What is the difference between 鍵 and 鑰?
I was about to learn the kanji for 'key' and I came across this problem. When I checked it in the dictionary it shows that both of them are read as kagi and both mean key/lock. So I'm asking you for help.
[Please be understanding about my English 'cause it's not my native language.]
In short, the difference between 鍵 and 鑰 is that you can safely forget the second one unless you prepare for the top level of kanji recognition test.
鍵 (on-reading: ケン) is the normal way to spell かぎ in kanji (and the only one to spell ケン "key" in compounds, such as [鍵盤]{けんばん} "keyboard"). It is a part of Jōyō list, and even the guidelines for Japanese newspapers explain that かぎ "key" is written with kanji 鍵. (Only do not confuse it with かぎ "hook," which kanji is theoretically 鉤 but it is now spelt in kana anyways, they even have the same pitch, [かぎ ]{HHL}.)
鑰 (on-reading: ヤク) is, unlike the one above, not a Jōyō kanji, or even a kanji allowed in names. It definitely exists, and is a possible way to spell かぎ, but no-one does that in modern texts, except maybe to show off. (I believe modern Chinese uses 鑰 in its words for "key" as in implement for doors, while reserving 鍵 for keys of piano.) There are some words that use 鑰 for its on-reading, but they are extremely rare.
Hence, 鍵 is your friend. Write it, and do not worry about 鑰.
鍵{かぎ} seems to have explained in the other answer. 鑰{やく} is difficult to read. I have never seen this one as a standalone character. So, I searched to a little bit.
I found a book called 秘蔵{ひぞう}宝鑰{ほうやく} written by 空海{くうかい}, a Buddhist monk, 1200 years ago.
So, a person who follows his Buddhism teaching/school may be familiar with the word. 宝鑰 seems to have the meaning of "the key to treasures" (宝の鍵。宝物庫を開く鍵。) according to goo辞書. (宝 is "treasure" in Japanese.)
I also found 印鑰{いんにゃく}神社{じんじゃ} which is a Shinto shrine (神社{じんじゃ}).
According to Wikipedia, shrines with this name seem to exist mostly in the Kyūshū region, but some in the Tōhoku region. Anyway, this shows that it is not only limited to Buddhism.
The meaning of 印鑰{いんやく} by itself is, according to goo辞書,
官府の長官の印と諸司・城門・蔵などの鍵。
The seal of secretary of government and "key" to the gate and warehouse, etc.
(印 is a seal which confirms or certifies something.) So, I also think it was used a long time ago in the legal system.
苗字{みょうじ}ネット lists the family name 鑰山{かぎやま}, but only 20 people have the name. (Similarly, 三鑰{みかぎ}さん 50 people, 鑰山{かぎやま}さん 10 people.) So, family names using 鑰 are very rare, but do still exist.
The difference is that 鍵 is the commonly used character in Japanese, being included in the 'Common Use' kanji list (常用漢字), while 鑰 is a rare variant which is almost obsolete in Modern Japanese.
So for the practical purposes of learning Japanese, you should learn 鍵 as the kanji which represents 'key/lock'.
