Using EdDSA/Ed25519 in the Internet X.509 Public Key Infrastructure
draft-josefsson-tls-ed25519-00

Abstract

This document specify algorithm identifiers and ASN.1 encoding formats for EdDSA/Ed25519 digital signatures and subject public keys used in the Internet X.509 Public Key Infrastructure (PKIX) for Certificates and CRLs.

Status of This Memo

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Copyright Notice

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1. Introduction

In [Ed25519], an elliptic curve signature system EdDSA was introduced, and a recommended choice of curve Ed25519 is chosen. Ed25519 was designed with performance and security in mind. EdDSA and Ed25519 is also described in [I-D.josefsson-eddsa-ed25519].

This RFC defines ASN.1 object identifiers for EdDSA and Ed25519 for use in the Internet X.509 PKI [RFC5280]. This document serves a similar role as [RFC3279] does for RSA (and more), [RFC4055] for RSA-OAEP/PSS, and [RFC5758] for SHA2-based (EC)DSA.

2. Requirements 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].

3. EdDSA ASN.1 Object Identifier Tree

The root of the tree for the object identifiers defined in this specification is given by:

	id-EdDSA OBJECT IDENTIFIER ::= { 1.3.6.1.4.1.11591.4.12 }
	

4. Subject Public Key Information Fields

In the X.509 certificate, the subjectPublicKeyInfo field has the SubjectPublicKeyInfo type, which has the following ASN.1 syntax:

     SubjectPublicKeyInfo  ::=  SEQUENCE  {
       algorithm         AlgorithmIdentifier,
       subjectPublicKey  BIT STRING
     }
     

The fields in SubjectPublicKeyInfo have the following meanings:

• algorithm is the algorithm identifier and parameters for the public key (see below).
• subjectPublicKey is the EdDSA public key.

The AlgorithmIdentifier type, which is included for convenience, is defined as follows:

      AlgorithmIdentifier  ::=  SEQUENCE  {
        algorithm   OBJECT IDENTIFIER,
        parameters  ANY DEFINED BY algorithm OPTIONAL
      }
      

The fields in AlgorithmIdentifier have the following meanings:

• algorithm identifies the cryptographic algorithm with an object identifier. This is the EdDSA OID defined below.
• parameters, which are optional, are the associated parameters for the algorithm identifier in the algorithm field.

5. EdDSA Public Keys

Certificates conforming to [RFC5280] may convey a public key for any public key algorithm. The certificate indicates the algorithm through an algorithm identifier. This algorithm identifier is an OID and optionally associated parameters.

This section identify the OID and parameters for the EdDSA algorithm. Conforming CAs MUST use the identified OIDs when issuing certificates containing EdDSA public keys. Conforming applications supporting EdDSA MUST, at a minimum, recognize the OID identified in this section.

The id-EdDSAPublicKey OID is used for identifying EdDSA public keys.

       id-EdDSAPublicKey OBJECT IDENTIFIER ::= { id-EdDSA 1 }
       

The id-EdDSAPublicKey OID is intended to be used in the algorithm field of a value of type AlgorithmIdentifier. The parameters field MUST have ASN.1 type NULL for this algorithm identifier.

The EdDSA public key MUST be encoded using the ASN.1 type EdDSAPublicKey:

       EdDSAPublicKey ::= OCTET STRING -- LE edwards point
       

where the value is the little-endian encoded edwards point. The DER encoded EdDSAPublicKey is the value of the BIT STRING subjectPublicKey.

6. Key Usage Bits

The intended application for the key MAY be indicated in the keyUsage certificate extension.

If the keyUsage extension is present in an end-entity certificate that conveys an EdDSA public key with the id-EdDSAPublicKey object identifier, then the keyUsage extension MUST contain one or both of the following values:

       nonRepudiation; and
       digitalSignature.
       

If the keyUsage extension is present in a certification authority certificate that conveys an EdDSA public key with the id-EdDSAPublicKey object identifier, then the keyUsage extension MUST contain one or more of the following values:

       nonRepudiation;
       digitalSignature;
       keyCertSign; and
       cRLSign.
       

7. EdDSA Signatures

Certificates and CRLs conforming to [RFC5280] may be signed with any public key signature algorithm. The certificate or CRL indicates the algorithm through an algorithm identifier which appears in the signatureAlgorithm field within the Certificate or CertificateList. This algorithm identifier is an OID and has optionally associated parameters. For illustration the Certificate structure is reproduced here:

   Certificate  ::=  SEQUENCE  {
        tbsCertificate       TBSCertificate,
        signatureAlgorithm   AlgorithmIdentifier,
        signatureValue       BIT STRING  }
	

Also recall the definition of the AlgorithmIdentifier type:

      AlgorithmIdentifier  ::=  SEQUENCE  {
        algorithm   OBJECT IDENTIFIER,
        parameters  ANY DEFINED BY algorithm OPTIONAL
      }
      

This document identify an AlgorithmIdentifier OID for EdDSA signatures. No parameters are defined.

The data to be signed is prepared for EdDSA. Then, a private key operation is performed to generate the signature value. This signature value is then ASN.1 encoded as a BIT STRING and included in the Certificate or CertificateList in the signature field.

The id-EdDSASignature OID is used for identifying EdDSA signatures.

       id-EdDSASignature OBJECT IDENTIFIER ::= { id-EdDSA 2 }
       

The id-EdDSASignature OID is intended to be used in the algorithm field of a value of type AlgorithmIdentifier. The parameters field MUST have ASN.1 type NULL for this algorithm identifier.

8. Acknowledgements

Text and/or inspiration were drawn from [RFC5280], [RFC3279], [RFC4055], [RFC5480], and [RFC5639].

Thanks to Ilari Liusvaara for ideas and discussion.

9. IANA Considerations

None.

10. Security Considerations

The security considerations of [RFC5280] and [I-D.josefsson-eddsa-ed25519] apply accordingly.

11. References

11.1. Normative References

11.2. Informative References

Author's Address

Table of Contents

• 1. Introduction
• 2. Requirements Terminology
• 3. EdDSA ASN.1 Object Identifier Tree
• 4. Subject Public Key Information Fields
• 5. EdDSA Public Keys
• 6. Key Usage Bits
• 7. EdDSA Signatures
• 8. Acknowledgements
• 9. IANA Considerations
• 10. Security Considerations
• 11. References
• 11.1. Normative References
• 11.2. Informative References
• Author's Address