No, the word Kephas is not Greek. The Aramaic word pronounced as Kefas comes from the Chaldee tongue and is in lexicons as follows:
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Strong's Number H3710
כֵּף
kêph
kafe
Strong's Definition:
From H3721; a hollow rock: - rock.
Total KJV occurrences: 2:
Brown, Driver, Briggs Definition:
1) rock, hollow of a rock Part of Speech: noun masculine
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The sound of the word is transliterated into the Greek, and an inflected ending given to it. That word is:
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Strong's Number G2786
Κηφᾶς
Kēphas
kay-fas'
Strong's Definition:
Of Chaldee origin (compare [H3710]); the Rock; Cephas (that is, Kepha), surname of Peter: - Cephas. Total KJV occurrences: 6
Thayer's Definition:
Cephas = stone
1) another name for the apostle Peter
Part of Speech: noun proper masculine
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Kephas does not mean "rock" in Greek. It means "petros" in the Greek mind that knows no English, but "stone" in the English mind that knows no Greek. The KJV translators took the word "Cephas" from the Vulgate, where about 400 A. D. Jerome transliterated the Aramaic/Greek sound by using the Latin alphabet. The English alphabet has a lot in common with the Latin, except for example "K" which does not exist in Latin, which uses "C" to express the "K" sound. So to pronounce "Cephas" as "Seefuss" betrays one's lack of a better education. The word written "Cephas" in English is pronounced "Kay-fahss"
So, I hope you have followed this discussion of your mistaken conclusions.
Don't confuse transliteration with translation, nor interpretation (which is giving the meaning of a word in one language with the words of another language) with either.
C'est si bon!
This will likely be of interest to you also re Képhas http://beggarsallreformation.blogspot.com/2010/11/built-on-sinking-sand-scriptural.html