To generate ssh key pair whose public key BASE64 encoding matches a certain substring

generate_ssh_key.sh

#!/bin/bash
# This script offers a fun way to generate an SSH key whose public key's BASE64
# encoding would end in a certain substring you specify. The script simply
# keeps trying until to find one. Because the probability distribution is
# geometric, so in practice, it is not "that bad" if you just want to match to
# last two characters.

# Ensure script is run from ~/.ssh
if [[ "$(pwd)" != "$HOME/.ssh" ]]; then
    echo "Error: This script must be run from ~/.ssh"
    exit 1
fi

# Check if username and machine name are both provided
if [ "$#" -ne 2 ]; then
    echo "Usage: $0 <user-name> <machine-name>"
    exit 1
fi

USERNAME="$1"  # Can be anything, no need to match the actual username
HOSTNAME="$2"  # Can be anything, no need to match the actual hostname
TARGET="IU"  # Ending substring that you try to match in the public key
TARGET_NCHARS=${#TARGET}

echo "Generating ed25519 keys until the public key ends in '$TARGET'..."

MAX_TRIALS=20000  # Upper limit to prevent infinite loop; adjust accordingly
COUNT=0
while [ $COUNT -lt $MAX_TRIALS ]; do
    # Progress bar
    if (( COUNT % 10 == 0 )); then
        printf "\rAttempts: %d" "$COUNT"
    fi

    # Remove any current keys of the same names
    rm -f ${USERNAME}-${HOSTNAME}-ed25519*

    # Create SSH key candidate quietly
    ssh-keygen -q -t ed25519 -C "${USERNAME}@${HOSTNAME}" -f "${USERNAME}-${HOSTNAME}-ed25519" -N ""

    # Extract last two characters of the base64 public key
    BASE64_KEY=$(awk '{print $2}' "${USERNAME}-${HOSTNAME}-ed25519.pub")
    LAST_TWO=$(echo -n "$BASE64_KEY" | tail -c 2)
    if [[ "$LAST_TWO" == "$TARGET" ]]; then
        echo ""
        echo "Success! Found a matching key after $COUNT attempts."
        exit 0
    fi

    ((COUNT++))
done

echo ""
echo "Failed to generate a key with '$TARGET' in $MAX_TRIALS attempts."
echo "Just try again. Thanks."
exit 1