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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.
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This Internet-Draft will expire on December 13, 2010.
Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.
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1.
Introduction
1.1.
Example
2.
Notational Conventions
3.
The HASH Command (HASH)
3.1.
User-PI usage of HASH
3.2.
Changing the HASH algorithm
3.3.
FEAT Command Response for HASH Command
3.4.
HASH Command Errors
4.
IANA Considerations
5.
Implementation Requirements
6.
Security Considerations
7.
References
7.1.
Normative References
7.2.
Informative References
Appendix A.
Acknowledgements and Contributors
Appendix B.
List of Non-standard Cryptographic Hash or Checksum Commands and Implementations
Appendix C.
Document History
§
Authors' Addresses
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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 ftpext@ietf.org (or apps-discuss@ietf.org if necessary). ]]
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Example of HASH client request:
C> HASH filename.ext
HASH server response with Positive Completion code and the requested hash using the currently selected algorithm:
S> 213 80bc95fd391772fa61c91ed68567f0980bb45fd9
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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.
In examples, the "C>" lines are commands from user-PI to server-PI, and the "S>" lines are server-PI replies.
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.
Syntax required is defined using the Augmented BNF defined in [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.).
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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-command = "HASH" SP <pathname>
As with all FTP commands, the "HASH" command word is case independent, and MAY be specified in any character case desired.
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 meant to be used for files transmitted in Image type mode (TYPE I) and Stream transfer mode (MODE S). The returned hash MUST be calculated over the raw octet data of the file irrespective of the selected data type, transfer mode or any other state affecting the transfer. In other words, if a client were to download a full file using TYPE I and MODE S and were to calculate the hash on the received octet data, it would be identical to the hash returned by HASH.
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The user-PI issues the FEAT command to query the server-PI about which algorithm is currently selected. This also reveals the other algorithms that the server supports. In this example, the SHA-1 algorithm is currently selected.
C> FEAT
S> 211-Extensions supported:
S> ...
S> HASH SHA-256;SHA-512;SHA-1*;MD5
S> ...
S> 211 END
OPTS HASH is an alternative method for the user-PI to query the server-PI about which algorithm is currently selected.
C> OPTS HASH
S> 200 SHA-1
In this example, we wish to select SHA-256, a different algorithm.
C> OPTS HASH SHA-256
S> 200 SHA-256
The user-PI requests the cryptographic hash of a file with HASH command. Server-PI replies with cryptographic hash of file.
C> HASH filename.ext
S> 213 f0ad929cd259957e160ea442eb80986b5f01...
Client downloads file. Client hashes the downloaded file and compares its hash to the hash obtained from the server. The HASH command could also be used to verify that an uploaded file is an exact copy.
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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
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A server-FTP process that supports the HASH command MUST include, in the response to the FEAT command [RFC2389] (Hethmon, P. and R. Elz, “Feature negotiation mechanism for the File Transfer Protocol,” August 1998.), a feature line indicating that the HASH command is supported, along 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. The order of hash algorithms is insignificant. 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-256;SHA-512;SHA-1*;MD5
S> ...
S> 211 END
The ellipses indicate place holders where other features MAY be listed, but this is OPTIONAL. The single space indentation of each feature line is REQUIRED by [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 ["*"] ";" )
hashname = 1*( hchar )
hchar = ALPHA / DIGIT / "-" / "_" / "/" / "." / ","
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The server-PI SHOULD reply with a 500 reply if the HASH command is unrecognized or unimplemented.
The server-PI SHOULD reply with a 501 reply to the OPTS HASH command if the user-PI has requested an unknown or unsupported algorithm.
The server-PI SHOULD reply with a 550 reply if the HASH command is used on a file that can not be found.
The server-PI SHOULD reply with a 552 reply if the user is not allowed to use the HASH command.
The server-PI SHOULD reply with a 450 reply if the server is busy, e.g. already hashing other files yet inviting the client to retry in future.
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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
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All conforming implementations MUST at least support the SHA-1 algorithm [FIPS‑180‑3] (National Institute of Standards and Technology (NIST), “Secure Hash Standard (SHS),” October 2008.). Implementations SHOULD NOT make any algorithm the default that is known to be weaker than SHA-1. Support for any additional algorithms is OPTIONAL.
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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 that 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.
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| [FIPS-180-3] | National Institute of Standards and Technology (NIST), “Secure Hash Standard (SHS),” FIPS PUB 180-3, October 2008. |
| [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. |
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| [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. |
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Thanks to John C. Klensin, Alfred Hoenes, James Twine, Robert McMurray, and Mathias Berchtold.
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[[ to be removed by the RFC editor before publication as an RFC. ]]
A number of similar checksum or hash commands exist, but are not formally specified, leading to non-interoperability and confusion. The commands, any specifications, and relevant details:
An incomplete list of FTP clients and servers that have implemented these commands:
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[[ to be removed by the RFC editor before publication as an RFC. ]]
Known issues concerning this draft:
-04 : June 11, 2010.
-03 : May 21, 2010.
-02 : April 16, 2010.
-01 : April 7, 2010.
-00 : October 19, 2009.
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| 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/ |