DSA in DNS: DSA KEYs and SIGs in the Domain Name System (DNS)
How DNS stores a DSA public key and a DSA signature, using the KEY and SIG record types.
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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AI ELI5
On this page
§1Why DSA Needs a DNS Format
The Domain Name System (DNS) is the global hierarchical, replicated, distributed database used for Internet addressing, mail proxy and other information. RFC 2535 extended it to carry digital signatures and cryptographic keys, so DNS can be secured and used for secure key distribution.
RFC 2536 fills in one piece of that: how to store US Government Digital Signature Algorithm (DSA) keys and signatures in the DNS. It assumes the reader already knows DSA and says that implementation of DSA is mandatory for DNS security.
The internet's phone book can now hold secret-code keys and signed notes. This document says exactly how to write one particular kind of key and signature in it.
§2The DSA KEY Record
A DSA public key is stored as a KEY resource record with algorithm number 3. The algorithm-specific part of the record data holds five fields, and the fields from Q through Y are the public key part.
T: 1 octet, a key size parameter with 0 <= T <= 8.Q: 20 octets.P,GandY: 64 + T*8 octets each.
Q is a prime with 2^159 < Q < 2^160, so it is always 20 octets. P is in the range 2^(511+64T) < P < 2^(512+64T). G and Y are quantities modulo P, so they get fixed-size fields as long as P. Every field is stored in big-endian network order. If T is greater than 8, the meaning is reserved and the rest of the record may have a different format.
During key generation a random number X with 1 <= X <= Q-1 is chosen. X is the private key, and Y is computed as Y = G^X mod P. The key's validity period is not in the KEY record. It is indicated by the SIG record that signs the KEY record at that domain name.
A key is a few big numbers lined up in a fixed order. One small number up front says how big the others are, and the secret number never goes in the phone book.
| Field | Size in KEY | Size in SIG |
|---|---|---|
| T | 1 octet | 1 octet |
| Q | 20 octets | not present |
| P, G, Y | 64 + T*8 octets each | not present |
| R, S | not present | 20 octets each |
§3The DSA SIG Record
In a SIG record, the signature part has three fields, in this order: T (1 octet), R (20 octets) and S (20 octets). The fields that come before the signature in the SIG record data are defined in RFC 2535, which also says how the data to be signed is determined.
The signer then follows FIPS 186: take hash = SHA-1 ( data ), pick a random K with 0 < K < Q, and compute R = ( G^K mod P ) mod Q and S = ( K^(-1) * (hash + X*R) ) mod Q. Because Q is 160 bits long, R and S cannot be larger than the 20 octets allocated.
T is copied from the public key. It is not logically necessary in the SIG, but it is there so that values of T above 8 can serve as an escape for extended versions of DSA or other algorithms specified later.
A signature is two numbers computed from the message and the secret key. Anyone holding the matching public key can check them, and a copy of the size number rides along in case the format changes later.
§4Speed and Size Trade-offs
Signature generation speed is roughly the same for RSA (see RFC 2537) and DSA. With enough pre-computation, DSA signing is faster, and DSA key generation is also faster. Verification is the weak spot: it is an order of magnitude slower than RSA when the RSA public exponent is small, as is recommended for KEY records used to authenticate DNS data.
DNS implementations of the time were optimized for small transfers, typically under 512 bytes including overhead. Larger transfers work, but the RFC advises keeping KEY record sets as small as adequate security allows, remembering that a secure zone also returns at least one authenticating SIG record.
Making signatures is quick, but checking them takes much longer than with the other main kind. Since the phone book likes small answers, keep keys no bigger than they need to be.
§5Security Considerations
Many of the general security considerations in RFC 2535 apply. Keys retrieved from the DNS should not be trusted unless they were securely obtained from a secure resolver or independently verified by the user, and that process meets the user's security policy. How strong a key must be depends on local policy.
The current DSA standard limits keys to a maximum of 1024 bits (T = 8), which may limit security. For particularly critical applications, implementors are encouraged to consider the range of available algorithms and key sizes.
DSA needs frequent, high quality random numbers (see RFC 1750). If manipulated numbers are used instead of random ones, very high bandwidth covert channels are possible, and leaking a whole DSA private key from only two signatures has been demonstrated. DSA is secure only with trusted implementations, including trusted random number generation.
A signature machine needs a fresh, truly unpredictable number every time. If someone tricks it into using a bad one, the secret key can leak out.
§6IANA Considerations
Giving a meaning to any value of the T parameter that this document does not define requires an IETF standards action. The unallocated values are meant to cover future extensions of the Digital Signature Standard (DSS).
The spare values of the size number are saved for later, and only a formal decision by the standards group can give them a meaning.