A v. an - The choice of which article to use, 'a' or 'an', is made by the sound of the first letter of the word following the article. The 'a' is used when the initial sound is a consonant, the 'an' is used when the initial sound is a vowel. The word 'SSH' starts with the "ess" sound, so the 'an' is used.
One common confusion is with acronyms and abbreviations, because that can change how you pronounce something. For example, if 'URL' is pronounced "You arr ell," it is "a URL". If 'URL' is pronounced "Earl" ("Urrl"), it is "an URL."
