| TOC |
|
The File Transfer Protocol does not offer any method to verify the integrity of a transferred file, nor can two files be compared against each other without actually transferring them first. Cryptographic hashes are a possible solution to this problem. In the past, several attempts have been made to add commands to obtain checksums and hashes, however none have been formally specified, leading to non-interoperability and confusion. To solve these issues, this document specifies a new FTP command to be used by clients to request cryptographic hashes of files.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”
This Internet-Draft will expire on October 18, 2010.
Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
1.
Introduction
1.1.
Example
2.
Notational Conventions
3.
The HASH Command (HASH)
3.1.
HASH Command Errors
3.2.
FEAT Command Response for HASH Command
3.3.
Changing the HASH algorithm
4.
Command Usage
5.
IANA Considerations
6.
Implementation Requirements
7.
Security Considerations
8.
References
8.1.
Normative References
8.2.
Informative References
Appendix A.
Acknowledgements and Contributors
Appendix B.
List of Implementations with Non-standard Cryptographic Hash Command
Appendix C.
Document History
§
Authors' Addresses
| TOC |
The File Transfer Protocol [RFC0959] (Postel, J. and J. Reynolds, “File Transfer Protocol,” October 1985.) does not offer any method to verify the integrity of a transferred file, nor can two files be compared against each other without actually transferring them first. Cryptographic hashes are a possible solution to this problem. In the past, several attempts have been made to add commands to obtain checksums and hashes, however none have been formally specified, leading to non-interoperability and confusion. To solve these issues, this document specifies a new FTP command to be used by clients to request cryptographic hashes of files. HTTP has a similar feature named Instance Digests [RFC3230] (Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” January 2002.) which allows a client to request the cryptographic hash of a file.
[[ Discussion of this draft should take place on apps-discuss@ietf.org. ]]
| TOC |
Example of HASH client request:
HASH filename.ext
HASH server response with Positive Completion code and the requested hash using the currently selected algorithm:
213 80bc95fd391772fa61c91ed68567f0980bb45fd9
| TOC |
This specification describes conformance of FTP Extensions for cryptographic hashes.
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 BCP 14, [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.), as scoped to those conformance targets.
This document also uses notation defined in STD 9, [RFC0959] (Postel, J. and J. Reynolds, “File Transfer Protocol,” October 1985.). In particular, the terms "reply", "user", "file", "pathname", "FTP commands", "user-PI", "server-FTP process", "server-PI", "mode", "type", and "ASCII", are all used here as defined there.
In examples, the "C>" lines are commands from user-PI to server-PI, and the "S>" lines are server-PI replies.
Syntax required is defined using the Augmented BNF defined in [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.).
| TOC |
A new command "HASH" is added to the FTP command set to request the cryptographic hash of a file from a server-FTP process.
The syntax for the HASH command is:
hash = "HASH" SP <pathname>
As with all FTP commands, the "HASH" command label is interpreted in a case-insensitive manner.
The HASH command keyword MUST be followed by a single space (ASCII 32) followed by the pathname.
The pathname argument should reference the same file as other file based commands such as STOR or RETR which the same argument would reference.
The text returned in response to the HASH command MUST be:
hash-response = "213" SP 1*HEXDIGIT CRLF
All hash values MUST be encoded in lowercase hexadecimal format.
The HASH command is useful for files transmitted in Image type mode (TYPE I) and Stream transfer mode (MODE S).
| TOC |
The standard negative error codes 500 and 501 are sufficient to handle errors involving the HASH command (e.g., syntax errors). Response code 550 is used if the file can not be found. Response code 552 is used if the user isn't allowed to use the HASH command. Response code 450 is used to indicate the server is busy, e.g. already hashing other files yet inviting the client to retry in future.
| TOC |
A server that supports the HASH command MUST advertise it in FEAT response [RFC2389] (Hethmon, P. and R. Elz, “Feature negotiation mechanism for the File Transfer Protocol,” August 1998.) with a list of all supported hash algorithms in a semicolon separated list. The hash algorithm that is currently selected MUST be marked with an asterisk. This command word is case insensitive, but it SHOULD be transmitted in upper case only. That is, the response SHOULD be:
C> feat
S> 211-Extensions supported:
S> ...
S> HASH SHA-1*;MD5
S> ...
S> 211 END
The ellipses indicate place holders where other features may be included, and are not required. The single space indentation of each feature line is mandatory [RFC2389] (Hethmon, P. and R. Elz, “Feature negotiation mechanism for the File Transfer Protocol,” August 1998.).
The IANA registry named "Hash Function Textual Names" defines values for hash types. Hash names should be presented in uppercase, but comparisons should be case-insensitive, e.g. MD5, md5, Md5 are all the same.
hash-feat = SP "HASH" SP hashlist CRLF
hashlist = 1*( hashname ["*"] ";" )
hashtype = 1*( hchar )
hchar = ALPHA / DIGIT / "-" / "_" / "/" / "." / ","
| TOC |
To query the current hash algorithm and to change it, the OPTS command as defined in [RFC2389] (Hethmon, P. and R. Elz, “Feature negotiation mechanism for the File Transfer Protocol,” August 1998.) is used with HASH as the first argument. If no second argument is passed, OPTS HASH simply returns the currently selected hash algorithm. To change the algorithm, a valid hashtype MUST be given as second argument. If the command is successful, all future calls to HASH until the next successful OPTS HASH command or until the session is reinitialized (REIN) will use the selected hash algorithm.
C> OPTS HASH
S> 200 SHA-1
C> OPTS HASH SHA-512
S> 200 SHA-512
C> OPTS HASH CRC-37
S> 501 Unknown algorithm, current selection not changed
hashopts-cmd = "OPTS HASH" [ SP hashtype ] CRLF
hashopts-response = "200" SP hashtype CRLF
| TOC |
Client requests the cryptographic hash of a file with HASH command. Server replies with cryptographic hash of file. Client downloads file. Client hashes the downloaded file and compares its hash to the hash obtained from the server. This command could also be used to verify that an uploaded file is an exact copy.
| TOC |
This new command is added to the "FTP Commands and Extensions" registry created by [RFC5797] (Klensin, J. and A. Hoenes, “FTP Command and Extension Registry,” March 2010.).
Command Name: HASH
Description: Cryptographic Hash of a file
FEAT String: HASH
Command Type: Service execution
Conformance Requirements: Optional
Reference: This specification
| TOC |
All conforming implementations MUST at least support the SHA-1 algorithm. Implementations SHOULD NOT make any algorithm the default that is known to be weaker than SHA-1. Support for any additional algorithms is optional.
| TOC |
Calculating a file's hash is a CPU intensive operation and can easily consume the available disk I/O resources. If the HASH command isn't implemented carefully, a server could be vulnerable to a denial of service attack. On an affected server a malicious user could, for example, continuously send HASH commands over multiple connections and thus consume most of the FTP server's CPU and disk I/O resources, leaving little room for other operations. To mitigate this risk, a server SHOULD cache the calculated hashes so that the hash of a file is only calculated once even if multiple hash requests are sent for that file.
The performance of commonly used hard disk drives is adversely affected by the amount of time the device needs to reposition its read-and-write heads. A server SHOULD therefore avoid hashing multiple files at the same time which are located on the same physical media and SHOULD instead hash them sequentially. The FTP server's right to refuse to calculate the hash is of course important to help against DOS risks. A possible solution is to use the 450 reply code of HASH to indicate that the server is already busy with another HASH operation.
In addition, the HASH command can be used to draw conclusions about the contents of a file. If the hash of a file on some server matches the hash of some known, local file, both files are likely identical. To prevent this scenario it suffices to limit use of the HASH command to users who would already be able to download the file.
Currently, some of the hash types defined in the IANA registry named "Hash Function Textual Names" are considered insecure. These include the whole Message Digest family of algorithms which are not suitable for cryptographically strong verification. Malicious people could provide files that appear to be identical to another file because of a collision, i.e. the weak cryptographic hashes of the intended file and a substituted malicious file could match.
| TOC |
| TOC |
| [RFC0959] | Postel, J. and J. Reynolds, “File Transfer Protocol,” STD 9, RFC 0959, October 1985. |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997. |
| [RFC2389] | Hethmon, P. and R. Elz, “Feature negotiation mechanism for the File Transfer Protocol,” RFC 2389, August 1998. |
| [RFC5234] | Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” STD 68, RFC 5234, January 2008. |
| TOC |
| [RFC3230] | Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” RFC 3230, January 2002. |
| [RFC5797] | Klensin, J. and A. Hoenes, “FTP Command and Extension Registry,” RFC 5797, March 2010. |
| [draft-twine-ftpmd5] | Twine, J., “The MD5 and MMD5 FTP Command Extensions,” draft-twine-ftpmd5-00 (work in progress), May 2002. |
| TOC |
Thanks to John C. Klensin, Alfred Hoenes, James Twine, and Robert McMurray.
| TOC |
[[ to be removed by the RFC editor before publication as an RFC. ]]
At least one previous Internet Draft [draft‑twine‑ftpmd5] (Twine, J., “The MD5 and MMD5 FTP Command Extensions,” May 2002.) attempted to address this issue (it only supported one hash, MD5).
An incomplete list of FTP clients and servers that have implemented multiple commands (XCRC, MD5/MMD5, XMD5, XSHA, XSHA1, XSHA256, XSHA512, SITE SHOHASH, SITE CHECKSUM, etc) that are not formally specified, leading to non-interoperability and confusion.
| TOC |
[[ to be removed by the RFC editor before publication as an RFC. ]]
Known issues concerning this draft:
-02 : April , 2010.
-01 : April 7, 2010.
-00 : October 19, 2009.
| TOC |
| Anthony Bryan | |
| Pompano Beach, FL | |
| USA | |
| Email: | anthonybryan@gmail.com |
| URI: | http://www.metalinker.org |
| Tim Kosse | |
| Email: | tim.kosse@filezilla-project.org |
| URI: | http://filezilla-project.org/ |
| Daniel Stenberg | |
| Email: | daniel@haxx.se |
| URI: | http://www.haxx.se/ |