Are al-Sitteer and al-Sattaar names of Allaah?
Question

What is the difference between al-Sitteer and al-Sattaar? Which of them is one of the names of Allaah, may He be glorified and exalted?.
Praise be to Allah.The names of Allaah are a tawqeefi matter [i.e., they can only be known through divine Revelation and sound texts of hadeeth, with no room for ijtihad], as is the view of Ahl al-Sunnah wa’l-Jamaa’ah, so He is not to be called by anything except that by which He called Himself or that His Messenger (blessings and peace of Allaah be upon him) called Him. 
Shaykh Ibn Uthaymeen (may Allaah have mercy on him) said:
The names of Allaah are tawqeefi and reason plays no role in knowing them. Based on that, we should limit our sources concerning them to what is narrated in the Qur’aan and Sunnah, and not add or subtract anything from that, because human reason cannot work out the names that He deserves. So we must limit ourselves to the divine texts and sound hadeeth with regard to that, because Allaah says (interpretation of the meaning):
“And follow not (O man, i.e., say not, or do not, or witness not) that of which you have no knowledge. Verily, the hearing, and the sight, and the heart of each of those ones will be questioned (by Allaah)”
[al-Isra’ 17:35]
“Say (O Muhammad صلى الله عليه وسلم): ‘(But) the things that my Lord has indeed forbidden are Al‑Fawaahish (great evil sins and every kind of unlawful sexual intercourse) whether committed openly or secretly, sins (of all kinds), unrighteous oppression, joining partners (in worship) with Allaah for which He has given no authority, and saying things about Allaah of which you have no knowledge’”
[al-A’raaf 7:33]
Because calling Him by a name which He has not called Himself by, or denying a name that He has called Himself by, is a transgression against His rights. It is essential to observe proper etiquette with regard to that and limit it to what is mentioned in the text. End quote. 
Majmoo’ Fataawa Ibn ‘Uthaymeen, 3/275 
Secondly: 
The name al-Sattaar is not proven -- as far as we know -- in the texts of the Qur’aan and Sunnah as one of the names of Allaah, may He be exalted. Shaykh ‘Abd al-‘Azeez al-Raajihi (may Allaah preserve him) was asked: Is al-Sattaar one of the beautiful names of Allaah?
He replied: I do not know of al-Sattaar being one of the names of Allaah, but one of the names of Allaah is al-Sitteer, as it says in the hadeeth: “Allaah is Hayiy (so Kind that He does not ignore the plea of the one who asks of Him), Sitteer (the One Who conceals (people’s faults and mistakes)).” But as for the name al-Sattaar, I do not know that it is one of the names of Allaah; rather it comes under the heading of description: it may be said of Allaah that He is sattaar, and description is broader than the names. End quote. 
Quoting from the tape Sharh al-Ibaanah al-Sughra li Ibn Battah. 
With regard to the name al-Sitteer, it was narrated by Abu Dawood (4012) that the Messenger of Allaah (blessings and peace of Allaah be upon him) said: “Allaah is characterized by modesty and concealment, and He loves modesty and concealment. When one of you washes himself, let him conceal himself.” It was classed as saheeh by al-Albaani in Saheeh al-Jaami’. 
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked in Liqa’ al-Baab al-Maftooh: Are al-Muhyiy and al-Sitteer regarded as names of Allaah? 
He replied: al-Muhyi is not one of the names of Allaah; rather it refers to one of the actions of Allaah. Allaah says (interpretation of the meaning):
“It is He Who gives life and causes death. And when He decides upon a thing He says to it only: ‘Be!’ ___ and it is”
[Ghaafir 40:68]
So muhyiy is the participle from the verb ahya (to give life), and it is one of His actions, not one of His names. 
With regard to the name al-Sitteer, it is mentioned in a hadeeth, but its authenticity needs to be examined. If it is saheeh, then it is one of the names of Allaah, because the view of Ahl al-Sunnah wa’l-Jamaa’ah is that every name of Allaah that is proven to be saheeh from the Messenger of Allaah is proven, i.e., it is proven to be one of His names. End quote. 
As for the difference between al-Sitteer and al-Sattaar, both of them are indicative of extreme concealment, and Allaah covers a great deal for His slaves. 
And Allaah knows best.