Network Working Group B. Kennish
Intended status: Standards Track April 8, 2015
Expires: October 10, 2015

Trusted Linker Download Redirection (TLDR)


This document describes an HTTP extension that allows user agents to verify downloaded data. It provides a standardised way for an HTTPS URL (assumed trustworthy) to redirect to a non-HTTPS URL and give the user agent extra information about the resulting output (e.g. a downloaded file.) Once that is retrieved, it can check whether or not the data has been modified since the trustworthy site checked it (e.g. altered during transit or as a result of the destination site having been compromised.)

Status of This Memo

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This Internet-Draft will expire on October 10, 2015.

Copyright Notice

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1. Introduction

Transport Layer Security (TLS/SSL) [RFC-5246] can add an often much needed level of security to the Hypertext Transfer Protocol (HTTP) [RFC-7230] (a combination normally referred to as HTTPS) but the encryption algorithms are often too much of a strain for busy public file download servers, especially when arguably the data stream does not really need to be encrypted (the files are publicly accessible anyway).

As a result, servers providing public file downloads, even potentially dangerous file types such as those with executable code, scripts, etc., often do not deliver the files using HTTPS. However, this makes the downloads vulnerable to Man-in-the-Middle (MitM) attacks; an attacker could change the data during transit to serve the client with malware.

Trusted Linker Download Redirection (TLDR) describes a process by which an (assumed trustworthy) HTTPS URL can redirect to a non-HTTPS download URL and give the user agent extra information about the file so that, once downloaded, the agent can check whether the file is the same one that the trustworthy site verified.

1.1. Conventions and 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 [RFC-2119]

Trusted Linker
The URL that provides the TLDR HTTP headers as it redirects the user agent to a new URL (using the "Location:" header).
The Link
The URL that the trusted linker redirects to. Will be identical to 'Final Destination' unless the Link redirects elsewhere.
Final Destination
The final URL (not necessarily using HTTP) that provides the file data (may be identical to "The link")

2. Appropriate HTTP status codes

The TLDR headers SHOULD only be sent by the Trusted Link web server with responses that use one of the following HTTP status codes:

Note that the permanent redirects (such as 301 "Moved Permanently" and 308 "Permanent Redirect") are not included in the list. These status codes instruct user agents to send all future requests to the URL provided in the "Location" header, thereby bypassing the Trusted Linker completely. It seems nonsensical to provide a TLDR service for the user agent's first request only!

3. Redirection with plain HTTP

Ideally, servers implementing TLDR should do so over the HTTPS protocol and not plain HTTP. If the headers are delivered without the safety that TLS/SSL provides, TLDR cannot detect Man in the Middle (MitM) attacks.

However, it does provides a bit of extra protection if used over plain HTTP assuming no MitM. For example, if the Final Destination server becomes compromised or for any other reason starts providing a different file to the one that it was before, the checksum will not match and this will be detected.

4. Checksum-related headers

These HTTP response headers are all OPTIONAL but a web server must provide at least one to be a Trusted Linker. It should provide as many of them as possible in order to give user agents the biggest choice. To avoid ambiguity, a web server MUST NOT return more than one TLDR header with the same name.

When more than one checksum is provided by the server, it is not recommended for a user agent to use more than one of the checksums provided. It should select one; preferably the checksum function considered most secure at the time. The method by which the user agent selects a hash function MAY be user configurable.

All checksums are calculated on the complete file contents. If the file is delivered to the client using any kind of transfer encoding (such as HTTP gzip, chunked, etc.), the checksums MUST be calculated on the decoded data. Any kind of transfer mechanism that results in ambiguous output data (such as "ASCII mode" FTP) is not supported.

The examples given below each show the server replying with a different value for the HTTP status code. This is just to illustrate the different codes that can use TLDR and bears no relationship to the checksum hash function used.

While the examples with the longer header values are shown split to multiple lines, this is to aid the readability of this document and it is recommended that the actual header values are kept on a single line.

4.1. Location-Checksum-MD5

Location-Checksum-MD5 = "Location-Checksum-MD5: " md5-checksum

md5-checksum should be a string giving the expected 128-bit MD5 message digest for the resulting downloaded file (see [RFC-1321]).

4.1.1. Example

HTTP/1.0 302 Moved Temporarily
Location-Checksum-MD5: ccca8352847856cd4c2df77ce675de2b

4.2. Location-Checksum-SHA1

Location-Checksum-SHA1 = "Location-Checksum-SHA1: " sha1-checksum

sha1-checksum should be a string giving the expected 160bit SHA1 message digest for the resulting downloaded file (see [RFC-3174]).

4.2.1. Example

HTTP/1.1 302 Found
Location-Checksum-SHA1: d98eaf66de93d9512958d6c7f5ed58d059dea53f

4.3. Location-Checksum-SHA256

Location-Checksum-SHA256 = "Location-Checksum-SHA256: " sha256-checksum

sha256-checksum should be a string giving the expected 256bit SHA-256 message digest for the resulting downloaded file (see [RFC-6234]).

4.3.1. Example

HTTP/1.1 303 See Other
Location-Checksum-SHA256: d9c30fccbfd25469b41c1e2c68b3b1a8f2a241e1808

4.4. Location-Checksum-SHA512

Location-Checksum-SHA512 = "Location-Checksum-SHA512: " sha512-checksum

sha512-checksum should be a string giving the expected 512bit SHA-512 message digest for the resulting downloaded file (see [RFC-6234]).

4.4.1. Example

HTTP/1.1 307 Temporary Redirect
Location-Checksum-SHA5126: a5555882da0419d98dd6b098e616aa66d7a5c13e54

5. Appropriate user agent responses

5.1. On starting a TLDR download

The user agent MAY display some kind of notification to the user upon starting a download using TLDR. However, it SHOULD NOT show any notification unless the Trusted Linker used HTTPS (because it might give the user a false sense of safety.)

5.2. On completing a TLDR download

Once the file has completely downloaded, the user agent calculates a checksum using its choice of the provided message digest algorithms and compares it to the one provided by the Trusted Linker. It then takes appropriate action depending on the result:

5.2.1. When the checksums match

If the original TLDR checksum headers were sent using HTTPS, user agents MAY notify the user that the downloaded file was as expected according to the Trusted Linker. However, if they do, they SHOULD clearly provide the user with enough information about the Trusted Linker URL so that they can decide whether or not they do actually trust it (e.g. the fully qualified domain name). See the Security Considerations section for an explanation as to why this is necessary.

If the original TLDR checksum headers were sent over plain HTTP, user agents SHOULD NOT give any positive feedback to the user because of the possibility of a Man in the Middle (MitM) attack.

5.2.2. When the checksums do not match

All the statements in this section apply even if the TLDR checksum headers were sent over plain HTTP. Any checksum mismatch is a cause for alarm.

If the checksums do not match, the user agent MUST give some kind of feedback to warn the user that something is wrong.

The user agent SHOULD also remove or quarantine the downloaded file to prevent it being used accidentally.

6. TLDR multiple times in a single request chain

It is possible for a Trusted Linker to redirect to a Link that redirects again. There could be any number of redirections and any of the hops along this redirect chain could also provide new TLDR headers. If this happens, a user agent SHOULD simply ignore these headers. The first hop in a chain of redirects that provides TLDR is considered the most trust-worthy.

7. In practice

The author has created a prototype Add-on for the Mozilla Firefox web browser which demonstrates TLDR in action. Links to download the Add-on and some example download links that implement TLDR can be found at a page on the author's website [TLDR-Demo].

8. Possible future extension

TLDR could be extended in the future to take advantage of other types of verification. Perhaps OpenPGP related HTTP response headers could be used, possibly by providing one or more trusted public key fingerprints along with the URL of a signature for the file. I hope to explore this idea in future revisions of this document.

9. TL;DR

The author is aware of the existing acronym "TL;DR" meaning "Too Long; Didn't Read" and feels that it is appropriate that this proposed standard has the same acronym as it aptly describes the way most people probably feel when they see instructions on how to manually verify the integrity of their downloads such as these instructions on The author would like to make it clear that he is not criticising these instructions in any way but instead pointing out that a manual verification process is quite lengthy and tedious, and therefore often skipped completely.

10. IANA Considerations

10.1. Header Field Registrations

[TO BE REMOVED: HTTP header fields should be registered within the "Message Headers" registry maintained at ]

This document defines a number of HTTP header fields, so their associated registry entries shall be added according to the permanent registrations below (see [BCP90]):

| Header Field Name        | Protocol | Status   | Reference   |
| Location-Checksum-MD5    | http     | standard | Section 4.1 |
| Location-Checksum-SHA1   | http     | standard | Section 4.2 |
| Location-Checksum-SHA256 | http     | standard | Section 4.3 |
| Location-Checksum-SHA512 | http     | standard | Section 4.4 |

The change controller is: "IETF ( - Internet Engineering Task Force". This specification defines a number of response header fields for the Hypertext Transfer Protocol (HTTP) that has been registered with the Internet Assigned Numbers Authority (IANA) following the "Registration Procedures for Message Header Fields" [RFC3864].

11. Security Considerations

The purpose of TLDR is to allow for an automated process of file verification assuming a Trusted Linker. Provided the Trusted Linker can be truly trusted, the method is sound.

11.1. Trusted Linker

The most important consideration is whether or not the the Trusted Linker is actually trustworthy. If it is not, then TLDR offers no benefit. As a result, user agents that implement TLDR SHOULD make it clear to the user which https:// URL is being trusted.

This is necessary because a string of redirects (using HTTP, JavaScript, or any other method) could trick the user into confusing the actual Trusted Linker for a different site:
   |=== redirects in some way to ===>
   |=== which then redirects to ===>
   |=== which redirects using TLDR to ===>
   (which is malware and downloads with TLDR)

In the example, without information to the contrary, the user may incorrectly assume that the Trusted Linker is but it is actually

11.2. Man in the Middle

As stated in Section 3, TLDR only provides detection of modification by a Man in the Middle (MitM) if the checksum headers are delivered via HTTPS. If they are instead delivered by plain HTTP, they can be modified in transit by the MitM such that the checksums verify a different file of the MitM's choosing which they can then deliver to the user agent.

11.3. Limitations

Although TLDR can help to detect when there is an integrity problem with a file download, it offers no means to retrieve a verified file should this situation arise.

How can a Trusted Linker obtain the file in a secure way such that they can be sure of its integrity before calculating the checksum? This is somewhat outside of the scope of this document but a few options include:

12. References

12.1. Normative References

[RFC-2119] Bradner, S., "RFC 2119 - Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997.
[RFC-7230] Fielding, R. and J. Reschke, "RFC 7230 - Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, June 2014.
[RFC-5246] Dierks, T. and E. Rescorla, "RFC 5246 - The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.

12.2. Informative References

[RFC-1321] Rivest, R., "RFC 1321 - The MD5 Message-Digest Algorithm", RFC 1321, April 1992.
[RFC-3174] Eastlake 3rd, D. and P. Jones, "RFC 3174 - US Secure Hash Algorithm 1 (SHA1)", RFC 3174, September 2001.
[RFC-6234] Eastlake 3rd, D. and T. Hansen, "RFC 6234 - US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.
[TLDR-Demo] Kennish, B., "Trusted Linker Download Redirection (TLDR)", September 2014.

Author's Address

Ben A. Kennish EMail: URI:

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