RFC 4402

RFC 4402 📜💖

Okay bestie, first the ID card 🎀 This is RFC 4402, called "A Pseudo-Random Function (PRF) for the Kerberos V Generic Security Service Application Program Interface (GSS-API) Mechanism" ✨. Nicolas Williams from Sun wrote it, it's a Standards Track doc, and it's dated February 2006 🌸. The title block doesn't list anything it obsoletes or updates 🙈

Status of This Memo 🦋

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Check 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 (2006) ✨

Abstract 🍓

This document defines the Pseudo-Random Function (PRF) for the Kerberos V mechanism for the Generic Security Service Application Program Interface (GSS-API). It's based on the PRF defined for the Kerberos V cryptographic framework, and it's for keying application protocols once there's an established Kerberos V GSS-API security context 🔑💅

Table of Contents 🧚

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................2
   2. Kerberos V GSS Mechanism PRF ....................................2
   3. IANA Considerations .............................................3
   4. Security Considerations .........................................3
   5. Normative References ............................................4

1. Introduction ✨

Okay so this doc specifies the Kerberos V GSS-API mechanism's [RFC4121] pseudo-random function, which matches [RFC4401]. The function is a "PRF+" style construction, and it's giving clean design 😭 For more info, check out [RFC4401], [RFC2743], [RFC2744], and [RFC4121] 📚

1.1. Conventions Used in This Document 💅

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]. Those capital words are the official requirement-level vibes, so read them exactly as [RFC2119] says 🫶

2. Kerberos V GSS Mechanism PRF 🔥

The GSS-API PRF [RFC4401] function for the Kerberos V mechanism [RFC4121] is the output of a PRF+ function. That function is based on the encryption type's PRF function, keyed with the negotiated session key of the security context. The key comes from the 'prf_key' input parameter of GSS_Pseudo_random() 🎀

This PRF+ MUST be keyed with the key indicated by the 'prf_key' input parameter, as follows:

• GSS_C_PRF_KEY_FULL -- use the sub-session key asserted by the acceptor, if any, or the sub-session asserted by the initiator, if any, or the Ticket's session key
• GSS_C_PRF_KEY_PARTIAL -- use the sub-session key asserted by the initiator, if any, or the Ticket's session key

The PRF+ function is a simple counter-based extension of the Kerberos V pseudo-random function [RFC3961] for the encryption type of the security context's keys. It works like this 🧮

      PRF+(K, L, S) = truncate(L, T1 || T2 || .. || Tn)

      Tn = pseudo-random(K, n || S)

Here, '||' is the concatenation operator, and 'n' is encoded as a network byte order 32-bit unsigned binary number. truncate(L, S) cuts the input octet string S down to length L, and pseudo-random() is the Kerberos V pseudo-random function [RFC3961] 💖

The maximum output size of the Kerberos V mechanism's GSS-API PRF is, necessarily, 2^32 times the output size of the pseudo-random() function for the encryption type of the given key. Big number energy 👑

When the input size is longer than 2^14 octets, as per [RFC4401], and it exceeds an implementation's resources, then the mechanism MUST return GSS_S_FAILURE and GSS_KRB5_S_KG_INPUT_TOO_LONG as the minor status code. No cap, that's the hard limit 🙈

3. IANA Considerations 🌸

This document has no IANA considerations currently. If and when a relevant IANA registry of GSS-API symbols and constants is created, then the GSS_KRB5_S_KG_INPUT_TOO_LONG minor status code should be added to that registry 🧚

4. Security Considerations 💀

Okay, this section is serious, so pay attention 😭🔥

Kerberos V encryption types' PRF functions use a key derived from contexts' session keys, and they should preserve the forward security properties of the mechanisms' key exchanges.

Legacy Kerberos V encryption types may be weak, particularly the single-DES encryption types. Be careful with those 💀

See also [RFC4401] for generic security considerations of GSS_Pseudo_random().

See also [RFC3961] for generic security considerations of the Kerberos V cryptographic framework.

Use of Ticket session keys, rather than sub-session keys, when initiators and acceptors fail to assert sub-session keys, is dangerous, as ticket reuse can lead to key reuse. So initiators should assert sub-session keys always, and acceptors should assert sub-session keys at least when initiators fail to do so. This is a big deal 🚨

The computational cost of computing this PRF+ may vary depending on the Kerberos V encryption types being used. In general, though, the computation gets more expensive as the input and output octet string lengths grow. (Note that the counter in the PRF+ construction allows for parallelization, which is kinda iconic 🦋) This means that if an application can be tricked into receiving very large input octet strings and being asked for very long output octet strings, that may be a denial of service attack on the application. So applications SHOULD place appropriate limits on the size of any input octet strings received from their peers without integrity protection 🛡️

5. Normative References 📚

• [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
• [RFC2743] Linn, J., "Generic Security Service Application Program Interface Version 2, Update 1", RFC 2743, January 2000.
• [RFC2744] Wray, J., "Generic Security Service API Version 2 : C-bindings", RFC 2744, January 2000.
• [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5", RFC 3961, February 2005.
• [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2", RFC 4121, July 2005.
• [RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API Extension for the Generic Security Service Application Program Interface (GSS-API)", RFC 4401, February 2006.

Author's Address 🎀

   Nicolas Williams
   Sun Microsystems
   5300 Riata Trace Ct
   Austin, TX  78727
   US

   EMail: Nicolas.Williams@sun.com

Full Copyright Statement 💜

Copyright (C) The Internet Society (2006).

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

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Acknowledgement 🫶

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