## January 29, 2022

### Yono SBI - Scan QR and contactless ATM cash withdrawal

Hi folks,

I had seen something, I want to do and I have done PoC sometime back. Which is contactless ATM cash withdrawal from SBI Yono Mobile app.

### Procedure

1. Visit the SBI ATM which supports QR Scan option on KIOSK ATM screen
2. Select the QR based Contactless Withdrawal option on the touch screen
3. Open the Yono Mobile app which is already registered with your SBI account and have enough funds to withdraw
4. On mobile app, select "QR Cash Withdrawal" option
5. Scan the ATM displayed QR from the Yono mobile app
6. Select the amount to withdraw on ATM machine
7. Enter the PIN or OTP when prompted
8. Money from ATM will be dispensed
9. This is almost contactless except, you need to touch the ATM screen twice to select the QR option and to select amount.

This way, we can have less touch points to stay safe and secure way to withdraw your money.

Hope you will use this option to withdraw money from an SBI ATM.

### Calculate the private key of RSA algorithm

Hi folks,

Today we are going to see how we can calculate the private key of RSA.

#### Inputs

Random prime numbers p and q

Public key (n, e)

#### Procedure

1. Compute n (Random modulus)

n = p * q

2. Compute e (Derived Number)

e = (p - 1) * (q - 1)

3. Form the public key

GCD(pubkey, e) should be 1

pubkey = 1 mod e

4. Private key

prikey = (1 + k * e) / pubkey

0 < k < e - Iterate until we get a number without fraction

### Real example

#### Inputs

p = 7, q = 17, pubkey = 11

1. n = 7 * 17 = 119

2. e = (7-1) * (17-1) = 96

3. pubkey = 11

GCD (11, 96) = 1

4. prikey computation

k=0, (1 + 0 * 96) / 11 = 0.09

k=1, (1 + 1 * 96) / 11 = 8.81

k=2, (1 + 2 * 96) / 11 = 17.5

k=3, (1 + 3 * 96) / 11 = 26.27

k=4, (1 + 4 * 96) / 11 = 35

prikey = 35

Hoping that this blog will be useful in understanding the RSA private key computation.