Introduction to Cryptography

Review of Hashing

Using one or all of the tools/command, get the hash of juan.txt

  • md5deep64
  • certutil -hashfile
  • Get-FileHash
  • md5sum

Cryptography

Cryptography is the use of specialized codes and algorithms to secure information, making it unreadable to unauthorized individuals.

In law enforcement, it serves as a dual-edged sword: it’s used to protect sensitive data and evidence, but it can also be used by criminals to obfuscate or encrypt their communications and digital evidence, creating a major challenge for investigations.

It is commonly used for secure communications to prevent eavesdroppers from listening, and/or reading sensitive information.

Keywords

secure, encrypted, obfuscated, authorized

Digital Signature

A digital signature is a cryptographic “fingerprint” used to verify the authenticity and integrity of digital documents.

In law enforcement, it serves as a legally admissible method to prove that an electronic document, like an arrest warrant or evidence log, has not been tampered with and originated from a specific, verified source.

Keywords

verify, verified, authentic, authentication, integrity, untampered, non-repudiation

Cryptography is the use of specialized codes and algorithms to secure information, making it unreadable to unauthorized individuals.

The process of communication in the presence of adversaries.

Types of Cryptography

1. Symmetric-key

Uses a single, shared key for encryption and decryption.

Example - Caesar Cipher

https://cryptii.com/pipes/caesar-cipher

2. Asymmetric-key

Uses a pair of mathematically related keys - a Public Key for encryption and a Private Key for decryption.

Example 1 - Certificate Authorities

PNPKI - https://dict.gov.ph/pnpki

Example 2 - Decentralized PKI

DIDs (The Standard) - https://www.w3.org/TR/did-1.0/

Example 3 - Web of Trust

PGP, OpenPGP, GnuPG

https://www.progress.com/blogs/the-difference-between-pgp-openpgp-and-gnupg-encryption

GPG - Web of Trust

Tool Installation

Install GPG4Win using the Windows Install link above.

Generate your KeyPair

Open Terminal on Windows, and go to C:\cert-ph directory.

gpg --full-generate-key
  • Type or select 9 for ECC (sign and encrypt) functionality, & press enter
  • Type or select 1 for Curve 25519 ECC algorith, & press enter
  • Type 1y for 1 year key duration/expiry, & press enter

Identify your KeyID

gpg --list-keys --keyid-format long

[keyboxd] ———

pub ed25519/C317FBCB5B193D17 2025-09-26 [SC] [expires: 2026-09-26] F9603BC8A86CC5EE8CBBD2FBC317FBCB4B170D17 uid [ultimate] Juan de la Cruz jdelacruz@example.net sub cv25519/CE719CFDB2DD0736 2025-09-26 [E] [expires: 2026-09-26]

Extract your Public Key

gpg --armor --export {KeyID}

—–BEGIN PGP PUBLIC KEY BLOCK—–
mENFaNadhBYJKwYBBAHaRw8BAQdA0NQHsKJBpXrDy6DC2n01LLOi6R1m3vo4QKTI oKLiI4m0KlJheW1vbmQgT2xhdmlkZXMgPGFyZGllb2xhdmlkZXNAZ21haWwuY29t PoiZBBMWCgBBFiEE+WA82Khsxe6Mu9L7wxf7y1sXDRcFAmjWnYQCGwMFCQHhM4AF CwkIBwICIgIGFQoJCAsCBBYCAwECHgcCF4AACgkQwxf7y1sXDRd2jAEAg6H4JKwh ZA+l8YyMXmCveNO6QWsTWFK63kHi+rj31q8BAPENOSGUcMugobzmEkeqogItrgjY hOfQmKx5XASwfIgFuDgEaNadhBIKKwYBBAGXVQEFAQEHQL88J9B5Bf9XrZYAivia 1Ek9gO6Tn7JgYkSB70/tqDB7AwEIB4h+BBgWCgAmFiEE+WA82Khsxe6Mu9L7wxf7 y1sXDRcFAmjWnYQCGwwFCQHhM4AACgkQwxf7y1sXDRezpgD/TI98JZMGtZeklb2U Wew7J8trhkKHXrPZvkHgXcZ77skBAOaxnPflXm8Pf4tpcT7Nh4Fi1G9G5fmKYk08 u2Bj9z8P =6GHj —–END PGP PUBLIC KEY BLOCK—–

Extract your Public Key to a File

gpg --armor --export {KeyID} > name.asc

Activity

Activity 1

ESTABLISHING YOUR WEB: Personal Emails

  1. Group yourselves into 4, trusted buddies
  2. Share to each other your personal emails
  3. Email to one another your Public Key, or your .asc file.

LOCAL WEB OF TRUST: Your Trusted Keys

  1. Download the Public Key shared to you.
  2. Import to your Web of Trust

     
    gpg -i jdelacruz.asc

CHECK TRUSTED KEYS: Verify

  1. Verify that the trusted key has been imported correctly.

    gpg --list-keys --keyid-format long

Activity 2

ENCRYPTING MESSAGES: Messages & Trusted Recipient

  1. Write down any sample message (non-sensitive) to a text file
  2. Encrypt the text file., and set intended recipient to you and 2 of your groupmate.

    gpg -sea -r {KeyID} -r {Recipient_KeyID} file.ext
  3. The command above will create a file file.ext.asc
  4. Email the file file.ext.asc to everyone on your group.

Activity 3

DECRYPTING MESSAGES: Only Trusted Can Decrypt

  1. Once you receive an email with encrypted attachment, download the attachment, and decrypt.

    gpg -d file.ext.asc

ENCRYPTING LARGE FILES

  1. Encrypt the file. With a detached signature; -b option

    gpg -bes -r {KeyID} -r {Recipient_KeyID} large_file.ext
  2. The command above will create a file large_file.ext.asc, this is the signature_file
  3. If uploading on a hosting service, be sure to upload the large_file.ext.asc for recipient to validate the decryption

Note: large_file should be the filename of your file. and ext is the extension of your file.

DECRYPTING FILES

  1. Verify & Decrypt

    gpg --version large_file.ext.asc large_file.ext
Info

The above commands will allow one to establish Confidentiality and Integrity of a file or data. The Availability of the public keys will discussed in a separated material.

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