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8. Security Considerations

This document describes the format of four DNS resource records used by the DNS security extensions and provides an algorithm to calculate a key tag for a public key. Other than the following items, the resource records themselves do not introduce security considerations. See [RFC4033] and [RFC4035] for the security considerations related to the use of these records.

The DS record uses a cryptographic digest, a key algorithm type, and a key tag to point to a DNSKEY RR. The DS record is intended to identify an existing DNSKEY RR, but an attacker could, in theory, generate a DNSKEY that matches all the DS fields. The probability of constructing a matching DNSKEY depends on the digest algorithm type used by the DS. The currently defined digest algorithm is SHA-1, and the working group considers that constructing a public key that matches the algorithm, key tag, and a SHA-1 digest as given in a DS record would be a sufficiently difficult problem and therefore such an attack is not currently a serious threat.

The key tag is used to help select DNSKEY resource records efficiently, but it does not uniquely identify a single DNSKEY resource record. It is possible for two distinct DNSKEY RRs to have the same owner name, the same algorithm type, and the same key tag. An implementation that uses only the key tag to select a DNSKEY RR might select the wrong public key in some cases. See Appendix B for further details.

The algorithm table in Appendix A and the key tag calculation algorithm in Appendix B include the RSA/MD5 algorithm for completeness, but use of the RSA/MD5 algorithm is NOT RECOMMENDED, as explained in [RFC3110].

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