computer science

Secure copy protocol

Secure copy protocol is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Secure copy protocol rather than just read about it. In short: Secure copy protocol (SCP) is a means of securely transferring computer files between a local host and a remote host or between two remote hosts. It is based on the Secure Shell (SSH) protocol. "SCP" commonly refers to both the Secure Copy Protocol and the program itself.

Key takeaways

Secure copy protocol belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Secure copy protocol to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Secure copy protocol from memory before moving on to harder problems.

Reference excerpt

Secure copy protocol (SCP) is a means of securely transferring computer files between a local host and a remote host or between two remote hosts. It is based on the Secure Shell (SSH) protocol. "SCP" commonly refers to both the Secure Copy Protocol and the program itself. According to OpenSSH developers in April 2019, SCP is outdated, inflexible and not readily fixed; they recommend the use of more modern protocols like SFTP and rsync for file transfer. As of OpenSSH version 9.0, scp client therefore uses SFTP for file transfers by default instead of the legacy SCP/RCP protocol.

Secure Copy Protocol The SCP is a network protocol, based on the BSD RCP protocol, which supports file transfers between hosts on a network. SCP uses Secure Shell (SSH) for data transfer and uses the same mechanisms for authentication, thereby ensuring the authenticity and confidentiality of the data in transit. A client can send (upload) files to a server, optionally including their basic attributes (permissions, timestamps). Clients can also request files or directories from a server (download). SCP runs over TCP port 22 by default. Like RCP, there is no RFC that defines the specifics of the protocol.

Function Normally, a client initiates an SSH connection to the remote host, and requests an SCP process to be started on the remote server. The remote SCP process can operate in one of two modes:

source mode, which reads files (usually from disk) and sends them back to the client, or sink mode, which accepts the files sent by the client and writes them (usually to disk) on the remote host. For most SCP clients, source mode is generally triggered with the -f flag (from), while sink mode is triggered with -t (to). These flags are used internally and are not documented outside the SCP source code.

Remote to remote mode In the past, in remote-to-remote secure copy, the SCP client would open an SSH connection to the source host and request that it, in turn, open an SCP connection to the destination. (Remote-to-remote mode did not support opening two SCP connections and using the originating client as an intermediary). SCP thus could not be used to remotely copy from the source to the destination when operating in password or keyboard-interactive authentication mode, as this would reveal the destination server's authentication credentials to the source. It was, however, possible with key-based or GSSAPI methods that do not require user input. More recently, remote-to-remote mode supports routing traffic through the client which originated the transfer, even though it is a 3rd party to the transfer. This way, authorization credentials must reside only on the originating client, the 3rd party.

Issues using talkative shell profiles SCP does not expect text communicating with the SSH login shell. Text transmitted due to the SSH profile (e.g. echo "Welcome" in the .bashrc file) is interpreted as an error message, and a null line (echo "") causes SCP client to deadlock waiting for the error message to complete.

scp program The SCP program is a software tool implementing the SCP protocol as a service daemon or client. It is a program to perform secure copying. Perhaps the most widely used SCP program is the OpenSSH command line scp program, which is provided in most SSH implementations. The scp program is the secure analog of the rcp command. The scp program must be part of all SSH servers that want to provide SCP service, as scp functions as SCP server too. Since OpenSSH 9.0, the program has been updated to use the newer, more secure SFTP protocol; an -O option is added for using SCP with old SCP-only servers.

Syntax Typically, a syntax of scp program is like the syntax of cp (copy): Copying local file to a remote host:

scp LocalSourceFile user@remotehost:directory/TargetFile

Copying file from remote host and recursively copying folder (with -r switch) from remote host:

scp user@remotehost:directory/SourceFile LocalTargetFile scp -r user@host:directory/SourceFolder LocalTargetFolder

Note that if the remote host uses a port other than the default of 22, it can be specified in the command. For example, copying a file from host:

scp -P 2222 user@host:directory/SourceFile TargetFile

Other clients As the Secure Copy Protocol implements file transfers only, GUI SCP clients are rare, as implementing it requires additional functionality (directory listing at least). For example, WinSCP defaults to the SFTP protocol. Even when operating in SCP mode, clients like WinSCP are typically not pure SCP clients, as they must use other means to implement the additional functionality (like the ls command). This in turn brings platform-dependency problems. More comprehensive tools for managing files over SSH are SFTP clients.

Security In 2019 vulnerability CVE-2019-6111 was announced related to the OpenSSH SCP tool and protocol allowing users to overwrite arbitrary files in the SCP client target directory.

Worked examples

Example 1 — a first encounter with Secure copy protocol

Start with the simplest possible case. Write down what Secure copy protocol claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Secure copy protocol before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Secure copy protocol ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Secure copy protocol

In research: Secure copy protocol appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Secure copy protocol in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Secure copy protocol is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cryptographic protocols, Cryptographic software, Network file transfer protocols, so understanding it makes those chapters shorter.
In everyday life: Look for Secure copy protocol outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Secure copy protocol in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Secure copy protocol means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Secure copy protocol out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Secure copy protocol in simple terms?

Why does Secure copy protocol matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Secure copy protocol?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Secure copy protocol.