DSA in DNS: DSA KEYs and SIGs in the Domain Name System (DNS)
The field layouts for DSA public keys and signatures in DNS KEY and SIG records.
Eastlake 3rd, D., "DSA KEYs and SIGs in the Domain Name System (DNS)", RFC 2536, DOI 10.17487/RFC2536, March 1999, <https://www.rfc-editor.org/info/rfc2536>.
@misc{rfc2536,
series = {Request for Comments},
number = 2536,
howpublished = {RFC 2536},
publisher = {RFC Editor},
doi = {10.17487/RFC2536},
url = {https://www.rfc-editor.org/info/rfc2536},
author = {D. Eastlake 3rd},
title = {{DSA KEYs and SIGs in the Domain Name System (DNS)}},
pagetotal = 6,
year = 1999,
month = mar,
} RFC text
Contents
Network Working Group D. EastLake
Request for Comments: 2536 IBM
Category: Standards Track March 1999
DSA KEYs and SIGs in the Domain Name System (DNS)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
A standard method for storing US Government Digital Signature
Algorithm keys and signatures in the Domain Name System is described
which utilizes DNS KEY and SIG resource records.
Table of Contents
Abstract...................................................1
1. Introduction............................................1
2. DSA KEY Resource Records................................2
3. DSA SIG Resource Records................................3
4. Performance Considerations..............................3
5. Security Considerations.................................4
6. IANA Considerations.....................................4
References.................................................5
Author's Address...........................................5
Full Copyright Statement...................................6
1. Introduction
The Domain Name System (DNS) is the global hierarchical replicated
distributed database system for Internet addressing, mail proxy, and
other information. The DNS has been extended to include digital
signatures and cryptographic keys as described in [RFC 2535]. Thus
the DNS can now be secured and can be used for secure key
distribution.
This document describes how to store US Government Digital Signature
Algorithm (DSA) keys and signatures in the DNS. Familiarity with the
US Digital Signature Algorithm is assumed [Schneier]. Implementation
of DSA is mandatory for DNS security.
2. DSA KEY Resource Records
DSA public keys are stored in the DNS as KEY RRs using algorithm
number 3 [RFC 2535]. The structure of the algorithm specific portion
of the RDATA part of this RR is as shown below. These fields, from Q
through Y are the "public key" part of the DSA KEY RR.
The period of key validity is not in the KEY RR but is indicated by
the SIG RR(s) which signs and authenticates the KEY RR(s) at that
domain name.
Field Size
----- ----
T 1 octet
Q 20 octets
P 64 + T*8 octets
G 64 + T*8 octets
Y 64 + T*8 octets
As described in [FIPS 186] and [Schneier]: T is a key size parameter
chosen such that 0 <= T <= 8. (The meaning for algorithm 3 if the T
octet is greater than 8 is reserved and the remainder of the RDATA
portion may have a different format in that case.) Q is a prime
number selected at key generation time such that 2**159 < Q < 2**160
so Q is always 20 octets long and, as with all other fields, is
stored in "big-endian" network order. P, G, and Y are calculated as
directed by the FIPS 186 key generation algorithm [Schneier]. P is
in the range 2**(511+64T) < P < 2**(512+64T) and so is 64 + 8*T
octets long. G and Y are quantities modulus P and so can be up to
the same length as P and are allocated fixed size fields with the
same number of octets as P.
During the key generation process, a random number X must be
generated such that 1 <= X <= Q-1. X is the private key and is used
in the final step of public key generation where Y is computed as
Y = G**X mod P
3. DSA SIG Resource Records
The signature portion of the SIG RR RDATA area, when using the US
Digital Signature Algorithm, is shown below with fields in the order
they occur. See [RFC 2535] for fields in the SIG RR RDATA which
precede the signature itself.
Field Size
----- ----
T 1 octet
R 20 octets
S 20 octets
The data signed is determined as specified in [RFC 2535]. Then the
following steps are taken, as specified in [FIPS 186], where Q, P, G,
and Y are as specified in the public key [Schneier]:
hash = SHA-1 ( data )
Generate a random K such that 0 < K < Q.
R = ( G**K mod P ) mod Q
S = ( K**(-1) * (hash + X*R) ) mod Q
Since Q is 160 bits long, R and S can not be larger than 20 octets,
which is the space allocated.
T is copied from the public key. It is not logically necessary in
the SIG but is present so that values of T > 8 can more conveniently
be used as an escape for extended versions of DSA or other algorithms
as later specified.
4. Performance Considerations
General signature generation speeds are roughly the same for RSA [RFC
2537] and DSA. With sufficient pre-computation, signature generation
with DSA is faster than RSA. Key generation is also faster for DSA.
However, signature verification is an order of magnitude slower than
RSA when the RSA public exponent is chosen to be small as is
recommended for KEY RRs used in domain name system (DNS) data
authentication.
Current DNS implementations are optimized for small transfers,
typically less than 512 bytes including overhead. While larger
transfers will perform correctly and work is underway to make larger
transfers more efficient, it is still advisable at this time to make
reasonable efforts to minimize the size of KEY RR sets stored within
the DNS consistent with adequate security. Keep in mind that in a
secure zone, at least one authenticating SIG RR will also be
returned.
5. Security Considerations
Many of the general security consideration in [RFC 2535] apply. Keys
retrieved from the DNS should not be trusted unless (1) they have
been securely obtained from a secure resolver or independently
verified by the user and (2) this secure resolver and secure
obtainment or independent verification conform to security policies
acceptable to the user. As with all cryptographic algorithms,
evaluating the necessary strength of the key is essential and
dependent on local policy.
The key size limitation of a maximum of 1024 bits ( T = 8 ) in the
current DSA standard may limit the security of DSA. For particularly
critical applications, implementors are encouraged to consider the
range of available algorithms and key sizes.
DSA assumes the ability to frequently generate high quality random
numbers. See [RFC 1750] for guidance. DSA is designed so that if
manipulated rather than random numbers are used, very high bandwidth
covert channels are possible. See [Schneier] and more recent
research. The leakage of an entire DSA private key in only two DSA
signatures has been demonstrated. DSA provides security only if
trusted implementations, including trusted random number generation,
are used.
6. IANA Considerations
Allocation of meaning to values of the T parameter that are not
defined herein requires an IETF standards actions. It is intended
that values unallocated herein be used to cover future extensions of
the DSS standard.
References
[FIPS 186] U.S. Federal Information Processing Standard: Digital
Signature Standard.
[RFC 1034] Mockapetris, P., "Domain Names - Concepts and
Facilities", STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, November 1987.
[RFC 1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[RFC 2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC 2537] Eastlake, D., "RSA/MD5 KEYs and SIGs in the Domain Name
System (DNS)", RFC 2537, March 1999.
[Schneier] Schneier, B., "Applied Cryptography Second Edition:
protocols, algorithms, and source code in C", 1996.
Author's Address
Donald E. Eastlake 3rd
IBM
65 Shindegan Hill Road, RR #1
Carmel, NY 10512
Phone: +1-914-276-2668(h)
+1-914-784-7913(w)
Fax: +1-914-784-3833(w)
EMail: dee3@us.ibm.com
Full Copyright Statement
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ISO Plain Language
On this page
§1DSA Data in DNS
DNS security adds digital signatures and cryptographic keys to DNS records. RFC 2536 specifies how to represent keys and signatures made with the U.S. Digital Signature Algorithm (DSA).
The RFC assumes familiarity with DSA and states that DSA implementation is mandatory for the DNS security system it describes. The surrounding KEY and SIG record structure is defined in RFC 2535.
This document defines the DSA-specific data stored in DNS records. The general DNS security record format comes from another RFC.
§2Fields in a DSA KEY Record
A DSA public key is stored in a KEY resource record using algorithm number 3. Its algorithm-specific data contains a one-octet T, a 20-octet Q, and P, G, and Y, each 64 + 8*T octets long.
The parameter T ranges from 0 through 8. The private value X is used to calculate the public value Y but is not placed in the public key record. Values are stored in big-endian network order.
The KEY record does not carry its validity period. A SIG record that authenticates the KEY records supplies that information.
The public key is stored as a fixed sequence of numbers. The one-octet T value sets the lengths of P, G, and Y; the private key is not included.
| Field | KEY record | SIG record |
|---|---|---|
| T | 1 octet | 1 octet |
| Q | 20 octets | Not present |
| P, G, Y | 64 + 8*T octets each | Not present |
| R, S | Not present | 20 octets each |
§3Fields in a DSA SIG Record
The DSA signature portion of a SIG record contains T, R, and S, in that order. T is one octet, while R and S are each 20 octets.
The signer hashes the data with SHA-1, chooses a random K in the range defined by the RFC, and computes R and S using the DSA procedure. T is copied from the public key; the RFC retains it as a possible extension point.
A DSA signature is represented by two 20-octet values, R and S, preceded by the key-size parameter T. The SIG record's other fields are defined by RFC 2535.
§4Performance and Record Size
The RFC says DSA and RSA have roughly similar signature-generation speed. With sufficient precomputation, DSA signing and key generation can be faster, while verification can be an order of magnitude slower than RSA with a small public exponent.
DNS implementations of the period were optimized for transfers typically below 512 bytes including overhead. The RFC recommends keeping KEY record sets as small as adequate security permits, remembering that a secure zone also returns an authenticating SIG record.
DSA can make signatures quickly, but checking them can take longer than checking RSA signatures in the comparison described here. Larger DNS record sets also increase the size of responses.
§5Key Trust and Randomness
A key retrieved from DNS should not be trusted unless it was securely obtained or independently verified according to the user's security policy. The appropriate key strength depends on local policy.
The DSA standard referenced by this RFC limits key size to 1024 bits. DSA also requires frequent high-quality random numbers; manipulated values can expose the private key, so trusted random-number generation is essential.
A DNS key needs secure verification before an application trusts it. DSA also depends on unpredictable random numbers; poor random values can expose the private key.
§6Future Values of T
Giving a meaning to a T value not defined by this document requires an IETF standards action. The unallocated values are intended for possible future extensions to the Digital Signature Standard.
The RFC leaves some T values undefined for later extensions. A standards action is required before an undefined value can gain a meaning.