#!/bin/bash

set -e

die() { 
    echo "$@" >&2
    exit 1
}

if [[ -z $1 ]] 
then
    die "Usage: $0 private_rsa_key"
fi

sshkey=$(< $1) 
if [[ $sshkey != "-----BEGIN RSA PRIVATE KEY-----"* ]]
then
    die "This does not appear to be an RSA private key"
fi

if [[ $sshkey == *ENCRYPTED* ]] 
then
    echo "Key is encrypted, using openssl to decrypt"
    sshkey=$(openssl rsa <<< "$sshkey")
fi

base64key=$(sed '1d; $d;' <<< "$sshkey") #the base64 data
decdump=$(base64 -d <<< "$base64key" | od -t u1 | sed -e 's/^[^ ]*//')
decbytes=$(echo $decdump | tr ' ' '\n') #decimal bytes, one on each line

readInt() { 
    read byte 
#    echo "read type $byte" >&2
    (( (byte & 0x1F) != 2 )) && \
        die "$byte doesn't encode an integer :O"

    length=$(readLength)
#    echo "read length $length" >&2
    exp="0"
    for((i=0; i<length; i++))
    do
        read b || die "could only read $i of $length bytes"
#        echo "read byte $b" >&2
        exp="($exp)*256+$b"
    done
    bc <<< "$exp" | tr -d '\n\\ '
}

readLength() { 
    local bytes length n i 
    read bytes

    n=$((bytes&0x7F))
    if (( bytes & 0x80 ))
    then
        length=0
        for((i=0; i<n; i++)) 
        do
            read byte
            length=$((length*256+byte))
        done
        echo $length
    else 
        echo $n
    fi
}

(
    read sequence # a sequence header
    (( sequence = 0x30 )) || die "expected sequence header"
    readLength > /dev/null # the sequence length
    unknown=$(readInt)
    n=$(readInt) #modulo
    e=$(readInt) #public exponent
    d=$(readInt) #private exponent
    p=$(readInt) #first prime
    q=$(readInt) #second prime
    exp1=$(readInt) # d mod p-1
    exp2=$(readInt) # d mod q-1
    c=$(readInt) || c=0 # p^1 mod q

    length=$(bc -l <<< "scale=20; v=l($n)/l(2); scale=1; v/1")
    echo "# Key is $length bits long"
    echo "# Input for bc:"
    echo "n=$n" 
    echo "p=$p" 
    echo "q=$q" 
    echo "e=$e"
    echo "d=$d"
    echo "c=$c"
    echo "exp1=$exp1"
    echo "exp2=$exp2"
    echo "scale=0"
    echo "define pmod(b,e,m) { if(e == 0 ) return 1; if(e == 1) return b%m; rest=pmod(b^2%m,e/2,m); if((e%2) == 1) return (b*rest)%m else return rest;  }"
    echo "define encrypt(message) { return pmod(message, e, n); }"
    echo "define decrypt(message) { return pmod(message, d, n); }"
    echo "define verify() { return n == p*q && (d*e)%((p-1)*(q-1)) == 1 && exp1 == d % (p-1) && exp2 == d % (q-1) && (c*q)%p == 1; }"
    echo "# End bc"
) <<< "$decbytes"