Text to one big integer through its UTF-8 bytes (big-endian), as in textbook RSA examples, and a number back to text.
Procedure
How it works
Public-key schemes such as RSA compute with numbers, not with text. The usual way to turn a message into a number is to take its bytes and read them as the digits of one number in base 256, the first byte being the most significant: Hi is 48 69 in hex, so $m = 72 \cdot 256 + 105 = 18537$. The PKCS #1 standard calls this OS2IP, and the reverse I2OSP.
Back from a number, divide by 256 repeatedly; the remainders are the bytes, from the last one to the first. In hex it is simpler still: each pair of hex digits is one byte.
In textbook RSA the ciphertext is $c = m^e \bmod N$, which requires $m < N$. Real RSA never encrypts the bare message: it adds randomized padding (OAEP) first, otherwise equal messages give equal ciphertexts and small messages can be recovered.
Everything is calculated in your browser. Nothing you type leaves your device.