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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.
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1.
Introduction
1.1.
Examples
2.
Notational Conventions
3.
The HASH Command (HASH)
4.
Command Usage
5.
IANA Considerations
6.
Security Considerations
7.
References
7.1.
Normative References
7.2.
Informative References
Appendix A.
List of Implementations with Non-standard Cryptographic Hash Command
Appendix B.
Acknowledgements and Contributors
Appendix C.
Document History
§
Authors' Addresses
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The specification for the File Transfer Protocol [RFC0959] (Postel, J. and J. Reynolds, “File Transfer Protocol,” October 1985.) 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] (Mogul, J. and A. Van Hoff, “Instance Digests in HTTP,” January 2002.) which allows a client to request the cryptographic hash of a file.
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Example of HASH client request:
HASH SHA-1 filename.ext
HASH server response with Positive Completion code and the requested hash:
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.
This document also uses notation defined in STD 9, [RFC0959] (Postel, J. and J. Reynolds, “File Transfer Protocol,” October 1985.).
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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The HASH command allows for requesting the cryptographic hash of a file.
The syntax for the HASH command is:
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] (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 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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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.
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None.
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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 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.
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| [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. |
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[[ 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.
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Thanks to John C. Klensin and Alfred Hoenes.
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[[ to be removed by the RFC editor before publication as an RFC. ]]
Known issues concerning this draft:
-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/ |