encoded structure must be decoded and processed into a canonical form (namely, DER encoding). Data encoding ambiguities also create opportunities for side channels.

13. IANA Considerations

This document implies no IANA Considerations.

14. Acknowledgements

Peter Gutmann suggested to document labels for Attribute Certificates and PKCS #7 messages, and to add examples for the non-standard variants.

15. References

15.1. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2315]	Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315, March 1998.
[RFC2986]	Nystrom, M. and B. Kaliski, "PKCS #10: Certification Request Syntax Specification Version 1.7", RFC 2986, November 2000.
[RFC4648]	Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006.
[RFC5208]	Kaliski, B., "Public-Key Cryptography Standards (PKCS) #8: Private-Key Information Syntax Specification Version 1.2", RFC 5208, May 2008.
[RFC5280]	Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008.
[RFC5652]	Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, September 2009.
[RFC5755]	Farrell, S., Housley, R. and S. Turner, "An Internet Attribute Certificate Profile for Authorization", RFC 5755, January 2010.
[RFC5958]	Turner, S., "Asymmetric Key Packages", RFC 5958, August 2010.
[CCITT.X690.2002]	International Telephone and Telegraph Consultative Committee, "ASN.1 encoding rules: Specification of basic encoding Rules (BER), Canonical encoding rules (CER) and Distinguished encoding rules (DER)", CCITT Recommendation X.690, July 2002.

15.2. Informative References

[RFC0934]	Rose, M. and E. Stefferud, "Proposed standard for message encapsulation", RFC 934, January 1985.
[RFC1421]	Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part I: Message Encryption and Authentication Procedures", RFC 1421, February 1993.
[RFC2015]	Elkins, M., "MIME Security with Pretty Good Privacy (PGP)", RFC 2015, October 1996.
[RFC4716]	Galbraith, J. and R. Thayer, "The Secure Shell (SSH) Public Key File Format", RFC 4716, November 2006.
[X509SG]	Gutmann, P., "X.509 Style Guide", WWW http://www.cs.auckland.ac.nz/~pgut001/pubs/x509guide.txt, October 2000.

Appendix A. Non-Conforming Examples

This section contains examples for the non-recommended label variants described earlier in this document. As discussed earlier, supporting these are not required and sometimes discouraged. Still, they can be useful for interoperability testing and for easy reference.

Figure 12: Non-standard 'X509' Certificate Example

-----BEGIN X.509 CERTIFICATE-----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-----END X.509 CERTIFICATE-----

Figure 13: Non-standard 'X.509' Certificate Example

-----BEGIN NEW CERTIFICATE REQUEST-----
MIIBWDCCAQcCAQAwTjELMAkGA1UEBhMCU0UxJzAlBgNVBAoTHlNpbW9uIEpvc2Vm
c3NvbiBEYXRha29uc3VsdCBBQjEWMBQGA1UEAxMNam9zZWZzc29uLm9yZzBOMBAG
ByqGSM49AgEGBSuBBAAhAzoABLLPSkuXY0l66MbxVJ3Mot5FCFuqQfn6dTs+9/CM
EOlSwVej77tj56kj9R/j9Q+LfysX8FO9I5p3oGIwYAYJKoZIhvcNAQkOMVMwUTAY
BgNVHREEETAPgg1qb3NlZnNzb24ub3JnMAwGA1UdEwEB/wQCMAAwDwYDVR0PAQH/
BAUDAwegADAWBgNVHSUBAf8EDDAKBggrBgEFBQcDATAKBggqhkjOPQQDAgM/ADA8
AhxBvfhxPFfbBbsE1NoFmCUczOFApEuQVUw3ZP69AhwWXk3dgSUsKnuwL5g/ftAY
dEQc8B8jAcnuOrfU
-----END NEW CERTIFICATE REQUEST-----

Figure 14: Non-standard 'NEW' PKCS #10 Example

Figure 15: Non-standard 'CERTIFICATE CHAIN' Example

Authors' Addresses

Simon Josefsson SJD AB Johan Olof Wallins Väg 13 Solna, 171 64 SE EMail: simon@josefsson.org URI: http://josefsson.org/

Sean Leonard Penango, Inc. 5900 Wilshire Boulevard 21st Floor Los Angeles, CA 90036 USA EMail: dev+ietf@seantek.com URI: http://www.penango.com/

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