QUESTION: The Gemara discusses whether the fact that a person recited Birkas Kohanim may be used as proof that he is a Kohen. The Gemara says that if a non-Kohen recites Birkas Kohanim, he transgresses an Isur Aseh. RASHI explains that the source for this is the verse which says that "they shall bless" the people, implying that only Kohanim may bless the people.
There is a widespread custom for a father to bless his children with the text of the Birkas Kohanim every Friday night and on Erev Yom Kippur. Why is this permitted? The Halachah is that one who is not a Kohen is not permitted to bless others with the priestly blessing.
(a) The BI'UR HALACHAH (OC 128) offers two answers. He cites the BACH who says that the Isur applies only when the blessing is recited with outstretched hands. Therefore, a non-Kohen is permitted to bless his child without outstretched hands.
(This implies that blessing one's child with outstretched hands is prohibited. It is said in the name of the VILNA GA'ON that one should bless his child with only one hand outstretched, but not two, since only Kohanim are permitted to bless with two hands outstretched.)
(b) Alternatively, the recitation of Birkas Kohanim was established as part of the prayer service. When one recites the verses of Birkas Kohanim outside of the context of Tefilah, he does not have intention to fulfill the Mitzvah of Birkas Kohanim and thus he is permitted to recite the verses.
QUESTION: The Gemara discusses whether a document (Shtar) may be used as proof that one is permitted to marry a full-fledged Kohen (a "Kohen Meyuchas").
The KETZOS HA'CHOSHEN (28:6) asks that there is a rule that testimony must be verbally stated and not written. How, then, can a document be used as testimony that one is a Kohen? The Ketzos ha'Choshen adds that although documents are legally valid, that status does not apply here to permit the document to serve as testimony.
(The Rambam explains that a Shtar is an enactment instituted by the Rabanan to help a person in need borrow money. (Without a Shtar, lenders would be very reluctant to lend money.) This power of a Shtar obviously does not apply to allow a Shtar to serve as testimony that a person is a Kohen, because such a case has nothing to do with borrowing money. Moreover, even according to the other Rishonim who say that a Shtar's power is mid'Oraisa, this applies only when the information in the Shtar was given to the witnesses by the writer of the document to establish a legal status based on the document. Here, though, the Shtar was given as proof that a loan transpired and not as proof that the lender is a Kohen (as that piece of information is irrelevant to the transaction).)
ANSWER: The Ketzos ha'Choshen answers that according to TOSFOS (24a) who says that mid'Oraisa normal families (Stam Mishpachos) are permitted to marry into the families of Kohanim as they are assumed to have valid Yichus, the entire need for testimony is only mid'Rabanan. Therefore, we may rely on written testimony (like in the case of Kiyum Shtaros, which is also only mid'Rabanan).