User's Manual

SEL-3022 Transceiver Instruction Manual Date Code 20050615
Wireless Operator Interface Security
The SEL Security Application
C.10
Cryptographic Manual—Do Not Copy
To produce a cryptographically secure signature of a message, NIST designed the
SHA-1 hash function to have the following properties:
➤ Given the SHA-1 hash function, H(m), and its output, h, it is
extremely difficult to derive a message, m, such that H(m) = h.
➤ Given a message, m, it is extremely difficult to find another message,
m', that produces the same SHA-1 hash output.
The first condition states that the output of the SHA-1 hash function used in the HMAC
authentication function does not give away any clues about the form, or classes, of
messages that would likely produce the same hash value. The second condition, known
as collision-resistance, states that there is no bias in the mapping of inputs to outputs
that would aid an attacker in finding messages that produce identical SHA-1 hash
values. Both conditions make it functionally impossible (given all realistic resources)
to alter a message in such a way as to produce the same hash value. The HMAC
specification provides a cryptographically secure way to combine the secret
authentication key and the protected message into the SHA-1 hash function input to
produce a key-dependent message fingerprint.
AES Overview
The AES encryption function uses a 128-bit-long secret key and scrambles the contents
of each frame prior to transmission to provide cryptographically strong data
confidentiality.
Encryption is the process of transforming a digital message from its original form into
a form that an unauthorized individual cannot interpret. The output of the encryption
process is a function of the message and an encryption key (see Figure C.3).
Figure C.3 Operation of the AES Encryption Function
This encryption process must be completely reversible by an authorized individual
with access to the secret decryption key. Authority to read a message is only granted by
sharing knowledge of the secret decryption key. Ideally, only individuals with
knowledge of the decryption key can reverse the encryption operation and interpret the
protected message. There are two main classes of encryption functions. Symmetric key
encryption relies on the same secret key value, K, to perform both the encryption and
decryption transformations. Asymmetric key encryption, on the other hand, uses a
different key for encryption and decryption. For example, asymmetric encryption
Encryption Key Decryption Key
Hi Bob!
*4gu>g
aIL8}2=h
AES
Encrypt
Hi Bob!
AES
Decrypt
Preliminary Copy