Introduction to
Encryption Technology
Mort Anvari
Encryption
What is it?
- The conversion of data into a form that cannot
be easily understood by unauthorized people
- The opposite is decryption, that is, changing
the difficult to read form back into the original
- It’s usually software, but can be hardware
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Cryptography
- What does it do for me?
- Symmetric Encryption
- Asymmetric Encryption
- One Way Hash
- Example: An Encrypted E-Mail
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What does it do for me?
Confidentiality: The data can only be read
by the intended recipients
Non-Repudiation: The data cannot be
forged. If data is “signed” by a person,
the data could only have come from
them. No more “I didn’t send that!”
Data Integrity: The data cannot be
modified without detection
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Symmetric Encryption
- Data is encrypted and decrypted
with the same key
- Fast
- Key must be kept secret
- Key must be sent Out of Band
- DES and IDEA are symmetric
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Asymmetric Encryption
- Uses 2 keys
- Data encrypted with one key can
only be decrypted with the other
- Public key is shared with all
- Public key can be sent In Band
- Private key must be kept secret
- RSA is asymmetric
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One Way Hash
- A “fingerprint” of data
- Any size data = same size hash
- Tiny changes in data produce
a very different hash
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Example: Encrypted E-Mail
Anne wants to send e-mail to Bob. The
plaintext message compressed to make
it smaller and the ciphertext stronger.
Plaintext
Compression
Small
Plaintext
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Example: Encrypted E-Mail
The plaintext message is run through a
hash algorithm to generate a
“fingerprint”.
Small
Plaintext
Hash Function
Fingerprint
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Example: Encrypted E-Mail
The fingerprint is encrypted using
Anne’s private key. This makes it into a
digital signature. It is then appended to
the plaintext.
Fingerprint
Anne’s Private
Key
Small
Plaintext
Signature
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Example: Encrypted E-Mail
A Random key is generated and the email is symmetrically encrypted using
that.
Small
Plaintext
Random Key
Ciphertext
Encrypted
with
Random
Key
Signature
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Example: Encrypted E-Mail
The Random Key is Encrypted using
Bob’s public key. The result is called a
“Strong Box”. Remember that only Bob
can read the contents of the Box.
Random Key
Bob’s Public
Key
A Box for Bob
Random Key
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Example: Encrypted E-Mail
The Box is attached to the ciphertext
and they are sent over e-mail to Bob.
A Box for Bob
Random Key
Ciphertext
Encrypted
with
Random
Key
To Bob
Internet
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Example: Encrypted E-Mail
Bob decrypts his Strong Box to get the
Random Key. Only Bob’s private key
can open the Box which was encrypted
with his public key.
A Box for Bob
Random Key
Bob’s Private
Key
Random Key
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Example: Encrypted E-Mail
Bob decrypts the ciphertext using the
random key which he got from his
Strong Box.
Ciphertext
Encrypted
with
Random
Key
Small
Random Key
Plaintext
Signature
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Example: Encrypted E-Mail
Bob decrypts Anne’s signature using
her public key. Since only Anne could
have encrypted it with her private key,
Bob knows the message had to come
from her.
Signature
Anne’s Public
Key
Fingerprint
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Example: Encrypted E-Mail
Bob runs the unencrypted message
through the hash function. If this
fingerprint is the same as the one from
the signature, the message was not
changed in transit.
Calculated
Fingerprint
Small
Plaintext
Hash Function
 or 
Received
Fingerprint
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Example: Encrypted E-Mail
Finally, the message is uncompressed.
Bob can read the message knowing for
certain that it’s from Anne, it’s what
Anne wrote and only the two of them
could have read it.
Plaintext
Compression
Small
Plaintext
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