Is singular correct, or is plural of "credential" correct for a single username & password pair?
This isn't a question you can answer by thinking about the grammar - it's just a case of conventional usage.
I've previously worked in the tech industry, and we would always say credentials, never credential. That would still be true even if the credentials were a single item like a certificate. [Edit to add: but Samuel has provided a counter-example to this in a comment on the OP.]
In wider English, credential is valid - it's used for example in this Wikipedia article - but it's still more common to use the plural (see Oxford Dictionaries for confirmation). For example, I could say that I have presented my credentials to someone if I have shown them my passport, even though I've only shown them one physical item.
Both are correct, and both have been used in both plural and singular form whether literally (when one has an actual proof of one's being authorised to do something) and figuratively in both plural and singular of a single piece of such evidence from the 1600s until the current day.
The plural use is far more common, as the OED confirms.
As such it's a matter of style, but if you don't have a rule from a style guide, you are probably better off using credentials, but also better off not criticising someone who uses credential for doing so.
In terms of providing proof of who you are with a user name and password, "Credentials" should be used, even if there is only one set of username & password.
When you log into a website, for example, you have to prove who you are. You do this with at least two pieces of information:
- Your User name, which is used to determine if you have an account
- Your password, which is used to determine if you can access the account
In some cases, you will have to provide one credential at a time. I've heard that Google and others are starting to do this, where you provide a user name first, then click into another page that will then ask for the password for that account. In these cases, each page would ask for one credential.
Other places may ask you for even more credentials, such as verifying a secret picture and phrase, answering security questions, etc.
By the way, you mentioned in a comment on your question that MSDN refers to it as a 'networkcredential.' I believe the class is called
NetworkCredential so that programmers know there is only one set of credentials in there, as naming the class
NetworkCredentials would make it seem as though the class is a collection of
NetworkCredential objects. This is common practice in programming.
If you read the description for the
NetworkCredential here: https://msdn.microsoft.com/en-us/library/system.net.networkcredential(v=vs.110).aspx, you'll see the description uses
Provides credentials for password-based authentication schemes such as basic, digest, NTLM, and Kerberos authentication.
Oxford Dictionaries Online confirms that credential does exist in the singular (achievement, quality or aspect of a person's background, especially when used to indicate their suitability for something) but it notes that it is usually used in the plural - which is my own experience.
And as Morton notes in his answer it would be usual to say that one had presented one's credentials even if it only involved one item.