Network Working Group A. Bryan
Internet-Draft T. Kosse
Intended status: Experimental March 23, 2010
Expires: September 24, 2010
FTP Extensions for Cryptographic Hashes
draft-bryan-ftp-hash-00
Abstract
The specification for the File Transfer Protocol does not include
methods to obtain cryptographic hashes of files. Cryptographic
hashes can be used to identify files and verify intregrity.
Unfortunately, because of the desire for this feature, multiple
commands that are not formally specified have been implemented in FTP
applications leading to non-interoperability and confusion. This
specification documents an optional command where FTP clients can
request the cryptographic hash of a file from a FTP server.
Status of this Memo
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. The HASH Command (HASH) . . . . . . . . . . . . . . . . . . . . 3
4. Command Usage . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . . 6
Appendix A. List of Implementations with Non-standard
Cryptographic Hash Command . . . . . . . . . . . . . . 6
Appendix B. Acknowledgements and Contributors . . . . . . . . . . 7
Appendix C. Document History . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
The specification for the File Transfer Protocol [RFC0959] does not
include methods to obtain cryptographic hashes of files.
Cryptographic hashes can be used to identify files and verify
integrity. Unfortunately, because of the desire for this feature,
multiple commands that are not formally specified have been
implemented in FTP applications leading to non-interoperability and
confusion. This specification documents an optional command where
FTP clients can request the cryptographic hash of a file from a FTP
server. HTTP has a similar feature named Instance Digests [RFC3230]
which allows a client to request the cryptographic hash of a file.
1.1. Examples
Example of HASH client request:
HASH SHA-1 filename.ext
HASH server response with Positive Completion code and the requested
hash:
213 80bc95fd391772fa61c91ed68567f0980bb45fd9
2. Notational Conventions
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], as
scoped to those conformance targets.
This document also uses notation defined in STD 9, [RFC0959].
Syntax required is defined using the Augmented BNF defined in
[RFC5234].
3. The HASH Command (HASH)
The HASH command allows for requesting the cryptographic hash of a
file.
The syntax for the HASH command is:
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hash = "HASH" SP <hash type> SP <filename>
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). Following the space, a hash type MUST be present. Another
single space (ASCII 32), MUST be followed by the filename.
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.
The filename 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 standard negative error codes 500 and 501 are sufficient to
handle all errors involving the HASH command (e.g., syntax errors).
Response code 550 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.
A server that supports HASH should advertise it in FEAT response
[RFC2389] with a list of all supported hash algorithms in a comma
separated list. The "C>" lines are commands from user-PI to
server-PI, the "S>" lines are server-PI replies.
C> feat
S> 211-Extensions supported:
S> SIZE
S> COMPRESSION
S> HASH SHA-1,MD5
S> MDTM
S> 211 END
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4. Command Usage
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.
5. IANA Considerations
None.
6. Security Considerations
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 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. 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.
7. References
7.1. Normative References
[RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol",
STD 9, RFC 0959, October 1985.
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[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.
7.2. Informative References
[RFC3230] Mogul, J. and A. Van Hoff, "Instance Digests in HTTP",
RFC 3230, January 2002.
Appendix A. List of Implementations with Non-standard Cryptographic
Hash Command
[[ to be removed by the RFC editor before publication as an RFC. ]]
An incomplete list of FTP clients and servers that have implemented
multiple commands (XMD5, XSHA1, SITE SHOHASH, etc) that are not
formally specified, leading to non-interoperability and confusion.
o Akamai NetStorage p17-18
http://pigdogslow.dyndns.org/NetStorage_UserGuide.pdf
o Cerberus FTP server http://www.softpedia.com/progChangelog/
Cerberus-FTP-Server-Changelog-1904.html
o FileCOPA FTP Server
http://www.filecopa-ftpserver.com/features.html
o FireFTP http://fireftp.mozdev.org/features.html
o Gene6 FTP Server
http://www.g6ftpserver.com/en/information#features
o GoldenGate FTP (Ftp Full Java Server)
o IceWarp FTP Server http://www.icewarp.com/products/ftp_server/
o JAFS http://www.sbbi.net/site/jafs/features.html
o MOVEit DMZ
o Nofeel FTP server http://www.nftpserver.com/history.php
o Null FTP
http://www.sharewareconnection.com/null-ftp-client-pro.htm
o ProFTPD module mod_digest
http://www.smartftp.com/oss/proftpd/mod_digest.html
o SmartFTP client http://www.smartftp.com/features/
o Starksoft Ftp Component for .NET / Mono
http://www.starksoft.com/prod_ftp.html
o RaidenFTPD32 FTP server
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o WS_FTP client / server http://ipswitchft.custhelp.com/app/answers/
detail/a_id/671/kw/xmd5/r_id/166/sno/1
o zFTPServer
Appendix B. Acknowledgements and Contributors
Thanks to John C. Klensin and Alfred Hoenes.
Appendix C. Document History
[[ to be removed by the RFC editor before publication as an RFC. ]]
Known issues concerning this draft:
o None known.
-00 : October 19, 2009.
o Initial draft.
Authors' Addresses
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/
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