rsa algorithm description:select p,q ; p,q must both be prime numbers
calculate n=p*q
calculte c(n)=(p-1)*(q-1)
select integer e such that gcd(c(n),e)=1;1<e<c(n)
calculate d ;d*e*modc(n)=1
public key ku={e,n}
private key kr={d,n}
encryption:plain text =m<n
cipher text :c=M^e *(modn)
decryption
cipher text =c
plain text : M=c^d*(modn )
EX:n=3*7=21
c(n)=(2)(6)
e=gcd(12,5)=1
therefore e=5
d =5*5mod12=1
25mod12=1
therefore d=5
then continue with plain text and cipher text.hope its understandable